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Almost Everything You Ever
Wanted to Know About CJ-3B Speed,
And Then Some

Oldtime by Ken "Oldtime" Bushdiecker
 

How fast can a CJ-3B go? How fast should it go? Oldtime, moderator of the CJ-3B Bulletin Board, goes back to basic concepts and principles here, and considers the specifications of the Hurricane engine. He then follows the force it produces, through the entire drivetrain of the CJ-3B, to determine the theoretical and actual speeds possible. It's well worth reading if you're considering drivetrain modifications, or if you just want to know what's happening when you step on the gas. -- Derek Redmond

Table of Contents

1 Speed
2 Universal Speed
3 Jeep Universal Speed
4 Universal Jeep Speed
5 CJ-3B Speed

6 The Force
7 Torque
8 Horsepower
9 The Crankshaft Revolution

10 Dynamometer Reference
11 Minimum Crankshaft Velocity
12 Maximum Crankshaft Velocity
13 Sustained Crankshaft Velocity
14 Flywheel Inertia
15 Clutch Pressure Engagement

16 Transmission RPM
17 Overdrive RPM
18 Transfer Case RPM
19 Propeller RPM
20 Axle RPM

21 Re-Inventing the Wheel
22 Tire Revolutions per Mile
23 Roll Resistance
24 Aerodynamic Resistance
25 The Jeep Speed Formula

26 CJ-3B Maximum Potential Speed
27 CJ-3B Sustained Speed
28 CJ-3B Minimum Speed
29 Gear Range
30 Center Of Gravity

The first five entries in this discussion of speed are intentional in nature.
They are not entered to delve off topic, but rather to show the importance of correct terminology and word usage, also detail. I suggest you read this    very    slowly    and attentively.

1 "SPEED"

Slow down! You are already reading too fast.
Look back above and notice the word "speed".
What do you see ?
Notice "speed" is not associated with any other word.
Therefore "speed" is relative only to your perception.
By perception I mean all sensory inputs.
Throughout this article we will mainly concern ourselves with visual reference.
Later I may use tactile and even audible reference.

Just what is speed ?
As a scientific definition: Speed is simply stated as a straight line distance divided by time.
Why a straight line distance ?
Because a curved line distance divided by time is referred to as velocity.
So perception of speed entails distance. How then do we comprehend a distance ?
In our world every object is describable by the space it occupies.
The three primary space dimensions are height, width and depth.
This allows for object description by way of geometry.
Physical geometry may be graphed as a mechanical drawing; an orthographic projection.
Draftsmen know that to produce an orthographic projection onto a blank sheet of paper one must use a reference.
Our initial reference, a single "point" is a dimensionless object having no property except location.
Besides location a point can designate time. Hence the saying; "a point in time".

It is interesting to note that any point whether it be a physical location or a time reference cannot be further identified without one or more additional points for cross reference.
In other words a single point in time absolutely must be compared with another point in time to be a relative consideration.
A single location cannot designate measurable distance.
Any two separated points allows an observable linear dimension.
Therefore SPEED is only notable by way of 2 or more reference points within 1 or more of the 3 primary dimensions.

Similarly the advanced concept of ACCELERATION which was first studied by Galileo Galilei is not definable without 3 or more reference points.

Galileo acceleration experiment

Also VELOCITY must include 3 reference points found in 2 or more dimensions.
 

Now what would someone observe if a single point location was to move ?
From its most basic motion that single point location is now seen as a straight line.
We find that the straight line has a measurable distance as compared to a rule.
Lets now look at this straight line from different perspectives (observational positions).
From two different perspectives we can again see a point.
The single point has been divided into two points identified as a beginning and as an end.
The beginning and the end are understood as two separate points in time.
We understand that a duration exists in between those "points in time".
We compare the elapsed time duration from beginning time observation to ending time observation with a standard time reference. A clock.
But technically time is defined as an unlimited, indefinite duration in which all things are considered as occurring.
So in our attempt to explain time we have standardized our concept into commonly observed segments.

Time occurrences are generally subdivided into past and future with the so called present tense existing only as the immediate and ever fleeting viewpoint.
This alludes to the typical concept of time as being sequential and directional.
Our example here is usage of the phrase "River of Time".
Time visualization or duration of observation is potentially effected by visual frequency reception.
Visual frequency reception is a bio-mechanical reference to the working of the human eye.
This entails optic nerve comprehension encompassing such features as rod and cone functions including the inherent frequency and duration of vision lapse intervals.
In other words the vision has an interrupted flow of continuity.
Therefore I say a definite time sequence will be visually impossible to prove.

Galileo acceleration experiment
Go observe a clock pendulum. One will observe the vision as bi-idirectional.
Yet still a directional time sequence is followed because in common practice we universally accept time as being uni-directional.
 
 
 
 
 
 
 

Anyway to cut this discussion off short, we have infused the static dimensional universe of height, width and depth into another dimension encompassing all, the 4th dimension of time.
This 3 dimensional universe thereby becomes dynamic rather than static; a fluid geometry.
Time lapse occurs and duration is observed when one compares any stationary reference point with a non stationary point.
We call the visual observation of sequenced time a motion.
So the key concept of motion is the observation between a mobile and a stationary reference.
This observation of motion as was detailed by Issac Newton is a basis upon which our mechanical concepts of the physical universe are based.
Speed then is the measure of straight line motion.
It is straight line distance divided by time.

In this article we will refer to our point to point distance as miles and time duration is segmented into increments called hours.
Speed will be expressed by the following standard: miles per hour (mph).
Gee we already knew this stuff, so what's next ?

"UNIVERSAL SPEED"

Universal speed is the word speed relative to that which is universal.
Universal speed then is the idea of a motion constant.
I previously stated that the key concept of motion is the observation between a mobile and a stationary reference.
I don't believe science can ultimately prove nor disprove any theory as to what if anything is actually moving. Why not?
Because... In order to do so science would have to define the central point of reference for the universe itself. The constant.
Next, that point must then be determined as either stationary or the only true point in absolute motion relative to every other point within a static universe.
Imagine it this way; every point in the universe moving in relation to a single stationary point.
Or imagine the opposite, a single point moving within and through a completely stationary field.
Of course this motion must also be in a straight line since we are addressing speed and not velocity.
Now let's get the Jeep involved into this discussion.

3 "JEEP UNIVERSAL SPEED"

The name and instruction plate located upon the dash of early CJ Jeeps explains this concept well.
Jeeps relative to universal speed, which is the motion constant.

Please now view and study this this plate.

Home

First you are warned "CAUTION" on the name and instruction plate.
The plate also indicates a "MAXIMUM PERMISSIBLE ROAD SPEED" of 60 mph with the transmission in high gear and transfer case in high range.
In my opinion, that information is potentially a very serious oversight by Willys Overland.
I ask you, why should we concern ourselves with road speed ?
If the earth is spinning at a certain velocity relative to another celestial body then it seems to me that Jeep gearing has little of nothing to do with the speed of any road upon this earth.
I say; In truth both the road and the vehicle are potentially in motion.
We need not be discussing the Jeep relative to the motions of the universe.
If we continue to envision with the universal outlook we may find that we never get anywhere. I mean that quit literally.
So lets rearrange our thinking with the same three concepts.

4 "UNIVERSAL JEEP SPEED"

Under this heading the word "Jeep" is positioned between "universal" and "speed".
Specifically the Jeep is intervening between our universal knowledge and our speed perception which we call motion thereby probing all relative factors.
We are now observing universal speed by means of Jeep study.
I say then that we shall concern ourselves with Jeep motion that is relative to universal or common belief.
That is our multipurpose vehicle typically compared to a roadbed that is universally or commonly perceived as stationary.
Speed then becomes the Jeeps location points on that road relative to the elapsed time between those observations.
I believe our heads will stop spinning now as I put the universal perspective to rest.

5 "CJ-3B SPEED"

CJ-3B This is simply the concept of speed specific to the model CJ-3B, our main focus.
The CJ-3B is our topic constant, yet its speed fluctuates as a motion.
Take a look back up to this subtopic heading directly above.
Now focus your attention toward the blank area between the word " CJ-3B " and the word " SPEED ".
What do you see within that void ?
I suggest that you do not inrject the word "restoration" into that void or we will have to redefine speed as cubic inches per year!
In time I will reveal some mysteries within that void.

Yes I am quite aware that I have left you within "the void".
I have left you devoid of specific direction.
I have given you no certain point to focus upon, only the experience of duration.

We have already covered the "And Then Some" concepts contained within the given title.
From here onward I will attempt to cover...

"Almost Everything You Ever Wanted To Know About CJ-3B Speed"

Among all that does exist within the void, I see two concepts. Resistance and Force.
Resistance opposes motion. Force compels motion. And so here I continue...

6 "THE FORCE"

In my estimation energy lies within all matter.
That is all matter contains latent or potential energy.
When seen as motion it becomes kinetic energy which is dynamic.
This power to motivate is properly identified as a force.
The force is within the fuel and is harnessed to motivate the Jeep. Why ?
Because the Jeep is commonly perceived as stationary or without force.
As you are aware the Jeep is composed of individual components and assemblies.
The engine assembly has no speed separate or different than that of our topic constant, the complete assembly, the CJ-3B Jeep.
So I will not be not be referring to engine speed as it is no more important than... well, lets say frame speed.

Mitsubishi Hurricane Likewise we are not truly concerned with the engine's power. That is its latent energy.
Instead we consider the force that our F-134 Hurricane engine harnesses from the fuel.
So fuel, (matter) enters into the engine.
That matter exits from the engine in a changed form.
Not disappearing but only re-manifest into another form of matter, the gaseous exhaust.
The engine does not create nor extinguish the matter.
The engine only functions to harness the motive force of fuel combustion.

In the end the so called "engine power" will be visually perceived as rotational output.
The force for propulsion is really a study of kinetics. It is also a matter of fuel combustion.
It is directly relative to the velocity and duration of the exploded gaseous expansion.
The engine's harnessed force output will be re-identified for the purpose of comparative measurements.
I will utilize the standard terms of Torque and Horsepower to discuss the engine's harnessed power.

7 "TORQUE"

Torque is a measure of force applied.
It is a static measure, a pressure exerted.
It is unlike hp in that it does not measure in respect to time.
To better understand the concepts involved here we will take a look at the human body.
Example: A man lies back upon a bench press.
A spotter lowers a 300 pound weight onto his outstretched arms.
That's all he can hold, his maximum force was applied.
Next we see that through repetition the same man can lift 100 pounds 12 times per minute. That's 1200 lbs per minute.
Torque output is akin to maximum amount held.
Horsepower is akin to the maximum total lifted per minute.

An engine's torque output is a factor of many engineered complexities.
Mainly it is a measure dependent on cubic inches of displacement.
The engine's torque output is measured and noted upon a dynomometer chart.
I believe for the internal combustion engine these tests must be determined under consistent conditions; such as "at mean sea level" because of atmospheric pressure affecting fuel combustion.
Testing at consistent temperatures must also be done and so forth.
Next we will consider this force which has been measured as torque in relation to time.

8 "HORSEPOWER"

Horsepower is a description of work performance.
One horsepower (hp) has been standardized to a rate of 33000 foot pounds per minute.
There are mainly three differing methods of describing hp:

Indicated hp is the theoretical efficiency of a reciprocating engine which is determined by
the pressure developed within the cylinders of the engine.

Brake hp is more commonly used to indicate the practical ability of an engine or the maximum performance
minus the power loss through heat, friction, and compression. This is often called SAE net hp.
American automobiles are generally rated in brake hp, as is our Jeep.

Rated hp is the hp an engine can produce through sustained periods of time.
This seems to be an undefined duration of time therefore it should be considered relative.

There is no actual test procedure used to determine the engine's hp output.
The actual work performed by an engine is never measured.
The hp rating for any and all engines is simply a calculation based upon measured torque.

Hp = torque x rpm divided by 5252
Below 5252 rpm an engine's torque rating is higher than its horsepower and
above 5252 rpm an engine's horsepower rating is higher than its torque.
At 5252 rpm the horsepower and torque ratings will be exactly the same.

So just how much hp does a CJ-3B need ?
Regardless of your specific objective, if you have enough force to break tire traction then hp is ample.
That said it is now obvious that needed hp is directly relative to available traction.
To make hp a valid consideration we assume traction is sufficient under all conditions.
It is up to an individual to determine there own preference for any specific rate of acceleration because hp overcomes the resistances to motion.
For now let's take a closer look at the motion.

9 "THE CRANKSHAFT REVOLUTION"

The crankshaft revolution is simply a complete engine crankshaft rotation.
It is a visual and a tactile output.
Rotation is a movement around a specific reference point or physical axis.
A revolution is a complete rotation; a progressive motion of a body around an axis so that any point of the body parallel to the axis returns to its initial position while remaining parallel to the axis in transit and usually at a constant non elliptical distance from it.
In order to assist us in our conception of engine hp we will be referencing the observable crankshaft velocity stated as Revolutions Per Minute (rpm).
As previously noted I will not refer to this observation as engine speed.
This visual rotational motion of the F-134 Hurricane engine crankshaft has already been measured as the applied force called torque.
We are next prepared to detail the charted measure.

Please study the following chart from F4-134 Engine Horsepower and Torque on CJ3B.info:

Dynamometer chart

 
Welcome back again. I trust you have had ample opportunity to study the Willys Engineering dynamometer chart for the F-134 Hurricane engine, and enough time to form a solid opinion as to what you observed. Now here is what I see:

10 "DYNAMOMETER REFERENCE"

Willys Engineering Dynamometer charts are a graph of the actual torque test results. The calculated hp is also placed onto this same chart.
This chart shows both net torque and gross torque.
There is not much practical use for the gross torque figures. Why not?
Because the gross torque figures are for a stripped engine with only the bare essentials of carburetor, oil pump and water pump.
The net torque figure shows needed engine additions including a fuel pump, the complete exhaust system, and a generator.
Upon inspection of the F-134 dynamometer chart one can readily see that the torque output is a fairly flat line.
The chart reveals that a maximum net torque of 108 ft lbs is achieved.
According to this chart that occurs when the crankshaft is revolving at about 2200 rpm.
Therefore 2200 rpm tells us when maximum torque is made available.
It indicates that good power is available at all velocities but in particular at around 2200 rpm.
So it is safe to say that 2200 rpm is the most efficient crankshaft velocity produced by this engine.
Also our chart reveals that as the crankshaft velocity increases, hp increases.
So the total volume of work produced increases as the crankshaft motion increases up to its maximum permissible velocity.


11 "MINIMUM CRANKSHAFT VELOCITY"

Minimum F-134 crankshaft velocity is stated as 600 rpm.
Some major factors affecting minimum velocity are fuel quality which includes, density (fuel charge), caloric value, rate of combustion,
and pressure also piston stroke of the engine and compression ratio, valve timing, valve area, valve lift, valve duration and flywheel inertia.
This is a study of both dynamic kinetics and mechanical engineering.
For the F-134 neither maximum torque nor maximum horsepower are reached at minimum velocity.
Minimum crankshaft velocity is specifically useful to increase engine longevity via motion reduction and is also useful for minimum vehicle travel speed.

12 "MAXIMUM CRANKSHAFT VELOCITY"

Maximum F-134 crankshaft velocity is stated as 4000 rpm.
It is affected by all factors already mentioned for minimum velocity operation. Plus it is also affected by ignition advance.
Fuel volume is unchangable. It is predetermined by cylinder volume.
Increased fuel density is what increases the crankshaft revolution.
The valve area and valve lift becomes more critical as more gaseous intake and exhaust is needed to handle increased gas densities and pressures.
Maximum torque is not available for the F-134 at 4000 rpm, but maximum hp is available at 4000 rpm.
At 4000 rpm crankshaft velocity stalls due to it's inherent mechanical limits.
Maximum crankshaft velocity is specifically useful for maximum potential work output and for maximum potential vehicle speed.
Some modifications can be made to this engine to increase velocity beyond the 4000 rpm rating but that is not our objective at this time.

13 "SUSTAINED CRANKSHAFT VELOCITY"

Barney Roos Barney Roos was the chief developing engineer for the concept Light Reconnaissance Vehicle (LRV) produced for the U.S. military.
He tested and retested the Willys Quad in an attempt to meet or exceed the original military specifications for the LRV.

There were no specific engine stipulations set by the army.
Being the chief developing engineer at W/O, Barney set the original standard for the L-134 from which the F-134 is a direct progression.
Rumor has it that under his influence the prototype L-134 Go Devil was to withstand 100 hours of continuous operation at the maximum velocity without any mechanical failure.
Therefore this engine was rated to maintain "brake hp" for 100 continuous hours of operation at 4000 rpm.
Perhaps it should be called "break hp". Ha Ha.

Few if any of us expect or demand such hard operation.
So what is the maximum sustained crankshaft velocity for the F-134?
Remember there is no predetermined "rated hp" figure for the F-134. It's up to you.
It's your personal concept of rated hp. If we gain one thing we lose something else.
In this case we are attempting to determine the reasonable engine longevity, properly termed the engine service life.
We know that if we lessen the crankshaft velocity we extend engine service life.

From the chart we saw that peak torque is reached at about 2200 rpm.
Therefore this engine will have its highest "G" force available at 2200 rpm.
Don't believe me? Take it to 2200 rpm and then accelerate it.
The most efficient velocity concerning engine work performance is 2200 rpm.
There is no sense in operating the engine below its most efficient velocity for sustained work.

Yet this engine's maximum hp is not produced until maximum attainable velocity is reached at 4000 rpm.
From sheer experience alone I suggest that the absolute maximum sustained velocity for this engine be set at 3100 to 3200 rpm.
Why? Perhaps my given maximum velocity seems unscientific.
Because I simply hear and feel this as my interperatation of the audible and tactile engine vibrations.

Now again look at our dynamometer chart.
What I call the point of convergence is simply the point where the theoretical "net hp" curve crosses the "net torque" curve.
It is roughly halfway from 2200 rpm to 4000 rpm. It is seen at just under 3200 rpm.
By coincidence my recommended sustained minimum of 2200 rpm and the point of convergence / sustained maximum crankshaft velocity of 3200 rpm is exactly the same as the PTO work velocities indicated in the Willys Motors Service Standards: minimum P.T.O. velocity @ 2200 rpm, maximum P.T.O. velocity @ 3200 rpm.

The F-134 engine is capable of maintaining this restrained work load for an extended time duration far exceeding the mere 100 hour rating that was initially set by Barney.
Just how long is a vague question. In the end the sustained crankshaft velocity is your choice.

More work (hp) per usage yields shortened longevity.
My suggested minimum sustained F-134 velocity is 43 net hp or 47 gross hp @ 2200 rpm.
My suggested median sustained F-134 velocity is 52 net hp or 55 gross hp @ 2700 rpm.
My suggested maximum sustained F-134 velocity is 57 net hp or 62 gross hp @ 3200 rpm.

The median sustained velocity given above could be described as my personal "rated hp" figure.
This figure just happens to coincide with Rick Grover's highly useful Willys Speed Calculator suggestion of 2700 rpm.
So let's call that good. Remember the force has been harnessed from the fuel and is being observed as engine crankshaft velocity.
Where now does the force reside ?

14 "FLYWHEEL INERTIA"

Newton's first law of motion states that an object at rest tends to stay at rest, and an object in motion tends to stay in motion unless acted upon by an outside force.
Inertia is the property of matter that causes it to resist any change of its motion either in direction or velocity.
Persons in an accelerating vehicle feel the force by way of the seat pressing against their back while overcoming their inertia so as to increase their speed.
As the vehicle decelerates the occupants tend to continue in straight line motion and lunge foreward.
If the vehicle turns a corner a package on a seat will slide across the seat because the inertia of the package causes it to continue its motion in a straight line.
Any body spinning on an axis such as a flywheel exhibits rotational inertia, a resistance to the change of its rotational velocity.

Inertia is directly proportional to mass and weight. Increasing mass/weight or speed yields more inertia.
The force we observed as crankshaft velocity is now realized as flywheel inertia.
The flywheel is little more than mass/weight and also a clutch mounting surface.
So the first function is to stabilize rpm fluctuations by putting inertia to work.
This is an additional way torque output is stabilized at all crankshaft velocities.
The extra mass/weight tends to flatten the torque curve on the dynomometer chart.
As stated the flywheel's second function is as a component of the clutch engagement.

15 "CLUTCH PRESSURE ENGAGEMENT"

The L-134 Go Devil was upgraded slowly over time to produce the F-134 Hurricane.
The force harnessed within these subsequent engines became greater.
All force exiting the engine by way of the flywheel face is initially forwarded to the clutch.
As the engine power output increased over time, driven disk pressure was increased by means of increased pressure plate spring pressure.
Also the standard 8-1/2" clutch was eventually increased to an optional 9-1/4" diameter with its increased surface contact area in order to deal with greater force and resistance.

SB 509 See Willys Service Bulletin 509.

The clutch friction surfaces can and do slip if the rated torque capacity is exceeded.
On one end the force compels a forward motion to the Jeep.
The Jeep's negative inertia along with frictional resistance opposes the force.
The clutch specifically absorbs and equalizes between the engine's force output and the vehicles inertia plus its inherent frictional resistances.

The pressure plate is rated from 147 to 216 ft.lbs. torque depending on the unit installed.
This is up to twice as much torque pressure as the engine output net torque of 108 ft. lbs.
That is why the clutch normally does not slip once inertia has been equalized between the Jeep motions and the engines rotational velocity.
 

Next lets put this thing in gear!
We will see that the first resistance to the clutch engaged force is the friction within the transmission.
And so we begin to do some number crunching in order to put our harnessed force into perspective.

16 "TRANSMISSION RPM"

The motive force exits from the splines of the engaged clutch driven disc hub.
Next it enters directly into the transmission via the main drive gear.
Barring clutch slippage, the input velocity for the transmission is always the same as flywheel velocity.
For our model of Jeep there were two differing transmission input ratios in use.

SB 600 See Willys Service Bulletin 600.

Since slightly more hp was realized by the 134 engine's progression, a slightly faster vehicle speed became feasible.
So in 1963 the transmission low gear ratio was increased.
This transmission change was done to offset the simultaneous axle change from the 5.38 ratio to the 4.27 ratio.
With this combination of 3 changes including the, engine, transmission and axle, the maximum vehicle speed would be increased without adversely raising the minimum (crawl) speed.

In the transmission this change was manifest by changing the main drive tooth count from 18 teeth to 16 teeth, plus also changing the driven cluster gear tooth count from 33 teeth to 35 teeth.
Remember these particular driving and driven gears can only be interchanged as a set.
 

Here then we have two different T-90 variants that were used in the 3B.
Below are the gear ratios for both the pre-1963 (early) Borg Warner T-90 A and the post-1962 (late) T-90 C:

T-90 A
INPUT
T-90 C
INPUT
OUTPUT
1st gear 2.798 3.339 1.00
2nd gear 1.551 1.851 1.00
3rd gear 1.000 1.000 1.00
Reverse 3.798 4.531 1.00

As you can see from the numbers, the velocity of the transmission output drive will be affected by gear selection.
The transmission velocity is to be understood as a numerical ratio; input to output.
Low gear ratios reduce a vehicle's resistance to the force by decreasing traveled distance.
The minimum output for the early T-90 A in 1st gear is ( 600 divided by 2.798 =) 214 rpm.
The minimum output for the latter T-90 C in 1st gear is ( 600 divided by 3.339 =) 180 rpm.

High gear is always a straight through drive without gear reduction.
So the maximum output for both T-90's in 3rd gear is 4000 rpm.
As you can now see the T-90 C has a greater gear ratio spread.

The motive force now exits the transmission via mainshaft gear where it encounters its next resistance.

17 "OVERDRIVE RPM"

Overdrive As of September 1st 1964 the now famous "Warn Overdrive" (seen here installed in my 1964 CJ-3B) became approved as "JEEP Special Equipment".

The installed overdrive replaces the transmission mainshaft gear.
Overdrive input velocity is always identical to transmission output velocity.
Overdrive output velocity is identical to its input velocity when in straight drive.
The overdrive is geared so as to reduce input velocity or increase the output velocity by 25% when engaged.
Here it should be always be understood as a ratio of 3 to 4. written as .75 to 1.
So it is essentially another two speed transmission with an opposite effect.
This gear unit when engaged can increase the motive force resistance rather than to decrease as underdrive gearboxes do.

Being a separate two speed gear unit it has the capacity to double all available gear combinations.
This multiple gearing advantage allows one to better control crankshaft velocity.
In doing so it helps keep the low hp F-134 nearer to its optimum power range relative to the work load required.
Since it is not a standard item we should take a look at the advantages and also the disadvantages of its usage beyond the obvious cost and installation concerns.

First I will list the 10 benefits originally claimed by Warn for its overdrive:

1) Increases engine life.
2) Cuts rpm by 25%.
3) Maintains comfortable highway speeds.
4) Increases off road speeds too.
5) Increases gas mileage.
6) Matches power to job more efficiently.
7) Reduces vibration at freeway speeds.
8) Cuts transmission rpm's by 25%.
9) Doubles the gear selection.
10) No interference with speedometer reading.

In response I claim the overdrive:

1) Can increase engine service life only if the engine crankshaft velocity is reduced due to potential 25% increase of vehicle speed.
2) Cuts rpm by 25% relative to a constant speed.
3) Can increase the speed of the vehicle.
4) Can be used with the transfer case in low range.
5) Can notably decrease fuel consumption due to decrease of crankshaft velocity.
6) Optimizes the engines powerband relative to the work load.
7) Reduces input velocity vibrations and increases output velocity vibrations.
8) Potentially cuts transmission velocity by 25%.
9) Doubles all available gear ratios.
10) No interference with speedometer reading.

According to Willys Motors Inc. this is recommended equipment for 5.38 and 4.88 to 1 axle ratios only.
Why only the axles with those gear ratios?
I surmize that it was considered as an option to upgrade the lower 5.38 geared Jeeps to that of the 4.27 versions.
The available hp of the Hurricane engine is not considered sufficient to hande the increase of overdrive resistance in addition to the higher axle ratio.
So either increase your 3B's maximum speed with the 4.27 final drive or use the 5.38 final drive with an overdrive.
According to Willys Motors do not use both speed increasers.
Of these two choices my opinion is that the overdrive easily rates more efficient than the 4.27 axle alone.
This is because of the compound gearing advantage that allows one to better maintain optimum engine velocity for the work load.

One concern is the potential increase of output velocity that will lead to corresponding increase of overall vehicle speed.
We find that engaging the overdrive will either increase the velocity of all components on the output side of the unit proportional to its use,
or it will lower the velocity of the components on the input side of the unit, or both.
It is important to remember that when engaged, a ratio exists between input and output velocity: the previously mentioned 3 to 4 ratio.
The transfer case that was once running a maximum of 4000 rpm is now capable of running 5333 rpm.
Thus increasing wear on the accelerated moving parts or reducing wear on decelerated input components.
Minimum output velocity for the overdrive is 214 rpm for pre-1963 or 180 rpm for post-1962.
Installed on our Jeep, the maximum output for the overdrive unit is 5333 rpm.

18 "TRANSFER CASE RPM"

The input velocity of the transfer case is always the same as the transmission output, unless an optional overdrive gearbox is applied in between these two gear units.
As we continue to observe the motion within, we enter what I consider to be the "Heart of the Jeep".
This is the one component that has always distinguished the Jeep from the rest of automotive crowd.

Transfer case We now observe The Dana Spicer model 18.

Here we find that the speedometer drive gears are built into our transfer case (numbers 9 and 12 in the diagram.)
For the CJ-3B there were two differring gear sets available.
This is dependant upon whether the 3B was equipped with the 5.38 axle (early) or the 4.27 axle (late).
The early speedometer gear set was 4 tooth drive with 15 tooth driven.
The later speedometer gear set was 6 tooth drive with 18 tooth driven.

Changing the transfer case input velocity in any way even including an optional overdrive will have no (zero) effect upon the speedometer's accuracy.
If either the axle gearing or tire size is changed on the output end of the transfer case, that change will affect the accuracy of the speedometer reading.
The transfer case output is either a 1 to 1 ratio or a 2.46 to 1 rotation reduction.

Minimum transfer case output is 87 rpm for early 3B's and 73 rpm for later 3B's.
That's as slow as the output goes during engagement at idling crankshaft velocity.
Crawl speed may be lowered but maximum vehicle speed remains unaffected by the transfer case.
Maximum output is still 4000 rpm without overdrive or 5333 rpm with an overdrive engaged.
Next we will forward the force toward the axle.

19 "PROPELLER RPM"

The propeller velocity is always the same as the transfer case output and also the axle pinion input.
Its minimum spin is 87 rpm for early T-90 A or 73 rpm for later T-90 C.
Its maximum is 4000 rpm, still the same as engine velocity without an overdrive installation.
With the overdrive engaged propeller velocity can be as high as 5333 rpm.
With an the overdrive engaged we potentially get a 25% propeller longevity decrease.
Simply put all things being equal 25% final speed increase yields 25% longevity decrease for all moving components beyond the overdrive.
The force observed deep within the the motions of the propeller makes for a very interesting subtopic all in itself.
But in this article we are not scrutinizing every minute detail of component motions and motive force transfer.
Rather we are seeing a more general view of the whole Jeep.
So now the motive force exits the cardan cross bearing caps and enters into the yoke of the axle's input pinion.

20 "AXLE RPM"

The axle pinion input velocity is always reduced to slow the final drive velocity.
This is accomplished with the ring gear.
The exact ratio is ring gear tooth count (driven gear) divided by pinion tooth count (drive gear).
Since the output of engine hp increased from that of the early Go Devils, a small increase in overall vehicle speed was deemed feasible.
The axle reduction ratio was changed in order to attain greater vehicle speeds.

SB 601 See Willys Service Bulletin 601.

Pinion/Ring Ratio :
Pre-1963 (early) 8 teeth 43 teeth = 5.375 to 1
Post-1962 (late) 11 teeth 47 teeth = 4.2727 to 1

If we compare the difference in ratios between the early axle plus an optional overdrive and the late axle without an overdrive we see that the total effect is very similar.
As already stated the overdrive ratio is .75 to 1. 5.375 x .75 = 4.03
Therefore if equipped with an optional overdrive, the 5.38 Jeep has the equivalent of a 4.03 rear axle.
It is seen that the 4.27 axle allows a .79 to 1 speed increase over that of the early axle.

Remember that as good as this axle change was for higher top end speed it did not give one the ability to multiply all available gear combinations like the overdrive unit.
That ability to multiply gear combinations is commonly called gear splitting.

One may notice that the physical mass of the axle shaft is much greater than that of the propeller.
This is a direct relation between the constant of the applied force and the decreased velocity from the ring gear, which amplifies the constant force by increasing time.
The velocity slows; therefore mass must be increased to hold the constant force in check.

Under direct engagement with the engine, the minimum axle output velocity will be figured as minimum pinion (input) velocity divided by differential reduction.
It will be seen as 16 rpm for pre-1963 Jeeps, and as 17 rpm for post-1962 Jeeps.

Under direct engagement with the engine the maximum axle output velocity will be figured as maximim pinion (input) velocity divided by final drive reduction.
It will be seen as 734 rpm for a pre-1963 Jeep without overdrive, or noted as 992 rpm for a pre-1963 Jeep with optional overdrive.
The post-1962 Jeeps without overdrive will have a maximum velocity of 936 rpm.

21 "RE-INVENTING THE WHEEL"

Speed From the axle shafts the harnessed force is transferred into the wheel hub assemblies, then onward through the molecular structure of the wheel itself toward the rim.
The force contained within the mass of the solid axle shaft becomes spread over a greater area, therefore the wheel is noteably thinner at it's outer perimeter.

In the transfer of motivating force the tire receives velocity from the wheel.
It is known that sufficient torque can spin a tire around a rim.
Here we understand that a larger diameter wheel such as our 16" has a slight advantage over an equal wheel of smaller diameter because the surface contact area of the tire bead is greater.

The tire is at the cutting edge of the ever ongoing perfecting of the wheel.
And so the wheel rotates the tire.

22 "TIRE REVOLUTIONS PER MILE"

Barring rim slippage, the tire velocity is the same as our axle shaft velocity.
The main duty of the tire is to transform the rotation from our wheel into the straight line motion that we call speed.

The 6.00 x 16" tire has a diameter of about 28.5" and a tread width of about 5.3", and is the standard tire for most early Jeeps.
The 6.50 x 16" tire has a diameter of about 29.5" and a tread width of about 5.5"
The 7.00 x 16" tire has a diameter of about 30.5" and a tread width of about 5.8", and is the largest tire that will fit a non-modified Jeep.

If you multiply the tire diameter by pi you come up with the tire circumference:
For the 6.00 x 16" tire the circumference is approximately 89.5".
For the 6.50 x 16" tire the circumference is approximately 94.2".
For the 7.00 x 16" tire the circumference is approximately 95.8".

If traction is adequate the Jeep travels its tire circumference for each revolution.
At this very point we are now observing Jeep speed.

There happen to be 63360 inches in one mile.
A basic mathematical calculation shows that the standard tire with its circumference of 89.5 should be rotating about 708 times each mile.
However the Jeep Service Manual indicates that the standard 6.00 x 16" tire rotates 730 times per mile.

So what is this discrepancy between basic mathematical figures and the service manual?
Mainly the tire diameter ( height ) is reduced due to compression under load.
There are other factors one could include to figure an exact distance traveled per revolution, including tire air pressure and the effective tire radius due to centrifugal force at differing velocities.
But for practical purposes I will simply multiply our given tire circumference by an average of .97 to 1.

Note that in Rick Grover's Willys Speed Calculator there is no compensation made to designate this affect.
Therefore I suggest your visual measure of tire diameter needs some adjustments for figuring exact speed results.

23 "ROLL RESISTANCE"

Racing Both the tire diameter and its width will affect the Jeep's speed.
Wide tires and to a lesser degree tall tires both increase the surface contact area.
This surface contact area is often called a tire footprint.
Wide tires in particular yield a substantially greater surface contact area.
Basically greater surface contact area yields more traction.
Traction is the result of cohesion as the tire conforms to fit the terrain.
And of course "full traction" infers zero tire spin in relation to the terrain.

So the wide tire with its greater surface contact area substantially increases vehicle roll resistance because of this cohesion.
This inherent increase of roll resistance must be overcome with more hp and or less speed (travel time).
Surface contact area of large tires can be too much for the small F-134 engine with its meager hp.
This will manifest as inability to get the tires up to operating speed.

Also acceleration and braking can be diminished.
This is due to the effect of the increased tire mass and its respective inertia.
So wide tires generally only excel at slower speeds.
The exceptions will be matters of limited traction or flotation.

A tall tire has less roll resistance than a wide tire of equal footprint.
It simply overcomes various sized obstacles easier.
That is the main argument in favor of tall narrow tires.
It allows the meager 72 hp to do its job better on most terrain.

Overly tall tires will affect the vehicle's center of gravity and therefore affect your speed potential in sidling conditions.
Besides they just simply will not fit a non-modified Jeep.

The cohesive bond called traction is the cutting edge.
Here we are not particularly concerned with the specifics involved at the cutting edge.
That is a matter better left solely to the study of tire design.
For now we will simply consider we have full traction and sufficient hp to produce our straight line motion termed ... "SPEED".

24 "AERODYNAMIC RESISTANCE"

Now for the the final resistance to the motive force.
Aerodynamics is specifically the branch of aeromechanical science which deals with gaseous motions and the associated pressure forces.
Here we are specifically concerned with the effects of pre-existing and self-generated wind drag.
The measure of resistance to motion presented by the vehicles form is called wind drag.

An ideal body shape giving the least resistance to straight line motion (speed) would of course be a slim cylindrical shape with a slender pointed cone both fore and aft.
Why aft? Because displaced air assumes less wave turbulence when the air flows back in to fill the void.
The CJ-3B is far removed from the efficient shape of a double ended canoe.
Its complex and abruptly displaced air flow is easily noted at speeds above 40 mph.
To get some feel for such a poor drag co-efficient one can simply compare driving the Jeep at 40 mph with the windshield up and also with the windshield folded forward.

It is estimated that the actual hp required to sustain the Jeep at 70 mph will be about double what it takes to operate at 50 mph.
Driving into a pre-existing headwind easily compounds the problem of wind resistance.

25 "THE JEEP SPEED FORMULA"

Willys Motors Inc. used the following formula to calculate vehicle speed:

MPH = RPM ~ (TF X GR) .....
TF is tire factor. GR is gear ratio.
The tire factor given for the 6.00 x 16 is 12.17.
Please don't ask me how W/O came up with the tire factor.
If we do the math, the Willys formula indicates a maximum speed of 61.15 mph.

Now here's my formula.....

Crankshaft rpm ÷ selected transmission gear ratio ÷ overdrive ratio if applicable ÷ transfer case ratio
X the number of pinion gear teeth ÷ the number of ring gear teeth X the tire diameter X pi
X tire radius decrease due to load (.97 average) ÷ inches per mile X minutes per hour = speed in mph.

Long before I realized this formula I observed through real world experience.
So let me now relate this short story which exactly validates the formula as given here.

Speed Many years ago, one particular morning found me nearly late for work.
I rushed out the door in an effort to beat the time clock.
I jumped into my CJ-3B which was in prime condition.
I fired it up and proceeded to transport myself in record time.
On my way to the shop which was well out of town lay this long, straight and narrow blacktop road.
Upon getting to the near end of that little used blacktop I let the "3B" have full reign.
The accelerator treadle was fully opening the throttle valve.
The road being well over a mile long gave me time to let the engine reach full potential.
I heard the engines audible full scream yet gave it no mercy.
Just over 3/4 mile into the surge I looked down at the speedometer.
The speedometer needle read beyond the 70 mph figure shown on the instrument.
I estimated it to be running just over 80 mph.
That's all the Jeep had to give and I did not cool it down till I neared the turn at the end of that runway.
A couple more turns and I was soon at work.
I was relentless that day, but hey I beat the clock.
Now let me tell you: that Jeep was equipped with 6.00 x16" NDT's and a Warn overdrive.
So now lets put this Jeep Speed Formula to work.

Crankshaft shaft rpm (4000) divided by transmission gear in high (1)
divided by overdrive ratio (.75) divided by transfer case ratio (1) times pinion gear teeth (8) divided by ring gear teeth (43) times tire diameter (28.5) times pi (3.1416) times radius decrease due to load (.97) divided by inches per mile (63360) times minutes per hour (60) = speed in mph ( 81.6 )

This is far beyond the speed posted on the Operation and Data tag.

Home

So, 81.6 mph is the the maximum speed a CJ-3B can go. Or is it?

26 "CJ-3B MAXIMUM POTENTIAL SPEED"

The speed previously designated of 81.6 mph is the maximum obtainable for a standard CJ-3B with an approved "Jeep Special Equipment" Warn overdrive.
Next I will hypothetically increase the maximum speed of a CJ-3B.

We now enter into the realm of a simple to install and easily reversible modification, apart from the Jeep approved service standard.
Suppose we have a standard CJ-3B with the taller geared, 4.27 to 1 ratio axles.
Suppose we installed a Warn overdrive onto this Jeep.
Let's simply race the F-134 to 4000 rpm with the clutch engaged to the drive train.
Have the transmission in high gear which is a straight through drive.
Have the transfer case in high range in series with taller geared axles.
Run the taller stock 7.00 x 16" tires.
Then we engage an optional overdrive to attain more velocity.

Here's the results...
Crankshaft shaft rpm (4000) divided by transmission gear in high (1) divided by overdrive (.75) divided by transfer case ratio (1) times pinion gear teeth (11) divided by ring gear teeth (47) times tire diameter (30.5) times pi (3.1416) time radius decrease due to load (.97) divided by inches per mile (63360) times minutes per hour (60) = speed in mph (110).

Airborne That's right, with the help of gravitational force and/or a sufficient wind in our favor so that the meager 75 hp of motive force is adequate, this post-1962 model CJ-3B equipped with an overdrive, can travel up to 110 mph.

That much speed is just simply too crazy for the short wheelbase Jeeps!
I certainly do not want to end this article in some high speed crash, so now I will begin to slow the speed back down.
 

27 "CJ-3B SUSTAINED SPEED"

Here we are again in the realm of a "rated" calculation.
Below are three charts, all based on previously detailed crankshaft velocities.
The main variables are axle ratios and the engine velocity which affects its service life.
All three of the axle ratios shown in the charts will work well with the F-134 engine.
The 4.88 geared axles listed were available on very early military Jeeps or some late CJ-5 and CJ-6 Jeeps (1965-72.)


Speed @ 3200 rpm (maximum recommended rpm)

Axle Ratio........28.5" Tires........29.5" Tires........28.5" Tires........29.5" Tires
......................................................................with overdrive....with overdrive

5.38...................49......................51.....................65.....................68
4.88...................54......................56.....................72.....................75
4.27...................62......................64.....................82.....................85


Speed @ 2700 rpm (compromise crankshaft velocity)

Axle Ratio........28.5" Tires........29.5" Tires........28.5" Tires........29.5" Tires
......................................................................with overdrive....with overdrive

5.38..................41......................43.....................55.....................57
4.88..................45......................47.....................61.....................63
4.27..................52......................54.....................69.....................72


Speed @ 2200 rpm (maximum efficiency)

Axle Ratio........28.5" Tires........29.5" Tires........28.5" Tires........29.5" Tires
.......................................................................with overdrive....with overdrive

5.38...................34.....................35.....................45.....................46
4.88...................37.....................38.....................49.....................51
4.27...................42.....................44.....................56.....................58

I like to drive about 62 mph.
That speed is sufficient for me to keep pace in most conditions found locally.
I like the compromise crankshaft velocity of 2700 rpm.
Remember, the Hurricane is not a large cubic inch engine.
Therefore, maximum efficient crankshaft velocity may not produce adequate hp.
From this chart I see that using 4.88 ratio axles with overdrive should be near ideal.

Hey, this speed of 62 mph is for my CJ-3B, now go find your own.

28 "CJ-3B MINIMUM SPEED"

How slow can the Jeep go? Obviously minimum speed will only be available with the transmission in low gear and the transfer case in low range.
Overdrive use will not be an issue. But the shorter 6.00 x 16" standard tires will help.

A Jeep's lowest gear combination is commonly referred to as the "CRAWL RATIO".
It is determined by the transmission 1st gear ratio x the transfer case ratio x axle ratio.

A pre-1963 CJ-3B will be 2.798 x 2.46 x 5.375 = 37 to 1 reduction ratio / crawl ratio.
A post-1962 CJ-3B will be 3.339 x 2.46 x 4.27 = 35 to 1 reduction ratio / crawl ratio.

Those are the lowest ratio combinations for these standard Jeeps.
An increased reduction ratio (numerically higher) of course yields a slower crawl speed potential.

Now let's use our imagination. If one interchanges the standard CJ-3B components around, combining the later T-90 C transmission with the earlier 5.38 axles, another crawl ratio becomes available.

This custom geared CJ-3B will be 3.339 x 2.46 x 5.375 = 44 to 1 reduction ratio/crawl ratio.

This trans-vintage combination of stock CJ-3B components yields the slowest crawl ratio available for an otherwise original CJ-3B.
Now add that slow crawl ratio to a set of 6.00 x 16" tires and that's as slow as the CJ-3B will go without further modification.

What exactly does this particular combination reveal about minimum speed ?
600 rpm ÷ 3.339 ÷ 2.46 x 8 ÷ 43 x 28.3 x 3.1416 x .975 ÷ 63360 x 60 = 1 mph.

So by now it should be apparent that we have ample hp for crawl speed.
And our crawl speed, just like our maximum potential speed, is highly dependent upon the gearing.

29 "GEAR RANGE"

What is a gear range ?
The gear range includes all gear ratios available from the lowest to the highest.
Even when all gearboxes are in straight drive and we shift into overdrive we still have our crankshaft velocity reduced at the axle. The axle is also called the final drive.

I have already detailed the crawl ratios.
But what are the high gear reduction ratios for these Jeeps?
They are exactly the same as the axle ratios unless an overdrive is installed.
So here I detail the high reduction ratios with the overdrive.
The high reduction ratio for a pre-1963 CJ-3B is figured as 1 X .75 X 1 X 5.375 = 4.03 to 1.
The high reduction ratio for a post-1962 CJ-3B is figured as 1 X .75 X 1 X 4.27 = 3.2 to 1.
The overdrive yields the same effect as a higher final drive.

Our pre-1963 Jeep equipped with an overdrive has gearing that ranges from
a low of 37 to 1 to a high of 4 to 1.
Our post-1962 Jeep equipped with an overdrive has gearing that ranges from
a low of 35 to 1 to a high of 3 to 1.
The low ratio to the high ratio equals the "RANGE SPAN".
Our pre-1963 Jeep has an overall range span of 33 ratios.
Our post-1962 Jeep has an overall range span of 32 ratios.

These are all very important numbers used for comparative purposes.
Remember that the "crawl ratio" indicates the low speed potential of the Jeep and the "range span" indicates the overall versatility of the Jeep.
With both the crawl ratio and the range span we encounter points of diminishing returns.
Examples:
A crawl ratio much over 100 to 1 would just be too slow to be of much practical use.
With an excessively high ratio beyond 3.5 to 1 the CJ-3B will simply run out of available hp or go too fast.
The high reduction ratio is most beneficial when given as a precise fraction. Example: 3.73 final drive.

So what about shifting?
Gear selection allows us to choose a number of preset divisions within the range span.
These preset divisions are the various gear ratio combinations available for use.
Velocity variation of our selected gear ratios is accomplished via throttle control.
This of course is done to optimize the engine velocity in relation to the workload.

How many sticks?With an overdrive behind the T-90, twelve ratios become available:

1) low range first gear straight drive
2) low range first gear overdrive
3) low range second gear straight drive
4) low range second gear overdrive
5) low range third gear straight drive
6) low range third gear overdrive
7) high range first gear straight drive
8) high range first gear overdrive
9) high range second gear straight drive
10) high range second gear overdrive
11) high range third gear straight drive
12) high range third gear overdrive
 

Transmissions are often defined as either close ratio or wide ratio.
A close ratio transmission is normally desired for the narrow power band engines.
Since our F-134 has a very wide torque power band (flat line on the dynamometer chart) we normally would not consider close ratio gearing as offering much advantage.
However the available hp is so marginally low that the addition of close ratio gear selection becomes very desirable.
To better understand why I emphasize the F-134 engine hp as being marginal, see Jim Allen's summary of how to evaluate your engine's torque output, in F4-134 Engine Horsepower and Torque on CJ3B.info.

The addition of the Jeep Special Equipment overdrive effectively turns the T-90 into a close ratio gearbox via gear splitting capability.
Plus it also increases the high ratio gear range.

When the transfer case is in low, the transmission ratios are 2.46 times closer.
With transfer case in low, an overdrive offers very little additional benefit because we are essentially splitting the gears twice, yielding extremely close ratios.
In fact gear splitting via overdrive in low transfer case range is simply overkill.

GEAR RATIO and RANGE SPAN CHART for various standard and modified CJ-3B's

Transmission......Transfer Case.............Axle........Crawl.....High Ratio........Range
low gear ratio.....low gear ratio............Ratio........Ratio......With O.D..........Span


2.798.........times.........2.46.........times....4.27........29 / 1........3.20 / 1........26 ratios
2.798.........times.........2.46.........times....4.88........34 / 1........3.66 / 1........30 ratios
2.798.........times.........2.46.........times....5.38........37 / 1........4.03 / 1........33 ratios

3.339.........times.........2.46.........times....4.27........35 / 1........3.20 / 1........32 ratios
3.339.........times.........2.46.........times....4.88........40 / 1........3.66 / 1........36 ratios
3.339.........times.........2.46.........times....5.38........44 / 1........4.03 / 1........40 ratios

2.798.........times.........3.15.........times....4.27........38 / 1........3.20 / 1........34 ratios
2.798.........times.........3.15.........times....4.88........43 / 1........3.66 / 1........39 ratios
2.798.........times.........3.15.........times....5.38........48 / 1........4.03 / 1........44 ratios

3.339.........times.........3.15.........times....4.27........45 / 1........3.20 / 1........42 ratios
3.339.........times.........3.15.........times....4.88........51 / 1........3.66 / 1........48 ratios
3.339.........times.........3.15.........times....5.38........57 / 1........4.03 / 1........53 ratios

6 398.........times.........2.46.........times....4.27........67 / 1........3.20 / 1........64 ratios
6 398.........times.........2.46.........times....4.88........77 / 1........3.66 / 1........73 ratios
6.398.........times.........2.46.........times....5.38........85 / 1........4.03 / 1........81 ratios

6.398.........times.........3.15.........times....4.27........86 / 1........3.20 / 1........83 ratios
6.398.........times.........3.15.........times....4.88........99 / 1........3.66 / 1........95 ratios
6.398.........times.........3.15.........times....5.38.......108 / 1.......4.03 / 1......104. ratios

Above: For variety I have included the ratios associated with the T 98-A transmission.
I have also included an aftermarket 3.15 low range gear set that could be installed
into a late Dana model 18 transfer case having the 4" index hole.

To read more on gear ranging I highly recommend The Novak Guide to Gearing & Gearing Math for Jeeps.

30 "CENTER OF GRAVITY"

In this final subtopic I am really right back to where I started this discussion of speed.
I am still attempting to put the observations of Galileo Galilei and Sir Issac Newton into perspective.
As I have explained, our final revolutions occured at the axles' center lines.
Therefore straight line motion of axles center lines becomes the basis of our speed observation.
But what if the motive force remains yet the axle center lines fail to move in a straight line?

First I will explain how even the Jeep's center of gravity can influence our speed.
Our Jeep's base rests upon the ground where the tire treads contact the terrain.
Yet the Jeep's center of gravity is located on a plane elevated above this base.
If this were not true, our Jeep would resemble a magic carpet.
It simply would not be bound to the earth.

As you know, any vehicle traveling into a high speed turn is susceptible to a side roll.
The vehicle's inertia regulates its motion in a straight line path.
Resistance to the straight line path is the tire tread in contact with the roadbed.
As tread ribs grip the pavement, a directional turn begins.
At a high enough turn velocity or deviation from the straight line path, the vehicle inertia becomes great enough to overcome the gravitational force.
There is no specific speed at which this occurs. It is a complex matter of the Jeep's mass/ weight, center of gravity, moment of inertia, deviation from a straight line path and also tread adhesion.
Needless to say the side rollover affects your Jeep's speed.

Slope Slope degree will be my last detail regarding speed influence.
When travelling sidling upon an inclined slope, one becomes most acutely aware of the Jeeps center of gravity.
Of course an ever increasing slope would produce a side roll all of its own.
It soon becomes apparent that the only viable method to approach a very steep gradient is either straight up or straight down the slope.
Why does this work?
Simply because the Jeep is longer than it is wide thus effecting a more stable base.
We have a longitudinal base vs. a transverse base.
Precise alignment of slope approach can eliminate the potential side roll.

In the most extreme cases I find that the Jeep must be positioned with the front end uphill.
This utilizes engine weight to better balance tractive force toward the up hill treads.
But even more importantly this places the major portion of the Jeep's weight further from the downhill pivotal plane.
If the engine weight is positioned downhill the weight and tractive force becomes accentuated at the lower pivotal plane.
This pivotal plane specifically is the center line of the low axle.

Reverse Attempting to reverse speed and back up an extreme incline under favorable traction conditions can be detrimental.
Under some conditions the weighted downhill tires have full traction.
If ascending such a steep slope in reverse, even a slight stubble or an increase of speed (acceleration) can produce lift.

When any uphill tread leaves the ground we have reached this condition of potentially grave consequence.
This is the dreaded end roll!
At this point the engine force continues to have its effect yet is no longer transformed into forward speed.
For a time it remains visible only as vehicle velocity without straightline speed...

THE END !!!


Thanks to Ken "Oldtime" Bushdiecker for giving us some of what Greg on the CJ-3B Bulletin Board called "the Zen of old Jeeps." -- Derek Redmond

Line drawings from How to Use Willys 4-Wheel Drive. Photo of engineers at work from Willys Engineering Calculations. Tire photo by Chuck from Spring Fever in a CJ-3B. Blacktop photo is a composite of a Death Valley background by William Warby and the Last CJ-3B Sold Stateside. Racing Jeep photos from Jeep Cross Off-Road Racing in Spain. Shifter photo from Six Sticks in Dixie.

Also on CJ3B.info, see Mach 1 in a 3B?


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Last updated 30 January 2018 by Derek Redmond redmond@cj3b.info
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All content not credited and previously copyright, is copyright Derek Redmond