From: Bjorn Brandstedt (super_deuce@hotmail.com)
Date: Sat Jan 17 2004 - 06:43:10 PST
About differentials...
Think of the differential as an adding machine with two inputs and one
output. It is really a very heavy duty precision mechanical analog computer.
On your vehicle, the inputs are the two half axles and the output is the
propeller shaft. The output turns as a result of the two half axles turning,
the sum of their rpm’s divided by 2.
For example, (disregarding the final drive gear ratio, which is 6.72:1 on
the deuce):
If the left side turns at 5 rpm and the right side at a rate of 5 rpm, both
turning clockwise, then the output turns at (5+5)/2=5.
If the left side turns at 6 rpm and the right at 8 rpm, both turning
clockwise, then the output shaft turns at 7 rpm. Remember that the output
shaft is the propeller shaft in the examples. Any axle may be considered
output or input depending on the driving source(s).
If the left side turns 5 rpm clockwise and the right side turns 5 rpm
counterclockwise then the output is zero, it won’t turn at all (“park”
position on your personnel car takes advantage of this fact by locking the
propeller shaft).
This is all fine and good if you only have one drive axle on your vehicle,
BUT when you connect the outputs from two differentials together those
outputs must be exactly the same or something will have to give.
The deuce has TWO differentials connected together at all times and THREE
when in all wheel drive.
This works fine when the truck is on slippery surface (off-road), which
allows the axles to adjust to any difference in turning rate.
When driving on hard surface with lots of traction, the “inputs” to the
differentials must be matched so that the “outputs” (propeller shafts) turn
at exactly the same rate. I know that the input and output nomenclature is
reversed here, but it is easier to explain this way, it works the same both
ways.
I have actually mounted an 1100 tire and a 900 tire on each axle without
causing drag. In other words, the forward rear axle had an 1100 tire on one
side and a 900 tire on the other. The rear rear axle had an 1100 tire on one
side and a 900 on the other. The outputs from the two differentials turned
at the same rate and when connected together the way they are, caused zero
drag (my deuce has single rear wheels).
This should give you an understanding of how the tires must be matched in
size for minimum drag.
Perhaps the best way to match tires is to measure their circumferences and
then mount them in such a way that each axle end up with the same total.
The tire pressure effects the actual rolling circumference, so tire
pressures can be used to match tires also.
If you drive off-road exclusively, tire matching becomes less important.
The differential is a key component in mechanical analog computers, widely
used in fire control computers onboard vintage war ships for example.
I hope I didn’t bore you with all this.
Have a great day!
Bjorn
Meadows of Dan, Virginia
>From: "enallin" <enallin@bedford.net>
>To: "Military Vehicles Mailing List" <mil-veh@mil-veh.org>
>Subject: [MV] Hey Dr. Deuce (or others), please elaborate....
>Date: Sat, 17 Jan 2004 07:07:40 -0500
>
>"REMEMBER
>
>The back end of all US Deuces is in AWD ALL the time.
>
>SIZE MATTERS!
>
>The tires that you ridin on in the back should be the same
>circumference within I believe 1/4 inch. If not, you get binding
>that acts through the ring and pinion gears and the ujoints.
>
>
>Steve AKA Dr Deuce"
>
>OK Steve, lets say you have 7 tires at 50% tread, and need one replacement.
>How do you handle this? Also, what if the two tires on a stub axle are
>different in amount of wear? Which goes inboard, which outboard?
>
>Thanks and God bless! Gene
>
>
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