# RWD differential help



## Joel (Jul 29, 2008)

Not sure where to put this, I did a quick search on google and didn't find much. All I found was that a differential is supposed to apply equal torque to both wheels.
I spun up the wheels again (on stands) with a 12V battery and I noticed that one wheel was spinning faster than the other. At one point, one wheel wasn't moving at all, I was able to push it with my foot to get it spinning again, but there seemed to be some sort of resistance.

Now I thought it might be rusty brakes or something, but when I put the gear into reverse, the wheels switched. (the right side had trouble spinning and the left spun just fine)

I put the gear back into 1st/2nd and it stayed that way.

simply put, I am not getting equal torque on my rear wheels. Is this normal because it is up on stands? (no load). (i hope so, i really dont want to take it to a mechanic)

-Joel
http://joelclemens.colinr.ca


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## Tubularfab (Sep 25, 2008)

Actually an open differential does anything but provide equal torque to the wheels. By open I mean any standard diff that is not a limited slip. A differential simply allows the wheels to turn at different speeds when going around a corner. If you jack up the car and spin the motor the wheel with the least resitance (bearing and brake drag) will spin the fastest.

I can't help but reference a movie here - My Cousin Vinny. Watch Marissa Tomei in that movie explain how a differential works.


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## Joel (Jul 29, 2008)

ah, thats somewhat encouraging, i'm not sure what kind i have. it seems to be the more common one found on trucks with RWD (engine and transmission is up front) (89 ford ranger)

I found this and it helped a bit.

(goes to http://web.mit.edu/2.972/www/reports/differential/differential.html)


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## Tubularfab (Sep 25, 2008)

On an '89 ford ranger I doubt you'd have a limited slip. There should be a little tag under one of the bolts on the rear cover for the diff that will tell you, though. It will have several numbers, one of which indicates the ratio (3.54, 3.73, etc) and if it is a limited slip it would have an L or LS right next to the number.

At any rate, I doubt you have any mechanical problems of note in your truck - the one tire spinning more than the other is not indicating any problems. As you mentioned rust on the brakes, etc will cause a little drag, and that's probably all you're seeing.

Jason



Joel said:


> ah, thats somewhat encouraging, i'm not sure what kind i have. it seems to be the more common one found on trucks with RWD (engine and transmission is up front) (89 ford ranger)
> 
> I found this and it helped a bit.
> 
> (goes to http://web.mit.edu/2.972/www/reports/differential/differential.html)


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## Coley (Jul 26, 2007)

The picture could be more accurate with the names of the parts.

What they call a Sun gear is actually an axle gear.

What they call a planet pinion is actually a spider gear.

A rear end has NO planetary gear setups in it, but auto transmissions do.

The two wheels will usually turn at different speeds, as mentioned, on an open (non-posi) rear end.


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## CNCRouterman (May 5, 2008)

An Open Differential WILL provide (nominally) equal torque to both wheels, Tubularfab understands it I think, but is mis-stating things a little. The reference to "My Cousin Vinnei" is quite good, but the bottom line is that the torque is the same to both wheels. The key here is that in order to have effective torque, you have to have traction (resistance). Net result is that for an open diff, your maximum effective torque is equal to the torque generated by the wheel with the poorest traction.

The reason one wheel turns and the other doesn't (or turns much slower) is because there are not equal amounts of drag from one side to the other, brakes, bearings, seals, etc.


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## kixGas (May 2, 2008)

Yep there is nothing wrong with your axle. Assuming equal resistance then both wheels will spin the same speed if on jack stands. This is rarely the case due to reasons stated above. When on the ground if they have equal traction then they will receive equal torque. However when you go around a corner then the outside wheel has to turn faster then the inside and the torque is split to give the biased amount to the inside wheel.


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