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From digest.v7.n1019 Mon Jan 26 10:22:31 1998
From: Pete Read <>
Date: Mon, 26 Jan 1998 06:56:14 -0500
Subject: <MISC> Re: Spinning Wheels, Differential Operation

Marc Edwards <> writes: >I will jump in on this one and agree with Craig's caution.
>I don't think many people think about the fact that, with a non-LSD diff,
>if one accelerates to 60 mph on the speedo while going nowhere, if one
>wheel is spinning and the other is dead stopped, the spinning wheel is
>likely going.....yes, 120mph...I think that my logic is okay here but
>I am willing to learn from others... Comments?

Chris Bourk <> comes back with a fairly strong comment which doesn't appear to help Marc's learning curve:

>...As far as the wheel going 120 mph when the speedometer is reading
>60, Well, NOT. I love the digest so many opinions, so little


Be careful what you write, it may be used against you -- but I do agree with your last sentence <grin>.

Even though it may be hard to believe, Marc is correct. If one wheel is not rotating while the other spins freely, such as on ice or snow, the speed of the spinning wheel is twice that of the speedometer reading.

The speedometer reading is taken from the rotational speed of the differential housing (impulse sending wheel mounted on housing), which is the average rotational speed of the two drive wheels.

Left wheel + Right wheel Rotations Diff Housing Rotations = ----------------------------------


Normally, when traveling in a straight line, the rotational speed of both drive wheels is the same. However, there are times when the two wheels on an axle travel at different speeds, such as around a corner where the outside wheel speed is greater than the inside wheel (outside wheel travels in an arc of greater radius than the inside wheel).

The differential allows the rear drive wheels to travel at different speeds while balancing the drive torque to each one. This is done through small spider gears, mounted on shafts rotating with the housing, which drive the side gears to each wheel.

If one wheel is held stationary, the other wheel rotates at twice the differential housing speed. The moving wheel rotates at double speed because of its own motion plus the extra forward motion from the differential spider gear rotating along the stationary side gear of the fixed wheel (spider gears don't rotate when the side gears/wheels are at equal speed, i.e driving in a straight line).

The equation above still works even when the differential housing doesn't move. Try jacking up both rear wheels, with the car in gear and the engine off, so the differential housing is locked in place. Turn the driver's side wheel forward one turn. The other, passenger side, wheel will also rotate one turn, but in the reverse direction.

1 (fwd rotation) + -1 (rev rotation) 0 diff rotations = -------------------------------------


A limited slip differential behaves the same way, but it's just much harder to turn the wheels because of the preload on the differential clutch plates.

In case you're wondering, yes, I've tried this test to confirm the theory. Too much time on my hands I guess.

Pete Read
'88 M5
Arlington, VA

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