After replying to something in danowats car thread, it got me thinking about something i have always found a bit strange about differentials.

WHY do they give drive to the wheel with least resistance?

Surely that is kind of the opposite of what you want, as a wheel with more weight and resistance on it would surely offer more grip than one with less load on it.

There is probably a very simple reason for why diffs work this way, but i don't know what it is, maybe someone could tell me.

I see you`re on about open diffs, watch this movie first.
Around the Corner 1937 Vintage Film on differential steering.

or something would break :hide:

and that video is great

An open diff doesn't send torque anywhere. Both wheels get the same amount of torque. If one wheel can't accept any torque (i.e. it is spinning and offers no resistance) then the other one will get that amount of torque.

There is no 'sending' of all the torque to the spinning wheel, because the torque is always the same.

open diff ftl


lockers ftw

Quote from tristancliffe :

An open diff doesn't send torque anywhere. Both wheels get the same amount of torque.

True, but if one of the wheels is spinning, then the rate of angular acceleration of the entire drivetrain, engine, flywheel, transmission, driveshaft, differential, rear axle, and spinning tire, will end up much being much greater than if neither tire was spinning, and the torque will end up being used to overcome the angular inertia of the entire drivetrain and that spinning tire at the rapid rate of drivetrain acceleration, and the torque delivered to the wheels will be much less.

This can be demonstrated by trying to accelerate a car with an open differential with one tire on wet pavement and the other tire on dry. Most of the torque will end up just spinning the drivetrain and spinning tire. With a limited slip differential, the rate of angular acceleration of the spinning tire is limited to about the same rate of angular acceleration of the non spinning tire, the otherwise rapid rate of angular acceration of the spinning tire and drivetrain is prevented, and most of the torque will end up going to the tire on dry pavement, and the car will accelerate much faster.

You just managed to say the same thing Tristan said but in a very convoluted and confusing way

Quote from AndroidXP :

You just managed to say the same thing Tristan said but in a very convoluted and confusing way

The point not made clear by Tristan, is that the torque delivered to the wheels is less if one or both of them are spinning.

Err, yes he did. Well maybe adding a word or two might've made it clearer:

Quote :

Both wheels get the same amount of torque. If one wheel can't accept any torque (i.e. it is spinning and offers no resistance) then the other one will get the same (low) amount of torque..

Ignore power and ignore angular velocity. Torque is all a diff cares about. Assume at least one wheel has grip.

Open diff: Each wheel gets 50% of the differential's output torque. Diff output torque is limited by the threshold at which 50% of the diff output torque exceeds one of the drive wheel's torque capacity. Otherwise, diff output torque is limited by engine output.

Locked: Each wheel gets anywhere between 0 to 100% of the differential output torque. Diff output torque is limited by the threshold at which diff output torque exceeds the torque capacity of both drive wheels. Otherwise, diff output torque is limited by engine output.

Limited slip (clutch-type): Generally somewhere between Open and Locked (e.g. one wheel may get, 33% and the other 66%), but preload complicates things slightly such that the diff is locked below a certain torque difference threshold.

And an engine doesn't, effectively, put out torque unless there is something (a resistance) to accept that torque - e.g. accelerating the vehicle, or spinning up the drivetrain.

You might know from a dyno that at 4000rpm your engine gives 100Nm of torque. But if both wheels are in the air then at 4000rpm the engine outputs zero torque at 4000rpm (ignoring drivetrain inertia, or the engine's own internal frictions and inertia).