Thread: Diff locks and traction control

Offtrack. My head hurts. How does a rear locker turn the d2 into rear wheel drive when one front wheel loses traction. If a front wheel loses traction the ETC gives the other wheel on same axle some drive...and to the same extent as if the rear diff is open. Cheers

Originally Posted by ozscott

Offtrack. My head hurts. How does a rear locker turn the d2 into rear wheel drive when one front wheel loses traction. If a front wheel loses traction the ETC gives the other wheel on same axle some drive...and to the same extent as if the rear diff is open. Cheers

Yes there is a thing in the middle called a CDL ,And what happens when that is applied with a rear locker and yes we know a 3wd

. .because locked CDL distributes torque equally front and rear so front gets same torque as rear and same axle ETC works as normal on the front. Cheers

I think off track was trying to explain the difference between:

torque, a force pushing against the moment of inertia in the component of the drivetrain (wheel) and

speed, the rotational velocity of the spinning component.

Things can can spin fast or slow, a locked rear axle makes both wheels spin at the same speed (velocity) but the amount of torque (force) being applied to each of the wheels depends on torque bias Of the drivetrain components.

Originally Posted by sierrafery

Please explain that better cos i'm stumped....
1. why it's impossible to spin a single wheel in this setup?
2. wouldn't the front diff engage if the TC puts brake on the slipping wheel?

With the rear diff locked the rear axle behaves as a solid beam, and both wheels rotate at exactly the same speed.
The input speed of a diff is equal to the average speed of the outputs, so if you have both wheels turning at 25 rpm, the diff input turns at 25 rpm (that ignores diff gearing).

If the CDL is locked then the front diff input must be rotating at the same speed as the rear diff input.
If one front wheel has traction and both rear wheels have traction then all three wheels must be rolling at the same speed across the ground.

The three "non-slipping" wheels must be turning at the same speed because they touching same ground surface and connected by the same chassis.
Rear Right = 25 rpm
Rear Left = 25 rpm
Rear diff input = 25 rpm
The locked centre diff means that
Front diff input = 25 rpm
Front Right = 25 rpm
Front left = "spinning"...

As Wikipedia states:

"The average of the rotational speed of the two driving wheels equals the input rotational speed of the drive shaft."

Given that the average of the diff outputs is equal to the input speed we know that

front diff input speed = (front right output speed + front left output speed) / 2

or

front left output speed = (front diff input speed * 2) - front right output speed

so

front left output speed = (25 * 2) - 25
front left output speed = 25

So under these conditions a single wheel cannot spin - it can only rotate at the same speed at the other three...

Originally Posted by ozscott

. .because locked CDL distributes torque equally front and rear so front gets same torque as rear and same axle ETC works as normal on the front. Cheers

That is true only if all wheels have equal traction.

Otherwise it's a locked shaft and can bias torque 0:100.

This is from the Wikipedia Differential entry...

"The torque applied to each driving wheel is the result of the engine, transmission, and drive axle applying a twisting force against the resistance of the traction at that roadwheel. In lower gears, and thus at lower speeds, and unless the load is exceptionally high, the drivetrain can supply as much torque as necessary, so the limiting factor becomes the traction under each wheel. It is therefore convenient to define traction as the amount of force that can be transmitted between the tire and the road surface before the wheel starts to slip. If the torque applied to one of the drive wheels exceeds the threshold of traction, then that wheel will spin, and thus provide torque only at the other driven wheel equal to the sliding friction at the slipping wheel."

It's worth also reading this article: http://oppositelock.kinja.com/your-differential-and-how-traction-works-1661277563

The amount of torque an open differential sends down each shaft is ALWAYS 50/50. This is due to the spider gears being pushed with equal force by the carrier because they are mechanically meshed. An open differential was designed to solve the speed differentiation issue, but its design isn’t capable of biasing torque across the differential and what that means is that what one wheel gets so does the other…always.

Less computer time... More time playing with spanners and four wheel driving needed...