Diff locks and traction control

In reply to offtrack's PM.

With CDL locked you have 50:50 F to R torque split (obviously this depends on wheel grip, but it assumes equal grip at both ends, you are correct in stating that velocity split would be an easier way to visualize it).

To put it simply, with CDL locked, the front and rear crown wheel will be spinning at the same speed, all the magic happens in the diff centers.

With a rear locker also engaged, both rear wheels are spinning at the same RPM as both front and rear crownwheels.

Now up the front, the torque will be split between the front wheels depending on grip, from 0 to 100% distributed side to side.
Assuming that the wheel with grip can only rotate forwards, then the fastest that the free spinning front wheel can rotate is double the RPM of the crown wheels (and hence rear wheels), this is when the wheel with grip is stopped. A rear locker effectively gives the front end better control and grip as a result.

To make it easy, think of it like this.
3 open diffs = 1 wheel positive drive
2 open diffs + CDL locked = 2 wheel positive drive (1 F and 1 R)
1 open diff + CDL locked = 3 wheel positive drive
all 3 locked = true 4 wheel drive

Hope that makes sense...

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

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Agreed, Much armchair driving and theorising going on here [bighmmm]

I have only had my D2a for just over 2 years and the drivetrain is Bog standard.
I have found that with the CDL engaged it is a very capable truck off road and is better than any of my previous Toyota, Nissan trucks.
However IF I had the spare quids I would fit front and rear diff locks in a heartbeat, Not because I desperately NEED them More of a "just in case" thing.
The logic is simple 1x locker with CDL = 3 wheel drive, 2x lockers = True 4 wheel drive [thumbsupbig]

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Originally Posted by MR LR

In reply to offtrack's PM.

With CDL locked you have 50:50 F to R torque split (obviously this depends on wheel grip, but it assumes equal grip at both ends, you are correct in stating that velocity split would be an easier way to visualize it).

To put it simply, with CDL locked, the front and rear crown wheel will be spinning at the same speed, all the magic happens in the diff centers.

With a rear locker also engaged, both rear wheels are spinning at the same RPM as both front and rear crownwheels.

Now up the front, the torque will be split between the front wheels depending on grip, from 0 to 100% distributed side to side.
Assuming that the wheel with grip can only rotate forwards, then the fastest that the free spinning front wheel can rotate is double the RPM of the crown wheels (and hence rear wheels), this is when the wheel with grip is stopped. A rear locker effectively gives the front end better control and grip as a result.

To make it easy, think of it like this.
3 open diffs = 1 wheel positive drive
2 open diffs + CDL locked = 2 wheel positive drive (1 F and 1 R)
1 open diff + CDL locked = 3 wheel positive drive
all 3 locked = true 4 wheel drive

Hope that makes sense...

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I agree with 95% of what you've written...

The idea that an open diff splits torque other than 50:50 is incorrect.
There are plenty of automotive engineering resources you can check for yourself if you doubt.

This is pretty good intro to diff mechanics and torque distribution...
Open vs Locked Differential - Torque Transfer - Explained - YouTube


The only other issue I have is that I said specifically that if three wheels have grip, a single wheel cannot spin.
Your example proposes two rear wheels with grip driving the vehicle forward and one wheel stopped and the other spinning at x2 the crown wheel speed.
That can only happen if one wheel is sliding and the other spinning.

If you look back you'll see my post starts with something like "If two wheels are spinning, then yes TC will activate".
Perhaps I should have said "slipping or spinning"...
cheers
Paul

When I first got my D2, I was using it for something towing a trailer, can't remember. I dropped two wheels into a boggy ditch. That was it. The car has CDL. All that meant was the car spun both wheels that were in the slippery stuff, and I needed help.. I have since learned how to use the T/C, but I much prefer the locker in the D1. T/C seems to me to be a little flawed, at least on the early cars. I know it was LRs foray into the area where they now flourish, i.e. TR, but I don't love it on my 1999 build D2.

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Originally Posted by johntins

When I first got my D2, I was using it for something towing a trailer, can't remember. I dropped two wheels into a boggy ditch. That was it. The car has CDL. All that meant was the car spun both wheels that were in the slippery stuff, and I needed help.. I have since learned how to use the T/C, but I much prefer the locker in the D1. T/C seems to me to be a little flawed, at least on the early cars. I know it was LRs foray into the area where they now flourish, i.e. TR, but I don't love it on my 1999 build D2.

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All you need now is a couple of trutracs/ATBs and you'll have the best of both worlds/

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Originally Posted by OffTrack

I agree with 95% of what you've written...

The idea that an open diff splits torque other than 50:50 is incorrect.
There are plenty of automotive engineering resources you can check for yourself if you doubt.

This is pretty good intro to diff mechanics and torque distribution...
Open vs Locked Differential - Torque Transfer - Explained - YouTube


The only other issue I have is that I said specifically that if three wheels have grip, a single wheel cannot spin.
Your example proposes two rear wheels with grip driving the vehicle forward and one wheel stopped and the other spinning at x2 the crown wheel speed.
That can only happen if one wheel is sliding and the other spinning.

If you look back you'll see my post starts with something like "If two wheels are spinning, then yes TC will activate".
Perhaps I should have said "slipping or spinning"...
cheers
Paul

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The torque vectoring of a diff assumes that both wheels have wheels equal grip levels. This is where theory clashes with reality.

Yes, my example with a wheel stopped proposes that both rear wheels are free spinning and the front wheel with traction is stopped and the free spinning wheel is going twice as fast as the rears. Which does happen in real life, usually when a vehicle is hung up on something.

Any difference in wheel speed will activate TC in a D2. Bit at the end of it all, you are better off with a locker in the back, than without. What you said about it turning into a RWD only is not true.

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Originally Posted by MR LR

The torque vectoring of a diff assumes that both wheels have wheels equal grip levels. This is where theory clashes with reality.

Yes, my example with a wheel stopped proposes that both rear wheels are free spinning and the front wheel with traction is stopped and the free spinning wheel is going twice as fast as the rears. Which does happen in real life, usually when a vehicle is hung up on something.

Any difference in wheel speed will activate TC in a D2. Bit at the end of it all, you are better off with a locker in the back, than without. What you said about it turning into a RWD only is not true.

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In the circumstances I described, it is an accurate statement.
It's actually a function of locking the CDL rather than being specifically related to the rear locker.

But regardless, in a situation where the rear has grip and one front wheel is lifted and the other front wheel is turning at the same speed as the rears there is tractive force applied only to the two rear wheels (or even one in the case of a locker).

The wheel without traction sets the level of torque output for both wheels on the axle.
If you have one wheel in the air the tractive force to the ground by the other wheel is close to zero.
That means there is no "positive drive" on either wheel of an axle fitted with open diffs when one of the wheels has no traction.

It shouldn't be controversial... it well known that cross-axled D1 with open diffs and locked CDL will "fail to proceed".

What the locked CDL does is make 100% of torque available to both axles.
With equal traction on all four wheels then the front:rear torque split is 50:50.
With no traction on one wheel of the the front axle 100% of torque is available to the rear axle.
One wheel lifted on each axle and you go nowhere as with the x-axle D1.

So a locked rear // locked cdl // open front diff will transmit 100% of available torque to the ground through the two rear wheels when one front wheel is lifted even if the other front wheel has traction. In this situation the vehicle is being propelled only by the two rear wheels.
The front wheel with grip is rolling across the ground because the vehicle is being propelled by the rear wheels.
As soon as the lifted wheel regains traction you are back to 4wd.

What the rear locker does give you is the ability to proceed if you have traction on either one rear wheel or both front wheels.
That is a step up over D1 style open diffs + locked CDL where you need traction on both rear wheels or both front wheels to proceed.
Less so over the D2 with TC .

The effect of locking a differential is to disable Traction Control on that axis.
Locking the CDL and front:rear disables TC. Locking a diff disables cross axle TC on that axle.

The D2 TC tries to balance torque between axles and it does this by looking at the average wheel speed on each axle.
When the CDL is locked, the crown wheels are rotating at the same speed, and the average wheels speed on both axles is the same.
So there is no front:rear speed differential for the TC to act on...

The only thing the TC can act on is difference in wheel speed across the axle.

A locker forces both wheels to rotate at the same speed, so with a diff locked the TC can't operate on that axle.
Using your example of the rear wheels spinning with diff and cdl locked the TC sees no difference in average wheel speed between axles and no difference in wheel speed across the rear axle. So the locked rear diff spinning both wheels won't cause the TC to operate.

The front wheels still observe the rule that the input speed is the average of the speed of the two outputs.
If one wheel is stopped the other wheel must turn at twice the crown wheel speed.
Because there is a difference in speed across the axle the TC will brake the spinning front wheel.

In that scenario you are relying on the application of active braking by TC to the one spinning front wheel to bias torque to the other front wheel.

But that is basically the exact opposite of what I was talking about in my original post.

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Originally Posted by OffTrack

[...]
The idea that rear locker, locked cdl plus open front diff is "3wd" seems to assume that if a wheel is turning it's applying tractive force to the ground.

That is not the case.

The wheel without traction sets the level of torque output for both wheels on the axle.
If you have one wheel in the air the tractive force to the ground by the other wheel is close to zero.
That means there is no "positive drive" on either wheel of an axle fitted with open diffs when one of the wheels has no traction.
[...]

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Absolutely true and what seems to be being missed. To be clear:
- rear axle locked
- CDL locked
- one front wheel in the air the other on the ground, open/unlocked diff
- both rear wheels on the ground and not slipping... vehicle moving forward....

Both front wheels will be turning, at the same speed, same speed as the rears, and neither front wheel contributing to the motion of the vehicle.
- Its not 3 wheel drive. Its at best two wheel (rears) driving.

In the above scenario (open diff at the front and one front wheel off the deck): You could even lift one of the rear wheels as well. Then, assuming the one rear wheel on the rear is not slipping:
- all 4 wheels will still be turning at the same speed
- the vehicle will still be moving forward at the same speed
- effectively 1 wheel drive. The other three not at all contributing to the forward motion.


In reality, in the wild, that balance wouldn't likely exist for long. The one or both rear wheels may lose traction and start slipping. (i.e. rotating at a speed that is not equivalent to the rate of forward motion - i.e. wheels rotating faster than the actual forward motion). Then you will start to see the front wheel in the air rotate faster than any of the others.

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Originally Posted by tact

Absolutely true and what seems to be being missed. To be clear:
- rear axle locked
- CDL locked
- one front wheel in the air the other on the ground, open/unlocked diff
- both rear wheels on the ground and not slipping... vehicle moving forward....

Both front wheels will be turning, at the same speed, same speed as the rears, and neither front wheel contributing to the motion of the vehicle.
- Its not 3 wheel drive. Its at best two wheel (rears) driving.

In the above scenario (open diff at the front and one front wheel off the deck): You could even lift one of the rear wheels as well. Then, assuming the one rear wheel on the rear is not slipping:
- all 4 wheels will still be turning at the same speed
- the vehicle will still be moving forward at the same speed
- effectively 1 wheel drive. The other three not at all contributing to the forward motion.


In reality, in the wild, that balance wouldn't likely exist for long. The one or both rear wheels may lose traction and start slipping. (i.e. rotating at a speed that is not equivalent to the rate of forward motion - i.e. wheels rotating faster than the actual forward motion). Then you will start to see the front wheel in the air rotate faster than any of the others.

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Thanks Neil,

I edited out the 3wd because I realised you could probably argue that two fronts with traction and one rear could constitute 3wd.
But then that would also apply to a full locked truck...

Talking about "positive drive" doesn't really explain the differences that well.
Perhaps better to talk about "minimum number of wheels with traction to proceed"...

3 open diffs: 4 wheels with traction
2 open diffs, locked cdl: 2 wheels on one axle
1 open diff, one locked diff, locked cdl: 1 wheel on locked axle or 2 wheels on open axle
2 locked diffs, unlocked cdl: 1 wheel on each axle
2 locked diffs, locked cdl: 1 wheel on any axle.

And 100% agree it's a dynamic situation.

cheers
Paul