Diff locks and traction control

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Originally Posted by OffTrack

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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.

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So the traction control doesn't look at each wheel as an individual item, it only looks at each wheel on a per axle basis?

I ask this because I don't understand the traction control system on a D2.
My brother's D2 has diff locks front and rear, but no CDL.
I'm telling him the CDL is more important in many respects than front and rear difflocks, and trying to get him to do the CDL lever upgrade to give him more grip in all situations.

Happened a few months back along a couple of tracks that got him 'stuck'.
First was a very steep rocky climb, where first up the rears had grip and fronts (both) spun.
I got out of the vehicle to be sure that the wheels spinning were both on each axle, and sure enough fronts first spun up(ie. front locker working), then as he pushed a bit more with a bit of a run up, then the rears spun as they slid on the wet rock, both rears spun which confirmed that rear locker was engaged.
What didn't work so effectively was the traction control, which allowed each axle(ie both wheels per axle to spin too long).

I thought that the traction control(considering that no CDL is fitted) would minimise spinning wheels on a per wheel basis, rather than on a per axle basis.
On a D2 fitted with factory CDL, it'd make sense that wheel spin could be assessed on a per axle basis.

The other 'slip up' we encountered on this same trip(different track) was a large bog hole covered in water. Depth up to just below bonnet line, must have been slushy inside the water hole. One axle in, one axle out situation.
On entry into the hole, rears still on the solid high ground, car had forward motion. Once the rears dropped in, but at the same time front axle drove out, the front axle now with less weight on it, even tho it's on solid ground had no drive, and the mushy ground inside the water hole no enough grip to drive the rears out easily.
For my liking there was wayy too much front wheel(s) spin before they finally bit and pulled the car out.
Traction control didn't appear to help .. but of course can't tell.
For me standing outside watching it all, it seemed as tho 'Jeremy Clarkson style brute force and powwerrrr!!!' is what got the D2 out, rather than good traction and or traction control.

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Originally Posted by OffTrack

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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
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Am sure that no one in this esteemed forum has overlooked this reality - with 3 open diffs all you need is to lose traction on one wheel and the whole torque/drive equation collapses to a zero sum. (additions like ETC change this....)

My defender (MY2013) doesn't have any ETC. Just three open diffs was how it was delivered to me brand new 4yrs ago. Even on-road all you have to do is drive hard through tight corners on wet tarmac to see one wheel lose traction. Sometimes the LAST thing one needs is to lose all effective drive forwards mid-slippery-corner! In the blink of an eye weight shifts with the loss of drive and the whole vehicle balance shifts.

I replaced the diff centres with Ashcroft ATBs a few years ago. They do make a difference on and offroad. But one must "know the beast" if one wants to have any shot at being able to predict vehicle handling outcomes! (Whether 3 open diffs or something different)

For example - it must be remembered that ATB's are effectively just like open diffs that are really REALLY lousy at allowing the differentiation action - whilst ever there is some load on both half shafts.

i.e. If there is no loading on the output shafts (e.g. one wheel in the air) then even ATBs act like open diffs.

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Originally Posted by AK83

So the traction control doesn't look at each wheel as an individual item, it only looks at each wheel on a per axle basis?

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No. It uses the ABS system to monitor each wheel. However, it compares the wheel speed across the axles and brakes the faster spinning wheel.

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Originally Posted by AK83

For me standing outside watching it all, it seemed as tho 'Jeremy Clarkson style brute force and powwerrrr!!!' is what got the D2 out, rather than good traction and or traction control.

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That seems to be the best way to get D2s TC to work best.

My understanding, and believe me, I could be wrong, is that the TC requires stored energy to operate the brakes for the TC to work, and that energy doesn't last long in prolonged low traction situations.

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Originally Posted by johntins

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That seems to be the best way to get D2s TC to work best.

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Not ideal, but I'm thinking if the wheel speed difference across an axle is how the traction control responds, then until brother does the CDL update, it'd be best to keep the front diff lock unlocked.
The thinking is that one wheel is more likely to spin up rather than both, and if the traction control responds to each wheel per axle, then it'd respond quicker than if both fronts were spinning up(as was the case on the two sticky moments we had).

At the time, I didn't think to test those two tracks with no diff locks at all, or maybe just the rear or just the front just to see if the ETC would respond differently.
As we were approaching both situations, it was a natural response to engage diff locks prior to the tricky section of those two tracks.

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Originally Posted by AK83

Not ideal, but I'm thinking if the wheel speed difference across an axle is how the traction control responds, then until brother does the CDL update, it'd be best to keep the front diff lock unlocked.

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Well, it basically renders the TC redundant if the diff is 100% locked, so it doesn't really matter ( I'm assuming the rear to already be locked in these circs. ). Edit: Hmm, no CDL.... Maybe not

What does matter is the loss of steering response with both diffs locked. So, I would prefer to let the TC do it's work on the front wheels.

I don't know if my particular car has issues, or whether it was just because it wasn an early build, but I find the TC underwhelming. But it DOES work if pushed. So does the HDC.

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Originally Posted by AK83

The thinking is that one wheel is more likely to spin up rather than both, and if the traction control responds to each wheel per axle, then it'd respond quicker than if both fronts were spinning up(as was the case on the two sticky moments we had).

At the time, I didn't think to test those two tracks with no diff locks at all, or maybe just the rear or just the front just to see if the ETC would respond differently.
As we were approaching both situations, it was a natural response to engage diff locks prior to the tricky section of those two tracks.

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If both fronts were spinning I would think the TC would react, as wheel speed front to rear would be very different, but my knowledge of the system is deficient here, so don't quote me....

Seriously though, the CDL would be the first mod I'd do to a D2, more so if the mechanicals are there and it's just a lever required. LR put far too much faith in their infant TC system, IMO.

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Originally Posted by johntins

..... as wheel speed front to rear would be very different, but my knowledge of the system is deficient here, so don't quote me ........

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If that were the case, then wheel speed difference isn't just accounted for on a per axle basis, they'd all be calculated independently.

In the steep rock climb situation, front wheels spun madly for 30sec maybe more or so that brother tried, and rears weren't moving.
It would be reasonable to assume that the ETC would have kicked in earlier if it accounted for wheel spin for each wheel totally independently.
So this idea that it's calculated for on each axle makes sense(from those observations).
In the water bog hole situation, I'm assuming that rear wheels weren't spinning, but couldn't tell as they were both submerged. No mad turbulent water churning lead me to think they weren't spinning up like the fronts were.

Agree that CDL sould take priority over axle diff locks, or that if axle diff locks were wanted, it should be done in unison with a CDL mechanism at the same time.
If the D2 already has CDL mechanism .. bonus!
Brother's car is as he bought it tho, and he's yet to mod it in any way.
But I question whichever previous owner(3 as far as we can tell) did do the diff locks, and their understanding of 4WDing.

Brother's car is a 07/02 build date ... dunno if that's late or whatever. It's not an update model tho.

I'm keen to get back one day and test out, primarily the steep rock track, with front diff unlocked tho to see if the ETC works more effectively.

Every time our conversations turn to the D2 tho, I'm always reminding him that he REALLY NEEDS to get himself the CDL mod hardware so we can fit it up ASAP ;)

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Originally Posted by AK83

If that were the case, then wheel speed difference isn't just accounted for on a per axle basis, they'd all be calculated independently.

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But that is the point. The ABS system does calculate each wheel independently, and that's what TC essentially is, ABS in reverse.

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Originally Posted by AK83

In the steep rock climb situation, front wheels spun madly for 30sec maybe more or so that brother tried, and rears weren't moving.It would be reasonable to assume that the ETC would have kicked in earlier if it accounted for wheel spin for each wheel totally independently.

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Was the TC light coming on in that situation? It should be lit, and you should hear it, rather like ABS.

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Originally Posted by AK83

Brother's car is a 07/02 build date ... dunno if that's late or whatever. It's not an update model tho.

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Oh oh, don't think that one will have it.... Wiser folk here will know.

My observations here are based on my experience with my own D2, which has CDL but no lockers. Also, a recent experience with a Defender with a faulty sensor, which was causing one wheel to grab until it faulted, which suggests each wheel is treated independently.

Haven't read any Land Rover literature on it but my assumption is traction control has to work across each axle, with each wheel on that axle considered with reference to the other on that axle.

There is no way to determine if a wheel is slipping without a reference point (ie - the other wheel on that axle). The mathematical definition of loss of traction would be speed of left > right, or vice versa.

So one wheel speed > than the other by some margin = grab wheel with brake.

Can't imagine it being any more complicated or clever than that.

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Originally Posted by ballbag

Haven't read any Land Rover literature on it but my assumption is traction control has to work across each axle, with each wheel on that axle considered with reference to the other on that axle.

There is no way to determine if a wheel is slipping without a reference point (ie - the other wheel on that axle). The mathematical definition of loss of traction would be speed of left > right, or vice versa.

So one wheel speed > than the other by some margin = grab wheel with brake.

Can't imagine it being any more complicated or clever than that.

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Don't forget front vs rear.

I think the build dates for CDL is in TGO