Additional Traction - at cross roads with options.

If this requires a new thread , please advise .
With regard to the tendency of a locked rear diff to make the rear of the vehicle slide down hill , ie sloping beach etc.
I have had this happen in the real world , but did not connect it with a locked rear . Could one of you learned gentlemen please explain why this happens .

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Originally Posted by dero

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I have had this happen in the real world , but did not connect it with a locked rear . Could one of you learned gentlemen please explain why this happens .

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happens with LSDs too.

Easy way to imagine it and why.

imagine a 2WD car with a LSD/Locker/ATB/Torsen diff, where on full power both rear wheels spin. Almost all will have a tendency to slip sidways at the rear as the rear wheels fight the spin to find grip.
Very few don't slip sideways, and usually those that don't have really well sorted suspensions.
But even perfectly sorted suspensions will still slip sideways with rears spinning(eg. racing cars, V8 Supercars, F1, etc.) as they light up the rears when they line up on the grid to warm the rears up .. etc. etc.

Same thign happens in off road. A side sloping track both rears locked so they don't slip relative to each other .. etc. if they just slip a little bit(due to low friction surface) they'll slip sideways with the grade of the slope.

I used to drive an old Frontera, had open front and LSD rear .. terrible off roader, but had 4WD all the same. Was ok on sand, but I think only due to the 10.5" tyres I had fitted, and it's rear LSD.
On side slopes it also used to slide sideways if the ground was of the low friction type(sand, very loose gravel, etc).

If only the one wheel spun in the above situation, ie. the diff unlocked, or open as normal, then that means the other didn't spin. The non spinning wheel provides the lateral stability that minimises or eliminates sideways stepping.

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Originally Posted by AK83

Trying to get my head around a real world situation as described here:

that is for the rears to have to rotate at 10k/h, and therefore force the unloaded front wheel to rotate at 10k/h, the implication here is that both rear wheels are spinning at 10k/h(obvious I know).
But in what situation would you find that the rears are spinning at 10k/h and the car not moving forward by any amount?

For the front ATB to stop working in such a situation would imply that both rear wheels are off the ground and spinning at 10k/h AND the car is not moving forward. ie. you're hung up on the chassis with the air under both rear wheels.

That's a fairly extreme off road situation!

As long as some momentum is maintained, and there is a small amount of load on both of the ATB driven wheels, it doesn't actually spin. As it tries too the other wheel takes up the torque immediately.

The term spin with an ATB is referring to the relative spin .. that is how much spin one wheel has relative to the other on the ATB driven axle .. it's not actually spin.

So back to the situation described above:

If the rears are in the air, and one of the front wheels needs to spin at 10k/h, that also means that this front wheel has to also be in the air for it to actually spin.

If you have three wheels in the air, CDL locked, using full lockers on both axles, and only one wheel capable of driving the vehicle, you then have all the torque of the engine driving that one front wheel.
The two scenarios most likely to occur are that the wheel with grip will most likely spin as it probably hasn't got enough friction to pull the 2T weight of the vehicle, or if it did, you most likely break an axle or CV having all the tractive effort on just that one wheel trying to pull the vehicle out.

Either way that's some pretty extreme off roading.

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If you had locked rears spinning at 10kmh and the the ground speed was 0kmh you would probably have one front at 0kmh and one front at 20kmh.

Lots of scenarios where this happens, seemingly every time I get stuck! :lol2: Anything from catching a diff on a rock, steps in a track especially if the step is one side or at an angle or big or there is a lot of load on the vehicle, low traction surfaces where there isn't enough traction to drive be that mud, loose rocks, soil and notably clay, climbing with cross axling with low traction etc. The rear wheels are not normally off the ground... normally, just loss of traction. Just takes a wet day on your holiday.

Your analysis of driveline failure and traction etc is really good! I remember doing the sums years ago and seeing that the engine and gearbox does produce enough torque to break an axle at will, and you're right also in that the torque limiting factor was traction. I remember also a reply by one of Australias best known rocke buggy builders to a comment that a particular driveline was indestructable, and replied that he could break it, just bind up a wheel in something and drive it. :eek2:

Interesting comments.
My pennyworth is, like others have said they have fitted, I have a Detroit Locker ( not to be confused with TruTrac) in the rear Salisbury & an Ashcroft ATB in the front. I think in the 3 years of usage they have done the Simpson Desert, Coorong sand travel, Billy Goat & Blue Rag a few times, Mt Pinnibar, - at least a dozen Vic High Plains trips plus numerous interstate trips etc etc.
Previous Land Rovers I've had have, had - ARB lockers Rear & later F&R - Maxidrive F& R & McNamara rear Rover locker.
Basically I fitted the Detroit Locker as an experiment, thinking it would be easy to remove & sell if I wasn't happy with it.
I won't be spending the money to upgrade to a lockable rear.
Whilst I'm not perfectly happy with the Detroit Locker ( uneven rear tyre wear & occasional clunk & Kangaroo hop) my decision to fit it was primarily based on cost & on that basis I won't be justified going to a full locker again.
However, If I win Tatts I would probably go selectable locker in the rear - sorry not ARB..... and leave the ATB in the front, All I can say is, with my current setup, I have easily driven up/ through obstacles that have taken vehicles in front of me a lot of trouble to negotiate.

Of course Tyres & tyre pressures are an equally important factor in traction.
David

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Originally Posted by Slunnie

Of course it does require wheel spin or on the road differences in wheel speed, otherwise you can't have torque differences on a differentiating diff.

Do you disagree with these quotes?

This is from Eaton, manufacturer of the Detroit TT


This is from Ashcroft transmissions



and this is from Quaife

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Some of what you quoted is quite accurate (and does not contradict what I have written). Some is classic marketing spin, dumbing things down for people not up to the real detail:
- “it’s like an open diff that’s not very good (at being open)” isn’t a good sales pitch.
- comments like “when a wheel spins the ATB delivers more torque to the other wheel” is much easier to understand even if not the whole truth. And such a comment:
- does not contradict what I wrote (I wrote that when wheel spin does occur more biassing happens).
- does not say that wheel spin is “needed” for an ATB to work.

Go go have a read at the torsen site. Torsen(R) Traction

It gives a more technically accurate description.

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Originally Posted by dero

If this requires a new thread , please advise .
With regard to the tendency of a locked rear diff to make the rear of the vehicle slide down hill , ie sloping beach etc.
I have had this happen in the real world , but did not connect it with a locked rear . Could one of you learned gentlemen please explain why this happens .

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In any situation except flat level absolutely straight line travel (assuming all tyres exactly the same circumference - not usually the case, manufacturing tolerances and wear etc) - just having a lockable diff locked will be forcing one of the wheels on an axle to scuff. I.e. breaking traction. Even if that’s measured in just millimetres... it’s still a forced breaking of traction, slippage, on one or the other tyre, for every centimetre the wheel rotates.

Tilt the surface and reduce the surface friction available (mud, wet grass, sand) and gravity assist exacerbates the tendency for that slippage to be in the down slope direction.

When turning the inside rear wheel is the slowest so you get wheel spin on that wheel because it is fighting the front diff

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Originally Posted by weeds

Didn’t realize Ashcroft don’t do sals......pretty sure I have a sals. Haven’t had a close.

Axle upgrade will be a given

Already have twin ARB compressor as I have air suspension.

Umm based on no Ashcroft for sals it might it might be down to

ARB or Detroit

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Check out if ashcroft don't do for rover. I have had issues with ARB. They just don't want to see you if you have a problem. TJM are also Australian and good customer service.

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Originally Posted by bachir51

Check out if ashcroft don't do for rover. I have had issues with ARB. They just don't want to see you if you have a problem. TJM are also Australian and good customer service.

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Ashcroft replied to my email confirming they don’t do Salisbury diffs.....

TJM website doesn’t show an option for Salisbury.

Think I mentioned earlier that my previous defer had front and rear ARB Diff Locks for 15+ years and I never had one issue over the 10 years that I owned it. Yeah the rear sucked the smallest amount of oil back when disengaging , no biggie and didn’t affect operation plus they are changed the design.

I’ve had plenty of other issues with my defers but I keep purchasing.

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Originally Posted by weeds

Ashcroft replied to my email confirming they don’t do Salisbury diffs.....

TJM website doesn’t show an option for Salisbury.

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TJM website is not very user friendly. But they do exist - I have purchased and installed one into a friends 200tdi defender.