Thread: Fitted Ashcroft ATBs front and rear

Originally Posted by 350RRC

And while we're on the subject...........

What are the differences between the centre diff in an LT95 TC and same in an LT230?

i.e. is there any way an Ashcroft LT230 ATB diff is going to work in an LT95?

TIA, DL

The short answer is NO. They are a completely different centre diff assembly. The only common parts they share are the cross shafts.

So I ended the first post in this thread with the words:
"Can't wait to get offroad and test on a muddy trail!"

It has now been 7mths and 10,000km since I had the Ashcroft ATBs fitted in both the front and rear of my 2013 110 Dual Cab. And yes, have seen plenty of muddy trails now!

Sorry if this refresher is too much like preaching to the choir, the real report kicks off a bit further down the page - What exactly are ATBs?:
- Automatic Torque Biassing (differentials). Sometimes the acronym TBD (torque biassing differential) is used.
- some well known diff brands, like "torsen", "Eaton or Detroit Truetrac" are ATB type differentials
- they are basically an open diff that is designed to be very inefficient at differentiating under load. Usually this is via varying arrangements of helical gears in the diff centre - instead of the usual side and spider gears.

What are ATBs not?:
- they are definitely not at all like normal LSDs that use friction disc packs
- they are definitely not at all like air lockers, or e-lockers
- they are definitely not at all like unlockers such as the "Aussie locker" or "Detroit Locker" (cf. the "Detroit Truetrac" which is an ATB )

Why are they considered "torque" biassing?:
(This seems to be key in understanding what's happening when you drive a vehicle fitted with ATBs)
- when the vehicle is coasting (straight line or turning) the diff centre is getting drive inputs from the engine/drivetrain and both wheels/axles all at the same time. In this scenario the ATB acts like an open diff. No magic happening at all.
- when you press the loud pedal AND the vehicle is traveling in a straight line AND there is no breaking traction at the wheels: the ATB acts like an open diff here too.
- when one of the wheels/axles is spinning freely (e.g. wheel lifted): the ATB acts like an open diff here too
- to see something happening that you wouldn't see with an open diff you need to be applying (positive or negative) input torque AND have the two driven wheels turning at different speeds (differentiation in action) and both with at least a little traction.

Practical Example: You are driving or engine braking (positive or negative input torque) in a straight line:
- Already there is some force pressing the helical gears against the walls of their pockets, but the helical gears are not rotating on their axes at all as there is no need for differentiation.
- The harder you are accelerating/engine braking, the harder the helical gears are pressed against the sides of their pockets and thus the more resistance to differentiation will already be there the instant you deviate from straight line travel (or either wheel breaks traction)

How does that translate to driving experience?:
- you will feel varying degrees of (gentle) steering resistance (tendency to straighten) depending on how hard you are turning and how much torque (positive or negative) is being applied to the driveline
- likewise on concrete floors (in car parks) you get audible feedback (tire squeal) proportional to how much the ATBs are resisting differentiation. (Which depends on how hard you are turning and how much input torque is present)

How does that translate to driving technique?:
- Basically just drive is good enough. Totally transparent operation with some small feedback cues (mentioned above)
- More advanced: Left foot braking (and more throttle to match) adds load at wheels which increases input torque which causes more resistance to differentiation. Can get you out of bother (e.g. A wheel in the air)

How does all this translate to performance on slippery vs high traction surfaces?:
- you might be tempted to think that if both wheels are on a slippery surface then one side or the other might break out and spin first, leaving you stuck like an open diff. It does not seem to be the case (when BOTH wheels are lacking traction)
- I suspect that what happens is both wheels have some (little - on a slippery surface) traction and so both sides of the diff put up some (perhaps little) resistance to turning (more, if more traction were available). The resistance the axles/wheels present to the turning forces requires torque to overcome, however small that is, the applied torque then applies proportionate pressure to the helical gears against their pockets as designed. Causing at least some resistance to differentiation. Plus there is some small amount of design inherent "preload" adding to the degree of resistance to differentiation.
- if the available traction (to both wheels) is very low, then the resistance to differentiation also needs only be low to keep both wheels turning.
- result is both wheels will get drive as they would if both were on higher traction surfaces.

Thus no real difference in action/performance between high & low traction surfaces when both wheels are on the same kind of surface. This can be seen and felt on very slippery trails. I have driven hard packed, wet, slick, steep and slippery clay trails with my stock AT tyres skating on the surface, no digging in: I have experienced climbing up slope, losing momentum until the vehicle makes no further progress but all 4 wheels are turning.

Similarly - in deep thick greasy mud. Like 300mm deep. I have experienced all 4 wheels driving regardless whether up hill, turning, side slopes, or engine controlled descents.

What about situations where there is uneven traction either side of an ATB diff centre?:
- no surprises here. If one wheel is totally tractionless and the other has significant traction you have very close to an open diff situation. You don't go anywhere. (Clue: left foot braking changes everything)
- however if the low traction side has any resistance to turning the wheel at all, then the resistance to differentiation rises in proportion to the torque needed to turn the lower traction wheel. Results in proportionally more drive going to the other wheel.
- how much drive (torque) the wheel with traction gets, depends on the design of the ATB (how bad it is at differentiating). Because any ATB (by design) is far worse at differentiating than an open diff, the torque to the wheel with traction will always be greater than any open diff can manage.

Real life examples here are deeply rutted slippery trails where you might drop one wheel into a hole/rut, side slopes, cross axle situations. In my experience the ATBs performed as expected. Where you would expect to be stopped with an open diff the ATB keeps you moving.

I agree with all the comments about the ATB, I have a true track in the front, and think it's an asset. If I had my time again I probably would have fitted ATB front and back. But I hadn't heard about them at the time so i fitted a maxidrive at the back. Yes it's a bit more complicated and expensive, but its bullet proof and reliable. And apart from the diff machining can be fitted at home in a well set up workshop.

I have an Ashcroft ATB for the TC center diff on the shelf at home. My transfer case has got 200k on it and has started to leak at the intermediate shaft.

I have a spare disco TC so the plan is to put new bearings and the Ashcroft ATB into the disco TC and then change the TCs out.

Originally Posted by simmo

I agree with all the comments about the ATB, I have a true track in the front, and think it's an asset. If I had my time again I probably would have fitted ATB front and back. But I hadn't heard about them at the time so i fitted a maxidrive at the back. Yes it's a bit more complicated and expensive, but its bullet proof and reliable. And apart from the diff machining can be fitted at home in a well set up workshop.

I have an Ashcroft ATB for the TC center diff on the shelf at home. My transfer case has got 200k on it and has started to leak at the intermediate shaft.

I have a spare disco TC so the plan is to put new bearings and the Ashcroft ATB into the disco TC and then change the TCs out.

All this sound great !
I have just fitted my rear ATB and hopefully will fit the front one next week, I cant wait to drive it in slippery conditions to see what occurs .