Thread: ATBs or Torque Biassing Diffs and how they work

Thanks for the explanation Neil.

Originally Posted by tact

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So to think that an ATB needs some wheel slip before it can work is completely wrong.

.....

Having looked at various options for myself and reading a bit about the different options available, I had trouble fully understanding the concept of spin with respect to the torsen/ATB design until I saw this video:



At the 2:40 mark they give an explanation as to this 'slip' that others have referred too, and my misunderstanding prior to the video description was also that it needed to slip to work .. which as you say is incorrect.

The slip that is needed is relative. Not actual wheel slippage, but slippage relative to each axle. Why I found that the 2:40 mark on that video made the difference for me.

That is, the wheel/tyre doesn't slip on the actual road, it's slips relative to the other axle.
I think this is where the misunderstanding happens.

Originally Posted by AK83

Thanks for the explanation Neil.



Having looked at various options for myself and reading a bit about the different options available, I had trouble fully understanding the concept of spin with respect to the torsen/ATB design until I saw this video:



At the 2:40 mark they give an explanation as to this 'slip' that others have referred too, and my misunderstanding prior to the video description was also that it needed to slip to work .. which as you say is incorrect.

The slip that is needed is relative. Not actual wheel slippage, but slippage relative to each axle. Why I found that the 2:40 mark on that video made the difference for me.

That is, the wheel/tyre doesn't slip on the actual road, it's slips relative to the other axle.
I think this is where the misunderstanding happens.

G'day Arthur. Good insights there! The video you linked to is a type 1 torsen. I still struggle to get my head around how they work.

I find the type 2 (like truetrac and Ashcroft) easier to understand and explain.

Have to be honest and say I struggle to work out how they work at all!

From the videos tho they work the same way .. in that they produce the same effect, just that they have different ways to do it.

The important point to note is the use of the term 'slippage', and people have often referred that to mean slippage of the wheel, as in the the same way that traction control works.
That for the TC to work, the wheel first has to slip, and the TC then brakes it to stop it slipping.

As long as the usage of the term slippage is clearly understood, I don't think the way that either the T1 or the T2 type diffs work matters much ... only that they don't let one wheel slip, or spin for them to work.

Wheel spin only happens in specific situations as you've already noted in your first post.

So for example, a full locker would be handy if for example an axle was snapped .. being std rover axles .. a good chance of that happening too.
I've thought about that as a future possibility, but then balanced it against what my normal needs would be .. and I still went with the ATB option.

Once finances are available, I'll start working on updating the axles to higher quality types to minimise .. etc, etc.

Originally Posted by tact

So I wanted to kick off a thread that discusses and dispels myths around how torque biassing diffs work.
One such myth goes like this:


A good read at the torsen site. Torsen(R) Traction gives the reader a good accurate steer regarding this. Especially this part:
"Torsen differentials are torque-biasing, meaning they distribute torque between the tires – biasing more torque toward wherever it’s best used – without requiring a loss of traction to operate."

I have also written at length in other threads about how the Ashcroft ATB (or a type 2 torsen) is also actually pre-emptive in operation, i.e. that it ( ATB ) is always in a state where it resists differentiation to some greater or lesser degree - and the degree or resistance to differentiation is directly related to how much torque is being applied to the driveline as a whole. Meaning: the more load on the driveline, or the more torque being applied to the driveline, the more the ATB will resist any differentiation. Even if driving in a straight line and there is no demand for half shafts to rotate at different rates (i.e. no differentiation being asked for).

So to think that an ATB needs some wheel slip before it can work is completely wrong.

If it helps to visualise.... look at the pic attached. Tow strap held in place with a pin. Put a bit of a load on that strap. The more load, the harder it is to twist or remove the pin from its place. Take the weight off and you can pull the pin out, or twist it round and round in its hole quite easily.

reciever hitch.jpg <---- Note: Not an actual ATB.

Same with the helical gear sets inside their casing pockets in an Ashcroft style of ATB.
In a straight line drive:
- the helical gears do not roll or rotate relative to the casing where they sit. (Seen at the 1:00 minute mark in the video linked to below)
- They just get pressed against the wall of the casing, harder or softer depending on how much/little torque is applied to the driveline as a whole.
- just like the pin holding the strap in place in the pic, if there is a lot of load on the drive line and one half shaft, one wheel, now wants to move faster or slower than the half shaft or wheel on the other side of the ATB.... its going to have a lot of resistance to overcome even BEFORE the wheel slip, or simple cornering differentiation, happens.

Eaton Truetrac video on youtube Exploded View - Inside the Eaton TrueTrac Differential - YouTube

In a bend, or when the wheel on one side of an ATB starts to slip:
- the initial resistance to differentiation has to be overcome (a little or a lot of resistance, depending on the load on the driveline, the torque applied)
- once there is actual differentiation taking place the helical gears do start to rotate in their pockets (seen at the 1:18 minute mark in the linked video above)
- depending on the design and engineering the amount of resistance to differentiation may increase due to the helical cut of the gears and friction against the pockets where they sit and are now also rotating

Point is... if there is load on the driveline (accelerating, decelerating, climbing a hill or descent with driveline engaged, not coasting) then an Ashcroft or torsen type 2 ATB will be resisting differentiation:
- regardless whether going straight, in a bend, wheel spin or not. Preemptively, whether it needs to resist or not. (i.e. Doesn't need wheelspin to make it work).
- and doing so (resisting differentiation) to a degree, greater or lesser, depending on the torque applied to the driveline as a whole.

And in case some miss it - there are plenty of conditions where there is not enough load on a driveline to support the application appreciable torque! And where there is bugger all torque in play, a torque biassing diff has nothing to do and behaves much like an open diff. Examples are:
- one wheel up in the air
- broken axle, clutch, gearbox, drive flange etc....
- neutral throttle

Great write up. So many people seem to get ideas of wheels spinning wildly with an ATB. It'd be challenging to visualise how they work if you didn't have a good understanding of how different gear types interact.

My experience reflects your explanation. One example, towing a heavy trailer on a slow gravel road, in 1st high, on a climbing bend, the steering became quite heavy due to the torque on the driveline. The front, centre & rear ATB's were doing there job. I.e. stopping the torque from taking the path of least resistance.

Originally Posted by Beery

Great write up. So many people seem to get ideas of wheels spinning wildly with an ATB. It'd be challenging to visualise how they work if you didn't have a good understanding of how different gear types interact.

My experience reflects your explanation. One example, towing a heavy trailer on a slow gravel road, in 1st high, on a climbing bend, the steering became quite heavy due to the torque on the driveline. The front, centre & rear ATB's were doing there job. I.e. stopping the torque from taking the path of least resistance.

There would be plenty of good examples how it works in real life, as per your example.

You just reminded me of something that happened and amused me in a car park. It illustrates the ATB action: Had to go round several floors looking for a spot. The facility had put some coloured surface on the floor that made for a lot of squealing with my tyres when in a turn.

I soon noticed a pattern. If I pressed the clutch pedal or slipped the transmission into neutral the squealing stopped while coasting in a turn. But if the wheels were driven, or if I backed off the throttle, loud squeals in the turn. In this kind of close conditions, of course I wasn’t hard on the throttle at all. Even though there wasn’t huge bucket loads of torque in play - the driveline quite lightly loaded, and nowhere near any wheelspin - the ATB effect was still very present. (The resistance to differentiation, even though mild, was causing rubber tyres to squeal on that surface coating)

Just to clear up any misconceptions about what type of Torsen we are putting in our Land Rovers. The Ashcroft, Quaife and Eaton are Type 2 as in this video. Except the Ashcroft and Quaife use six pairs of pinion gears instead of three. (Just realised now that Tact has a link to this video in his original post)

For anyone having trouble visualising how these things work, watch this video a few times, then start it again and pause it a few seconds in.
Looking at the paused video, if you can imagine, the crown wheel of the diff is attached to the outer casing of the diff below and the axle shafts into the splines of the side gear facing the camera and on the other side.

The force from the crown wheel turns the outer casing and transfers that torque to the axles by pushing those gear pockets against the helical worm gear sets which in turn get pushed against the teeth of the side gears.

So you can imagine that when lots of force is pushed against the gears from the outer casing, they're all going to 'bind up' and not want to spin on their own axis.

The person in the video is able to spin one side gear easily because there's no force being applied to the gear pockets and worm gear sets.

If this is how good they are and if you could believe the vid why would you bother honestly
Just one positive locking diff would have walked that
Read the comments
Discovery 2 off road axle twist with Ashcroft ATB Diff (useless) - YouTube