Brake testing with LSD

[missingsid]

Never thought I'd say that since 1980.

So I need to put my S1 V8 on the brake rollers to ensure my disc conversion is safe. However I have a Torsen torque sensing diff in the rear axle so my local garage says they can't test it.

I can see why this could be an issue if the balance is out across the axle causing different wheel speeds and the diff locking.

I could use the old method of the tester on the floor as some garages still have them.

Any other ideas and comment's greatly accepted.

[Snagger]

Whether fitted to an axle diff or centre transmission diff (like the Borg Warner units in RRs), LSDs and ATBs should not be run on brake rollers.  You need the brakes done by “Tapley” test, where they lay a decelerometer on the floor and drive the vehicle and try an emergency stop.  It is much vaguer as it only give a deceleration rate for the vehicle, not a reading for each wheel, but is adequate for MoT.
 

An alternative would be to remove a half shaft from each axle.

[miketomcat]

The IVA brake test is done on rollers. The only reason the mot is not done on rollers is because some dipstick tried to do it with a diff locked and did a lot of damage. 

In this instance I wouldn't do the single wheel test on the rear but I see no reason why both wheels on the rear axle couldn't be tested at the same time. Yes they more than likely will stop at different times but the diff should cope with that. It doesn't help but my mot man would do it, he put my ibex on the rollers for me before I took it to the IVA.

Mike

 

[missingsid]

I think I will give Nige a call to see what he says as well.

[landroversforever]

I think the issue with the rollers for an LSD is I’m sure they turn the wheels different ways, then do the reverse. 

[missingsid]

Ow, I just picture all the worm gears being driven against each other from the wrong direction. But then again that is what happens when it goes around corners and how Torsens work. Time to look at the videos again.

[Nonimouse]

Just get them to use a Tapley Meter

[Escape]

I wouldn't expect any trouble with a TorSen style diff on a normal brake roller. As long as there is no input torque to the R&P there shouldn't be any locking action. Over here the standard rollers check both wheels on an axle at the same time, but for permanent 4wd with a limited centre diff (like the P38) they test one wheel at a time with the other free to turn backwards so the propshaft doesn't turn and there is no torque on the centre diff. Of course if a tester forgets to change the roller set up, the car is likely to drive out of the rollers.

[FridgeFreezer]

Pretty sure my MOT guys always do a tapley test on anything 4x4 as the rollers can jigger it up... also fairly sure a lot of the time the tapley meter stays in the cupboard and they just check that it stops nicely

[missingsid]

Used a Tapley tester some years ago, the MOT tester was set up by his pals to do it and was terrified at the prospect of sitting in a 1955 V8 LR with no seat belts knowing that we only had room in the car park for me to floor it then stamp on the brakes!

Escape, I did wonder as per your comment as without power from the propshaft the diff does not drive the prop with both wheels lifted as per an open diff.

I have some testing and investigation to do as I have seen differing views on how effective these diffs are from they are a semi open diff to they are the fastest locking diff type? I have not used mine in anger yet as it has only been down a private road to test the brakes.

[Escape]

I'm fitting a TrueTrac to a customer's RRC next week (as soon as all the parts arrive), I'll play around with to get an idea how it would behave on rollers. Because of the centre viscous a standard roller test will be out of the question anyway, but I don't see a problem with the wheels turning in opposite directions for the test. Mind you, I don't actually have brake rollers, so it wont be a conclusive test.

Filip

[Bowie69]

My MOT chap has new rollers fitted, depending on set up he can  choose how they operate.

So even for a viscous or torsen centre diff vehicle you can roller brake test it.

[pat_pending]

I seem to remember that when I tried testing an LSD equipped car in the rollers, it kept trying to jump out whenever you applied the brakes.

Either decelerometer test it, we have a modern digital one that sits on the passenger seat, or pull a half shaft.

[Escape]

As promised I played around a bit after fitting the TrueTrac. I was surprised to find quite a bit of resistance trying to turn a rear wheel with the handbrake applied. I must add that was before I filled the axle with oil and it was second hand unit the customer supplied, so might just be a bit stiff from sitting. As it is, I wouldn't be comfortable putting in a the rollers.

[landroversforever]

3 minutes ago, Escape said:

As promised I played around a bit after fitting the TrueTrac. I was surprised to find quite a bit of resistance trying to turn a rear wheel with the handbrake applied. I must add that was before I filled the axle with oil and it was second hand unit the customer supplied, so might just be a bit stiff from sitting. As it is, I wouldn't be comfortable putting in a the rollers.

What year RRC? viscous centre?

[Escape]

A 1989, so a Borg Warner with viscous centre diff. But I had the rear jacked up, just to see how it would feel if I turned one wheel. The other one did turn the opposite way, but it took some force to get it moving. Not as much as a viscous, I was able to turn the tyre by hand.

[missingsid]

Thanks Escape for looking and replying.

So I tried my Torsen out yesterday.

It does not lock up when a wheel is lifted as expected but this is what I found.

Under power in 2wd at first I felt the grounded wheel try to snatch backwards for a split second, then the lifted wheel rotated forwards but I could feel it pulse as the LSD tried to grip through gear wind up. On tarmac conditions it will wind up. It is designed that if one wheel is rotated faster than the powered wheel I.e. like turning a corner it rotates fine.

Conclusion.

Still not sure it really depends on what the rollers do so I will stay off them.

[Escape]

I was interested in the results myself as well. I'll try and repeat the test next a mate comes by with his Classic, also with a TrueTrac in the rear but that one is known to be good and well used.

As you don't have a viscous centre diff in your S1 there shouldn't be any effect on a normal roller, with both rollers turning the same way. If anything, it will compensate somewhat for differences between the left and right brake, so the result could only be better than expected.

I just remembered the L320 and L322 have a TorSen centre diff. I've done gearbox work on both types and clearly remember the front prop flange being easy to rotate with the box in N and rear wheels on the ground. Which is what I'd expect from a TorSen, so I think the last one I fitted is just a bit stiff from not being used and should hopefully free up now that's it's turning in a nice oil bath again.

[Snagger]

11 hours ago, Escape said:

As promised I played around a bit after fitting the TrueTrac. I was surprised to find quite a bit of resistance trying to turn a rear wheel with the handbrake applied. I must add that was before I filled the axle with oil and it was second hand unit the customer supplied, so might just be a bit stiff from sitting. As it is, I wouldn't be comfortable putting in a the rollers.

Don’t forget, ATBs are supposed to be sensitive to torque differentials, and small differential in torque should not cause enough internal friction for it to “lock up” (they don’t actually lock, but you get my drift).  Turning a wheel by hand is a tiny amount of torque, so it’d be behaving as much like an open diff as it is capable of.  Turn the wheel with a motorised roller and the friction and biasing will be much, much higher.  
 

Some ATBs may need a little left foot braking or ETC  to trigger them when a wheel is lifted, but that tends to be worn units or earlier, more basic types.  Trutracs have a preload in one direction of rotation, which is why they are built as front or rear axle units, while Ashcroft (and I believe Quaife) are preloaded in both directions, so are not axle specific.  If the preload springs are worn out, or the axle driving in the opposite direction to its preload configuration, then you need the brake on the spinning wheel to trigger the ATB into its bias.  So, if you turn a Trutrac in one direction with a single wheel lifted and the associated prop shaft free to rotate, you may get an open diff behaviour, but not in the opposite direction.  With the other brands, I think you’ll get the same “sticky” or stiff open behaviour turning by hand, but not once you increase the torque to the levels the rollers will give.

[Bowie69]

I believe,but worth investigating as not 100%, that the locking up of a truetrac only occurs due to input torque to the pinion, not wheel-applied torque.

Need to dig out out these torsen style diffs work, again.

[Escape]

2 hours ago, Bowie69 said:

I believe,but worth investigating as not 100%, that the locking up of a truetrac only occurs due to input torque to the pinion, not wheel-applied torque.

Need to dig out out these torsen style diffs work, again.

That is my understanding as well.

[cackshifter]

Complete aside and OT and puerile but every time I look at this topic I have a mental image of stoned MOT testers.

[Bowie69]

7 hours ago, Bowie69 said:

locking up of a truetrac only occurs due to input torque to the pinion, not wheel-applied torque.

Thinking about this more, if it did react to wheel torque, then it would cause some 'interesting' braking moments on differing surfaces.

[landroversforever]

My concern still with an LSD and a brake tester is the fact that there is no normally driven scenario where wheels on the same axle would be turning different ways in the way a roller tester spins one each way.

[Escape]

1 hour ago, landroversforever said:

My concern still with an LSD and a brake tester is the fact that there is no normally driven scenario where wheels on the same axle would be turning different ways in the way a roller tester spins one each way.

A standard roller has both wheels turning the same way and tests both brakes at the same time. The ones with opposing rotation are only used for permanent 4x4s with a viscous centre diff or similar LSD. At least that's how the tests are done over here. So with a Series just leave it in RWD, Defender difflock off and there shouldn't be a problem. There are other cars out there, both FWD and RWD that come as standard with an LSD or TorSen diff: hot hatches, sportscars, pickups... AFAIK those are tested on a standard roller and I've never heard about problems.