Quattro and MOT brake test.....2WD???
 

The 8 went in for its annual MOT today - another pass +++ :D.

I never normally pay much attention during the MOT test and just let them get on with it, but today when they were doing the brake test I noticed that the rear wheels were not moving when the fronts were on the rollers, and vice versa when the rears were on the rollers.

Is this normal? Shouldn’t the other set of wheels not on the rollers try and jump the car forward?

I mentioned it to the tester and he said it’s normal. Shouldn't they raise a 4WD car? Surely it’s not good for the transmission?

Usually with four wheel drive cars, the tester will select a mode on the brake tester that rotates the wheels on each axle in opposite directions. This prevents the axle 'winding up' the centre diff and the wheels not in the rollers +++

Thanks Adrian. I guess they wound up my diff today :(

Hopefully there’s no long term damage.

My tester used to drive my D2 down the lane to test the brakes as they couldn’t do 4wd on the machine

Proper quattro with a torsen diff is absolutely fine on 2wd brake rollers as the torsen can easily accomodate the small amount of rotation. Haldex though, not so much, as the rear diff clutch would be locked up so it would either destroy the clutch or the diff. That would need the wheels rotating in opposite directions as Adrian described. I've never seen that happen though.

Most MoT testers I know just do a road test, some with a special weight-gyroscope sort of thing to measure the braking force, - my current tester particularly as he had to pay for a new diff on a TT quattro when they first came out!

Similar to MJ's reply I think.

My tester puts a pendulum type of device in the passenger foot well and then goes for a test drive.

You don’t generally notice the contra-rotating wheels unless you’re in a position to see both sides of the car at the same time while testing that axle.

My tester showed me it running in that mode before applying the brakes

It’s a common feature (my dad used to sell roller brake testers 25+ years ago)

If garages don’t have the appropriate mode they should use a Tapley meter on the road, as mentioned earlier

I've never really given this any thought before. In my 15+ years of owning AWD 8s, all my MOTs have been done on a standard 2WD brake tester. Never had any issues as a result.

Also, if all the wheels are free turn (such as when the car is on a lift), I understand why turning the wheels by hand will result in wheels turning in opposite directions, due the way diffs work. However, I don't understand why it would be necessary to have them turning in opposite directions on a brake tester. After all, they turn the same way on the road.

But that would surely only test the braking efficiency as a whole, not the individual braking efficiency at each wheel? In my experience, conventional brake testers often highlight potentially dangerous issues with a single caliper.

Inertia Tester is recommended by the Ministry for 4 wheel drive if the Test Centre does not have a 4 wheel roller.

Because turning the wheels in opposite directions will result in zero rotation of the output shaft of the diff. Turning the wheels in the same direction results in the output of the diff turning. With Torsen it makes no difference as the geared centre diff will just be driven by one axle instead of the engine. No big deal for a short time. Something with a viscous centre diff (Subaru?) would probably also be fine.

Think of a TT though, where the rear wheels are directly coupled to the front wheels by way of a clutch pack, or indeed a Land Rover where they are directly coupled by the transfer box. If you turn one axle forwards the output from that diff must turn the other axle forwards. If that other axle is on the ground something has to give. In a TT the clutch pack breaks - what happened to my MoT guy. I expect a Landy would either drive off the rollers, stall them, or just lose traction.