Miles965uk

Hey,
I just read on the 964C4 you cant use a 2wd rolling road becasue you could damage the transmission or 4wd system and my car has the 993TT 4wd...

I just went to have an MOT done and to get the brakes tested they put it on a set of rollers just for 2 wheels only.... testing front and rear seperately.. brakes and handbrake force against it.

I saw there was some 5kg drag on the wheels when just rolling without the brakes even being applied!

Can anyone just let me know if this is safe for the transmission to use it on a 2 wheel roller? and the 5kg drag... is this to do with the 4wd or should there be no resistance at all when you spin the rear wheels for example... if not then im thinking of the caliper sticking possibily...

Felix

The drag is from the viscous coupling and is normal.

The rollers used for brake testing are slow enough and the duration of the test short enough that it shouldn't harm the viscous coupling. That said, I tell my MOT man that the car is AWD and he immediately says he'll test the brakes by road testing the car using their decel meter. His test route is short, twisty and 30 mph speed limit and I've known him for years so i have no qualms about him doing so. He always tell me the same thing about how good the brakes are!

I have another car with a torque sensing (Quaife) LSD and that one also needs to be road tested as the LSD affects the roller readings. Any MOT tester with experience will want to road test a car with an LSD and probably most with AWD as well. The 993tt has both!

Miles965uk

great stuff- thanks for the quick reply.... i can sleep better now. so i guess it was ok but in future ill get him to do a road test or put the put the other end in air?!

PorschePhD

NEVER NEVER use a 2WD dyno on a AWD setup from Porsche. The viscous coupler will over heat and pop. I have seen it happen to many times. Use a AWD or actual road test.

tlark

Ditto, my car had to have the v-coupler replaced before I purschsed this b/c the bonehead dealer that had my car did this.

Felix

The title of the post and the question within it are somewhat different. It's pretty universally agreed here that 2WD chassis dynos (US-speak) and rolling roads (UK-speak) can damage the viscous coupler in a 993tt (or 993 C4) if the front prop shaft is not disconnected. The same applies to some 4WD chassis dynos. If in any doubt run the car in 2WD (GT2 wannabe) mode.

UK MoT's (annual roadworthiness inspection on any car 3 years old or older) use low speed rollers to test foot and hand brake system efficiency, front to rear bias and side to side variation. These are low speed short duration tests that shouldn't harm the VC. Having said that, the VC and LSD will throw off the results so a clued in tester will quickly realise this and use a road test instead.

Miles965uk

ok thanks for the clafication Phelix. i thought a full rolling road would be harmfull but you think this brake test did any harm? what i mean is a fast rolling test will do harm but why not a slow one? is it either just a small amount of harm is being done or something doesnt trigger something else to do harm...... hope im not in trouble, ive just got the car!

Felix

I should think everything's fine. The quick test is to jack up a front wheel and try to turn it. If it turns easily the VC's dead; if it turns but with some resistance the VC's still doing its thing.

The next time an MOT tester wants to put your car on the brake rollers direct him to section 3.7 as here He should know better!

Miles965uk

Hey thanks alot for that. Jesus you have to be so careful that these things are done right!
Cheers alot

VerySideways

I'm with phelix on this one (hello mate) - the VC would not have gotten too hot on a short stint on the low speed rollers used for the MOT, so i reckon you'll be alright.

By the way Miles, lovely car!!

VS

Jean

I don't know how long it takes in the UK MOT perform this, but here ( not really MOT inspection) it is only less than 10 seconds, and they hardly even roll.
Stephen is (obviously) right concerning the dyno though.
Are you driving it Miles? How is it going?