AP Suretrak LSD behaviour

[Shaun_E]

There are lots of discussions on here about not putting LSD equipped cars on the brake rollers for an MOT and in fact the MOT manual does say don't do it. For the past 2 years my car has gone on the rollers so today at my MOT, I asked the tester why. He surprised me by saying that he hadn't realised that mine had an LSD as it hadn't behaved like one when he was turning the rear wheels during the checks of the rear. He said he usually spots them during that check. Because of the unusual behaviour, it wouldn't affect the brake test - seems the MOT instruction is because of the effect on the test rather than any likelihood of damage. He wondered if the diff behaves differently at different speeds and maybe the roller speed wasn't enough to activate it.

 

So the question is: what is it about an AP Suretrak that makes it behave differently from, say, a ZF?

 

Its definitely an AP diff as a)I have the original purchase docs for the car b) the diff is stamped with an AP part number and c)it does rather excellent donuts.

 

Yellow SL #32

[shn7]

Good question. I've wondered this too as when my car was MOT'd last year the tester suggested that it didn't have a LSD for exactly the same reason.

 

Steve.

[martinwhitcher]

I believe that the AP diff behaves like a "normal diff" ie. when turning one wheel the other wheel turns the the other way, whereas an ZF turns the same way as per typical LSD

 

Martin

 

MW 51 CAT

Superlight No.171

now known as:Superlight DVA 207

[shn7]

Ah but the question I was wondering is, Why?

 

Steve.

[Peter Carmichael]

The AP design gives a torque-biasing effect. The diff is more locked up when there is lots of load going through it. In the MOT test, at least one shaft is always free to move, so the diff behaves as an open diff would.

[Shaun_E]

OK that makes sense. Just to be sure, does that mean that mean it is fine to put on the rollers for a brake test? Does the Quaife diff behave in the same way then?

 

Yellow SL #32

[caterhamnut]

More importantly Shaun, they look really cool inside - see here

 

www.mycaterham.com

here

91,000 miles -1st 1.6k Supersport, '95 Motor Show car

[caterhamnut]

Oh yes, and if you ask an expert how it works, they kind of think, and then say - 'well, its sort of magic......'

I think, briefly, I understood how it worked when I was looking at the constituant parts, but I soon lost it, and its a b*gger to explain!! 😬

 

www.mycaterham.com

here

91,000 miles -1st 1.6k Supersport, '95 Motor Show car

[Shaun_E]

Angus - I looked at your site again to remind me of what it looked like but, as you say, trying to understand how it works is a different matter 😳. They do look kind of funky inside though.

 

Yellow SL #32