Impreza's and brake testing.....The definitive answer....
 

I know there's been threads on this in the past but none really gave a definitive answer.

So, can Impreza's be brake tested on the normal roller machine at MOT time or should they be done on the road using a decelorameter in the foot well ???


Matt.

No normal roller can damage diff so decelerometer is official device for testing brakes for 4wd vehicles. There is however rollers which are designed for 4wd cars with counter rotating rollers.

As above most places do not have the rollers for four wheel drive so it is the decelerometer.

I test cars at our dealership in the rollers, But ours are the latest 4wd enabled. ordinarily I'd only recommend a decelerometer test if 4wd rollers are unavailable

Cars are done on rollers for mot over here.

What's the exact reason why they can't be done on normal rollers ??
I've seen things like range rovers etc tested on the normal rollers for example.

I can't see a problem if the car has an open centre diff, not for a quick break test anyway

I would only test with deaccelerometer!!

If not 70 foot brake 30 hand brake always passes he he

I think it's more the reason they shouldn't be done on rollers, if I can explain.

Permanent (or locked) 4WD means that both axles will rotate at the same speed - or more precisely each will TRY to rotate at the same speed, so if one eg. the front is driven then the other will try to keep up. If in this example the rear is prevented from doing so then the centre diff has massive strain - one end says 'go' and the other refuses, so it quite likely breaks.

As said, an open diff wouldn't be affected like this, and to add to the confusion certain 'intelligent' configurations wouldn't be either.

Early examples of 4WD, AWD etc. wouldn't be that 'clever' and would almost certainly be affected, later or more sophisticated ones probably wouldn't.

Imo the golden rule is simply this - ALWAYS use a Tapley (decelerometer) on such vehicles, to avoid any possibility of issues arising.

Oh - and ensure that the tester knows it's 4/AWD of course :)

Last time mine went for mot the tester just ran it up the road " yep that stops "

:lol1:

I think you might be getting a few requests....... :D

Glad i saw this post, mines booked in down a local place on wednesday.

I'll see what they say.

Yes, one up for the op I think :thumb:

Just to add to what I said (or make it obvious I suppose) you might find they say something like "it's OK, we've MoT'd dozens of Subarus". They could well have, and all been ok on the rollers but the critical thing is how the 4/awd (and diff) operates, so 999 could be OK but one with something different might not.
(It could for example have the DCCD locked if it's snowing at MoT time).

There was a thread on here a while back from someone who thought their car had been ragged after having work done and MoT'd. Someone posted about the need to use a Tapley, suggested what the problem was (NOT the clutch slipping but a 'jarred diff', to use my words) - and were right.
Still caused a lot of consternation - accusations flying back and forth and iIrc the threat of legal action against SN.

Although this is a bit sneaky (but very tactful in the circumstances) I'd suggest recording yourself asking the tester or testing station if they have a decelerometer and if they say 'no' mention your concerns because of what you've read.
A secret recording won't get their hackles up and any reasonable business should be able to put your mind at ease with an explanation.

If it goes t!ts up you will have a good case (no such thing as watertight unfortunately). You won't just have to rely on 'they should have known'.

I have a dashcam, i'll leave it running ;)

Don't the diffs allow so much play? I ask for the following example...

Drive a Subaru in a relative tight circle (say 20m diameter) for a while, surely the diff would go? Two inner wheels will have a substantially less distance covered than the two outer...?

An Mot tester done my old one on the rollers not long after the centre diff shat itself. Vosa said tapley only should be used not the rollers.

I'd like to see some posts from actual mechanics who use this site.
Like all the other threads about it, some say you can't but don't really know why and some say you can.

Not to mention braking hard enough to lock the front wheels whilst the rear still rotate.

At our place ALL 4x4's go out with the Tapley, no exceptions, no comebacks. Better safe than sorry.

Open diffs allow for this I.e. the differential in rotational speeds

An open centre diff together with open diffs on each axle will send all power to a single wheel if it is slipping

My point being the different distances covered between inner and outer wheels. Is it different to axles ie front and rear?

Yes, but an open center diff allows for the difference

Would said center diff allow for the front wheels to be in motion briefly whilst the rears are still.

See the point I'm getting at. Actually the point's laid out well in my original reply :lol1:

Yes, they could rotate forwards, backwards and sideways

+1, it would be nice to see some replys from the likes of SC etc

My Mot station recently done mine on the rollers with my classic WRX. Newish rollers, I mentioned to the tester and he was confident that no issues would arise.
My thoughts are that it was that short that it can't do any harm, and that Ive probably done more damage to my drivetrain arsing about in the car drifting about and doing donuts etc. All Imo of course, and I'm not saying he was right or wrong.

The lack of response from the big names only proves my point.
Nobody really knows.

I'm surprised Ali B hasn't commented, he's usually up on these matters.

I commented before on it in previous thread. As stated above by rb5 stu all mots in northern ireland are done in government run test centres and all cars are put on rollers. The rollers have a 4wd mode that means only one roller is driven the other roller rotates freely hence allowing the test to be completed

if these rollers broke awd transmission & was proved rest assured itd be all over the news as everyone in n.i. would be claiming for new gearboxes, diffs etc

youll always get someone saying they should never be put on rollers no matter what but fact is it can be done if the brake roller tester has a 4wd mode on it.

That's what diffs are for.

Centre diffs have a different function.

Read my lips: