P38A Rolling road brake test

most mot stations take it out for a run with the deceleration thingy...

wammers said:

A P38 must not be brake tested on a two wheel brake tester unless the rear propshaft is removed and the transfer box and gear box are in neutral. Same applies less transfer neutral to Disco 1, Disco 2, Defender and Freelander. Must not be tested on a four wheel rolling road unless ABS and traction control are disabled. A Tapley meter is so much easier.

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We use a two wheel brake tester at work. I've questioned if it is likely to do damage, but was assured it is fine and considering how regularly these vehicles get sent for RBT (at minimum, twice a year), if it was a problem I imagine it'd materialise by now.

Bix said:

We use a two wheel brake tester at work. I've questioned if it is likely to do damage, but was assured it is fine and considering how regularly these vehicles get sent for RBT (at minimum, twice a year), if it was a problem I imagine it'd materialise by now.

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I can only quote Land rovers view on two wheeled testers. Their instructions for using one are quite explicit. Rolling roads were not about when i did tests, the Tapley meter use was standard. I have had several Land rovers over the last few year and none have ever been brake tested on the rolling road at any of the different testing stations i have used. The one you have maybe an all singing and dancing version, normally they are not in the vast majority of cases.

Well, I never thought this would generate such a response, I thought I'd get a couple of answers and that would be it.
Anyway, here is my take on it.

On the P38, as I understand it, it is a limited slip centre diff, therefore, it will allow some slip, especially at the low speed that brake testers work. Because of this, I'd be surprised if it damaged an otherwise healthy car. However, that said, there will certainly be SOME load applied to the static side of the drivetrain and I suspect this is the reason for not using it.
Obviously, if it was a fully locked centre diff it would do lots of nasty things. I think land-rover are simply being cautious as there is potential for damage.
It is also written in the testers manual that 4x4 vehicles don't have to be tested on rolling brake tester and can be tested with the decelerometer.
I'm more concerned about the torsional load on the propshaft when they do the parking brake test!

Wazzajnr said:

Well, I never thought this would generate such a response, I thought I'd get a couple of answers and that would be it.
Anyway, here is my take on it.

On the P38, as I understand it, it is a limited slip centre diff, therefore, it will allow some slip, especially at the low speed that brake testers work. Because of this, I'd be surprised if it damaged an otherwise healthy car. However, that said, there will certainly be SOME load applied to the static side of the drivetrain and I suspect this is the reason for not using it.
Obviously, if it was a fully locked centre diff it would do lots of nasty things. I think land-rover are simply being cautious as there is potential for damage.
It is also written in the testers manual that 4x4 vehicles don't have to be tested on rolling brake tester and can be tested with the decelerometer.
I'm more concerned about the torsional load on the propshaft when they do the parking brake test!

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Ham fisted testers have been known to snap half shafts testing the transmission brake. On old Jags that could never pass the hand brake test using the Tapley meter, i used to park them on an slight, hill if the car did not move it passed.

It is a requirement that all VTS's have a properly calibrated decelerometer as part of the mandatory testing tools and equipment. The vehicles exempt from the roller brake tester are outlined in the Testers manual.

allanw said:

It is a requirement that all VTS's have a properly calibrated decelerometer as part of the mandatory testing tools and equipment. The vehicles exempt from the roller brake tester are outlined in the Testers manual.

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Obviously there are some who don't read it.

I've always used a Tapley meter on permenent 4 wheel drives and would't attempt to RBT one ( the MOT tester is liable for any damage caused)

Moron said:

I've always used a Tapley meter on permenent 4 wheel drives and would't attempt to RBT one ( the MOT tester is liable for any damage caused)

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Sense at last.

wammers said:

I can only quote Land rovers view on two wheeled testers. Their instructions for using one are quite explicit. Rolling roads were not about when i did tests, the Tapley meter use was standard. I have had several Land rovers over the last few year and none have ever been brake tested on the rolling road at any of the different testing stations i have used. The one you have maybe an all singing and dancing version, normally they are not in the vast majority of cases.

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Oh they are far from all singing all dancing. This is MoD equipment, so over priced crap. I can only assume using an RBT is a legacy from the days when Land Rovers had selectable 4 drive, we have a Turnkey BrakeSafe which is used for brake testing trailers or when the RBT is down but never anything else. I wouldn't use a RBT on my own vehicle if it has perma 4drive, but trying to get change to happen in this place is like trying to get blood out of stone. Which is a pain as the Turnkey is much more convenient to use.

There is also a plate brake tester.
Tester drives onto plates and brakes, reverses and goes again.
Mcconechys in forfar use this method.
Hand brake is lift clutch and see if it moves.

With all the test data being zapped direct to DVLA I can see a time when the Tapley meter wont be acceptable unless it has Bluetooth or broadband or some such thing