MOT brake test results question

[leorest]

The Wedge has just passed again!
Figures quoted for breaking efficiency were
Service brake 70% (50)
Handbrake 30% (18)
Numbers in parentheses were the quoted minimum pass level.

When I asked what 70% is a measure of I got a poor answer.
Without being funny, to be a competent tester (and he was) does not require full understanding of the physics involved, so I didn't embarrass him further

I've just Googled for half an hour and still not answered my question so hope someone out there can help.
What does 100% represent?
Leo

[GreenV8S]

Maybe 100% is the braking force required to lock the wheel on the test surface?

[jeremyc]

Check out the VOSA site and search for 'brake efficency': there is a PDF file that tells everything, equations an' all.

[leorest]

Thanks
So...
(Total brake effort / Vehicle Weight) * 100 = % efficiency
Does that mean that 100% = 1 g
Leo

[GreenV8S]

leorest said:

Thanks So... (Total brake effort / Vehicle Weight) * 100 = % efficiency Does that mean that 100% = 1 g Leo

Only approximately, since they test front and rear wheels separately. In the real world you would get loads of weight transfer so the weight on the front wheels goes up and the weight on the rear wheels goes down under heavy braking. In a static test on the rollers you don't get this weight transfer so each wheel just carries its static weight, presumably it is this static weight which is used in the formula given above.

[leorest]

GreenV8S said:

Only approximately, since they test front and rear wheels separately. In the real world you would get loads of weight transfer so the weight on the front wheels goes up and the weight on the rear wheels goes down under heavy braking. In a static test on the rollers you don't get this weight transfer so each wheel just carries its static weight, presumably it is this static weight which is used in the formula given above.

In this instance the test was done by driving the car at 15mph on the road and the measurements being taken buy a box of tricks placed in the passenger foot well (Tapley?). This obviously takes into account weight transfer.
Considering the crap origins of the front brakes and the state of the disks I'm impressed that they achieved 0.7 g. Even more impressive is the handbrake holding a third of the weight of the car. Luckily my drive is less steep than 1:3
Thanks for the help
Leo

[steve_D]

My god. Not seen or heard of the Tapley meter for donkeys years.
Used to use one on a regular basis when I worked in a garage as a lad.

Steve

[agent006]

Tapley meter is still standard practice for testing permanent 4wd cars i believe. Although i did have to remind my tester last year what "quattro" means.

[steve_D]

agent006 said:

Tapley meter is still standard practice for testing permanent 4wd cars i believe. Although i did have to remind my tester last year what "quattro" means.

Have they gone electronic or is it still a drum that revolves inside a window?

Steve

[leorest]

steve_D said:

Have they gone electronic or is it still a drum that revolves inside a window? Steve

Couldn't tell, it was a small wooden box with a handle on the top.
Leo

[steve_D]

Sounds like the same one.
You stand it in the passenger footwell, open the box and set the instrument level.
There is a rachet lever which you release.
Get going to a steady 30mph and reach over and set the rachet. You then stamp on the brakes.
A drum inside the instrument rolls forward under braking and locks in its highest reading.

If you can get it to fall over you don't bother with a reading.

Steve

[leorest]

steve_D said:

If you can get it to fall over you don't bother with a reading.

...in the good old days they used a house brick

[steve_D]

I know I'm old but I don't go that far back.

Is that a 'Hand Made' house brick?

Steve

[leorest]

Yep
The tallest one you can find
L

[nighthawk]

The old tapely meter still gets frequent use on 4x4s and LSD equipped vehicles, always nice to get out of the garage for a wee drive

The handbrake only needs to be 16% on a vehicle with a dual circuit vehicle and 25% on a vehicle with a single system.

[ifc63]

nighthawk said:

The old tapely meter still gets frequent use on 4x4s and LSD equipped vehicles, always nice to get out of the garage for a wee drive The handbrake only needs to be 16% on a vehicle with a dual circuit vehicle and 25% on a vehicle with a single system.

Now I'm worried. The Camaro was tested last year on a normal roller tester, but has a limited slip diff. Would thishave harmed the diff in any way. The new Camaro is nearly due its test and I'm taking it to the same garage. Should I take a house brick with me?

[leorest]

ifc63 said:

Should I take a house brick with me?

Yes and if he starts to put it on the rollers hit him with it
Seriously though. I'd check with the dealer or factory and make sure you let them know when you book the MOT.
Leo

[ifc63]

Well the Camaro went for the MOT on Wednesday. Told the tester ( an old acquaintance of mine ) that I had been advised not to have it roller brake tested. No problem was the reply, and out comes the floor mounted brake tester. "Oh no, left hand drive, I'm not keen on driving that" was the next comment. I jump back in the driver's seat and we set off to test the brakes. 1/2 a mile later after showing him it brakes as well as it accelerates, even in the damp conditions of last Wednesday, a rather nervous MOT tester returned to print my new certificate.
Thanks for the advice everyone, it certainly pays to read most of the threads on here.

[Mr E]

ifc63 said:

1/2 a mile later after showing him it brakes as well as it accelerates, even in the damp conditions of last Wednesday, a rather nervous MOT tester returned to print my new certificate.

Good lad.

Did the same thing with 4 new discs (325mm vented), 4 pot calipers and very recent TRD pads. On recent Goodyear Eagle F1 GSD03s on a nice warm sticky road, I managed to trigger the ABS from 30 odd mph.

It's fair to say she passed. And I detached my left retina.