Deciphering MOT brake calculations

[MrSteve]

I was looking at the print-outs from my Citroen LNA's last MOT and realised I didn't understand it all. It passed, but I was wondering if any of the figures mean I should be fiddling with the brakes to get them better for the next MOT.

 

Service brake force is 559kgf, giving an efficiency of 59%. Am I right in thinking 50% is magic total to pass?

Parking brake force is 190kgf, giving an efficiency of 20%. No idea what the pass percentage is for this one!

 

Total weight is given as 950kg, which seems a bit heavy for a car with a kerb weight of only 706kg (I think they add on 140kg for driver, fuel etc.?). Will this heavier weight help or hinder the brake efficiency calculations?

 

Imbalances are 19% at the front (186kgf left, 215kgf right), 29% at the rear (left 86kgf, right 72kgf).

Handbrake says 94kgf left, 96kgf right, no imbalance.

 

It does have a tendancy to pull gently to the left under braking, but I always take the pads out to clean them and grease them, and check each piston for movement- all 4 pistons move in and out ok.

 

The pedal also pulses a bit when braking, which I put down to a shit brake disc and front bearing change job that a garage in Kendal did a few years back. It didn't pulse before they changed the discs!

 

Is anyone able to give me some pointers on what to do?

[DSdriver]

I thought that kerb weight included fuel i.e. the weight of the car on the road. Perhaps all the extra weight is wob

[scaryoldcortina]

The extra weight is for the driver and luggage/passengers..

 

Now, the brake calculations are pretty easy. Each wheel is measured in turn, and its brake force reading recorded. The readings are added together, and then divided by the brake test weight of the car to give an efficiency reading.

 

Pass for service brake - 50%

Pass for parking brake - 16% (or 25% on a car with single circuit brakes)

 

Imbalance is calculated by measuring both front wheels brake force, then subtract the low reading from the high one and divide by the high one x 100 (to get percent) so if you got 200 and 180 that would be 20/200x100=10% imbalance. Pass is 25%.

[catsinthewelder]

My VW used to pull to the right under braking quite worryingly. Changing the flexible brake hoses has improved it no end. It was a right swine of a job though as the unions were properly seized after 37 years.

[Albert Ross]

Not forgetting that if more than half of the braked wheels actually lock up, then the total effort achieved could actually be UNDER the 50% and still pass. I did a ZX like this a few years ago..... Within a week the owner traded it against a Meriva whicg she still has. The ZX got fragged.

[CIH]

Can I haz hijack ?

 

A "friend" has "modified" his previously open diff so it is now permantly locked for sick drift yo etc can that still be brake tested on rollers or does it have to be skid pans/roadtest ?

[Albert Ross]

It cannot be roller tested, as any increase/decrease will not register. I would suggest that a standard diff should be fitted for the annual inspection. The thing being, it MAY damage MOT equipment...... And I wouldn't like to pay for a set of brake rollers.

[scaryoldcortina]

Re - locked/lsd diffs, we get this quite a lot (bloody morgans!) and what we have been advised to do by vosa is to roller test the front brakes (for balance) and then do the efficiency readings with the tapley (decelerometer) in the car park.

[Craig the Princess]

Re - locked/lsd diffs, we get this quite a lot (bloody morgans!) and what we have been advised to do by vosa is to roller test the front brakes (for balance) and then do the efficiency readings with the tapley (decelerometer) in the car park.

 

What is the story with the decelerometer, our local garage seem to use it on most of our chod.

[scaryoldcortina]

^ If you don't know the weight of the car (and the computer can't find it because it's pre 1980) then you have to decelerometer test to determine efficiency.