Any MOT testers on here?

[TTManUK]

Just had one of my cars MOT'd today and am trying to understand the brake test results, more specifically the efficiency part.

My front brakes have an efficiency rating of 41%, and the rears 29%. To a layman such as myself, that does not sound like the brakes are working very effectively, yet the car passed. Disks, pads, fluid and hoses have all been recently changed, the car seems to brake OK. Why are these figures so low? Is the theoretical maximum efficiency 50% for each axle, or 100% for each? I got advisories for corrosion on the inner side of the front disks, I would have thought if anything, the fronts would be less efficient due to this (although I think its likely this is just due to car has been sitting for weeks before driving a short distance for its MOT and shouldnt be much more than surface corrosion). Can anyone educate me?

 

[190]

Brake efficiency in percentage terms is basically equivalent to expressing stopping power as 0.41 g and 0.29 g and you are right to consider those figures poor. The problem is the MOT pass criteria is very poor so most cars easily pass even when the brakes are borderline inadequate. In the real world 60% efficiency should be the target minimum.

If you are talking about how well should a vehicle stop with both brakes, then 0.8g is a reasonable maximum with the front brakes accounting for most of that.

I'm not an MOT tester, just a motorcyclist that feels it necessary to understand braking theory.

 

[TTManUK]

Is that 60% total, or 60% for each axle? In my report, the total force efficiency is 71% - I'd like to know what I can do to get that closer to 100%, assuming that is possible. As mentioned, I've changed basically all the components, so not sure what else I could do try to improve my results other than maybe a caliper rebuild?

 

[Doodle]

MOT inspection manual: cars and passenger vehicles - 1. Brakes - Guidance - GOV.UK

Brake condition and operation, service brakes, secondary brakes, parking brakes, anti-lock braking system (ABS), electronic braking system (EBS) and brake fluid rules and inspection for car and passenger vehicle MOT tests.

www.gov.uk

Section 1.2 will explain exactly what they're looking for.

100% would not normally be expected for a road car.

 

[grober]

Vehicle test procedures

What happens at each stage of a vehicle test (MOT), from arriving at the test centre until you leave.

www.nidirect.gov.uk

 

[190]

Is that 60% total, or 60% for each axle? In my report, the total force efficiency is 71% - I'd like to know what I can do to get that closer to 100%, assuming that is possible. As mentioned, I've changed basically all the components, so not sure what else I could do try to improve my results other than maybe a caliper rebuild?

I was suggesting 60% total should be the pass criteria. If you had 71% then that's quite good because 80% is generally as good as it gets unless it's a F1 car which can do over 200%.

Adding individual axles together is not as simple as it seems because in real world braking there is weight transfer to the front and a car in use is not quite the same weight as it is on the rollers.

 

[TTManUK]

I read the above, it makes more sense now, the percentages aren't relative to max braking performance, they are relative to the weight of the car. So with the exact same brake setup, efficiency would be improved by reducing weight.

Thanks gents

 

[Fastcar155]

Hi , a friend of mine was a MOT tester and and he says it down to the tester on the day.

 

[Petrol Pete]

"Hi , a friend of mine was a MOT tester and and he says it down to the tester on the day..."

How does that work ? surely the car is on the rollers and the tester presses the brake pedal down . The brake either work or they don't . Some cars with pressurised systems will feed each caliper accordingly. And those relying on the vacuum from the brake booster with the engine running will simply pass or fail regardless of 'the tester on the day'.

But I have heard of MB's failing parking brake tests due to the tester being a short **** who did not press the foot pedal down hard enough .

 

[Spider10]

Hi , a friend of mine was a MOT tester and and he says it down to the tester on the day.

As said before the car is on the rollers and goes green if is in the parameters of a pass red if not so how can it be upto the mot inspector
I bet your friends name is Dave

 

[Wolfie1]

Im a tester, for brake efficiency it is calculated automatically when the figures are added for each wheel whether they lock up and then calculated against the weight of the vehicle, the formula for working it out is on the dvsa link above from the testers guide. Your brake efficiency can appear very low because the wheels may have locked up ie the brakes have outperformed the traction the tyres are able to provide

 

[zipdip]

Well I think the op is worried over nothing,he gave us all a clue in his post as to why his brakes in the MOT were not great and the answer is that all he has to do is take that car out and do about 20 stops braking from 60 mph hard that will bed the new pads and discs and if he is still worried put the car on the brake test machine at a garage ,I bet the results will be right up there with the best.

 

[ALFAitalia]

The will fail the test if the brake efficiency is below 50 per cent (58 per cent for cars first used on or after 1 January 2012)or he parking brake (hand brake) efficiency is below 16 per cent.

Your are perfectly acceptable and normal.

 

[Spider10]

Genuine question ,how can you test rear brakes ? Car is on rolling road in park I presume so how does the rear wheels spin?

 

[190]

Genuine question ,how can you test rear brakes ? Car is on rolling road in park I presume so how does the rear wheels spin?

It's the rollers that do the driving so I presume an auto can't be left in park to do a roller test. The brake performance is measured by how much power is needed to rotate the rollers. As I mentioned earlier the pass criteria is not particularly high espaecially for older vehicles, they are looking for even and smooth retardation as much as anything and don't necessarily apply sufficient braking to cause a wheel to lock. The only reason I know any of this is talking to the tester while my bike was on the rollers. It's 44 years old and the brakes are not at all powerful by modern standards but it always passed. He seemed more interested in the smoothness of application even down to any variation of braking effort during a single rotation of the wheel.

 

[Spider10]

The car is in park on the rollers

 

[whitenemesis]

Car is on rolling road in park I presume so how does the rear wheels spin?

The car is in park on the rollers

Do you presume or do you know?
I always presumed the transmission was in Neutral, allowing the wheels to rotate freely (driven by the test rollers)

 

[Spider10]

The transmission was in park

 

[whitenemesis]

The transmission was in park

Then it can't be tested as the rear wheels cannot rotate (the parking pawl will prevent movement)

 

[Spider10]

That was my thoughts exactly
Thanks for confirming