Little doubt about brakes.

CiniO

I had my car NCT'ed today. (mazda 6).
It passed, but results somehow worried me a bit.

Brake test:

Front axle: 3.35kN (nearside), 2.57kN (offside), imbalance 23%
Rear axle: 2.01kN (nearside), 1.73kN (offside), imbalance 14%
Service brake effort: 70%

Parking brake: 1.83kN (nearside), 1.30kn (offside), imbalance 29%


Generally speaking imbalance between left and right is quite big.
I had a little pre-nct check and had my brakes tested few weeks ago. Front seemed perfect (both about 2.9kN) and rear were very bad as left side was about 1kN and right side about 2kN. The same with handbrake - left twice weaker than right.
However I got rear brakes fixed (new discs and pads) and results after were very good (both rear service brake and handbrake at nearly the same force on both sides).

Now on NCT it looks bad, and I'm wondering how come brakes could deteriorate so much during few weeks and less than 2k miles traveled?

23% imbalance should on the front, should be possible to feel while severe braking that car is pulling to the left. I tried few times (today after NCT), but it goes exactly straight while heavily braking. I checked my handbrake as well and it looks it works with the same force on both sides.
My testing method is just to accelerate to about 50km/h on loose surface (gravel) put car on neutral and start pulling handbrake lever with constant speed. I pull it until both rear wheels lock down.
Then I get out of the car, and look at the traces if they both locked on the same spot. If the didn't, it would mean there is imbalance between them, but fortunately they both locked at exactly the same spot.

It all looks a bit strange. Is it possible, that NCT equipment for testing brakes is not precise and makes such big errors?
Or maybe when ABS comes in force while testing, there might be some abnormalities. However if this was the case, it wouldn't affect handbrake test at all, while on NCT results hand brake had nearly 30% imbalance.

What should I think?

kerten

Just guessing here but could it be tester error of positioning car not straight because front, rear and handbrake braking power is higher on nearside consistently.

Easiest way to confirm is going back to first place and do another test to see if result differs from nct results.

Also brake imbalance may not be visible at low speed emergency brakes if you have good tyres as grip can compensate imbalance.

CiniO

kerten wrote: »

Just guessing here but could it be tester error of positioning car not straight because front, rear and handbrake braking power is higher on nearside consistently.

I didn't notice that.
Could be the reason

Easiest way to confirm is going back to first place and do another test to see if result differs from nct results.

Unfortunately it's not possible, as first place is over 2500km away.

Also brake imbalance may not be visible at low speed emergency brakes if you have good tyres as grip can compensate imbalance.

I remember I had once front brake imbalance of about 40% some 10 years ago in some car, and while doing road emergency braking test, you could feel, car was pulling a lot to the side. However it was car without ABS. Maybe ABS on my Mazda is compensating this effect.

.... wouldn't worry, had an ole Merc retested the same evening it failed on the handbrake, the service brake results weren't too similar tbh across two test lanes.

CiniO

RoverJames wrote: »

.... wouldn't worry, had an ole Merc retested the same evening it failed on the handbrake, the service brake results weren't too similar tbh across two test lanes.

Can you remember around how much was the difference?

.... I might have the ole test results, stay tuned

Apologies, just had a looksee, only have the original fail sheet, must have passed the pass one onto the lad I sold the car to.
On a more positive note I notice there's £30 in my passport that I'd forgotten about.