NCT brake test

aero2k

Hi all,

Just put my ’00 9-5 Aero through the NCT. It failed for an insecure CV boot which is easily fixed. However the brake performance is of concern as it appears to have deteriorated dramatically since the previous test as per the following numbers:
Test on 5 Jun ’08 88676 miles

Nearside Offside Imbalance
Front 3.11 kN 3.97 kN 22%
Rear 2.70 kN 3.19 kN 15%

(car weight 1701 kg) brake effort 78%

Test on 21 Jan ’10 100913 miles

Nearside Offside Imbalance
Front 2.79 kN 2.82 kN 1%
Rear 2.33 kN 2.32 kN 0%

(car weight 17421 kg) brake effort 60% (below 55% is a fail)

Between the two tests the following work has been done:
Fitted front & rear suspension dampers (Koni FSD)
Fitted new front & rear flexible brake hoses (fitted Goodridge braided hoses)
Fitted 4x new tyres at 98,000 miles (Hankook Ventus V12 Evo 225/45 17 94W)
replaced caliper seals front & rear
replaced caliper pistons (front only)
changed brake fluid (Ferodo)

The discs and pads are the same for both tests and there is plenty of friction material left on the pads.
I am assuming that because of the extremely low % imbalance numbers that the possibility of leaks, sticky caliper pistons, or contamination of friction material is more or less ruled out. As the brake force has been reduced on all 4 wheels, it appears that the two most likely causes of the poor test results are low tyre grip or insufficient pedal pressure (I suppose the servo could be faulty but I presume I’d have noticed this before). Braking performance feels better than before, as does tyre grip. Should I demand a retest? Any other observations / suggestions?

(sorry about the formatting, couldn't figure out the table tool)

Cosmo K

aero2k wrote: »

Test on 5 Jun ’08 88676 miles
(car weight 1701 kg) brake effort 78%

Test on 21 Jan ’10 100913 miles
(car weight 17421 kg)

Not sure what the problem with your brakes is, but maybe it has something to do with all the extra weight your car has put on over the last two years......:eek::eek:

BrianD3

Good question. The thing with NCT equipment is as with any other measuring equipment there are errors involved. If you had the car tested 10 times in a row on the same machine by the same tester, you might get quite different results. I'm not sure if how hard the tester presses the pedal affects this test. Also if your tyres were wet and/or cold going into the test perhaps that will have an effect.

How were your handbrake kN readings? If they were also reduced, then it would seem to me that a brake hydraulic or servo problem is unlikely with a friction issue or a measurement bias from the equipment more likely.

Also if your car was the same weight in the 2nd test as it was in the 1st test, your % efficiency would be slightly higher. I presume the 2nd weight is 1742 kg, not 17421 kg that would be a heavy SAAB if it was the latter!

aero2k

Cosmo K wrote: »

Not sure what the problem with your brakes is, but maybe it has something to do with all the extra weight your car has put on over the last two years......:eek::eek:

must be the Maxol E5 I use, all that extra octane....

aero2k

BrianD3 wrote: »

Good question. The thing with NCT equipment is as with any other measuring equipment there are errors involved. If you had the car tested 10 times in a row on the same machine by the same tester, you might quite different results. I'm not sure if how hard the tester presses the pedal affects this test. Also if your tyres were wet and/or cold going into the test perhaps that will have an effect.

How were your handbrake kN readings? If they were also reduced, then it would seem to me that a brake hydraulic or servo problem is unlikely with a measuremnt bias from the equipment more likely.

Also is your car was the same weight in the 2nd test as it was in the 1st test, your % efficiency woudl be slightly higher. I presume the 2nd weight is 1742 kg, not 17421 kg that would be a heavy SAAB if it was the latter!

Sorry about the typo!
Yeah, I accept the point about the weight difference, but I reckon that would only make 3% or so difference. Similarly, I'd expect maybe +/- 5% between machines. However the reduction between tests is 23% (no, not a typo, 18/78 = 23%). I'm confident that the brakes are fine - they stop well, however I don't like paying €50 just to get a piece of paper for the windscreen, I like to be able to rely on the results as part of the service history of the car.
I'm leaving the handbrake out of the equation as it's completely separate on Saabs - rather than using the pads there are shoes inside the body of the rear discs that are operated by the cable.

BrianD3

aero2k wrote: »

I'm leaving the handbrake out of the equation as it's completely separate on Saabs - rather than using the pads there are shoes inside the body of the rear discs that are operated by the cable.

Handbrake data should still be useful in that case. If your handbrake force is also reduced from the previous test, then it might point to something that the handbrake and footbrake have in common (tyres, testing equipment etc.) OTOH If the handbrake force is not significantly reduced, then it might point to a servo or hydraulic problem with the footbrake.

aero2k

BrianD3 wrote: »

Handbrake data should still be useful in that case. If your handbrake force is also reduced from the previous test, then it might point to something that the handbrake and footbrake have in common (tyres, testing equipment etc.) OTOH If the handbrake force is not significantly reduced, then it might point to a servo or hydraulic problem with the footbrake.

I should have pointed out that while the handbrake works, it takes a fair pull on the lever, in other words it "feels" worse than 2 years ago. (I think it needs a new cable, it's on my to-do list. I've heard other owners complain about the handbrake in these cars) The footbrake, on the other hand, "feels" much better than before, hence the concern about the test result. The servo seems to be fine and there are no leaks, so I doubt there's a hydraulic problem. (Also I think a hydraulic problem would make the brakes "feel" bad - they feel fine)

daveharnett

aero2k wrote: »

the reduction between tests is 23% (no, not a typo, 18/78 = 23%).

This was my experience too. I failed a visual inspection for the rear hoses. Replaced them, bled them, and retested the next day on a different lane. The retest found braking force about 20% lower on all corners.

The testing conditions were consistant (similar weather, same drive to the nct center). Even if we assume that I didn't bleed properly the results for the front axle (which wasn't touched from one day to the next) shouldn't have changed should it?

Bottom line is that regardless of NCT's claims about regular calibration, the brake test does not seem to be uniform/accurate.