New brake discs fitted - runout measurement or not?

trex

trex

Member
Is the A2's bearing assembly prone to developing runout? I just bought myself a set of new Brembo discs in order to replace my old scarred ones, and even though this seems as a pretty straight forward DIY job, i'm concerned if i should choose a garage to do this, because of the runout that discs+bearings can develop. I don't have the specialized tool needed to measure this (some sort of pronged micrometer), and even though i doubt the new disks have any problems, i'm not so sure coupled with the rest of the assembly they won't cause problems. I'm also changing the pads to Ferodo Thermo Quiet.
Any mechanics out there willing to give a reassuring/cautionary piece of advice?:)
 
I fitted mintax pads in september. Rubbed like hell for the first 1000 miles (i was convinced it was wheel bearing, so I had it changed with no benefit!). Now it runs fine.
Moral of the story for me? IF you are confident that everything went on straifght, bits that needed cleaning were cleaned, and silicone grease was in all the right bits then don't worry if the brakes rub for the first month after fitting.

That was a £220 lesson and several dis-assembblies to check the brakes for me!
 
I'm certain that the pads will take some time to set in, but what i'm worried about is the lateral movement that discs might have once fitted.
 
What is run out?

I don't think there are any adjustments in a brake setup, so whatever you have now you will have the new discs, yes?
 
Black Pearl said:
What is run out?

I don't think there are any adjustments in a brake setup, so whatever you have now you will have the new discs, yes?
Click to expand...

It's the lateral movement of the disc (rather like a buckled bicycle wheel).

It can be due to uneven brake disc wear, warped disc, or as the OP says, wheel bearing.

I've not inspected the A2 bearings yet, but on my old Mercedes the bearings were tapper bearings, which could be adjusted. Does anyone know whether this is possible on the A2? Either way, it's still a specialised job, since the tolerance on tapper bearing adjustments is very small indeed.
 
trex said:
I'm certain that the pads will take some time to set in, but what i'm worried about is the lateral movement that discs might have once fitted.
Click to expand...

I think you are worrying about something which is very unlikely to happen. Even a garage would not automatically check the disc runout unless there was a problem with the 'feel' of the brakes on the road test.
The wheel bearing are not adjustable. Worn bearings tend to become noisy long before any free play becomes apparent.
If you have worked on brakes before you will find the A2 discs and pads very easy to replace. Key things to remember are
You need a 7mm allen key to remove the sliding caliper pins
May also need an impact driver if the countersunk disc locating screw is rusted
Unplug the pad thickness warning wiring loom (only on one side (the passenger side) on UK cars) before removing the caliper
Note the inner and outer pads have different length locating clips. Check and note on dis-assembly.
Be meticulous in cleaning the hub face and mating face of the new disc.
Thoroughly degrease the disc 'braking' faces
Rotate the disc on the hub a couple of times to ensure it seats correctly before fitting the countersunk screw.

Cheers Spike
 
That's why i went for new discs, in order to skip on the skimming and useless extra work needed when refurbishing old ones. That and the fact i'm too poor to buy/reuse crappy stuff. :D
Will bite the bullet and go for a non-measured runout but an extra cautious assembly, fingers crossed i won't feel any problems after a few thousand kilometers.
Will the medical type alcohol do the job in cleaning the protective hub face coating on the disc?
 
I finally had the time to fit my disc and pads. After correctly installing the pads and connecting the wear sensor, the warning light on the dash did not go away. Do i need to reset it via vagcom?
The pedal now feels spongy. Do i need to bed the pads and disc via the 10 hard stops from 100 to 15 km/h, or just wait a few hundred kilometers and use the brake gently? There seem to be mixed opinions out there...:confused:
 
do you mean the yellow warning lamp?

thats the cable on the nearside for low Brake pads.
do its probably not back together properly.

Did you take the cap off the reservoir when you pushed/ twisted the caliper back?

maybe Low fluid so check that as well
 
Yes, it's the yellow warning lamp. I connected the sensor plug correctly, it even popped when i inserted it.
I didn't take the top off when pushing the pistons (come to think of it, that can't be too good...), but i checked the level and it's a tad over the maximum limit. Could this be the issue?
 
spike said:
I think you are worrying about something which is very unlikely to happen. Even a garage would not automatically check the disc runout unless there was a problem with the 'feel' of the brakes on the road test.
Click to expand...

Agree with Spike 100%. If you're at all worried about potential problems with a bearing (which incidentally should be self-evident) in terms of 'rumble' noise up to 40mph, then jack up the car and hold the wheel at a 10-to-2 position and rock back and forth. It should be as tight as gnat's chaff with no lateral movement.

I fitted Brembo discs and Bosch pads. Secret is meticulous cleaning on disassembly, grease where necessary (copper-ease and Lockheed red grease) and torque to specification. Job done.;)
 
Just a few updates: found the problem with the yellow warning not going out with the new pads. It seems that the previous owner (or her mechanic more likely :) ) managed to break the end part of the plug that connects to the plug from the pads, and bodged it up using sealant tape. This lets the two pins inside it move freely so they didn't always connect proper. Managed to make them touch and now i have my thermometer back on the dash, yay!
As for the bedding in, i went for the good old "10 consecutive brakes from 100 to 10km/h" approach. In between the ten braking sessions i allowed the discs to cool while slowly getting back up to speed. Brakes are now less spongy and they've got that reassuring feel back. The discs lost their machining grooves and are now all shiny and ready to bite. :)
Only thing i hate is the pedal and the way it feels. It used to be gentle in the first short part when depressed and then turn hard for the rest of the stroke, but now it's soft for the better of the first half and gets hard only very near the floor. It's what i would call more progressive, but not to my taste really.
Any ideas on how to change this setting? I'm pretty sure air get into the piping via the filling reservoir as i didn't even need to top up (normal if you think about it), but could it make it's way when i pushed the pistons back in order to fit the new pads?
 
If you're going to clean brake parts, only ever use brake cleaner. It comes in a pressure pack, and is designed for the job. Alcohol shouldn't leave any residue, but store-bought alcohol is never 100% pure.

Brake cleaner is cheap and proven, so don't skimp.

http://www.halfords.com/webapp/wcs/..._productId_274611_langId_-1_categoryId_165750

As for run-out, I don't know of anyone that has ever measured them on a road car in a regular commercial garage. As per previous advice, fit them correctly and do a gentle test drive. Remember not too cook your pads with much consistent heavy braking, and they will really stink for the first little while until they have cured.

I used take measurements on my racing cars in-between meetings to know when the discs were cooked. Any more than 0.003" and I'd replace them.
 
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