Brake judder

[RollingThunder]

I fitted a set of Brembo discs to my 940 about a month ago and have had varying degrees of brake judder ever since. I'd assumed that this was down to me not fitting them correctly, but having pulled the wheels and discs off today, they were/are a good installation. However...

Although the discs are new (from GSF) they seem to have a slight throw - both of them are out by I'd guess 0.5mm (I held a pencil resting on the caliper and spun the discs observing the clearance between disc and pencil tip).

Would this be enough to cause a judder? I checked the discs when they were cold, but the judder does get notably worse after using the brakes a few times. Would I be right in assuming that the warpage can get worse when hot?

Thanks in anticipation!

Rather than guess I'd go and get yourself either some feeler blades, a run-out gauge or (for measuring disc thickness only) a micrometer. Problem is < 0.5 mm is going to be impossible to tell by eye etc. 0.5mm would be a lot.

If you get yourself feeler gauges, measure the thickness of the gap between a fixed point on the caliper and the disc (say mid way into the disc) at four different points and check the maximum difference between the measurements. This is your overall run-out which - crucially - includes hub runout as this may be where your problem is coming from. Another possibility is that you didn't clean the mating surface on the hub sufficiently leading to too high a runout.

Oh, by the way all measurements are pointless unless you secure the disc to the hub properly. Do this by refitting a couple of wheel nuts with some washers between them and the disc. Tighten them up, not necessarily to full torque, but pretty tight. Hold the disc in place by using a long bar between two wheel studs and the ground to prevent the disc from turning.

If you find the runout is out of spec, it is then either the hub face or the disc. You can check the disc with a micrometer. If it is ok (which I would expect as it is brand spanking new) then the problem is the hub or the mating face. You could check this with a runout gauge but that would be a more expensive option. I'd go for the feeler blade test first, that's cheap.

If you find the runout is ok then post again and can advise more on what to check with pads/calipers etc. If you haven't bedded in the brakes this could have caused some warping but I'd say this was unlikely at this stage. If you suspect runout out of tolerance, I'd do the simple check as I say above (feeler blades probably aren't too much more expensive to buy than a pencil )

And wow this is my 1000th post! I think I should get out more ....

Pete

[CTCNetwork]

Hi,

Originally Posted by peteS40

And wow this is my xxxth post! I think I should get out more ....

Pete

You and me both I think..
And you are still purple...!!! Group colour issue deffo..

Des. . .

Originally Posted by CTCNetwork

Hi,

You and me both I think..
And you are still purple...!!! Group colour issue deffo..

Des. . .

Ha ha... yeah the colours don't change very often I'm not sure they happen automatically. And I thought I'd been thanked a lot, but I see you've been thanked even more than me - not far off once in every 6 posts! U must be way too helpful...

Pete

[Andy_Gibson]

0.5mm is a lot of runout on a disc but the only real way is using a DTI gauge and measure the runout.
I take it you removed all the rust off the mating face of the hub first, I always use a rotating wire brush in a drill to remove all rust

[RollingThunder]

Thanks for the thorough reply Pete Deserve another 'thanks' for that, as do you Andy - I checked the spec after your comment and the max runout allowed is 0.05mm. Obviously I'll need to get a runout gauge to measure this - I didn't think the tolerance was so fine!

Yep, I cleaned the hub face with a wire brush on a cordless drill, etc - its very clean and shiny.

Never had this problem before - spoilt a good day's work! (I changed the auto selector bulb - what a pain!!)

Cheers,

[CTCNetwork]

Hi,

Would it be that you have actually got a dodgy set of disks?
And would GSF be prepaired to change them?
What info, measurements would they be looking for to assess whether they are badly made and up for an exchange..?
Have you asked them this?

Des. . .

[Andy_Gibson]

Have the brakes made a judder ever since fitting the discs?
Did you replace the brake pads as well, if using old pads are they worn uneven

It does sound like you've bought some faulty discs though