Brake Judder

[alan c]

I get a fairly heavy vibration when braking, i have already changed the front brake pads as they were worn and when doing that I checked the discs and they run true. System was bled and pedal feels good when stationary.

At same time there was some play in the rack so adjusted that as well.

Vibration is only evident after a few braking events, ie once everything is warm. Car does stop but lots of vibration.

Any suggestions on what else to look at, nothing else is obviously loose.

The brakes are the AP 4 pots on vented discs as fitted to HPC's

Alan

[DJ.]

I had brake judder in the past and it turned out to be a warped rear disc, so I would have a look at them just in case. 

[SM25T]

Could be pad material bonding to the disc surface after a hot fast stop .... with foot left on pedal after stopping ? Try cleaning it up with a few hard braking sessions from 70 down to 30 ... but not stopping afterwards .... drive on to cool the brakes down .... then repeat. Check nothing behind first !

[Mankee]

I had some mad brake judder through the wheel once. Turned out to be a loose lower wishbone rear fixing. Replaced the lot with new nylocs. Never a problem since. Bit of an unlikely cause of brake judder, but there is an outside chance nonetheless.

[Simon.Rogers]

Can you feel it when just resting on the pedal.  Feels like a warped disc.  Touch not touching, touch not touching?

I get it regularly on the hire car when people have not been braking hard enough.  Take it out for a blast and ten minutes later all sorted.

As SM25T said its pad pick up.

[alan c]

Thanks have managed some heavy braking sessions today and it's much better but not yet eliminated - will have try some more tomorrow. 

Never had this issue with the car before so don't know what I have done differently to the past. Combination of AP rear brake set up with useless hand brake caliper and under dash hand brake probably doesn't help. Then when it started I probably made it worse by braking gently.

I will probably fit new discs anyway when we get home if I have not managed to eliminate the vibration as I have new discs already and I assume I could get the current ones skimmed to really clean them assuming they are not to worn.

Thanks again for the advice

[SM25T]

Discs don't warp. Skimming will remove the pad build up ... and some metal.

[7 wonders of the world]

Discs do suffer from variation though where sections of the disc have expanded beyond their elastic limit and hence this portion remains thicker than the remainder of the disc.

Measure the disc thickness at 8 places with a micrometer using the mic frame to get a consistent depth, a couple of thou will be enough to put a kick in the pedal with increases to a judder at higher speed.

Chief cause of this is sitting with the brakes on whilst the discs are HOT, in tintops big automatic cars in town traffic suffer worse, frequent 40-0 stops , little air flow and localised overheating due to people not selecting park / hand brake.

[alan c]

Update on this problem, still getting vibration under braking so over last few weeks with some help pads, discs and wheel bearings have been replaced and problem still exists.

Managed to get the car on some MOT rollers and braking effort visibly varies but only on front brakes not the rears.

So next step is remove the calipers and send them back to AP for a refurb - does anyone have any experience of their service?

[SM25T]

Could there be any dirt or corrosion between the hub and the disc. A fraction of a millimetre here equates to a lot of run out at the perimeter.

[JNC]

Have you tried changing the fluid ? If it's moisture retentive there won't be a positive pressure ! It can cause pulsing, 

[Pendennis]

Master cylinder seals worn?

[OliverSedlacek]

Sounds like contamination of the disk surfaces to me. The modern brake pads are much more sensitive to this than the older type of pads.

[alan c]

Discs, pads and wheel bearings have all been changed and this has made no difference.

Brake fluid was not that old and system has been bleed twice in process of changing all of the above.

We have tested brakes at an MOT station on the rollers wheel by wheel front and rear and it only occurs with front wheels so cannot be master cylinder.

I am taking the calipers off tonight  and will send back to AP for a refurb and I was asking if anyone had experience of the AP refurb service.

Alan

[SM25T]

Are you sure the contact area on the hubs to discs is spotlessly clean

[7 wonders of the world]

Assuming the wheels / tyres are the same size - swap them front to rear and retest

[alan c]

Hi,

Apologies I thought I had posted a reply yesterday but if does not seem to have appeared.

Disc to hub area was clean, issue first appeared with discs having been in situ for some time and I am sure it was all clean when we put the new discs on.

Front and rear wheels are different widths so cannot try that but wheels are balanced no issue with vibration except when braking. 

Calipers now off and in way to AP for a refurb if that does not solve the problem I will replace all the suspension bushes but there is no play when you do the normal pushing and pulling of wheels and suspension.

[Pendennis]

When the judder occurs do you also get pedal travel?

[alan c]

Not been aware of any variation in pedal travel

[Paul Deslandes]

Given that you've changed all the obvious components, I'd be looking for run-out of the rotating assembly.

While your calipers are away, can you mount a dial gauge on the uprights and measure the movement of the outer radius of the disks to see that the whole disk and hub assembly doesn't waver as it rotates.  This might occur if one of the bearings isn't seated properly in the hub or if the hub is distorted in some way.  I'd also check the stub axles for wear, potentially causing the bearing inners not to sit properly.

As has been said, a very small amount of movement translates into a big variation in braking effort.

I'm struggling to see why the calipers might cause this problem unless a piston is stuck or reluctant which would accentuate any run-out issue.

Which type of hub do you have, the old one with the felt seal or the later sealed bearing type?

Paul

[alan c]

Hi Paul

I should be able to borrow a disc run out gauge, fairly sure my employers makes/sells them so will check that.

It is not something that gradually appeared so I don't think it's something that has worn. Car was fine then it was off road for a few months while I had the engine rebuilt and problem appeared as soon as I put it back on the road. There was no work done on brakes or suspension etc while it was off the road.

If it is a bearing/hub/axle issue wouldn't I feel something at other times?

I have the older style hub with felt seals

[7 wonders of the world]

Are you sure you haven't got an out of shape tyre?

[BlackSeven]

Last time I saw this issue (on a tin top) it was eventually traced to a faulty tyre.  Failing that I'd recommend checking all front suspension bushes and joints for security, wear or damage. Rob

[Paul Deslandes]

Hi Alan

I must say that would seem to rule out the bearings and hub etc.  I think if you had an out of shape tyre you'd feel it when driving along, with or without the brakes although I wouldn't rule anything out totally.

It would be worth checking the run out on front and rear disks as it's easy enough to do.  Otherwise wait for the calipers to come back and see if that's fixed it.

Paul

[alan c]

Hi I am fairly sure it's not the tyre and what are chances of both front tyres going out of shape? We tested the car on some MOT rollers at a friendly garage and you can see braking force varying on the gauges on both wheels, we tested together and individually and problem was the same.

We cannot detect any play in suspension, any play was less than the other sevens we had available for comparison. Having said that if calipers come back and problem persists that has to be the only remaining thing not changed.

I will update this thread when I make progress, AP will advise on timescale once they have inspected. Depends on what parts are required.