Brake Shudder - Over 60mph only

[scam75]

Afternoon all

As it says in the title, I am getting brake shudder through the pedal above 60mph, nothing at all under 60mph. It is more pronounced when dabbing the brakes, firm application shows a bit of shudder but not much. There is no shudder or wobble at all off braking at any speed and wheels are all balanced ok.

Discs have been changed all round in my ownership, never cured the problem. Front calipers are recently refurbed and rears were new about 5 years ago.

We did notice a rear wheel slightly out of true on the rolling road last week, passenger side. It is balanced fine though.

Alastair has also cleaned up the hubs and mating surfaces which helped maybe 10-20%. We had gauges on the discs a while back and there seemed a little bit of run out on the drivers side.

What could be causing the issue? The wheel, a wheel bearing, a disc or a caliper issue maybe?

Suggestions welcome!

Stuart

 

[PSH]

sounds like a warped disk to me... have you recently done a track day or just hard braking from speed and held the brakes on after stopping, that's how disc's get warped as they can't cool evenly?

Pete

 

[eddieedmo]

discs or sticking/stuck piston.

Nick []

 

[944 man]

Isnt this caused by the disc not being mounted true?

 

[PSH]

ORIGINAL: 944 man

Isnt this caused by the disc not being mounted true?

I got the impression that these disc's were fine until recently, hence why I suggested warped disc's, however if this problem occurred straight after fitting new disc's, them yes, mounted out of true is possible cause.

Pete

 

[scam75]

Yeah, had problem for a while, new discs didn't cure the issue but improved it a bit. It's the why over 60 and not under 60 that is puzzling me, makes me think the discs are ok and quite possibly not mounted true, maybe another clean up of the mating surfaces is in order.

Can I rule out a wheel bearing, and does the speed thing suggest I should be looking at the front or rear? The front hubs were completely refurbed and cleaned up which makes me think rear.

Cheers again

Stuart

 

[PSH]

Hi Stuart

I would rule out the wheel bearings first before looking at the disc's, they are very easy to adjust, no more than a few minutes so worth doing.

Pete

Edit:.. I'm talking about the front bearings of course, these are more likely to be the cause of the symptoms described

 

[scam75]

Hi Pete

The front bearings are quite new and adjusted up ok. It's the rears (!) that may be guilty although there is no play evident on either side if you grab either wheel. Could the rear bearings be testing out ok with a traditional wheel wiggle test but somehow be out of running true slightly? That's what I don't know.

Stuart

 

[bmnelsc]

When sealed bearings (like your rears) start to go - you often get some noise - often low frequency - like a groaning or growling. Usually more pronounced while turning. Also a galled or flat spotted bearing may not fail the wiggle test (you may not be able to put enough torque on the bearing at rest) but will show itself at higher speeds.

Next obvious question is have you checked all your bushes front and rear - and ball joints/tie rod ends/steering rack - perhaps some play there that doesn't show itself until higher speeds....

 

[david924s]

Have you swopped the wheels front to rear if not staggered widths, it could be the tyres slightly out of round or a flat spot. I once had an XR2 that had warped discs so Frods replaced them three times under warrenty each set were warped from new so in the end they skimmed a new set and the problem was solved. But I would have thought warped discs would be noticeable all the time when braking, although it could be a misalignment of the disc like mentioned before.

 

[scam75]

All the steering components are pretty much new, I have new castor blocks and front arb is polybushed. At the back I have new droplinks and the rest of the bushes are ok.

The rear wheel bearings are original as far as I know. It's either them or a slight mis-alignment of a disk, most probably a rear one. Or a disc!

Will be up on a ramp tomorrow. Will inspect the rear bearings and mating surfaces I think and see if there's anything to go on.

David - Tyres have been changed a couple of times and made no difference, may be that a slightly warped disc was supplied (it was done by a local indy just after I got the car who subsequently gave me reason not to go back not long after), the rears have been on about 3 years now, may just chuck a new pair on before I go changing wheel bearings and messing about with hubs just to completely rule them out.

Cheers

Stuart

 

[scam75]

Well pretty convinced it's a rear wheel bearing now. I get a vibrating noise and sensation when turning right which becomes evident from about 50 upwards on the motorway or any spirited round about negotiating! Double checked the fronts, which are new, and they have no play.

I will bite the bullet and get both rears done, pretty sure they are original anyway.

Stuart

 

[Paul 290T]

Stuart,

I'd buy (£20 from eBay) or borrow a clock gauge. This is the instrument you need to measure lateral run out. Mount it so that its pin is perpendicular to the brake disc surface which is contacted by the pad. Slowly rotate the disc and record the highest and lowest values of deflection of the gauges pin. Subtract one from the other, the difference should be less than 0.1mm.

This is the definitive measurement that will tell you if your discs are off true. I think you need to know this before going off to replace any components or have hubs skimmed etc.

However, should they have been mounted off true and used for enough miles the high spots on the outer and inner sides of the disc (they're 180° apart) will have become more worn than the part of the disc in-between the high spots. This creates brake disc thickness variation (BDTV).

When you brake the uneven disc thickness forces the pistons in and out twice per revolution. At a certain road speed the frequency of this force input to the cars suspension will match the suspensions natural frequency - the frequency where a small input force results in a large vibration.

To detect BDTV you should use a micrometer.

I went through this problem many years ago when my local Indy fitted new front discs. Not at first, but a few months later I got brake judder. I found the discs to be out of true, refitted them in tolerance, and after a few hundred miles driving wore the discs back into thickness variation spec, thus curing the vibration that occurred when braking. I didn't think I would wear them back in, but it was worth a try.

If you're really interested email me and I'll send you the table of results I got for my disks when I measured them.

 

[scam75]

Thanks Paul, interesting stuff. I have had this issue ever since I got the car, over 3 years now. New discs didn't resolve the issue, I am sure all my mating surfaces are good also. Front hubs were refurbed and rear surfaces got a good cleaning up. The fact my car is acting odd when turning right, the faster, the more odd, I think it is wheel bearing related. Plus the rear bearings have never been done. The vibration/noise when turning right has maybe only been happening about a month but it may well be worn rear bearings becoming more worn, and would explain the brake issue above 60mph which has always been there.

Cheers

Stuart

 

[Paul 290T]

I new of an S2 owner that had the probelm. it turned out that his hubs had somehow become warped. Rather than having them machined he replaced with new. Not sure if this what you had done to your hubs.

 

[scam75]

Did it cause my cornering symptoms (described above) or just the brake shudder above 60 problem?

Cheers

Stuart