humming brakes

spannermonkey · 16 Jun 2018

Since owning the my C2S, ive always has some issue with brakes, namely vibration and humming noise braking from high speed, and a slight pulsing feeling when coming to a halt. Had the discs skimmed a while back and the pulsating went, so assume they were warped. Tyres were also replaced due to wear - no effect on braking issue.
Recently, the low speed pulsing came back and a dti confirmed a bit of run out on both discs. Replaced front discs and pads with new.

100 miles later, and my first high speed stop, discovered the humming noise is still there. Basically, if i brake hard-ish from say, 80mph, pulling off a slip road, the humming noise gets louder as the speed is scrubbed off. Once down to town speeds, the brakes are fine and quiet. Braking force is good, pulling nice and straight. No signs of overheating.

Any ideas what might be going on?

Endoman · 16 Jun 2018

What make are the pads? Could be pad lift or a sticking piston. Pull the pads and look for tapered wear. Only use Textar pads if solely for road use.
Pagid yellows squeal like a stuck pig.
Another possiblity is worn front suspension bushes setting up a harmonic vibration.

spannermonkey · 17 Jun 2018

Endoman said:
What make are the pads? Could be pad lift or a sticking piston. Pull the pads and look for tapered wear. Only use Textar pads if solely for road use.
Pagid yellows squeal like a stuck pig.
Another possiblity is worn front suspension bushes setting up a harmonic vibration.

pagid, standard compound. This isn't a squeal, more of a rumble. Pads and discs are brand new.
Its definitely heat related. When i brake hard the noise gets louder until i ease off the pedal. :?:

Endoman · 17 Jun 2018

I would have said load rather than heat. Low frequency noises are most often wheel bearings. Heavy braking will alter the radial loading of the front wheel bearimgs especially if the lower wishbone bushes are worn, the front wheels will toe out slightly. That alters the slip angle of the tyre, tyre looses grip and abs is triggered.
Check for loose caliper bolts (another possibility) they should be replaced when calipers removed.
I'm totally ocd about brakes having had a few brown trouser moments on track.

stuttgartmetal · 17 Jun 2018

You have plate lift
Do they lock on when parked ?

Caliper off refurb.
About a grand

Endoman · 17 Jun 2018

Great article from Jackal:- http://www.jackals-forge.com/lotus/968cs/968_platelift.htm

spannermonkey · 18 Jun 2018

brakes (front) are all new (edit; discs and pads new) and were inspected during refresh so id be surprised if there was lift. No locking when stationary.
I might give the rears a look over too; Ive been assuming the fronts were to blame due to the accompanying wheel wobble, but I suppose that could be a red herring.

Endoman · 18 Jun 2018

Golly, if you have wheel wobble then it does suggest wheel bearings.

AVI_8 · 18 Jun 2018

stuttgartmetal said:
You have plate lift
Do they lock on when parked ?

Caliper off refurb.
About a grand

Jeez the words of doom!
Plate lift is easily identifiable, did the new pads slide in easily? If so any plate lift is not going to cause a problem.
If the pads had to be forced in then plate lift is likely.
The calipers won't need to be rebuilt just because of plate lift unless the seals are leaking or if there's a seized piston/s
It's only a few hours of labour to cure plate lift.

guards red · 19 Jun 2018

Hi,

The brakes do make more noise than other cars due to the cross drilling. Have to been out in another 993 or a car with cross drilled brakes?

It may be a side effect of the brakes fitted, depending on how noisy they are.

Regards
Gr

9xxnick · 19 Jun 2018

Have your wheels been refurbished/repainted?

wilsonny · 19 Jun 2018

AVI_8 said:
It's only a few hours of labour to cure plate lift.

man it took me more than a few hours to cure plate lift on mine... if those torx screws have been in since new get ready for some intricate cutting off... I've done all four, one of them took me all day just to get those four horrible bolts out without damaging the calipers or pistons. I'd recommend the very best torx bit set you can find and a cordless dremmel with cutting disc for when it all goes south. refer to the usual jackal site for the procedure.

jev911uk · 20 Jun 2018

wilsonny said:
AVI_8 said:

It's only a few hours of labour to cure plate lift.

Click to expand...

man it took me more than a few hours to cure plate lift on mine... if those torx screws have been in since new get ready for some intricate cutting off... I've done all four, one of them took me all day just to get those four horrible bolts out without damaging the calipers or pistons. I'd recommend the very best torx bit set you can find and a cordless dremmel with cutting disc for when it all goes south. refer to the usual jackal site for the procedure.

Yes those torx screws are little tinkers!! Key is to use some heat to soften the threadlock that is used on them, they mostly wind out fairly easily then.

You need quite a small flame and some alloy heat shields to protect the rubber seals but well worth the hassle v's stripping the torx heads trying to break the threadlock without heat.

All that said I'm not sure this is OP's problem, do you have the correct anti squeal parts fitted?

https://type911shop.co.uk/epages/de...ca1-377e-432e-84fd-bdccf2206766/Products/4299

Pro Calipers · 20 Jun 2018

AVI_8 said:
stuttgartmetal said:

You have plate lift
Do they lock on when parked ?

Caliper off refurb.
About a grand

Click to expand...

The calipers won't need to be rebuilt just because of plate lift unless the seals are leaking or if there's a seized piston/s
It's only a few hours of labour to cure plate lift.

Calipers will never leak with worn seals, but fluid seals last only 50k or 5 years.
They are made of rubber so will not last forever as most thinks that they do.
I will say that all 996, 993 ... going down the list will need overhaul with new seals.

9xxnick · 23 Jun 2018

9xxnick said:
Have your wheels been refurbished/repainted?

OP - this may not seem relevant, but trust me that it may be if your wheels have been repainted.

If paint has got onto the face of the wheel where it mates with the disk, this causes the clamping force of the nuts to be uneven across the disk's bell. When it heats up this allows the disk to distort - temporarily at first, then permanently as the "high points" get worn down. New disks will cure the problem temporarily (few hundred to a thousand miles, dependent on brake use) and it will then recur.

This is by no means the only mechanism that can cause brake vibration, but it is absolutely and demonstrably one possible cause.

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