Braking vibration

[dj.fitz]

My 1994 993 has recently developed a "shudder" and steering wheel vibration when braking. It is at it's worst between about 60- 50 mph but is noticeable throughout the range and with both light and heavy braking. I think it might be time to replace the disks and pads but would be interested if anyone knows any other likely cause?

Cheers.

 

[bones]

I would say that is almost definitely a sticking caliper but if you are replacing or rebuilding those then you will as a matter of course look at the disks and pads too.

 

[dj.fitz]

Thanks for the info. I'll let you all know what the problem is as soon as I pin it down. Pretty sure it's not a wheel balance problem though. The car is set up really well and the problem definately only occurs with breaking.

Thanks once again for the help.

 

[Jonathan Collins]

All of the above could cause your problem, however this fault is becoming so common on other makes of vehicles and it is usually due to warped discs, it makes me wonder if the cause may be these asbestos free pads, or maybe poor heat treatment in disc manufacture who knows? The fault is so common on Peugeots that all dealers have one of those machines for skimming the discs on the car, I had some done at a peugeot dealer(through a friend) for £20, problem solved. If you can find a friendly pug dealer & get them resurfaced it would at least tell you if that is where the fault is.If resufacing the discs cures the problem you could then change everything later if you wanted to. Hope this helps.

 

[Guest]

Maurice

You're right! The holes are chamfered to reduce the risk of the disk cracking. That's why it's not adviseable to have std disks drilled. Skimming them must reduce the ability of the disk to shed heat and can only shorten the life of the disk. Given the relative 'cheapness' of discs (around £100 + vat from an OPC) why risk skimmng old ones?


I also had a bad disc vibration but that was after 160 miles at brands using Ferodo track pads that ate the discs! If the vibrate then the discs are probably warped and it's time for new ones.

 

[Jonathan Collins]

I sugested cheap skimming to eliminate the the discs from the equation.I have played hunt the vibration before and it is not always so straightforward. O.K. it was only a Vauxhall but new discs & pads suspension lower arms, shock absorbers, anti rollbar bushes and tyres did not cure a similar fault! Turned out to be the steering rack which when taken to be reconned was told that it was within its wear limits just my experience.

 

[jcosh]

As we're still on the subject of brakes, can anybody offer any advice for brakes that squeek at lower speeds. It only seems to happen once they've warmed up a bit. Brake performance seems fine. Just a bit annoying/embrassing.

 

[Jonathan Collins]

At the risk of sounding pedantic, how much would it cost (in labour and time off the road) to have the run out checked. I presume you mean with a dial test indicator, these guages are notoriously tempremental in most workshops due to misuse or accidental damage.As I have already mentioned manufacturers tolerances are not always to be relied upon either, although I would imagine Porsche to be better than most.If money isn't an issue I would personally just change the discs and pads, and while you are there clean and regrease the hub bearings.You will have to buy a couple of hub oil seals as well but hey whats another tenner on top of the hundreds you have just spent

 

[Guest]

I had new discs & pads from my local OPC. They had covered 1200 miles and one track day before they began to shake. My OPC had them skimmed and they are now OK, but not a good advert for porsche parts.

Skimming is OK so long as you stay above the minimum thickness, it is only the same as wearing the discs down .

 

[LK_991GTS]

Rallen, I wonder if it was the track day that helped cause your disc warping.

I read somewhere that drivers who know how to avoid this happening warm their brakes up before using them in "anger", but I am not sure of what the correct warm-up procedure should be for a track day. Does anyone know?

 

[Melv]

"disc warping"

Not so much warming the discs up, but doubly important is letting them cool for a lap after a track session.

Mel

 

[ClubSport 911]

Skimming must affect ability to disipate heat and shorten life,

Why ?

An engineering student, will know exactly how brakes work....they dissipate heat. Quicker to heat up (small, thin) = quicker to cool down. Ever seen those mega motorbikes capable of 175+ MPH ? Very very thin discs, loaded with holes..even worse...the newer brakes look more like disc cutters.

Thoughts ?

 

[38F3E1]

I have just had the exact same symptoms. The outer edge of the inside face of the front disks were a little rusty which causes the problem. Same problem occured to the rear disks as well.

Talking to the engineers where I had the problem fixed this is a common problem on 993 cars. Does not happen on 996, 964, etc. For some reason this rusting occurs even on cars that are well looked after / garaged / etc. It is very difficult to see without the car jacked up and typically the outside face will look fine even though the inside face is rusted.

Regarding skimming - there is a minimum thickness that Porsche recommends that disks are allowed to wear down to safely before being replaced. All that skimming does is reduce thickness in the same way as normal wear and tear. You have to make a judgement as to whether there is sufficient thickness left after skimming to have any worthwhile life left in them.

I had my rears skimmed and the fronts replaced as they were too thin to skim. Braking is perfect now.

Ian.

 

[38F3E1]

I can see your argument Maurice but, as always, it depends ....

Let's say you get 20000 miles out of a set of disks so with 1mm wear allowed per side that would give 0.05mm per 1000 miles per side. If a skim takes 0.1mm per side then that is equivalent 2000 miles of use.

Each person has to take a subjective opinion whether to replace or skim depending on how the car is used. Now for me this particular car only does max 3000 miles per year, as it is not a regular driver, and with the rears skimmed they still have 3+ years use in them so for me it was not a difficult choice for the rears. As the fronts were more worn and wear quicker anyway replacements were the correct option for those. If I did 12000 miles per year or used the car for track days I would have made a different choice.

The reason I posted is because this was very fresh in my mind from last week when my car went in for it's service and I ended up having a long conversation with the mechanics on the subject. They were very much "we see this a lot on 993 cars and believe it to be a minor design flaw in the model" (they all have them hey ... 996 and RMS anybody?). I suspect again because the mileage is low on my car the disk edges rust faster than they get worn down as would not be the case with a higher mileage car. I am sure the symptoms are caused by unequal pressure from the pistons but get the impression it is to do with the design rather than sticking (all four of my disks had similar symptoms and none of the pistons were siezed or sticking).

So I don't think 'junk them' is always the right answer.

Regards.
Ian.

 

[vic cohen]

i am not a great fan of skimming but u make a lot of sense as to the way u ve come to the decision u have . Most of the problems referred to ,especially at low speeds are the result of either using worn pads dissipating too much heat on track days or [as Melv said] not doing a cooling down lap/or properly warmimg up either! I suspect not many peolpe have 'real' problems with 993 brakes until they try Track Days ,theres no reason they should have problems then if they go through common sense procedures but a little care is needed to save uneccessary wear and tear.....................i believe u have to start going very quickly ,or have no idea how to brake properly on a circuit,before OE kit becomes superfluous