Track day brake behaviour

[James_G]

While I can't make either of the Silverstone dates being mentioned on this forum I did manage to get myself over there on Friday for a great day of driving. Amongst other things I had an illuminating ride in a 968 CS, which served to answer some questions I had.

The key benefits appeared to be much better brakes, due no doubt to the owner's recent splurge on the ProMax Big Brake conversion kit (928GTS Big blacks to you and I) and increased grip due to 17" wheels and wider tyres. <Whispers> Didn't feel any faster</Whispers>

Two questions then.

Firstly the owner of said 968 CS managed to lock up one front wheel during a particularly smokey display of late braking. I wasn't in the car at the time but he told me that the ABS was only working on one wheel and the other got a nice flat spot. Purely out of mechanical interest, is an ABS system calibrated for a specific amount of braking force, or will it operate no matter what the quality of your calipers? Or has the 968 owner got a failed ABS pump?

Secondly my totally standard S2 "small blacks" worked great but as the day wore on I was getting more and more juddering after less and less laps. However a 100 mile trip across country back home at the end of the day and they were smooth and judder free once more. Is there a simple explanation for this? Uneven expansion rates in the disk perhaps?

Anyway, since everyone likes a photo here's a shot of the 968, with my car just visible in the background.

 

[Peter Empson]

I've experienced the same many years ago, can't help with the explanation but I'd always assumed it was build up of material or temporary warpage.

I've not experienced this for a long time however and don't currently wish for bigger brakes (I have the same as you and use Porterfields pads and Castrol SRF), they lasted perfectly for 50 mins on track the other day so I think are quite up to my needs []

<edited to add> Oops, I thought I read you had medium blacks...

 

[EddySpaghetti]

Hi James,

Did the 968 belong to a guy called Rich Carter? He and another guy from work caleld Malcolm were doing a track day recently.

I was talking to one of them on Sunday and he said he had tried out a black 944 S2 on the track the other day, owned by a guy who also had a 330 Beemer - had to be you no?

Malcolm was the guy I was chatting to, he gave me a go in his 996 C2 on Sunday which was pretty nice - although everything (bar the immediate acceleration) felt a bit 'light and flimsy' compared to the 944T.

Cheers anyhow, let me know if you plan more track days - I have a voucher to use that I got as a present and would be good to combine with another 944 owner if you'd be up for it.

Eddy

 

[Diver944]

I used to get the same thing when I tracked my S2 with the 'small blacks' and road pads. As the day wore on I would get quite a bit of judder under heavy braking. Next day it would be back to normal.

It could be pad material burnt onto the disk, but I was always very anal about doing a full cool down lap and not touching the brakes as I came to a stop in the pits. Then leave the car in gear and dont put the handbrake on.

 

[James_G]

ORIGINAL: EddySpaghetti

Hi James,

Did the 968 belong to a guy called Rich Carter? He and another guy from work caleld Malcolm were doing a track day recently.

I was talking to one of them on Sunday and he said he had tried out a black 944 S2 on the track the other day, owned by a guy who also had a 330 Beemer - had to be you no?

Hi Eddy

Yes, that was me. Nice to meet those guys and I was particularly taken with Malcolm's 996 as it looked great in that Seal Grey. Felt sorry for Rich because I don't think he was the one driving when the flat spot issue happened, but I couldn't understand why the ABS hadn't done it's job, hence the question.

Anyway, we all agreed that the 6 lap chase session we had was great fun, and that the Silverstone South circuit was a nice configuration.

Be good to catch up again in the future so will keep my sig updated with future dates. Great to hear about your Europe trip too.

Cheers

James

 

[James_G]

ORIGINAL: Diver944

I used to get the same thing when I tracked my S2 with the 'small blacks' and road pads. As the day wore on I would get quite a bit of judder under heavy braking. Next day it would be back to normal.

It could be pad material burnt onto the disk, but I was always very anal about doing a full cool down lap and not touching the brakes as I came to a stop in the pits. Then leave the car in gear and dont put the handbrake on.

Peter/Paul

You've both described exactly the same as what was happening to me and even though I was good with cool down laps. I'm running Pagid OEM pads which last far better than the no-name ones in there before but maybe I need something a bit more focused. Glad to hear it's not just me that it's happened to anyway!

Cheers

James

 

[edh]

sounds very much like pad material on your discs - try 2 things

1. better cooling to the discs

2. better pads - mintex 1155 or similar


As for the ABS, I'm sure it doesn't care what brakes you are running

 

[sawood12]

I always thought it is a feature of one piece disks where the disk can't expand as it wants because it is asymmetrically constrained by the bell/hub of the disk so the disk expands unevenly, i.e. warp temporarily. Two piece brake rotors with floating disks get around this problem as the disk can expand independently of the hub/bell.

 

[edh]

Warped discs are very rare - unless they have been mounted on hubs that aren't completely clean & flat.

Have you checked your pads? - do they still look in good condition or are they starting to fall apart?

 

[Peter Empson]

Not strictly related, but an interesting anecdote nonetheless, a few years back I was at a trackday and a 944 owner was swapping to track tyres from his road ones after a session or two. The cold wheel wouldn't fit flush to the hub, which I can only imagine had warped to the shape of the (very) hot wheel. As soon as the hub had cooled off it went on fine.

 

[ikillcopiers]

I had brake problems aswell before, culminating in charging down a straight,
coming up on a hairpin to find the brakes up to scratch![&:]

True brown trouser moment....
Donal (VanHireBoys) in the passenger seat started laughing as he thought
the sideways action was intentional!

What was happening with me however, was the brake fluid was starting to boil
(smelled like burning plastic).

Now run her with DOT 5.1 as opposed to DOT 4, no overheating issues since.
Running Mintex pads.

To be fair, my car is pretty much standard power at the moment,
so there isn't much more want with regards to braking yet.......

 

[James_G]

Thanks for the replies all. A quick visual inspection shows that the pads are fine, but I'll be taking the wheels off tomorrow so I'll have a proper look.

So far then it seems that there's nothing "wrong" as such with the brakes, just maybe need a pad upgrade if I want to do a decent session on the track.

Fluid by the way is Castrol Super Response, one down from SRF in the Castrol pecking order and I don't experience any fade, just this juddering.

 

[Super_Marv]

It'll be a uneven distribution of brake pad material across the disc. It usually is. It's very very hard to warp a brake disc IMO.

Read this:

http://www.stoptech.com/tech_info/wp_warped_brakedisk.shtml

Interesting that the ABS didn't work on the 968....I've got the same problem with mine at the moment! Got it booked in with my local Bosch specialist in a couple of weeks...

 

[Hilux]

Warped discs are very rare

It's very very hard to warp a brake disc IMO.

Both statements are true. I`m not saying it doesnt happen but over many years and track days I`ve never heard of pads warping so much they were unuseable.

You have to imagine the heat and effort needed to do that as the heat is evenly distributed, cool air tempers the steel evenly and the pads tend to track it straight.

as the day wore on I was getting more and more juddering after less and less laps.

Sounds like material transfer. As Paul says, always cool down your brakes whenever possible and never leave them on when really hot as this deposits pad material onto the disc.

When on track you brake harder so the `bump` or material deposited is more noticeable and hit harder by the pad edge due to higher clamping rates involved so it grips momentarily on the disc and then on the material and pulses/vibrates. On a road the pad `kisses` the material and wears it back to metal over a period of time.


Edited to say I`ve just read Marv`s link and it appears I`m not wrong (which is nice)

 

[sawood12]

I agree that warping disks is hard and is very unlikely to be the cause of vibration here - especially with our disks as they are much thicker than other cars (typical Porsche engineering) - however the temp distribution on a disk is not even at all. You have very high temps at the point where the pad is in contact with the disk - as soon as the disk surface passes through the pads and leaves contact with the pad it is cooling as it rotates around until it comes into contact with the pad, so you've got one side of the disk trying to expand, the other side of the disk trying to contract and added to this you've got the outside face of the disk being prevented from expanding due to the hub of the disk and the inside face of the disk trying to expand at a different rate. In essence huge internal stresses and forces are going on in the disk material when being used in anger, and those forces are acting in opposite directions thereby amplifying the magnitude of the stresses. It is not unheard of on lesser cars with inferior brakes, for brake disks to literally explode with heavy track use due to all these different forces acting in different directions - I believe Skylines are susceptible to this and if you are tracking them it is advised to upgrade the brakes.

On another note - I used my ABS properly in anger the other day when some idiot decided to pull out from a row of parked cars I was already passing when there was not enough room for both cars to pass eachother - the result was a full emergency stop from about 40 - 45mph. ABS kicked in immediately and I kept the pedal pressure on until I came to a standstill, so the ABS was continuously working for a good few seconds. Luckily we both stopped and no accident occurred, but I did notice that once the ABS kicked in it felt like my brake pedal dropped a couple of inches. It wasn't on the floor and felt hard, but it did drop. As soon as I released the pedal and depressed it again all was tickety boo, and all has been fine since. I wonder if this is normal? The only other time my ABS has kicked in is on slippery surfaces at slow speeds and light brake pedal pressure where the ABS only kicks in momentarily.

 

[333pg333]

Vented discs are a big improvement over solids. However I would go for the slotted ones over the drilled as they do crack quite quickly. I think you will eradicate this symptom you describe with vented rotors.

 

[Diver944]

ORIGINAL: sawood12

but I did notice that once the ABS kicked in it felt like my brake pedal dropped a couple of inches. 

That's how mine feels too. I test it occasionally when there is no traffic around, it's nice to know that its working []

 

[paulf968]

some extra points to add from my experiences:

1) if you do have caliper plate lift then there is a strong chance that one or more pads might bind onto the disc - local overheating occurs and this leads to uneven deposits causing juddering.
2) even if you have no plate lift and use your brakes in a spirited fashion on a trackday then the pads reach temps of say 500 degrees whilst the calipers are about 150 only. So there is potential for differential expansion that will trap the pads. I do grind a bit off the edges of the pads to ensure a loose/sloppy fit against the plates.
3) a sticking piston or two might be a problem as well, temp indicating paint on the disc can help to spot this or get the pistons to move over their full range by pumping out onto worn pads and then pushing back a couple of times.
4) make sure your discs run true when installing them, -- a cheap dial gauge indicator and magnetic stand from Screwfix is ideal for checking. NB but make sure the disc is firmly in place by using the wheel nuts to clamp it -- the little screws are not enough on their own. (cheap dial gauge and base are items 58566 & 47525 at www.screwfix.com)

 

[Hilux]

I do grind a bit off the edges of the pads to ensure a loose/sloppy fit against the plates.

Good shout, me too and I use coppergrease on the sliding surfaces.

 

[sawood12]

Not sure having a 'sloppy fit is a good idea'. The caliper plates are spring plates to capture the pad with some spring tension. The problem comes when corrosion lifts the plates such that they bind and prevent freedom of movement. Having a sloppy fit of the pad in the caliper doesn't seem like a good idea to me. Porsche could have easily designed them withought the spring plates at all providing plenty of play if that was a good idea, but they didn't design them that way. If you're having to modify your pads in any way other than a dab of Copperslip, then there is something wrong that needs seeing to.