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Post by **Zamani** » Mon Mar 14, 2005 1:03 am

I think that is what Jim K did for his M3 rotors. I'm pretty sure machine shops can do this.

Jim K · Post by **Jim K** » Mon Mar 14, 2005 4:12 am

No problem for the machine shop. Goes without saying that good centering on the lathe chuck is essential, so that there won't be any imbalance problems during use. Before you turn them down, ask the machinist to check for any previous (factory) rotor balance marks and if so,compensate accordingly. My M3 rotors WERE factory balanced and I tried to replicate this by similar localized grinding after the diameter reduction from 315 to 309mm. I did NOT however rebalance the rotors by themselves-I thought it superfluous-and everything turned out ok-no vibration at all, up to 220kph.
Maurizio, does your coefficient apply to cast iron, as it is markedly different than steel!
Regards,
Jim K.

Maurizio · Post by **Maurizio** » Mon Mar 14, 2005 4:34 am

Jim true,

first did not have numbers for cast iron handy. But checked it and cast iron is 11*10^-6*K^-1
It was just to give an idea, I have no clue what temp a brake disk gets, 400 K increase was also a best guess…

Jim K · Post by **Jim K** » Mon Mar 14, 2005 9:21 am

Hi again Maurizio,
Brake disk core temperatures reach 920*K during track days and disc surface peaks reach up to 1080*K -thats VERY hot!! The special cast iron formulations used for disc manufacture are chosen to contain disc expansion and warping to a minimum. It would really be interesting to measure disk growth on track days! I must find someone brave enough to want to remove the wheel nuts, right after 15 or so hot laps, heheh!

Jim K.

Maurizio · Post by **Maurizio** » Mon Mar 14, 2005 10:39 am

A nice experiment for the coming summer ...

So resume a caliper clearance of 2.5 a 3 mm would do the trick!

Post by **Michael** » Mon Mar 14, 2005 11:47 am

Gotcha - I'll go for 3mm as I plan to heat those disks up pretty well this summer

I'll let you guys know if I find a shop that can/will take this on.

Cheers,

Mats · Post by **Mats** » Mon Mar 14, 2005 11:57 am

Jim, I'm your man...

I used to do some work for a endurance team and have changed my share of hot wheels. We didn't have hats on the valves because they melted from the heat radiation.

Post by **Michael** » Mon Mar 14, 2005 6:22 pm

I found a machine shop that can take this on - I just have to decide exactly how much to trim off... Looking at either 2 or 3 mm since I don’t want the top of the pad to protrude beyond the outer diameter of the disk. Since there is no interference when the brakes are cold, I just need to know how much the disk can expand when it’s at its hottest.

Should know how this works tomorrow.

Cheers,

Jim K · Post by **Jim K** » Mon Mar 14, 2005 10:22 pm

The outer edge of the disc should always be flush with the edge of the pads! I'm sure this was the situation where these brakes were before and there should have never been any noise issues. If you haven't removed the rotors yet, why don't you try something out: Apply some paint to the rotor circumference (around the cooling holes, the part of the rotor visible behind the caliper spring), Drive the car until you hear the noise again and then remove the wheel to see if the paint is scraped away! This will prove the case about expanding rotors fouling the caliper.

Jim K.

Post by **Michael** » Tue Mar 15, 2005 12:12 am

Yes - I thought of doing that (I use that quick drying typist correction fluid for such tests) - but then I noticed that the edge of the disk actually has a coating of Aluminium from where the caliper material was being worn away – and the inside of the caliper (now removed) has a matching wear groove which is rather conclusive. I called Larry (who made the kit) & he confirmed that (upon reexamination of the CAD drawings) the measurements for the brackets were marginally off which confirmed my findings.

I can adjust the pad to match the new dimensions of the rotors by modifying a small metal plate which governs how far the pad drops down into the caliper bore, or just file a notch into the brake pad backing slightly - so I think this plan should work nicely.

Cheers,

Jim K · Post by **Jim K** » Tue Mar 15, 2005 8:21 am

Really Michael, it baffles me, WHY do you have to do ANYTHING to the pads? They are the ones originally designated for these calipers , right? Then, the manufacturer sure as hell has everything falling in the right place regarding dimensions,clearances etc. ALL you should have to do is design the adaptor brackets so that the disc edge is flush with the pads' edge and THAT'S IT!! As an example, the Brembo Reds I put on the 1,8T are specified for 320mm max disc diameter and I'm using 309mm (reduced from 315mm).
The ONLY way then the disc would foul the caliper, is if the disc is much BIGGER than the max recommended for the specific caliper (is it??). I would seriously rethink everything before any caliper/pad mods! If need be, redisign the brackets! I'd be very wary of altering caliper/pad relationship, as the central pressure point would shift, with unpredictable results on pad/disk wear.
Jim K.

Post by **Michael** » Wed Mar 16, 2005 1:02 am

That might very well be true - the disk I was sent is larger than the 951 disk. Porsche disk is 298 mm and the new Alfa disk is approx 311 mm. I'm not sure if this is outside of the recommended disk size for the calipers … probably difficult to get that data now. Had I designed the system myself, I’d have gone for disks exactly the same size as the 951.
Ahh well - hindsight is 20/20

Post by **Michael** » Sat Mar 19, 2005 10:01 pm

Good times

I had the disk outer diameter machined down by 2.5 mm and on reassembly, its awesome! I'd say the car brakes better than the Porsche now (its lighter, lower and stiffer) ... and no grinding noises, vibrations etc

.
These are the brakes that Alfa should have fitted in the 1st place.
I did not need to change the location of the pads in the calipers - the tops of the disks sit flush with the tops of the pads, heeding Jim's advice on this issue which made total sense.

The pedal does go down a bit further than before, but that was expected. Zamani, I understand you installed a 164 Master Cylinder - was it to address this issue? If so, did it help? Is it a direct bolt on?
I could potentially use a Porsche 944 Turbo master cylinder (its lighter - aluminium too) but I have not checked to see if it would be a bolt on proposition. I'm not too worried about extending/changing the brake lines, which I'd probably need to do for either master cylinder change.

Cheers,

Jim K · Post by **Jim K** » Sat Mar 19, 2005 11:39 pm

Way to go Michael! Now you can outbreak most of those Jap crapmobiles on the track!!!

Jim K.

MD · Post by MD » Sun Mar 20, 2005 3:02 am

Congratulations on a good result. So what is the pad formulation that you wound up using, Portafields?
Hows is the balance front to rear?

Brakes