MJG911

hopefully I'm wrong, but I think that foam on the caliper will trap the rock in there and scrape up the barrel of the wheel.

flickroll

That certainly is a possibility and I hope you are wrong too . I also ordered some .38” foam and I hope that results in very little clearance and maybe would mitigate that possibility. I hope it works because other solutions are expensive so this is worth a try I think. I don’t mind being guinea pig on this as it might help others in the future if it works.

flickroll

Was out of town over the weekend and when I got back I took a closer look and got more precise clearance measurements. The narrowest gap between the caliper and wheel is a mere 0.225"! The end of the caliper is curved outboard, and so as you move toward the face of the wheel the clearance increases, with the measurement at the greatest point 0.410". The damage to my caliper and wheel took place in the 0.225" region. So at this point not too sure what to do. I have some 0.380" foam on the way and I am inclined to use that, and use a razor blade to trim it about 0.200" tall at the narrowest wheel/caliper clearance area, making the cut so it is parallel to the wheel. Open to suggestions and other ideas.

AutoQuest Motorsports

Most of our track cars on the 981/991 we are running specific 19in wheels over the factory PCCBs, leaving about 1-2mm of clearance between the calipers and the barrel. Even with the tightest of clearances, we still see occasions where some debris gets in between the caliper and barrel and causes some cosmetic damage. For track cars it is what it is, for street cars I know it is quite an annoyance. I would just make sure that you don't set your expectations too high, as there is always some little piece of something that might be just the right size to cause you a problem. You're simply playing a numbers game, unfortunately you got the short end of the stick early. Sorry to hear about your problem and best of luck on your solution! If what you come up with ends up being a cure-all, major kudos to you : )

flickroll

John, thanks for your insight. I'm still going to give it a try as it seems to me there is a difference between the 19" track wheels and the 20" wheels with the foam mod. For the 19" wheels, any debris that gets in there is still going between the caliper and wheel, so damage can happen. With the foam mod, anything that gets in there is between softer foam and the wheel, so I hope that difference is enough to allow this to work. One potential problem is that a price of debris just the right size could possibly get stuck between the foam and the wheel. I'll give it a try and see what happens. If it does not work I'll just have to live with it and chalk it up to life with PCCB's. Cheers!

JRitt@essex

We (Essex/AP Racing) are currently working on some complete AP Racing Radi-CAL brake solutions for the 991 chassis, particularly the 991 GT3. They are similar to what the 911 RSR runs in IMSA. Our goal is to fit them inside 19" wheels. You can see details in this thread:
https://rennlist.com/forums/991-gt3-...t-finally.html

We would like to offer them the other trim levels of the 991, but we have yet to get our hands on a car for measurement. Please let me know if something like the above would be of interest to you, and if you could help us with a car. We just customers ship us a 718 Boxster S and a 981 Cayman GT4 as well, and we have complete front and rear kits on the way for both of those cars. You can PM me for more details. Thanks!

4pipes

Flickroll, maybe try the foam just on the leading edge of the caliper to allow debris to exit.

flickroll

That's what I'be been thinking about doing. The problem with the foam dam is the curved shape of the caliper which makes it difficult to have a tight fitting piece which is parallel to the wheel barrel. What it really needs is a cast piece that is custom made for the caliper, with a very tight gap between the dam and the wheel. I might be able to make one out of epoxy but am going to play around with the foam first.

zx5go

Why not just lay a bead of high temp RTV the needed thickness?

flickroll

That’s a great idea zx5go, thanks for the suggestion. Need to think about the best way to implement that.

4pipes

Build two stiff paper dams and tape them to the caliper. Then fill the damp, let dry and remove the paper.

flickroll

That works, just need a release agent for the RTV; think Amazon has some. Another idea is to get some balsa wood or foam and carve it to make a scale model of what I want. Once the balsa is configured properly cover it in packing tape and make a mold from epoxy. Once set, remove the balsa and I have a mold. Go from there.

arter

Just put oversize foam in the gap. Running the wheel will quickly wear down the excess to a perfect fit.

Although I dont expect the foam to help......

flickroll

Arter, that may be the best idea yet and it is easy to try, so thank you. I can pretrim the foam with a razor to get it close, and then let running the wheel finish it off. I think I will do a slight variation, and put a couple of layers of blue painters tape around the circumference of the ID of the wheel barrel. It has just a little more roughness than the wheel which is good, it protects the wheel during the running in phase and it also should provide just a little clearance once the running process is completed and the painters tape is removed.

Regarding whether it will work or not, I think it will work fine to deflect the bigger pieces of debris. However, very small particles such as sand might get through and/or get stuck and maybe cause an even bigger problem. So I think I will try it only on the damaged wheel/caliper first to see what happens (since they are both going to get fixed anyway), and keep my fingers crossed the other side won't take a hit during the proofing phase. I don't think it will take long to see if this will work or not.

mmuller

I would be most concerned about the foam not standing up to the heat. Brakes can easily get to 900F on a road car and well beyond 1300F on a track car. If you have all wheel drive then the heat loads are higher. I personally have never seen foam rated that high.

I would be concerned about foam or rtv breaking down in the heat and then coating the very expensive carbon rotors, which will then require replacment.