In reply to codrus:
No brake ducts yet, at least according to Internet Whizzdom I shouldn't quite need those yet.
In reply to codrus:
No brake ducts yet, at least according to Internet Whizzdom I shouldn't quite need those yet.
Still trying to fix this issue and it's beginning to look like it's kicking my behind...
Didn't do a full rebuild on the calipers yet, but they're cleaned up and seem to be moving much better now.
I also bought a new set of rotors - not Centric this time, but Stoptech. Probably the same blank, only that these are slotted which may or may not help. Spent even more time cleaning up the hub, fitted the rotor dry to avoid any uneven copper grease application and...
... 0.006" runout fresh out of the box with all five lug nuts torqued down, still three times the max runout the shop manual lists. Worse, the runout doesn't change if mount the rotor in different orientations.
At this point in time I'm kinda refusing to believe that two different brands of rotor have the same runout out of the box on the same hub. That's too much of a coincidence for my liking.
Anybody got any suggestions as to what I may be overlooking? I've cleaned both the mating surface of the rotor and the hub, made sure there aren't any visible deposits anywhere. And this is not exactly the first time I'm fitting brake rotors to a car, either, but seems to be the first time I'm stumped by it.
Halp! Preferably before I push the damn thing outside and set fire to it.
Oh, and have I mentioned yet that I'm supposed to be on the way to Thunderhill right now for the last time trials event of the season, and my points lead in my class is unlikely to survive a no-show?
Well, it looks like I finally managed to track down the reason - it appears the hub has a little over 4 thou runout at its out edge. I'm surprised that only translates into 6-7 thou at the out edge of the rotor.
Time to put the new rotor back into its box, slap the old one back on and do quick service on the other caliper. Not much else I can do about that right now.
That's an interesting find!
It might have been a red herring unfortunately. Clearly the hub and rotor are running out of true, but after the cleanup and caliper service with the correct silicone paste/rubber grease the brakes work well without any noticeable judder on the street. Get some heat in them and things get much worse than before in a hurry.
We had our last event of the season a Thunderhill East today and after 3-4 laps at distinctly non-competitive pace the judder was back, much worse than before and the car started pulling left under hard braking - at that point I decided that throwing the towel and limping it back home was probably the smart choice.
After 1/2h of cooling down the brakes were good enough again for street driving, so at least i got home with the car in one piece. I'm a tad stumped by now, I still suspect it's an issue with either the hub or the caliper. I'll try to enlist the help of one of the few mechanics I trust around here to get to the bottom of this. As of right now, the RX8 isn't of much use to me if I can't drive it on the track.
Yeah, that exceeds my mechanical diagnostic skills I'm thinking... I can't think of anything you didn't already cover.
Dumb question (that may have been answered if I re-read from top to bottom): When you're completely up to temp (when you get the judder), do you have any other vibration issues, or is everything perfectly smooth except for the brakes?
Perfectly smooth going down the front straight at T-Hill at 90mph+ until I had to brake, then it felt like the rotor wants to shake itself and the wheel lose. Got worse the more heat I got into the brakes or the hub.
Come to think of it, that's been the pattern since the problem first made an appearance - it'll be fine initially (maybe with a little rubbing brake pad noise from the runout) and as the brakes warmed up, it became more and more pronounced. The work I did on Saturday seems to have improved matters inasmuch as it's less of an issue in low heat street driving situations and much worse in high-heat track driving.
Still sounds like uneven pad material deposition from here. My own experience was that juddering/noise was worse on the street than track though. It would eventually go away as the cold track spec pads cut through the embedded crud, but it took weeks.
Have you used the matched pad and rotor set from new, or did you change pad specs on the same rotors?
In my opinion if it's the latter - time to either cut the rotors (not recommended) or start over with a matched set from new.
Matched set of pads and rotors since new - both only have been on the car for about 4k miles and 5 1/5 track events.
I've pulled the problematic rotor off a couple of times and haven't found any signs of pad deposits. Doesn't mean they're not there, though. One thing I did notice was that the paint on the pad backing plates is showing signs of bubbling as if it was about to start melting/burning. That surprised my a little as the pads are supposed to be able to deal with very high temperatures (they're Hawk Street/Race pads, which is essentially a streetable race compound for people who can live with even more dust than you get from HP+).
IMHO, there is no such thing as a genuinely dual purpose pad that can do both hard track sessions and also be usable on the street. If you're bubbling the paint on the backing plates, you're getting them very hot. How many track days are you getting out of a set?
I don't know why they bother painting track pad backing plates. One track day on my DTC-60s leaves them looking like this:
To me this suggests that you're getting them too hot. Some combination of higher temp pads, brake ducting, or bigger rotors with more heat sink/rejection capacity is what I'd look at. You might consider bringing track pads to the track with you and changing them there.
So far I've been able to run same set of pads for the above mentioned 5 1/5 events and about 4k miles of road driving. That includes 2 events at Laguna Seca, which is notoriously hard on brakes. Front pads are roughly 1/2 worn by now.
The backing plates on the pads don't look anything like yours above, they just have small bubbles, but more than I'm used to seeing on the other cars I've tracked. The pads should be up for this sort of abuse, after all Hawk specifically rates them for operating temperatures up to 1200F - a little less than the DTC-60s but when comparing specs they look pretty much like some variation of the DTC-30s.
I've been seriously considering bigger front brakes with Wilwood or similar calipers so I can just drop full race compound pads in there at the track and have saner pads for street use. However before I spend $2k-ish on front brakes I'd prefer to understand what the current problem is before trying to solve it by throwing more money at it.
Before you go replacing things, you might want to check ball joints, wheel bearings etc. Your hub run out could be a symptom of a loose bearing.
Just spoke to my trusted mechanic and his first guess was that it's the wheel bearing/hub going bad based on my description of the symptoms and what I've tried so far. Got the car booked in with him for early next week for proper diagnostics and hopefully a fix.
Diagnosis confirmed - RHS front hub is on the way out, LHS not far behind. New hubs on the way, but they probably won't make it in time for a track day at Sonoma this weekend.
Still, one can hope, if UPS drops them off by Thursday there's a chance it'll get fixed in time.
Hrmpf.
So, new hubs. And just to be on the safe side, we threw on the new rotors and pads. Everything checked and double checked.
Fine on the road during normal driving, 2-3 laps of Laguna Seca at a decent clip even in 60-ish degree weather a couple of weeks ago and the brakes judder again. Not as bad as they used to, though. Unfortunately they also do that during street driving when harder braking is called for - like going down one of the more interesting passes around here that requires a couple of spirited applications of the brakes. The good news is that the judder goes away when the brakes cool down, but I'm not very happy with that.
If I do keep the car (I'm currently trying to put a deal together for a pre-lit Westfield 7 replica and one of my motorcycles that's just sitting around in the shed), it looks like it's time to add some brake ducts and see if those help.
I'm still suspecting pad deposits...
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