Let's Talk Track Brakes |
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Let's Talk Track Brakes |
jdogg |
Aug 10 2007, 08:09 AM
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#1
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Member Group: Members Posts: 431 Joined: 28-January 03 From: Youngsville, NC Member No.: 204 |
OK, so I've had my car for almost 6 years now, and it has been through a LOT of changes over those years...It started as a stock class 2.0 AXer, then to street prepared, the to SM2 with the addition of the -6. I had done a smattering of track events over those years, primarily focusing on AX, but last fall the track bug bit and bit hard!! I've done about 8 of 'em since October last year, and just can't get enough.
But this brings me to my question(s)/issues. As my driving is improving, I find I am being much harder on the car (a good thing), but some issues are coming to light, most notably the brakes. I have the much hyped 914club billet hubs on my car, utilizing M calipers, SC vented (not cross drilled) rotors, Carbotech XP10 racing pads, brand new 19mm MC, etc. The brakes work very well...once the pads get some heat in them they will stick your eyeballs to the windshield with no fade issues. The issue I am having with this set up is the pedal softening and the travel growing after a couple sessions. At first I suspected the MC, so I replaced it...then after bleeding the system and leaving it under pressure with my power bleeder, I noticed some fluid seepage around the RF caliper. The calipers were rebuilt last fall and have about 8 track weekends on them, but I went and got a new rebuild kit and rebuilt it. I thought that maybe that was a potential source of air and a possible reason for the pedal travel issues. I spent the past weekend at Carolina Motorsports Park (CMP) in lovely east bumfuck middle of nowhere no cell service one stoplight Kershaw, SC. A very challenging layout, tight and technical, and very hard on equipment with a lot of hard braking zones, so I figured it was the perfect venue to test out my brand new MC and freshly rebuilt caliper and see if I could keep a pedal all day long. After session 2 on Saturday the pedal travel had indeed moved again. So I bled the brakes (heel and toe with a low brake pedal is nearly impossible) and the pedal came right back where it needed to be. I resigned myself to the fact that I was just going to have to bleed 'em every two sessions. Well, it was successful, uneventful rest of the weekend with plenty of bleeding (although it was hot, really fucking hot...103 on Sunday with lots of humidity). Fast forward to Monday night when I go to unload the car from the trailer....no brakes!!! WTF?!?! Seem I have blown the left front caliper out now too, and this one leaked fluid all over the damn place.... So does everyone need to bleed their brakes that often to keep a firm - high pedal? Is rebuilding calipers every 2 or 3 events whether they need it or not going to need to become a regular maintenance item? Is there a better way? |
davep |
Aug 13 2007, 11:31 AM
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#2
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914 Historian Group: Benefactors Posts: 5,144 Joined: 13-October 03 From: Burford, ON, N0E 1A0 Member No.: 1,244 Region Association: Canada |
Cryo'd rotors would be a good idea. The rotor should not be larger in diameter than where the pads touch them (it adds excess weight, and worse increases the inertia).
Possibly part of your problem is not cooling the brakes properly before parking. Cool down laps on the track or off of the track are necessary to get the heat out of the brakes. If you do not do so, a hot rotor will warp, and also cook the piston seals. I have seen dust seals that are totally disintegrated. They probably should not be used. If you use a brake duct to get air into the center of the rotor, then at least use metal ducting; I understand that the vinyl ducting is not even legal for most clothes driers. Second, consider a fan setup to force air through after you park it to ensure an even cool down; I'm not suggesting to build it in, just a little rig to blow air into the scoops. I strongly believe that more damage is done while parked than when on the track. I have it on good authority (from an endurance racer) that oversize master cylinders are very useful in preventing low pedals. Part of that comes from the pistons being knocked back excessively by the rotor ( thermal expansion, wobble, warp, flex, whatever). He used a 23 mm (back in 1971 to 1973), standard on the GT was the 20mm MC. Also, brace the MC to reduce the floor flex. He never had problems with the "S" or 908 caliper variant. |
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