Brake problems

At the VIR races I experienced braking problems in both races. During the race the pedal would get REALLY long after a hard stop such as at the end of each straight. The next application of the brakes required pumping to get effective braking.

I currently have 2 two piston calipers on each front disk for a total of four calipers on the front end of the car. (See photo). At the back I have a single four piston caliper. All caliper pistons are 1.75 inch bore diameter. There is a 0.700 master cylinder pushing the fronts and a 0.750 pushing the rear. Even before I go out on the track with the amount of caliper piston area I’m pushing the pedal is long but firm and gives good modulation until the problem occurs. It seems to be heat related but I’m not totally sure.
http://i118.photobucket.com/albums/o...2caliphers.jpg

After getting home and thinking about what happens on the track and considering some of the suggestions made by various people I’m plan to trouble shoot it as follows.
1. Replace the front rotors with solid disks. The ones on the car look like Swiss cheese. (By design I might add). I believe the number of drilled holes has compromised the mass of the rotor and they can’t reject the heat effectively. If you look closely at the rotors you can see stress cracks radiating from the drilled holes. I consider this to be a sure indication of to much heat. Anyone have a different opinion? See the photo below.
http://i118.photobucket.com/albums/o...otorcracks.jpg

2. Take off two of the front calipers and run with just the remaining two on the front. The theory being the disk can’t dissipate the heat generated by four calipers fast enough. I’ve not seen any other car with the four caliper set-up except the Rakavon. They did however win the Runoffs!
3. Follow up on the suggestion made by Leon (Mitchell Racing Services) that the amount of caliper piston showing might be too much and I should be using shims to get the correct spacing. Correct meaning little, if any, piston showing out of the caliper bore. See photo – if you look hard you can see the amount of piston sticking out of the bore. This needs to be shimmed.
http://i118.photobucket.com/albums/o...herpistons.jpg

I’m open to other theories, opinions and suggestions. Ted S.

My first question is.. are you using silicone brake fluid? The reason I ask, is WAY back when when I raced a LOLA 5 ltr. car in A/SR and a few Can-Am races we tried silicone 'purple somethng' brake fluid. That didn't work. Long story short..you will always have moisture find its way into the brake system. They two don't mix. I pissed away about $40.00 bleeding the system with the silicone brake fluid. Finally one of the REAl pro teams told me to go back to Girling or anything but silicone. Never a problem again with the long prdal..no fade.it just worked for me. Probably more than you wanted. Mike Canan

I know nothing about F500 brakes. But I think those cracked rotors need to go! Nothing more dramatic then seeing a rotor fly apart in the braking zone.

The fluid issue above is a good one. From your initial description sounds like a classic case of boiling fluid or air in the system.

Those pistons do seem to stick out quite a bit and it looks like reasonably new pads.. Hmm. Are those calipers the correct size for those rotors? If there's any Wilwood number on the calipers you might check to see what thickness rotor they are intended for. With a new(r) set of pads, the pucks should be close to flush with the caliper face.

One thing I've noticed on the Girling master cylinders. You can have a "firm' pedal. But if you apply hard pressure on the pedal you may notice some continued travel of one or both the rods inward into the cylinders. A sign of fluid leaking around the piston seals. But I'm not sure that would cause 'pumping' to be required.

Ted, can you elaborate on the need to have two calipers on each front disk? Is it because of the small size of the wheels/rotors? Is that standard for an F5? With the light weight of an F5 I wouldnt think two calipers would be needed on each side.

Take care.

Tom

Does sound like moisture in the fluid or air in the system. Have you tried bleeding the brakes after a secession? Was there air in the lines?
Possible sources for air- bad/ old crush washers or sleeves in the banjos. Worn seals in the masters, or calipers.
I also wonder if the two caliper setup is creating to much heat (the drilled rotors would help get rid of some of that heat but cracking is no bueno) and your fluid is boiling... what type are you using?

Your problem is definetely heat related - you are boiling the fluid.

What pads are you using? Different compounds transfer heat to the pistons at different rates, with the carbon metallic being usually the worst. A different pad type might help.

What material are your pistons made of? For you, stainless would be best (Ti if you can afford it) to keep the heat from reaching the fluid. If you already have stainless pistons, try machining notches into the end where it contacts the pad - decreasing the contact area will decrease the heat transfer rate.

Possibly also, with the small rotors you are stuck with, they are just getting too hot (partially because of the lack of mass from all the drilling), which then increases the pad temps, etc. Rotors with a lot less drilling will probably help.

You mentioned you were considering solid rotors. Are there slotted ones available? Those would allow out gassing and give you some weight reduction while maintaining resonable mass for heat dissipation.

Clarification of my original reply regarding silicone fluid in the brake system. IF you are using silicone fluid this applies..if not disregard. DOT 'whatever' brake fluid will mix with moisture. Silicon fluid will not, hense you will will have a pocket of moisture that will boil when the brakes are used in racing conditions causing you to have no effective brakes at all. You won't notice it at all until the braking system is heated up. Hope you find the problem!

Thanks for the input everyone has provided so far.

To answer some of the questions:
1. Pad compound is PFC 01 with one race on them. What pad material might be better?
2. Wilwood Dyna lite singles designed to fit disk thickness of 0.25 to 0.375. Mine are a size in between.
3. Caliper pistons are stainless.
4. Using Wilwood 570 fluid.
5. Brakes were flushed with new fluid and bled after the Saturday race. Small amount of air seen on the left front. Sunday race still had the same results.
6. There is virtually no run out on the disks minimizing pad knock back.
7. Master cylinders are AP 0.700 front and AP 0.750 rear. They appear to be in excellent condition. Will check seals.
8. The only need for two calipers per wheel is to try to improve braking performance.???
9. Crush washers were new before this race weekend.

I plan to replace the drilled disks with solid/slotted ones to increase mass and hopefully have more efficient heat dissipation. Ted S.

Hi Ted,
I bought a Raptor DE3 from Bob Gelles, he had two calipers per side up front also. I got rid of two as it was designed for one per side and knew the PO, prior to Gelles, and he was damn fast in the car. I never had any braking problems with just two calipers on the front, never boiled brake fluid. Also ran solid rotors. The brake lines for the four caliper setup makes it possible to have air in the calipers if not bled carefully. The line came into the lower side of the rear piston and out the top onto the front caliper. If they didn't use the bleeder on the caliper closest to the MC in the system bleed it first then the one furthest from the MC. Make up some spacer shims to get the pistons back in the bore further; the Wilwood singles tend to cock the piston when they are out of the housing bore very far. With four calipers on the front you are going to have alot more pedal travel as you are moving twice the volume of brake fluid vs a two caliper setup; makes it more critical to not have any air in the system as you will run out of MC piston travel. Check on the F500 board, I seem to remember some guys were using some type of ceramic spacer between the pad and piston.
Regards, Doug

Ted my son and I had the same problem the first time out in our KBS. I switched to Wilwood pads, replaced fluid with Motul, and put in more rear bias. The problem was gone. We used drilled rotors. I think the biggest fix was more rear bias. It would really haul down after that and braking was very smooth.
Dave

A bit late to the party on this one but as your local Wilwood dealer I'll toss out some thoughts.

1. Fitting twin two pots to the front of 1.75" design is too much front brake. You have way too much area for a good feel in a .700mc. And right out of the gate you have a 2:1 bias which is probably less rear than you can actually use. I think you're making the front bakes do the majority of the work here.

2. With that mix of mc and piston sizing you're likely putting the bias blocks/bar in quite a travel and part of your soft pedal may be related to the volume requirements followed by the bending/binding of the bias bar at what I believe would be some extreme angles. The .700 part will have a longer travel under same/same caliper requirements anyhow. And you've doubled this on one end! I'd also suspect that the pedal is over center which is bad for leverage and a potential failure point.

3. Those rotors probably need to go...borderline disaster waiting to happen. I'd agree with your thoughts on loss of rotor mass being too extreme. Less holes or smaller holes perhaps but nothing real good is going to come of those in the pic as you know.

4. Pads. The ideal pad for this in the Wilwood line is probably the H compound. It's noisy when cold but wears better than A and will hold heat well.

5. Pads II. The DLS caliper uses some very small pads. That means both less time before replacements and their thin design promotes more heat transfer to the caliper. A move to a smaller bore four pot would help here as the pads would be much larger.

6. Calipers, the DLS can be had now with a forged ss piston. Not the stamped cup design of old. If you don't have the forged ones you'll find under high pressure the stamped parts deflect some and this will also give a softer feel and less response.

7. Fluid, I don't believe you're boiling this on an open wheel car of this weight. Assuming you are only bleeding the upper screws on the DLS that caliper and fluid should hold up fine. The pads on the DLS are thicker than the more common LD19 pad. And with the ss piston (old or new) and an aluminum body the whole assembly is more efficient than the LD. As RP points out however a poor choice of pad compound combined with too light a rotor can put too much stress on the caliper to do work it's not really intended to do. The rotor is what should be cycling the heat. So if you are in fact boiling it now some improvements elsewhere should prevent this from happening in the future.

8. What to do?? Replace questionable rotor for safety. Get proper Poly H compound pads. Then I'd swap the mc's around. Why? Yes the .700 will provide greater pressure to the rear but you'll be ok with that. And the .750 will be better matched with the demands of the "four pot 1.75" front. Might take a bit of a knob tweak to get the balance where you like it. Verify the pedal performance- angles and depressed balance between the blocks.

If you still find the feel less confident than you like you have two options. 1. You can move to a larger bore mc up front still to bring some feel back to the system. .875 or 2. Pull that second caliper off the front and swap the mc's back to .700 front and .750 rear with only ONE caliper per rotor. Aside from pad life decrease up front you'll still end up with the same braking forces overall and maybe even some better braking totally given the rears will be able to do something to help.

In fact...pull one caliper now before you do the mc swaps and run it a few times like that, it's easy enough. A one caliper 1.75" set up on similar sized rotors is well within the range of most single seaters.