Braking Issues

[planet6racing]

OK, here is a weird one...

I'm having a problem with the brakes on my Saturn and I'm hoping you all can help. At my last race, coming into turn 8 at Road America (a left hand turn at the bottom of a hill), my brake pedal turned to mush and didn't really inspire confidence. The first time this happened, it really freaked me out, but by the end of Sunday, I just accepted it, braked earlier, and dealt with it. Pumping helped a little, but the pedal definitely had much more travel in this corner.

I also noticed this problem a little in turn 5 (another left at the bottom of a slight descent), but everywhere else on the track the brakes were rock solid and perfect.

Things I eliminated and why:

1) Pad fade- If it were this, pedal travel should be about the same, but I just wouldn't slow down. Plus, it was at the end of a fairly long section without brake use.

2) Air in brake lines- Should be consistent throughout use, but it may still be a possibility.

It seems as though it has something to do with coming downhill into the braking area, but I'm not certain what it could be. Any help will be appreciated!

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Bill
Planet 6 Racing
bill (at) planet6racing (dot) com

[gsbaker]

Bill,

This is weird. Is it happening in T6?

I'm going to go with air. Since the brakes are solid everywhere except downhill left-handers, I'm guessing (and that's all that it is) the brake fluid is getting sloshed into the right front corner of the master cyliner and, somehow, letting air into the line.

Of course, I haven't had a master cylinder apart in many years, so I could be all wet.

Did you check the fluid level? If the level is good, is there any danger in overfilling it just to check?

Gregg

[planet6racing]

The level is within the factory specs. It was lower than I liked, but I figured if it were letting air in, it would stay in and get progressively worse. But it didn't.

And, no, it wasn't happening in 6. Rock solid through that as well as 12, 14, 1 and 3.

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Bill
Planet 6 Racing
bill (at) planet6racing (dot) com

[David Ferguson]

You might be experiencing pad "knock-back" this can occur when you've had some hard-cornering (particularly in both directions) before getting on the brakes. For purpose-built cars, floating rotors are the solution.

You may want to check wheel bearings/hubs for play. Any play can allow the rotor to move relative to the caliper and "push" the pads back. The first part of the brake application has to move the pads back into position, and you feel the extra travel in the brake pedal

[JeffG]

The rumble strips at RA can contribute as well if you run them. 6 and 7 both have significant bumps at track out with no braking for 7.

Jeff

[Mark LaBarre]

The 6-7-8 combination aren't that far apart. Sounds like they're just getting hot.

[924Guy]

Yeah, I'd be mindful of knockback - from rumble strips, but also possible if your bearings are on their way out. I know that SCR went through wheel bearings at an alarming rate - just watch for grease spraying out of the seals. They kept spare hubs with new bearings on hand for that occasion. Knockback is characterized by an immediate recovery of braking ability after pumping the pedal once or twice to take up the gap.

As for the possibilities of overheating - you didn't make any mention of pad or fluid selection. Cooling ducts and rim selection will also play into these. What's your setup?

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Vaughan Scott
Detroit Region #280052
'79 924 #77 ITA/GTS1
www.vaughanscott.com

[chuck baader]

924Guy...I'll hijack the thread for a moment. The grease spraying out of the seals is caused by pressure/temp buildup in the bearings. To prevent this, drill a .040" hole in the bearing cap and install a .020 or .030" aircraft rivet in the hole. The rivet keeps dirt out, but lets hot air out also.

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Chuck Baader
#36 ITA E30 BMW
Alabama Region Divisional Registrar

[m glassburner]

Everytime I see grease my bearings are done...the bearing cap??? you mean seal?? And how do you keep the junk from drilling out of the bearing...or is that considered collateral damage?

[Greg Gauper]

I'm in the 'pad kick-back' camp on this one. The true physical problem can be difficult to find. I used to have the same problem with corner specific kickback problems at Blackhawk i.e. the brakes would work find at 1, 3, and 3d, but the pedal would be on the floor for 6 unless I gave the pedal a tap. There was just something funny about 3 consecutive left handers (exit of 3d, 4, and 5) that effected the brakes. I think the 'Hurry Down' combo at RA (6,7, and 8) is contributing to the problem, either side to side transitions, or due to the curbing/rumble strip, etc.

Loose wheel bearings or a slightly out of round rotors can contribute to the problem. On my Honda, I also found that as the pads wore, the amount of caliper flex got worse and worse with heat and pad wear, primarily because with single piston calipers, as the pads wear you have less guide pin engagement, and this causes more flex. Even with only 20-30% pad wear, the pedal never feels as good as it does with new pads. For me the simple fix is to shim up the pads as they wear so that as much of the guide pin is engaged as possible. I use old brake pads ground down to various thicknesses i.e. 1/16th, 1/8th, etc, and as the pads wear, I pull out a thin shim and replace it with a thicker on. Note: I use the shims on the outboard pad.

I bring up the shim idea because it's possible that kick-back MIGHT be minimized with increased guide pin engagement. Anything to minimize pad movement helps.

[planet6racing]

Me and curbing at RA do not get along. I really try to stay off of them as they really upset my car. That said, I always tried to avoid them in the 6-7-8 combo, so I don't think they are effecting the brakes.

Wheel bearings were brand new for that weekend.

The odd thing is, if it is pad kickback due to a slightly out of plane rotor, I don't get it in 12. From 8 to twelve I don't use the brakes (just a chicken lift for the kink) and the brakes are rock solid in 12...

I've thought about the shim idea as well, but everywhere else, the brakes are perfect. Maybe I'll just wait and see what happens at the Farm. If it does it again, I'll worry about it then.

I'm more concerned about it being due to the downhills and going to RA for the ARRC. There is plenty of runoff room there, right? Eh, I have all winter to rebuild it!!

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Bill
Planet 6 Racing
bill (at) planet6racing (dot) com

[chuck baader]

The hold goes in the dust cap on the outside ofthe outside bearing, not in the seal. CB

[lobster]

I also have had knock back problems, with old 4 piston Corvettes, rotor runout had allot to do with it, it was a wild and scary feeling, also have run across only twice the master clyinder rear cup letting a little air in both were on aluminim masters and very hard to find. Good luck Glenn

[gsbaker]

Originally posted by planet6racing:
...I'm more concerned about it being due to the downhills and going to RA for the ARRC. There is plenty of runoff room there, right? Eh, I have all winter to rebuild it!!

Oh yeah Bill, you'll love T10 at Road Atlanta with no brakes. Think of T5 at Road America, just more of it--something like ten miles of full throttle approach. And when you crest the hill it feels like someone cut the cable on your elevator.

There is a large patch of runoff though, so you should be fine. Just bring a shop vac for the kitty litter.

Gregg

[John Herman]

Bill, there is one corner at Gingerman where I have that same problem. I think you are just overheating the brakes, and there is not enough airflow before you get to 8 to cool everything off (though you think there should be). Once I get through the corner, and go down the short straight, the brakes are fine, even though I'm going much faster (but obviously much more airflow). I have experimented with a different compound brake material, and the problem went away. There were no other changes made to the car, and the problem/solution was quite repeatable.

[planet6racing]

Interesting...

But, my question is this: When I bed in the brakes, I get considerable fade of the pads. While the car doesn't stop as well, the pedal still has the same firmness as the amount of travel to engage the pad is the same. I do push harder, though, to compensate for the decrease in friction due to the overheated pad. So, based on this, is it truly pad fade?

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Bill
Planet 6 Racing
bill (at) planet6racing (dot) com

[Knestis]

That is pretty much the textbook definition of pad fade, sounds like.

One issue that hasn't been addressed here is the whole vacuum-assist puzzle. The old Ren-slow Cup cars were persnickety as hell on a left foot brake, because of interactions between throttle position, vacuum, and the boster's effect on pedal travel and feel.

There was a valve in there that was supposed to manage the system but it was designed without considering that the driver might be on the throttle and brake at the same time.

Are there any variables relating to throttle position, in the corners where it feels wacky?

K

[joeg]

Are their drums in the rear?

If so, out of adjustment.

If not, air in the hydraulics or worn-out pads.

[benracin]

To bad you didn't have another try at the track. I remember chatting with you about the ALMS at RA I saw on Speed where a dude in his porsche tapped the pedal with his left foot prior to getting his right foot off the pedal and on the brakes. Maybe that is due to kickback or he was just being safe and getting rid of the issue in case it just might be there. Well if it shows up again, before you get to the brake zone, try reaching over real quick with your left, giving it a decent tap, then nail the brakes.

Giddy up! Or, just keep them as they are so that I can drive on by. That sounds like a better option to me.

[planet6racing]

Well, hmm, there shouldn't be a vacuum assist problem (that was last years problem!), I've got discs in the back, and I can't say that I've noticed a relationship between throttle and occurrence.

I guess I'll keep the kickback in mind in case it rears its ugly head again. Other than that, I just don't know...


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Bill
Planet 6 Racing
bill (at) planet6racing (dot) com