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Old 11-06-2013, 02:11 AM   #1
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Default Best Course of Action For Me to Get Better Rear Brake Bias

All,

Currently have the FM stage 1 BBK on the car - 11" wilwood fronts with 11" rears w/1.8l NON sport calipers. Street tires on 15x8 6ULs, Stock everything else (going to get Tein Flex's this month).

So I seem to be going against the grain here, but the brake bias sucks. I have the prop valve on the rears, screwed all the way in (highest knee point possible, pretty sure it just eliminates it at this point). My front tires lock way too soon, I can't trailbrake without understeer, it is just downright scary for me when pushing the car because I can't rotate it with the brakes. I fear if I ever have to race in the rain, it would be even worse.

Does no one else have this issue? Maybe everyone that runs a BBK is also using slick/r compounds? (More mechanical grip would promote more weight transfer and require more front bias).

So, what would be the best course of action for me budget wise? The way I see it:

1) Swap prop valve to the front brakes. I haven't seen this before, but in theory should work? Best of all free! Kinda crazy sounding idea, knee in the pressure might make braking more unpredictable.

2) Buy sport brake rear calipers with 2mm larger diameter pistons. That should roughly give me a 8% gain in rear brake torque for the same pressure, maybe change bias a little less than that. Cons are it might not be enough, cost will be about $260 total.

3) Get carbotech XP8 for the rear (keep wilwood BP-10 on front). That would be a mu increase of maybe .14 compared to the current r4-s. TECHNICALLY that should increase brake torque by a whopping ~33% (Ff=N*mu), but the XP8's need to be heated more so I think it would be less than that with a stock miata. Maybe a 20% increase? Sounds much better, probably would be enough. Cost would be around $200 w/rotors. Sounds like a good compromise perhaps?

4) By FM 4-piston rears. Yeah I don't have the money for that.


So I would like a math and reality check on the pad compounds, for those that have done it was the change really that great? Man, I wish I had a proper datalogger and some pressure sensors to calculate actual brake bias...
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Old 11-06-2013, 01:18 PM   #2
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The prop valve on the front is an awful idea. Would have figured the reason why would have been drilled into you with FSAE (hysteresis is bad mmkay). And adjusting the prop valve for trail braking wont help much either since trail braking should be happening below the knee point or you're just asking too much of the front tires. So that is either a driver issue, trying to break too hard during trail brake, or a basic bias issue that the prop valve cant fix. Fix that first and you should be much closer to what you need for straight line threshold braking.

The short term fix is the pads, but those cost almost as much as sport calipers. But if you end up doing both and it still not right your could have just gotten the FM bits for the same money and saved some unsprung mass. Clear as mud.
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Old 11-06-2013, 01:24 PM   #3
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Originally Posted by cardriverx View Post
wilwood BP-10
Found your problem. Put some decent pads in that thing.
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Old 11-06-2013, 02:02 PM   #4
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Originally Posted by Savington View Post
Found your problem. Put some decent pads in that thing.
in both the F/R? I ask because I have a similar issue since I upgraded the size of my rotors.

I'm running 11" F and 10.9" R with ULTs both F/R and have the prop valve cracked all the way down and still lock up too fast in the front.
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Old 11-06-2013, 02:07 PM   #5
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Yeah, I would have said to pick a brand of pad and use the same compound front and rear for a baseline, and then stagger the compounds if necessary after that point. The prop valve should be for fine tuning and not overcoming huge inequities, imo. But if you haven't run the same compound front and rear you don't have a baseline for adjustment.
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Old 11-06-2013, 02:07 PM   #6
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Found your problem. Put some decent pads in that thing.
BP-10 is in the front, it has a low mu so that's how I want to keep it.

Like I asked, I currently have the r4-s pads in the rear so I could switch to xp8's to get a higher mu. Do you all think this would be sufficient?
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Old 11-06-2013, 03:05 PM   #7
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Yeah it was pretty much a joke, but free is so enticing haha.


Quote:
Originally Posted by Leafy View Post
The prop valve on the front is an awful idea. Would have figured the reason why would have been drilled into you with FSAE (hysteresis is bad mmkay). And adjusting the prop valve for trail braking wont help much either since trail braking should be happening below the knee point or you're just asking too much of the front tires. So that is either a driver issue, trying to break too hard during trail brake, or a basic bias issue that the prop valve cant fix. Fix that first and you should be much closer to what you need for straight line threshold braking.

The short term fix is the pads, but those cost almost as much as sport calipers. But if you end up doing both and it still not right your could have just gotten the FM bits for the same money and saved some unsprung mass. Clear as mud.
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Old 11-06-2013, 07:28 PM   #8
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Originally Posted by Braineack View Post
in both the F/R? I ask because I have a similar issue since I upgraded the size of my rotors.

I'm running 11" F and 10.9" R with ULTs both F/R and have the prop valve cracked all the way down and still lock up too fast in the front.
The ULTs are nice with 10" F and 10.9" R. The fronts still lock up first but you can feel the rears doing more work. This is with a 2002 non-abs prop valve.
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Old 11-06-2013, 08:19 PM   #9
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XP8/XP10 F/R
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Old 11-08-2013, 12:26 AM   #10
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I am just going to suck it up and buy XP8's for the rear, ill let you know how it works.

To recap, i'll end up with Wilwood's BP-10 in the front, and carbotech XP8's in the rear. Hope this gives me enough rear bias!

Quote:
Originally Posted by Braineack View Post
in both the F/R? I ask because I have a similar issue since I upgraded the size of my rotors.

I'm running 11" F and 10.9" R with ULTs both F/R and have the prop valve cracked all the way down and still lock up too fast in the front.
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