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Old 05-02-2012, 03:44 PM   #1
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Default question on proportioning valve with big brakes

I'm nearly complete with my track-day car build. 2004 NB/turbo. I've been making all of the necessary safety upgrades and engine output and reliability upgrades.

Now moving on to brakes. I've got a set of Hawk blues for the OEM calipers that I had used in past track day sessions. After reading up on a few of the threads, it looks like it could be pretty easy to cook these, even if ducted properly.

I'd like to get the willwood big brake kit. In either the 12" or 12.75 rotor size. I understand that the stock master cyld is heavily biased towards the front and it is "recommended to use a proportioning valve to re-establish proper bias".

What I don't get, (admittedly I'm scandinavian, so it might be genetic) is if the proportioning valve goes on the rear line, and if the valve can only turn the braking pressure down, then how is this going to help the brake biasing troubles caused by increasing the brake pad swept area and increased rotor leverage of the front? Do you also change out rear the master cyld size along with the proportioning valve? Or, are you somehow adding a proportioning valve to the two front lines?

Don't flame me, I'm Norwegian.
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Old 05-02-2012, 03:56 PM   #2
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The brake bias is from the factory proportioning valve on the rear line, not the MC itself.

You are replacing a fixed valve with an adjustable one, that's all. The ABS cars have more rear bias than the non-ABS cars.
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Old 05-02-2012, 03:56 PM   #3
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There are no 12/12.75" Wilwood kits; they simply wouldn't fit under any wheels.
Our 11.75" kit is the largest front Wilwood kit on the market for Miatas.

Our adjustable proportioning valve replaces the stock proportioning valve. Your factory proportioning valve biases the brakes too much toward the front once you upgrade to bigger brakes. By replacing it with the adjustable proportioning valve you can dial this bias in right where you want it, since you are replacing your factory valve (and therefore can add more rear pressure than it would have from the factory, adding more rear brake). It does nothing to alter brake pad swept area, not sure where you are getting that from. There is only one master cylinder for both front and rear brakes, which remains unmodified (unless you go to a sport master, but not required).
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Old 05-02-2012, 03:58 PM   #4
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id argue it was too much to the front to begin with... espeically when you upgrade the front rotors.
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Old 05-02-2012, 04:21 PM   #5
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Quote:
Originally Posted by gtred View Post
What I don't get, (admittedly I'm scandinavian, so it might be genetic) is if the proportioning valve goes on the rear line, and if the valve can only turn the braking pressure down, then how is this going to help the brake biasing troubles caused by increasing the brake pad swept area and increased rotor leverage of the front?
It's a little counterintuitive, so you need to forget everything you think you know about how a bias valve works before reading this explanation.

The ideal braking pressure applied to the front and rear calipers is a non-linear curve. As you start braking, both axles (front/rear) support approximately the same weight, so both can support the same brake pressure. As the car stops harder and harder, more of the weight shifts to the front of the car, which requires the rear brakes to do less (and require less pressure). If taken to the extreme, you end up with a car standing up on its front tires only, with the rear brakes requiring no pressure at all.

The ideal curve is the blue line, and the brake pressure applied to the REAR brakes is the white line. You can see the "knee" point - it's the poiint at which the bias valve begins to restrict the rear line pressure. The area under the ideal curve is where the front tires lock first, and the area above the ideal curve is where the rear tires lock first.



As you turn the **** on the bias valve in, it pushes the knee point further up, and therefore closer and closer to the ideal curve, which optimizes the balance between the front and rear brakes.

The stock bias valve does the same thing - the Wilwood valve just allows you to adjust that knee point and get it much closer to the ideal curve.

Remember that when you adjust the valve, you don't ever want to go above the ideal - if you're in a hard braking zone and you go over a small bump which reduces rear grip and ends up locking the tires, you'll spin the car. I will typically run a lot of rear bias on smooth tracks with very little trailbraking, and dial the bias back a bit on bumpy tracks with long, sweeping braking zones.

On race tires with our 11.75" kit in the front and 10.9" Sport rotors in the rear, I typically suggest starting at 5-6 turns from fully in, and then adding more rear bias to taste. On street tires, you'll want to run more rear bias than that, but exactly how much will depend on your application, pavement, etc.
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Old 05-02-2012, 04:22 PM   #6
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Uff-da. OK. I think I understand now. Yes, I meant to say 11" not 12. I guess my post didn't make it clear enough, I meant to infer that the big brake kit gave you bigger pads and more leverage on the front.

So the factory proportioning valve in the master cyl is replaced with one that gives more bias to the rear. Tusen Takk/ thanks
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Old 05-02-2012, 04:27 PM   #7
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Originally Posted by gtred View Post
Uff-da. OK. I think I understand now. Yes, I meant to say 11" not 12. I guess my post didn't make it clear enough, I meant to infer that the big brake kit gave you bigger pads and more leverage on the front.
The leverage doesn't come from the larger pads - friction force is a function of the force being applied to the brake pad and the friction coefficient of the pad itself. If you increase the surface area of the pad, you decrease the pressure per square inch, and the net result is no change.

The increase comes from the additional radius of the rotor - same thing as opening a door from the middle of the door vs. the handle on the edge.

The factory prop valve is separate from the master, and our valve replaces it. For the best pedal feel, we suggest upgrading to the NB Sport master cylinder and brake booster - it's slightly larger and the resulting pedal feel improvement is drastic.

Theseus and Rover both run the same brake setup - NB Sport master/booster, TSE/Wilwood bias valve, TSE 11.75" fronts, M-Tuned BRK 10.9" rears, DTC-60 pads front/rear. I have yet to drive a Miata with better brakes than our cars.
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Old 05-02-2012, 04:36 PM   #8
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Sav do you recommend the sport calipers for the rear to go with the sport rotors? How about the 1" master on sport booster config?
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Old 05-02-2012, 05:29 PM   #9
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Thank you for taking the time to school me on brakes!
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Old 05-02-2012, 06:27 PM   #10
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I'm enjoying these recent threads on braking systems, and input from guys like Andrew who have btdt and are here to write the book and school the rest of us.

Much like the "Suspension Hierarchy" thread, we should get a "Braking Hierarchy" thread going, put all this info in one place, and get a mod to sticky it.
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Old 05-02-2012, 07:42 PM   #11
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Quote:
Originally Posted by k24madness View Post
Sav do you recommend the sport calipers for the rear to go with the sport rotors? How about the 1" master on sport booster config?
Sport calipers will add another ~10% to the rear bias, which is always a good thing. I would put the sport calipers on a 1.8 bracket and use the M-Tuned BRK adapter, though, since you can't get a DTC-60 in the Sport pad shape.

I think a 1" master is too much. The 15/16" Sport master is perfect IMO.
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Old 05-02-2012, 08:34 PM   #12
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Have you guys tried using the standard pad on the sport brake bracket and caliper? I was swapping out my parts to use the M-Tuned adapter and 1.8 bracket today. I noticed I was already using a non-sport pad in my factory sport brakes due to a possible shipping error. I've been using it for a year without catching the error. See post in Track Brakes thread for pics.
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Old 05-02-2012, 08:46 PM   #13
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So what is the effect of sport brakes all around on an early NA master and proportioning valve? Feels pretty good.
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Old 05-02-2012, 11:10 PM   #14
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Quote:
Originally Posted by olderguy View Post
So what is the effect of sport brakes all around on an early NA master and proportioning valve? Feels pretty good.
Ho-hum bias, a long/soft pedal, and expensive brake pads.
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Old 05-02-2012, 11:24 PM   #15
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Do you get new NB Sport master/boosters from MazdaSpeed, or do you source used pieces?
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Old 05-02-2012, 11:27 PM   #16
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I source them both used, but if a new master cylinder makes you warm and fuzzy, you can order one new from Mazdacomp.
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Old 05-03-2012, 12:37 AM   #17
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Could we go over briefly the benefits, or lack thereof, of the proportioning valve for those of us with ABS cars. Seems to me it would still be beneficial to have it to get the car neutrally biased, so that the ABS doesn't have to kick in unnecessarily.

Edit: this is assuming the TSE rotors in front. So in my (soon to be) case it would be TSE front BBK + stock mazda sport rears.
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Old 05-03-2012, 04:04 AM   #18
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Quote:
Originally Posted by Mobius View Post
Could we go over briefly the benefits, or lack thereof, of the proportioning valve for those of us with ABS cars.
I'm interested in this too. I put the donor car's ('04MSM) complete brake system including ABS in the '90 when I built it and planned to put the ABS on a toggle to use in slippery conditions. So far things have been pretty hairy braking from high speed with the ABS off (~130mph). I thought it was just that the car isn't fully sorted yet but with the ABS on but not engaging noticeably, everything feels more planted. I've only had 4 days on this build so there are plenty of variables I haven't even discovered yet...

Is this in my head or a real phenomenon?
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Old 05-03-2012, 03:11 PM   #19
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Quote:
Originally Posted by Mobius View Post
Could we go over briefly the benefits, or lack thereof, of the proportioning valve for those of us with ABS cars. Seems to me it would still be beneficial to have it to get the car neutrally biased, so that the ABS doesn't have to kick in unnecessarily.

Edit: this is assuming the TSE rotors in front. So in my (soon to be) case it would be TSE front BBK + stock mazda sport rears.
Even with ABS, you still need to adjust your proportioning front to rear for optimum brake balance. Do this by disabling your ABS (whether you have a switch wired or simply have to pull a fuse), threshold braking to adjust the prop valve to the balance you desire, then re-enabling ABS.

Quote:
Originally Posted by mr_hyde View Post
I'm interested in this too. I put the donor car's ('04MSM) complete brake system including ABS in the '90 when I built it and planned to put the ABS on a toggle to use in slippery conditions. So far things have been pretty hairy braking from high speed with the ABS off (~130mph). I thought it was just that the car isn't fully sorted yet but with the ABS on but not engaging noticeably, everything feels more planted. I've only had 4 days on this build so there are plenty of variables I haven't even discovered yet...

Is this in my head or a real phenomenon?
This is because of the EBD (electronic brake distribution) starts working to fix your crappy bias once you enable ABS. This is less than optimum, and your system would benefit from the addition of a proportioning valve. You would do the same as I described above to set up the prop valve, switching your ABS off to set it then enabling ABS when you feel necessary.
EBD (as well as ABS) uses the wheel speed sensors to adjust based on slip, meaning your setup has to be doing something wrong (wheel speed differential) before it'll kick in. Eliminate this system design error by incorporating a properly adjusted proportioning valve and you improve your braking. Of course, the EBD and ABS are still there (assuming you have it switched on) in case you make a mistake or track conditions change (for instance you have bias set for a dry track, but there happens to be a wet patch right in your braking zone).
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Old 05-03-2012, 03:29 PM   #20
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Thanks. ABS is not something I would have spent extra money on but I had everything from the donor and was going to be swapping the booster, master and sport brakes anyway so I figured I would take the 6#weight penalty and see how it did.
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