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Discussion Starter #1
So I have installed the rear bias Proportioning valve on the NA we have been tracking. We currently likely have an overkill for the brakes.

Fronts are Wilwood with Corrado Brembo brand rotors running DTC 70's with tears being stock 97 with DTC 70.

The PP valve is few turns away from full. I am not sure what exactly I am looking for in the adjustment. Most of the braking is in the front already and taking more away from the rear seems counter intuitive. Help me understand the use of the PP valve. Thanks


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Chinchilla Rancher
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From my understanding, having more front biased braking helps to reduce the potential of the rear brakes locking up and inducing a potentially uncontrollable skid/slide. So less in the rear is a good thing, but all of this depends on your purpose for the car and associated driving style/needs.
 

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Aren't the Corrado rotors 11"? You're probably front brake biased, making your proportioning valve useless as a tuning device. Keep shifting the balance rearward and keep taking note of how the car handles under braking in a straight line and trail braking. If you max out the valve, then you need larger diameter rear brakes.

A proportioning valve reduces the slope on the input versus output pressure curve at a certain knee point. So without talking about transients and pre-lockup forces, it basically limits pressure to the rear. If you're front biased, turn the proportioning valve outwards to give more rear bias. Do this until it oversteers on either straight line braking, or too much on trail braking that you can handle, and then turn it 1 or 2 turns frontward.
 

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Discussion Starter #4
full forward on the valve away from firewall is full pressure to rear. I tried and nothing happens.


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There is no one real setting that will work for every surface. Ideally you are changing your brake bias every time you are at the track depending on surface condition. The goal is to have the front tires lock JUST before the rear tires lock. Too much rear bias and the car will be unstable in braking zones while threshold braking. For a street car running a proportioning valve, it is much safer to have slightly more front bias than rear so you lock the fronts first.

As far as where you are in the adjustment range, that will depend on your set up. Have a heavy rear bias? The adjuster will be closing off rear line pressure.
 

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StopTech Program Manager.

True and not true. The system bias/balance has a baseline as measured against the stock system with no changes. The stock system uses a non-adjustable bias valve, ensuring the rears can never lock up before the fronts. It is POSSIBLE to have particular road/tire combinations with front/rear brake pad choices that move bias to the rear enough that you are able to lock them first, but highly unlikely. OEs design their systems with a factor of safety built in to preclude exactly this situation (for liability reasons), which means that with pads of the same compound/friction, it should be quite difficult to lock up the rears first. For performance driving, this also means that technically some brake ability has been left on the table.

Adjustable proportioning valves don't do what is commonly thought. They can't reduce pressure to the rear of the car (it's not a regulator), but they can limit how quickly that pressure builds after a particular pressure has been reached. It's an orifice - an adjustable opening that allows hydraulic fluid through at a particular RATE, not pressure. If you placed pressure gauges on all four hydraulic corners and depressed the brake pedal to the floor and held it, all four corners would reach the same pressure but the rear would reach that pressure at some delay after the fronts.

http://stoptech.com/technical-support/technical-white-papers/proportioning-valves

While particular companies have made the adjustable prop valve mainstream, in most cases data logging would be needed in order to make adjustments accurately enough to show improved lap times. Some serious racing teams have this ability but almost nobody in the consumer market does. Making adjustments on the fly might stop the rears from locking up first but unless the resulting stopping distances are compared to a baseline, most people would have no idea if that setup actually improved or hurt braking performance under all dynamic conditions. Maybe it was improved in a straight line dive but some situations could be diminished.

So in the context of racing setups for the Miata, it is very common to run different friction compounds front and rear. In most cases the fronts are a higher friction compound than the rears, which moves MORE work/bias to the front. This would be bad for braking performance if the stock prop valve is used. If an adjustable valve is used, the capability for improvement is there but that doesn't mean people are setting it up correctly. When adjustable prop valves are used in this situation, it is possible to get more work out of the rears (as compared to the OE non-adjustable valve). On an otherwise stock system if you are opening the adjustable prop valve all the way and nothing is changing, it is likely because the friction compound you are using in the rear is too weak. Getting more work out of the rear as compared to the stock system will generally shorten stopping distances. But this is where taking accurate data is absolutely necessary and almost nobody does this.

There are plenty of other ways to manipulate brake bias - dual master cylinders, caliper piston sizes, friction choices, rotor sizes, etc. The best solution is a dual master with a balance bar because it maximizes both front and rear brake work. Consider that companies like Brembo and StopTech don't even offer an adjustable prop valve and there is a reason for that; the advantage is not worth the cost to all but the most serious and well prepared race teams - who use a dual master anyway.
 
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