Miata front/rear brake proportioning: Stock vs aftermarket proportioning valve?

Following up on a brake improvement plan for the Molvo, I figure I will install an aftermarket brake proportioning valve. The car is a bit longer in wheelbase and a bit more nose-heavy, so I figure there is something to be learned here.

What I wonder about is how the factory brake propotioning works vs an aftermarket valve.

My understanding was that the factory valve sent the same pressure to the front and the rears until some sort of threshold was met, and then it proportioned more to the front than the rears?

An aftermarket valve limits pressure to the rear from the start giving the front and rear different slopes of pedal travel to pressure from the start.

Will the aftermarket approach not give less braking in initial pedal application?

Just overthinking stuff here I suppose.

Pete

Not sure what gen your car is but for NA/NB cars, the valve was 50-50 (dummy valve) for ABS equipped and had the typical knee in the valve for non-ABS cars.  This is considered "safe", in that for panic stops with lots of pressure, the rear will never lock up.  ABS cars have other means to do so, so their valves are dummies.

The cheap way to fix this on non-ABS cars is to buy a pair of brass unions and plumb out the stock valve. Works a treat and costs less than $5.

Or you can install an aftermarket valve, which you will end up setting to one end of its adjustment, effectively doing the same thing.

Andy Hollis said:

Not sure what gen your car is but for NA/NB cars, the valve was 50-50 (dummy valve) for ABS equipped and had the typical knee in the valve for non-ABS cars. 

Are you sure?  My '91 with ABS seems just as crazily front-biased as my '90 without ABS.  Flyin' Miata's chart doesn't seem to back up this statement, either.

Andy's statement is correct for 2001+ ABS cars. They use electronic brake distribution. You don't want to install brass unions and remove the prop valve on the earlier cars unless you have weak rear brakes or low traction - or you're installing a replacement one elsewhere. If you're running sticky tires and you have good rear brakes, you will have too much rear bias in that case and that's an unstable situation.

Earlier cars had a typical ball and spring prop valve with varying knee points. They're all equal front and rear until that point, then they run slightly different slopes. The general idea is to approximate a curve. This information was extracted from the factory manuals.

https://flyinmiata.com/pages/tech-stock-brake-proportioning-valves

An aftermarket valve like the Wilwood has an adjustable knee point that lets you reshape the curve a bit. Since it's 50:50 until the knee point, it will behave the same as the factory parts (and Andy's unions) until you get to that point. It's a good way to fine-tune for your particular setup.

James Walker Jr wrote a few great white papers for Stoptech, and here's the one on proportioning options: https://www.centricparts.com/media/technical_bulletins_docs/Brake_Proportioning_Valves.pdf

In reply to Andy Hollis :

Thanks for the info and the new term to describe what I expect a stock valve to do: "knee". 

The Miata is stock non ABS 1991 brakes. The chassis has been stretched 9" and the car has gained about 300 lbs of weight with the body swap and V8 engine.

The immediate goal is to install the proportioning valve that I already have. Pretty standard Wilwood part that runs the front brakes straight through and the rears through the proportioning valve.  So what you are saying is that this factory piece does not do much in the way of creating a "knee" to the front rear pressures? I was hopeing to bring a little more rear brake power to the party since I am expecting less weight transfer due to the longer wheelbase.

Subbed, as at the last track day i noted the tendency of my 99 to try to swap ends under hard braking 

Andy gives good advice on some things, but I wouldn't recommend following it here. Your Wilwood part is the easy answer and will get you the best results.

Your front weight bias means you need more front brake bias than the stock Miata.
Your long wheelbase means you need less front brake bias than the stock Miata because you have less weight transfer.

The 1991 already had a lot of front bias, but if you've played games with other parts you may have quite the mix of factors. You can math your brain out or you can install an adjustable valve and spend a bit of time setting it up. Just keep in mind that you'll want to set it up under maximum traction conditions, as that's when you'll want the most front bias. If you set it up in low traction (cold weather, rain, snow) you'll end up with rear brakes that lock too easily when you have the maximum traction. For a track car, you'll probably want to tweak it on the day - mine is adjustable from the driver's seat for that reason.

In addition to my 2000 NB, I also did this on my 92 NA.  Worked great for autocross, first with Hoosiers in CSP trim, and then later for STS with street tires.  Solved the whole flat-spotting front lockup issue which was costing me dearly when Hoosiers were on the car.

But the adjustable valve is certainly a more flexible option.

In reply to Keith Tanner :

Then I must have the wrong Wilwood valve. The one I have provides a jumper for the front brake line and a in-line valve for the rears. How could there  be a front/rear knee point?

Screen capture from the FM video on brake proportioning valves. I bought the one shown in the pic.

I do have one of these in my spare parts pile, might it be a better option?

It's not a front/rear knee. It's only the rear pressure that is affected. Front stays at 100% all the time, and that's what the union does. The rear stays at 100% until the line pressure overwhelms the spring (that's the knee point, referring to the change in the slope of the graph) then it goes to a percentage. That lowers the relative amount of line pressure in the rear, which means less braking torque. Turning the knob preloads the spring, which affects how much line pressure is needed to open the valve.

To make this work, you need rear brakes that are capable of locking up before the fronts. If your rear brakes are too weak to do that, you might as well just remove the valve like Andy did.

Keith Tanner said:

 The rear stays at 100% until the line pressure overwhelms the spring (that's the knee point, referring to the change in the slope of the graph) then it goes to a percentage.

That is the piece of information I was missing. I did not realize that the valve had a spring-load function built in. I thought it was just a restrictor; like a faucet valve.

So the next question would be how does the other valve that I posted a picture of work? does it just move the "knee" point further or lower on the scale?  When would it be the correct choice over the single circuit version? 

EDIT

Upon closer inspection, is it possible that this is two independent systems in one casting separated around the red line?

Maybe. That combo valve might have an internal shuttle valve that triggers a light when there's a big pressure differential between two circuits - aka, blown line. I don't know if it's just between the two front circuits or if the rear is also involved. 

Hmmm, I have some fairly serious rear brakes and I'm still full rear through my proportioning valve on my NA, though I also have AFCO calipers on 11.75x1.25" brakes on the front so maybe just still too much front. I have the stuff put together to put the same calipers and rotors on the rear of my car but was waiting to put together the mk60e5 before I swapped....

I also don't run a proportioning valve, everything is plumbed directly into the oversize Wilwood master cylinder. Stock 1.8 brakes in back, 949 big brake kit in the front. Balance is perfect. I think if the front brakes were smaller, it might be that I would want more bias towards the front. 

It's not like the proportioning valve will hurt anything, even if you end up running it completely open. I bought one when I got my master cylinder, but ended up just trying it without it. 

My car gets used for a wide variety of purposes, from track use on sticky tires to rallying in a hurricane. There's no one correct bias for all of those situations, and running without a prop valve means you have a fixed front/rear bias at all levels of decel. That can work if you are only ever experiencing one, consistent level of braking and traction. My Locost was set up that way and it rarely got used for anything but track use - although when I did drive it on the street, it had a tendency to lock the fronts very easily. The way to make it work is to give the car a considerable amount of front bias from component selection so you never find yourself in a situation where you're getting rear lockup, acknowledging that anything less than a max decel event will not really work the rears.

You're leaving braking performance on the table if you don't fully utilize the rears, though. Both single stop distance and heat management suffer. I've managed to get my rear brakes hot enough to change the color of the anodizing from black to pink.