Too much rear brake bias
 

I’ve been running a 15x8 rim with RA1s, and Dunlap Direzzas prior to that on my 2003 NB. For pads, I’m running G-Loc R10 in the front and R8s in the rear (it’s worked well). With that set up, I don’t seem to have much rear brake bias under threshold braking (e.g. turn 10A at Road Atlanta), with just a little rear movement, but nothing unsettling.

I’ve now upgraded to 15x9 rims with NT-01s. I ran Barber last weekend and I’m getting more rear brake bias and the rear moving around more under threshold braking. It was wet last weekend so maybe that make it worse? I put on my 15x8s with new Direzzas on Sun (my new “rain” set up) and seemingly got less movement, although I got limited runs in the rain.

In any event, what are the options for getting less rear brake bias? Thoughts so far are:

• · Flyin Maita adjustable brake proportioning valve - https://www.flyinmiata.com/complete-...oning-kit.html
• · Less rear brake pads – G-Loc R6.
• Other options?

Thoughts appreciated.

Get the Wilwood from Trackspeed with steel fittings. I've got brass on mine and am always worried.

Typically you want more rear bias in the wet than in the dry, since you can't brake as hard and get less forward weight transfer. Obviously, YMMV if you're changing setups.

Wilwood now makes this prop valve with inverted flare seats, but it has 3/8-24 threads, so it unfortunately won't thread onto your stock rear brake line. You're going to deal with at least one adapter on the prop valve no matter what, so the TSE is a nice choice.

Thanks for the replies. Will running less rear brake pads also help?

It will, but it won't give you the flexibility that a prop valve will.

I'd also recommend the setup that Trackspeed has... they make it very easy to push the "easy" button with the adapters.

Upgrading fronts to a bigger set is not an option?

I ofund the extra braking is a massive improvement on track. Not to mention far better pad wear in the long run.

Our steel adapters have been discontinued. For all the accolades they received here and elsewhere, the extra cost translated into nearly zero sales for us. We have to make the fittings in big quantities and couldn't justify it any further.

Are they all brass now? What's the downside to brass? Thx.

They are much easier to break if you are not careful during installation.

Can confirm, have broken the brass fitting. Had to put the stock fittings back in and re-bleed.

I'm not following why you need more rear bias. As afm pointed out, you added more front grip. Unless your rears are actually locking up, I think the extra grip is transferring the weight quicker to the front, unloading the rears and making the rear feel light.

I want less rear bias. The rear end is getting squirrelly when threshold braking in a straight line.

I could be wrong here, but I don’t think a prop valve will fix your problem. IIRC, my Wildwood valve has the same proportioning as the stock valve when it’s wide open. Tightening the screw only adds more rear bias, you can’t get less than stock.

The wilwood reduces the bias to the rear brake from 100% to 43% of the front. Im not sure if that percentage is pressure, flow, or just orifice opening size though, but either way it will reduce rear brake bias. Its also like $60 for the ones with fittings and installs in a light afternoon as long as you dont do anything stupid like break a bleeder like I did...

Unless the class you are trying to compete in doesnt allow it, the Wilwood prop valve is the cheap and easy button.

I got that pretty much all backwards...

i have the same problem. with the willwood set to minimum rear pressure, i can stil lock up 225 rears before front. xp10's and 12's. thinking I just need a whimpy rear pad.

Run bigger front brakes?

:shrug:

This is incorrect

Strike that reverse it...

The OEM prop valve reduces pressure at a set ratio above a certain pressure. So it's linear up to 800psi, but then at 900psi front, the rears only get to 850, at 1000psi front the rears are only 900psi, etc. (Numbers are made up but the theory is correct)

The Wilwood prop valve reduces pressure at a set ratio above a certain pressure, but you can manually adjust that pressure (known as the 'knee'). So if you want the rear brakes to do more, you increase that knee point so that the prop valve doesn't reduce rear pressure until you've added more front pressure.

There are lots of charts and actual figures for this stuff available in the FSM

Also, OP, your post makes little sense. In the rain, you need MORE rear bias, not less. You are not braking as hard, which means you are transferring less weight onto the front tires, which means more weight is left on the rear tires during braking. Thus, you need more rear bias.

So the same car in the rain, with no bias changes, will not become more unstable due to brake bias issues. If a car is unstable in the rain due to bias, it's really unstable in the dry.

My money is on the NT01s being quite poor in the rain, which they are.

Interesting. Thank you. I have Road Atlanta coming up late this month. I will leave it be and see how it feels. Hopefully it will be dry!