Drop an anchor! Miata rear brake bias...or lack of

As I discovered last night during a hairy downhill stop, the combination of crappy all-season tires and a low knee point in the prop valve makes my 1990 Miata a bit hard to stop in the wet. Currently investigating options to eliminate this issue and (hopefully) increase safety.

1. OEM Value pads up front, and Hawk HP+, HPS, Stoptech Street, Porterfield R4S, AX6, etc on the back. Looking for a higher bite from the rear without locking them first. Not track driven, needs to work when cold. Like not to spend an exorbitant amount, up to $70 for the rear if need be?

2. Gut stock prop valve. Seen this mod discussed on miata.net, seems to work favorably with the right brake pad combination, and all I'd want to be sure of is no rear brake lockup in the wet. With my crappy tires, I think that may be a non-issue.

3. Prop valve swap. Could be an option to swap to a 2002+ prop valve, but the cost difference between it and a FM adjustable is fairly minimal.

4. Better tires. Would like to...next year. Too close to winter now and I haven't had issues with the tires for cornering grip. Just braking...which leads me to believe my rears aren't doing any work.

I should mention the car will be driven in the snow and ice (with full set of snow tires). All the more reason to have more rear brake bias, but I don't want a pad that will lock up too quickly.

Thoughts? I think I'll gut the prop valve this evening and test while it's still raining...see what I think.

Why's it hard to stop in the wet? what is it doing?

With winter fast approaching buy OEM quality pads frt and rear instead of wasting time and money experimenting in conditions that will change very soon.

Another option (maybe the best option) is to install this piece in place of your stock proportioning valve, but it requires some fab work:

http://www.wilwood.com/MasterCylinders/MasterCylinderProd.aspx?itemno=260-11179

Robbie wrote: Why's it hard to stop in the wet? what is it doing?

Front tire lockup under moderate braking. I can't feel the rears doing any work...if it's possible to feel the rears.

I thought to blame the tires, but they corner just fine.

Option 1 works well for autox (on the 1.8 brakes). I never had any problems with it on the street either. Then again, didn't drive that car in the snow.

1.6 cars have tiny brakes. I upgraded to 1.8 rotors and added hawk hps pads and it helped a ton. I tried running just 1.8 in the rear and hotter pads in the rear and it helped the bias a bit but 1.6 Miata just have bad brakes.

In reply to OSULemon:I think you answered your own questions (crappy tires!). And I've always thought stopping was the most important function of tires.

Have you ever noticed how much more effective, larger, vented etc., front brakes are designed to be than the rears? Simple reason. With the weight shift the front tires do the vast majority of the braking as the rear end is unweighted.

With too much rear brake you'll find your Miata stopping like a pickup with no weight in the bed ie: sideways or backwards! Stomp on the rear brake on a motorcycle. That usually makes your rear tire sound like a siren and does nothing to slow down, but maybe it'll try to slide out from under you. Same deal with a car.

Personally I'd not change anything until I had good and predictable tires.

the quick and easy test is to 'drag' the parking brake by bringing it up a few clicks and then trying another emergency stop.

If you stop better, then maybe you do need more rear bias.

Might want to adjust the rear brakes. I've been told that is something that should be done on Miata's on a regular basis, especially if you don't have anti-lock brakes. Different people have told me different intervals from every tire rotation to every oil change. Easy to do. Remove a plug and adjust with an hex (Allen) key.

Easy button here is Hawk HPS front pads and HP+ rears. I wouldn't mess with the proportioning valve until after you do that.

Just swapping to those pads should make your braking about as good as it can be with less than ideal tires.

Have you bled the brakes and adjusted the rears properly? Try that as well.

Rupert wrote: In reply to OSULemon:I think you answered your own questions (crappy tires!). And I've always thought stopping was the most important function of tires. Have you ever noticed how much more effective, larger, vented etc., front brakes are designed to be than the rears? Simple reason. With the weight shift the front tires do the vast majority of the braking as the rear end is unweighted. With too much rear brake you'll find your Miata stopping like a pickup with no weight in the bed ie: sideways or backwards! Stomp on the rear brake on a motorcycle. That usually makes your rear tire sound like a siren and does nothing to slow down, but maybe it'll try to slide out from under you. Same deal with a car. Personally I'd not change anything until I had good and predictable tires.

I understand the consequences of too much rear brake. I'm attempting to find the point at which the fronts lock just before the rear in a worst case scenario: dry day, hot tires, grippy stretch of road. If I change to a stickier set of tires, I would need less rear brake bias.

Never had I felt the rears come close to lockup in the dry, and I'm not impressed with the stopping distance either. This leads me to believe I may be leaving some available braking on the table with the rears. I will do some tests with the handbrake to see if the tires are the limiting factor. They shouldn't be THIS crappy.

I will bleed, adjust, and report back with findings before I proceed any further.

When is the last time you had the rears apart? I has an issue where my rear sliders seized and I had to replace the brackets and calipers as I couldnt separate them, even with a sledge in a bench vise.

They do have too much front bias though... I hear a lot about HPS/HP+ rear and junk front pad-wise from other STS miatas though. (not allowed to play with bias valves)

I think you have a lot of low hanging fruit before doing brake mods.

I have a 1.6L and was able to get the brakes so well adjusted that a pro driver had to re-think his approach to corners, because he was able to dive so deep. I will confess my car is lightened.

Tires are first. You are hydroplaning.

Then testing. Pull the handbrake and see if you have anything at all.

Then comes rear brake adjustment. There is a strange adjustment on the rears of the 1.6Ls which everyone seems to miss. It's easy to adjust, but hard to find. There is a cap on the back side of the rotors covering a set screw. I use a mirror to see. Remove it, adjust until they catch, then back off a little.

If your fluid hasn't been changed since 1990, do that too. They respond well to fresh fluid.

Then come the pads. How are yours? I use OEM replacements, but am very happy with the Hawk street performance. While doing the pads, you need to clean and lube the sliders. Also check the rotors and true them.

THEN you look at bias. I swapped to a OEM unit from a 1996. Changes the knee point enough to balance things well, and was virtually free from a junkyard. Flyin Miata has a really nice adjustable one if you want something more. The later prop valve would not make me happy- too much. But again, my car is light.

Never tried gutting the prop valve.

After all of that, I would consider mis-matched pads, but you've got several steps first.

My vote is that your rears have seized so that you are getting little or no braking out of the rear. Not unusual in early cars in my experience. Insert hex key into rear of caliper and see if it moves the piston. If not, rebuild(if you can find the kit) or just replace the rears.

SVreX wrote: I think you have a lot of low hanging fruit before doing brake mods. I have a 1.6L and was able to get the brakes so well adjusted that a pro driver had to re-think his approach to corners, because he was able to dive so deep. I will confess my car is lightened. Tires are first. You are hydroplaning. Then testing. Pull the handbrake and see if you have anything at all. Then comes rear brake adjustment. There is a strange adjustment on the rears of the 1.6Ls which everyone seems to miss. It's easy to adjust, but hard to find. There is a cap on the back side of the rotors covering a set screw. I use a mirror to see. Remove it, adjust until they catch, then back off a little. If your fluid hasn't been changed since 1990, do that too. They respond well to fresh fluid. Then come the pads. How are yours? I use OEM replacements, but am very happy with the Hawk street performance. While doing the pads, you need to clean and lube the sliders. Also check the rotors and true them. THEN you look at bias. I swapped to a OEM unit from a 1996. Changes the knee point enough to balance things well, and was virtually free from a junkyard. Flyin Miata has a really nice adjustable one if you want something more. The later prop valve would not make me happy- too much. But again, my car is light. Never tried gutting the prop valve. After all of that, I would consider mis-matched pads, but you've got several steps first.

Great suggestions. I'll go through them all!

FWIW, its well known that Miatas of that era with everything working great will still lock up the fronts way before the rears with equal pads. Hence all the workarounds suggested in this thread.

Nothing wrong with fresh fluid, bleed, and adjust, but I wouldn't jump to the conclusion that there is a problem with the OP's braking system or tires.

FWIW, I had major problems locking 225 RS3s on 9s on the front of the STR Miata when were foolish enough to run the same pad front and rear. Swapping to autozone E36 M3ty pads up front made a huge difference.

Flyin Miata has a chart of different Miata prop valves, but the FC RX7 valve is also in that range, the exact numbers can be found in the FSM, available to download from various places online.

1.6 Miatas have wonderful brakes--If the brake system is in good shape. I thought I was going to need to upgrade to 1.8 sport brakes to make my '92 happy on the track. Not needed. Not even close.

The recipe: Toss old calipers, rotors, and pads. Consider it an opportunity to work on your shot-put mojo! Replace slide pins if the are pitted/worn. Replace calipers with rebuilt stuff. Replace anti-rattle clips with new. Lube slide pins well with NAPA high temp ceramic brake grease (the purple sticky stuff).

For Street, run NAPA rotors and whatever stock pads. For track, run Hawk DTC-60 in front and DTC-30 in back. They're shockingly good. (I have no financial interest in Hawk. However, after trying several flavors of track pads and being pretty Berkleying frustrated the level of braking performance, I'm very, very happy with these DTCs).

I run a set of rotors for track and a set for street. I mark the rotors and pads backs with pink paint marker so I can tell what rotor goes where, and which pad is inner/outer. Add your fav high temp brake fluid and serve.

You already know what varying traction levels will do to your ideal brake bias. If you want the car to work well on snow and hot race tires, you're best off with an adjustable prop valve. Or you set the car up for one condition and acknowledge that it will be poor in others.

I don't like mixing brake compounds because their interaction changes with temperature. I prefer to adjust bias with rotor diameter, piston diameter and a bias valve. Generally, you'll always be looking for more rear bias so you'll want larger rotors and maybe a set of Sport rear calipers. You can always turn them down with the prop valve. Definitely do not handicap the end that's working - putting crap pads on the front because you're getting front lockup is the wrong direction.

Of course, you do want to make sure everything's working. I've never been convinced that adjusting the rear calipers does anything, but the slider pins are problematic.

I second the 1.6 brakes being great. Like everyone else my slide pins were completely seized. This was a California car as well, I would start with that low hanging fruit.

Keith Tanner wrote: You already know what varying traction levels will do to your ideal brake bias. If you want the car to work well on snow and hot race tires, you're best off with an adjustable prop valve. Or you set the car up for one condition and acknowledge that it will be poor in others. I don't like mixing brake compounds because their interaction changes with temperature. I prefer to adjust bias with rotor diameter, piston diameter and a bias valve. Generally, you'll always be looking for more rear bias so you'll want larger rotors and maybe a set of Sport rear calipers. You can always turn them down with the prop valve. Definitely do not handicap the end that's working - putting crap pads on the front because you're getting front lockup is the wrong direction. Of course, you do want to make sure everything's working. I've never been convinced that adjusting the rear calipers does anything, but the slider pins are problematic.

Thanks, Keith. I found a lot of your posts while searching for the answer, and at this point I agree that pad swapping is not the way to go in this case. I also came to understand that it's preferable to have more rear brake capacity than needed, then use the prop valve to turn it down.

However - the reason I felt I could get away without a adjustable prop valve is because I only run low-traction all-season tires for commuting. My other car runs ultra summers, and it's just fine.

So, if I'm doing rotors/pads/calipers (which seems likely at this point), I could go ahead with a 1.8 swap, since the reman calipers come with brackets...

BTW, the 1.6 cars seem to have the capacity for fantastic pedal feel. I don't know why, there's nothing in the specs that is different than the non-Sport 1.8 cars. But when set up well, they feel great. They can still use more rear bias, though.

Keith Tanner wrote: BTW, the 1.6 cars seem to have the capacity for fantastic pedal feel. I don't know why, there's nothing in the specs that is different than the non-Sport 1.8 cars. But when set up well, they feel great. They can still use more rear bias, though.

Unfortunately, my 14" wheels won't allow a Sport upgrade...although it serves as an excuse to swap wheels/tires...

I could go ahead and swap to the 1.8 in the rear? Brackets are $20 each.

Otherwise, my current plan is all 1.6: Centric Premium rotors all around, Stoptech Street Performance F/R, refresh/evaluate the calipers while I'm in there and replace as needed. Plus fluid flush/bleed, pin lube, etc...

Yup, you could go to 1.8 brackets in the rear. You just need the matching pads and rotors. Note that you have the possibility of also using the Sport calipers on 1.6 or 1.8 brackets. That gets you a slightly bigger rear piston (1.25 vs 1.375", if memory serves) which will give you more rear bias.