Need more rear brake bias
 

I have upgraded to the larger 94+ brakes all around, everything is new with Carbotech xp10 fronts and xp8 rears (Emillio's recommendation). Fronts were locking up so I installed a willwood adjustable prop valve, in the line to the rears, and a straight through adapter to the fronts. Even with the prop valve set to full brake to the rears, I still have too much front, and not enough rear brake, fronts will lock up. I have trouble understanding how people put larger brakes on the fronts, without upgrading the rears.

On the track. when I get the brakes good and hot, they are better, but still I can feel them not grabbing enough in the back. After some laps, I pull into the pits, check temps front and rear, I read ~150 deg c front and ~100 c rear. The temps remain proportional as they get more heat, after more laps.

I have considered trying different pads, but I am unsure if I should change front or rear, or what to change too. I have given thought to changing the prop valve to the front line, but that seems unsafe to me.

Every component of the brakes is new, other than the steel lines. SS hoses and DOT5 fresh fluid. Calipers, rotors and master are new. The ebrake will slow the car, I am sure the rears are grabbing, but the fronts are working much better. Suspension has been all gone over with new components everywhere. Yoko AO48's 205 50 15 tires are new, 2 events, and hold corners great, but I fear late braking into corners.

I welcome ideas, comments and questions. Change pads, move prop valve, slicks on the front(kinda extreme), I need to keep from locking the front tires.

Ron

great one asshole.


I had this same problem on the street with 11" rotors in front and 10" in rear with HP+ pads. Bias all the way to rear and it still wasn't enough...

Rear sport brake conversion? I assume the brakes have been bled sufficiently?

Same problem here.

It seems a fix might be to put rear calipers from a 2001-2005 sport brake car. They fit any of the brackets and the piston bore goes from 32mm to 36mm giving more clamping force.

http://forum.miata.net/vb/showthread...=363284&page=3

Bob

if what you say is true then you are probly gonna have to put the prop in the line going to the fronts and redirect pressure that way. If it cant be exerted on the fronts it should by default go to the rears eventualy.

The sport / MSM rear brakes are a substantial improvement over the standard 1.8 brakes. Bigger rotor, bigger piston, and bigger pad. You get a lot more clamping force and fade resistance.

I need to confirm this brake caliper change-up. If I read this post, and the one referenced on M.net, the 01-05 sport rear caliper will fit my 94 bracket, rotor and pad? That sounds very much like what I am looking for. If its not enough I can update then fully to the sport rotors, brackets and pads, if the larger rotors will fit either set of my wheels, Konig Helium 15x6.5 or Enkei RP-F1-15x7. Goodwin says no to the Enkei's but yes to the Helium's. I can live with that, but wish it was the other way around.

If the calipers are a direct swap, and they do the trick, I will be very happy. I suspect both sets of wheels will work with this change-up.

I suspect Emilio recommended the pad selection to control this bias problem, so I don't really want to change the selection.

Thanks for the help so far guys.

Ron

Rumor has it that the calipers are interchangeable between brackets all versions. I think it was Keith that said this I have not personally varified. The pads you use depends on the brackets not the caliper.

On my brakes sport rotors would be a downgrade from the rotors I’m currently using. I have 11.75” front and 11.44” rear rotors now but my caliper relocation bracket works with 1.6l brackets on the rear.

Bob

The caliper should swap this way but that's not the path I would recommend. I think you would be a lot better off with your 1.8 caliper on a sport bracket and rotor (caliper would be held further out on the larger rotor). This would give you more mechanical advantage. It might give a better pedal than the sport caliper as well.

I went from 1.8 to MSM brakes (same as the sport as far as I can tell, but they have different part numbers) and the difference was dramatic. I did all 4 at once, so I can't offer a direct comparison, but I don't have a rear bias problem. The sport/MSM rotors are larger diameter in the rear than the front.

Won't fit in some wheels though. FWIW the front calipers and the Master cylinder all have bigger bores for the 2001+sport brakes.

Bob

This is true. They're also heavier. I've tried on several different stock 15" wheels and not had fitment issues. 6UL and most of the popular stuff all clears. Also I run the stock master and have good pedal feel.

To the OP:

You mentioned the E-Brake slows the car in your initial post.
E-brake, when properly adjusted, should lock the rears as far as I know.

I may be far off the mark here, but have you made sure to seat the rear pistons correctly when you installed the rear calipers?

I mean, there will be excessive piston travel otherwise, pretty much rendering the rear brakes useless.

Just a thought.

I think you were on the right track identifying what temperature your brakes were running at. Next would be to see if you were exceeding the maximum temperature vs coefficient of friction. Generally the coefficient of friction will increase with tmeperature but at a certain temperature the coefficient of friction will begin to go down.

Here is a good site with operating temperature vs coefficient of friction numbers for a lot of common race pads including XP8's and XP10's.
http://www.ricks2k.com/products/Carbotech.php
http://www.ctbrakes.com/brake-compounds.asp

If your temps seem about right, even after your full rear bias adjustment. Then I would consider the sport rear rotor. However, I would be reluctant to put the sport calipers with your stock master cylinder. I believe that will actuaclly reduce clamping force because you'll have the same line pressure with a larger piston which should result in less pressure. Keeping your calipers with a larger rear rotor sounds more ideal unless you get the sport master cylinder as well.

You could always decrease front tire pressure a hair...lol...that will help get the rears to lock up a lil sooner, but not because the rears are working more.

You got that backwards

Same line pressure pressing against more caliper piston area will creat more clamping force

Sort of the oposite while at the master cylinder if you go bigger on the MC the same pedal force will produce less PSI

Go bigger on everything and nothing really changes but the operating pressure of the brake fluid.

Bob

Knowing Pascal's Law somewhat, the line pressure unchanged with a larger piston, will exert more force, but the master piston will have to travel further, to fill the volume, and maintain the pressure. This should give more clamp force.

The lapping events I am involved in, is usually 10 laps, 9 turns, on a 1 mile road coarse. At the end of the 10 laps is when I measure the temps, with a proper probe, not a raytech. I am after 10 laps, only coming into the working heat range of the pads. As more lapping sessions happen, the temps will climb proportionately, but far from the max rating of the pads, front or rear. The hotter they get, the better they work. This seems to be more so for the rears, as more laps, and I don't feel the fronts trying to lock up. The front wheels certainly have much more dusting at the end of the day.

A full rear brake upgrade is an option, and if its too much brake, I can dial it back with the prop valve. Rotors, pads, calipers, brackets and a new set of wheels, may be a little out of the budget at this time.

New calipers alone are close to max budget, and I will need them either way. Sounds like a good thing to try, I highly doubt it will make braking worse.

I really wonder if a different pad selection would also be a step in the right direction as well. What I don't read (understand) is if the xp10 would give better rear bite, or if that's the wrong direction.

To Godless Commie, The brakes are installed and adjusted correctly, I don't need or want to lock the rears with the ebrake handle. chpmnsws6, Yes the brakes are bled properly.
Braineack, Your comment requires no reply, other than acknowledgment LOL :laugh:

Out for a drive today, I used the ebrake to put some heat into the rears, about 10 hard applications from about 70 mph. The bias was much better, with the rears close to up to temp, and the fronts still basically cold. I don't think this is a major rework, to get it working right.


Thanks for giving me great food for thought on all this guys.

Ron

Please don't get me wrong. I was not suggesting you should lock your rears with the E-brake.

What I was trying to say was that the E-brake, in the case the rear caliper pistons are seated properly, are in fact capable of locking the rears.

Properly adjusting how far the rear pistons are located from the rotor surface means eliminating the unnecessary (excess) piston travel, which translates into less pedal movement.
If the rears are not adjusted by removing the adjuster cap bolts and turning the allen head inside the caliper, since the front pistons adjust themselves automatically, the fronts make contact with the rotor surface much sooner than the rears do.
I have had the same problem once, too. That's when I had someone else (the dealer) replace my brakes.
I have not let anyone else touch them since.

Does the "slack adjuster" need adjusted after the first mile or two? I noticed on our driveway, the parking brake let the car creep forward with the bigger MT Radials on.

How about swapping the pads around front to back and see what that does to your bias? And maybe fine tune from there with the bias adjuster.

Seems like the cheapest (free) and easiest thing to try. If it helps, and you want to, go from there in changing brake hardware or pads on one end to get the balance you want.

Front to back? Please explain for the uneducated like myself :)

Ha, just realized that probably doesn't work due to different fitments. :)

But I meant just trying the compounds on opposite ends, ie XP10s in the rears.

Front serving prop valves are generally not a good idea. There is a modulation concern related to hysteresis and friction in the prop valve.

More capable rear brake hardware would help fundamentally.

I've installed full sport rears onto earlier Miatas with great results. Around 30% more rear braking action leads to stronger braking overall along with markedly better trail braking capability. It also reduces some of the workload up front.

If you are rules or wallet limited pad swapping comes to mind, however the bias shift varies with conditions.

Anyone have any guidance for installing the prop valve? FM kit?

NA with 1.8 front and NB Sport Rear brakes:

http://www.pbase.com/sjmarcy/image/129125192.jpg

I will double check with Emilio next time I see him. he recommends XP10/8 on most Miatas w/o prop valve, and that combo is the most popular Spec Miata setup. but he also recommends same compound front and rear, which is what I run on my Rotrex Miata.

Search around, a few of us have done it different ways. FM's kit is by far the cheapest and easiest way to put in a proportioning valve. I'm not going to lie, some of there stuff is a little spendy, yet this kit is only charging you $28 over the price of the valve from a place like summitracing.com, and for that you're getting 3 custom made adapters to take all the headache and tube flaring out of the project. 28 dollars VERY well spent.

Check out my build thread and Keith's Targa build for an in-cockpit mounting option, otherwise follow FM's instructions to mount it in the stock proportioning valve's location.

P.S. You're doing okay for a newb but you're also bumping a few slightly old threads, try to search a little more. If it's a "dead" thread (older than a couple weeks), the original question has most likely been answered. Make sure to fully read and comprehend the entire thread and check out any linked sites/threads/pictures.

just be careful with those damn fittings. let's just say I need another one.

Thanks Curly. I think I have seen what you're talking about on the Targa build site...remote knob mount looks nice. Been up in the air with this mod in general for street car (daily drive) due to changing tires for track and then street again / setting the prop. Don't want to find the rear end in front of me on the 405. Would need to be diligent about it is all.

Which one do you need?

the metric to sae ones that FM make.

I know this is an old thread but I wanted to bump it up because I am in a similar situation with my brakes, other than the having already installed an adjustable prop valve part.

Current brakes on my '90 are 1.8's all around with the XP10/XP8 combo. 1.6 Master/Booster/Prop valve. The car is very difficult to trail brake with almost any steering input. For the SoCal guys, squirrely into Off ramp and the Sweeper on CW13 at BRP.

I'm wondering if putting in a 1.8 prop valve will help me out? Was recommended this by an SM driver at BRP one weekend but haven't really confirmed it. Also, have heard of people removing the spring in the OEM valve all together to have the pressure linear to the rears all the time. Does this work?

Overall my braking is good, in a straight line. Just trying to see if I can get the car more planting in trail braking situations. Often feel like its making taking the correct line through a corner more difficult.

Thanks guys.

get an adjustable prop valve and XP12s for the rear.

Is this typical to run a more aggressive pad in the rear versus the front? Is it to offset the lack of capacity in the rear brakes? Just goes against the typical XP10/XP8 combination. Thanks.

its not typical. We are still undoing everything that SM incorrectly did to us that and years of m.net engraining things into us.

I'm sure many others will disagree, but in my experience the miata needs help with rear bias...


Two things I know for fact:

With similar sized rotors F/R and even pads, the car needs more rear bias.
With upgraded front rotors and even pads, the car needs more rear bias.

The fix for me was sport rear calipers, with a larger piston, XP10's F&R. The prop valve lets me dial in for conditions now.

The XP10 front and XP8 rear is not at all correct imo. 10's front and 12's rear maybe.

Ive not run carbotech stuff for several years but they had my pad of choice for many years but Ive never run the xp10 on a miata. I would consider trying a xp8 up front be because xp10 seems pretty aggressive up front however It should be fine if that was recommended

when you say you need more rear bias, what exactly is happening to make you say that?

you are only running 205 up front

what suspension do you have?setup? suspension can have a profound affect on braking performance.

My thought is I need more rear bias. Car gets very unsettled during trail braking where the back end will come around if I brake too hard or turn too much while braking. The handling of the car is very neutral otherwise in both high and low speed corners. Current setup below.

Koni Sport w/ FM springs
FM front sway, 12mm OEM rear sway
205 RA-1's
Front Alignment: -2.25 camber, 0 toe, 5 caster
Rear Alignment: -2.5 camber, 3/32 toe in
1.8 Brakes with XP10/XP8 combo and Centric blanks

my understanding of the bias controller is that it it does not increase pressure over what is available so the most bias you can get in the rear is equal to the front. so you need bigger rear rotors, more aggressive rear pads or less aggressive front pads. based on those temps you sited i would personalty try a less aggressive pad up front. Im sure the recommendation was sound but sometimes an overly aggressive front pad is just about the worst thing ever if you are hammering hard.

suspension is also a factor, what is your ride height front and rear? measured from where? a free option is to perhaps raise the front a little or drop the back. if suspension is good I would just get different pads up front.

Would removing the spring in the stock rear brake proportioning valve basically make the pressure sent to the rear brakes linear? No knee point.

If so, would that be the same thing as a Wilwood valve turned all the way to "more brake"?

correct. you can go from 100% of the pressure to 57% of the pressure.


btw, what size are the miata brake line nuts? 10mm? seems like wilwood sells a prop valve drill for 10mm bubble flare nut/tubing.

would be nice not to have to use brass adapaters.

Sorry, I have to dig this old thread out. I am just wondering, what would the brake biase be if I remove the stock proportioning valve?
I am running wilwood in front and sports brakes at the rear. Car is is used mostly for circuit racing.

I run unions between the lines instead of a brake prop valve setup. Works just fine and it's cheaper...but the wilwood prop valve setup from FM is nice for those rainy days where you don't wanna die.

Big Brake Kit -

He'll sell just the rears. I have a set on the way.