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 :)