v8r brake kit + brake bias
 

I got a decent price on the full v8r brake last year so I swapped out my 11.75" front/sport rear setup for it. The brake modulation is much worse (almost no modulation) and I tried forcing myself to use it for 5 track days to see if it was just me. I'm still 1-2 seconds slower than my previous brake setup. I also installed the fat NB booster at the same time which is probably also contributing to the problem and have since swapped to a smaller NB booster, but haven't tracked it yet. The calipers are standard 4 piston dynalites with 1.75" pistons up front and 1.38" in the back. I'd really like to keep the 11.75" rotors in back since the rotors and pads are more affordable, are the same as the front, and they last longer than having a stock caliper in the back. I've gotten several suggestions:

1 No booster.
2 No booster + dual master.
3 Try 1.62" pistons up front (Emilio mentioned this in another thread).

I hacked up someone's spreadsheet to see the effects of going to a smaller piston up front. It's linked below and I'm not sure if I got all the parameters right. The bias concerns me even with a prop valve. I suppose I could play with the rear pad, but then I can't swap front and rear pads. I wonder if the smaller 1.62" pistons up front will solve the modulation problem. Any comments on the spreadsheet or suggestions on how to fix the problem?

https://docs.google.com/spreadsheet/...hl=en_US#gid=1

Just curious, what size MS (7/8", 15/16", 1")?

1" wilwood. That's what comes with the v8r kit.

Try swapping pads. Not sure the effects at the track, but it really helped with modulation on the street going to a less aggressive pad.

I had the same pads (Cobalt XR2s) on the old kit which had no modulation problems. Pads are expensive enough that I'd rather just give up and go back to 1.38" pistons up front, a 15/16 master, and a sport rotor in the rear since I know that works fine with all the pads I've tried (xp12s, xr2s, dtc-60s). Also, I don't want to limit my pad choice.

Huh, just checked out the v8r kit and it seems to have all the right components.

Do you still have the original (11.75" front/sport rear/OEM booster&MC) kit? I know it's a lot of work, but I might try swapping one component at a time. Try the sport brakes in the rear first, and if that doesn't fix it swap back to the 11.75" rear and try the OEM booster.

Seems like the front is the same, so I wouldn't think that's the problem, although I could be wrong.


Sidenote:wrote all that before looking at piston size. Is the v8r kit different piston size up front than all the other 11" kits floating around? And your old 11.75" kit?

All other kits I've seen put 1.38" up front. They all seem to work well with the 15/16 master. The v8r kit has 1.75" up front and 1.38" in back with the 1" master. I'm hoping the 1.62" piston up front will work better for me. I guess a bigger master may also be worth a try, but I don't have the time or money to swap every part one by one. I'm looking for 1-2 options that are easy to do before giving up and putting the 1.38" back up front and going back to oem rear.

I put the big NB booster up front which was a mistake. Changing the booster is easy enough, but from talking with other people I doubt it's going to solve the problem. Here are a few choices:

(1) 1.75f,1.38r, 1" master, big NB booster, xr2s - doesn't work for me
(2) 1.62f,1.38r, 1" master, msm booster, xr2s - easy to do and looks ok on paper aside from rear bias.
(3) ???
(4) 1.38f,sport rear, 15/16, msm booster, xr2s - very similar to the setup I used to have. I know this works, but I hate giving up the 11.75s with dynalites in the back.

I need a test track :( If it was summer I'd go out at 4am and do some testing on a "private" road.

Have you talked to Shandelle at v8r? Or contact this guy who has them and track them.

http://forum.miata.net/vb/showpost.p...15&postcount=8

I did a big brake kit on a 323 GTX I hav very similar to my miata and to get the bias right for that car since it is way front heavy I used 1.62" if I remember right up front. The brakes worked good but never had the tremendously awesome pedal feedback feel that the Miata had with similar sized everything else and 1.38" front pistons.

I’m wondering if the giant caliper pistons aren’t jacking with brake feel. Seems like the lower system pressures required to generate brake force should make them touchier with a harder pedal but now I’m thinking the opposite is true. My guess is the fluid system has a nonlinear spring rate to it the higher the pressure the stiffer the spring maybe and the feedback in feel is better.

Bob

4 x 1.75s on a 1" is dicey. Still a lot of displacement. Combine that with a 4 x 1.375 rear and while the bias might be within a reasonable range the overall displacement is going to be great and cause a very long pedal.

Most kits in 'my world' run about 4 x 1.625 fronts and 4 x 1.125 or 1.25 rears and work quite well on a 1" or 1.063 bore. *Assuming the rotors are larger also. Keep in mind that going small on rotors while looking for a lot of overall brake torque you cannot skimp much on piston area- you need it.

Would a 1.125" master make sense? The pedal will have to travel less and the wheel torque will be reduced. Both of these seem like good things.

Changing to the larger bore will help in the total displacement of course and balance out the mis matched volumes. It will not change the bias as it still is only one cylinder feeding all four.

The big negatory in my mind is that it will raise the required pedal effort through the roof. No way to determine how much booster value you have but from a static point the pedal effort would be huge.

My current pedal effort is definitely too low so that might not be a bad thing.

Another interesting suggestion I've gotten is to change the pedal ratio by adding some holes in the pedal. That sounds a bit dangerous if it weakens the pedal.

Forgive me for being blunt and all but why don't you put the right calipers on the car and not stew over all this other stuff? The cause has been pretty well documented by others and all this other stuff is a band-aide for just doing it right.

I'd roughly say that the fronts should be 1.375 and the rears maybe 1.0 or possibly 1.125 but that's based upon caliper options also. If you're going to keep the 1" mc then a 1.625 front and 1.0 rear would still work.

Any way you cut it a 1.75 / 1.375 combo is not the right set up for your needs.

Holes in the pedal or holes in the piston? Important difference. One does something, one doesn't.

Nothing wrong with being blunt.

A few reasons:
- The one already stated: same rotors and pads in the rear is very convenient. I'd have to look what options I have from wilwood that have a smaller piston if I want to keep the rotor and bracket.
- I'm trying to understand the effects of various brake components beyond some basic spreadsheet math and I don't have the time or budget to swap everything at the track.

I'm having problems finding the well-documented cause. Too much piston area? A link or a description of where to look would be appreciated so I can read about it.

It's a mis match of piston area vs master cylinder volume. The combo you're speaking of "could" work with a lot of these changes to the mc etc but it's the long way to get there in my mind. (I thought this was covered earlier comparing specs of other kits etc, maybe that was the other brake thread)

Depending upon the caliper you're using there are some options. The Forged DL (cotter pin) caliper can be replaced by the billet version Lug Mount Dyna Pro and offers a variety of piston sizes much more in tune with what you want to do. One of only a few options of 4 x 1.00 parts in fact.

Sharing pads is a fine idea but you may find that in this criteria your options or pricing may dictate that is just doesn't work or make sense. A 4 x 1.00 is nearly the same as a 2 x 1.375 for example. You'd just not have as large a pad and shorter life.

Seems like you would want the front and rear brakes operating temps to be similar through the range of temps they see during usage. Proper bias with calipers based on my experience with the car seems to be with 1.38 4 pistons in the front and 1.38 single pistons in the rear. essentially it is near a 2 to 1 ratio of brake force needed front versus rear.

I’m not sure running the same size pads and rotors front and rear is optimal. With race pads you might have the rears below optimal temp too much and they could wear even faster as a result. I know the 11.75” vented rotors in the rear are overkill seem to work good for the front however. The .35” solid disks I’m using are 2.7 lbs lighter per side than a vented ultralight and they have plenty of cooling for the rear.

I know running my 11.75” 1.38” Dynapro up front and 11.44” .38” thick non vented rear rotors with 1.375” piston sport calipers with 1.6l small pads results in front and rear pads wearing out at roughtly the same rate. Bias and modulation is excellent.

Based on this knowledge the hot race setup without a parking brake I think would be Dynapro radial calipers up front with 1.38” pistons and 11.75” X .81” thick vented rotors and Dynapro Single 1.38” piston in the rear with 11.75” X .38” thick non vented rotors. I’m fairly confident that this would work nicely with a 1” master cylinder and one of the stock boosters. It would also match thermal capabilities with the braking tasks front to rear to maintain a more constant balance. I haven’t checked but it wouldn’t surprise me if the Dynapro single pads had real close to the same area as stock size 1.6l pads. Seems like the wear rates would be similar I think they may be a little thicker.

Question I have is if you were going to switch to a non boosted dual master cylinder setup with this using a wilwood pedal assembly what size master cylinders would you use?

Bob

Nice comments Bob.

For the most part I'd agree. Experience on the other hand says that a 1" bore with this combo on such small rotors could make for some good work outs. Maybe....I honestly don't know what the stock bore was to compare it.

Twin mc's can be calculated with a good spread sheet and knowing exactly what pistons are chosen for the calipers and all. That's the easy part- install it will be the work!

Stock bore on a 2002+ or sport brake car is 15/16th. earlyer cars were 7/8ths.

I think its pretty common consensus amung those with experiance that the common 1.38 wilwood fronts with stock 1.375 rear pistons with stock master and booster feels borderline over boosted. Going to a 1" master reduces pedal movement a bit making heal tow much better and nobody incliding me ever complains about the pedal force required Its actually quite light.

The V8R basically doubles the caliper piston area. Seems drastically different but I’ve never driven with them.

Bob