Brake bias tuning

[EMWHYR0HEN]

I'll start off with my setup:

 

AZ Z car fronts (Wilwood) w/ "street" pads

Z31 rotor + 240sx caliper w/ poterfield R4-S (street) pads in the rear.

15/16 master cylinder

Rear proportioning valve

All stock valves and switches removed.

 

Everything is working properly and the brakes have been bedded. After many hard stops I've come to the conclusion that my rear brakes don't get enough pressure even with the bias valve all the way open. The car stops well under moderate to light conditions , but under threshhold braking the fronts tend to lock a bit too early and I feel as if I can't maintain maximun grip.

 

Iv'e considered changing my rear pads to a more aggressive one but I don't like the idea of having pads that "bite" at different teperatures.

 

The only thing left is for me is to switch to a dual master cylinder setup or change my current rear setup. Am I heading the the right direction?

 

I'm liking the idea of the dual master setup because I would be able to switch out master cylinders easily to fine tune the front/rear bias. My only concern is getting rid of the brake booster because, I will be driving it on the street once in a while to track events and weekends. With the booster eliminated is the pedal effort going to feel just as hard as my current setup with the booster off?

 

I'm keeping my eyes on this pedal assembly:

http://www.wilwood.com/Products/005-PedalAssemblies/002-FSMP/fsmdmcbp/index.asp

[JMortensen]

Based on my own experience trying to adjust bias with Toy fronts and 280ZX rears, I think you're going in the right direction by switching to duals. You may consider modding the stock pedal box or buying one of Coffey's pre built brackets rather than buying that pedal assy and then having to figure out how to mount it.

[Zmanco]

FWIW, I had a similar situation with the toyota in front and 240SX in the rears. While I would prefer the dual master setup, my budget didn't allow it. In the end I experimented with different pads in the rear and found that with Hawk HP+ in the rear and Axxis metal masters in the front, I get lock up in the rear first with the prop valve all the way open. I know the metal masters don't have the best reputation, but for me, I've had no issues with them and have never had any fade even at Pueblo where I was seeing 120 at the end of the straight before braking hard to about 70.

 

And once I had them balanced I was quite surprised how much later I could brake. I had always thought of the rears as just going along for the ride. Obviously they don't do as much work as the fronts, but the benefit of having them do as much as they can (before lockup) was surprising to me.

[1 tuff z]

myron, i've been working on the same thing for a few years now. factory prop valve and safety valve removed, mm front calipers w/13" rotors, 240sx rears, no prop valve at all. hawk hp+ rear and had been running the kvr pads that ross sells with the setup up front.

 

that set up has worked very well at the track but i can lock the fronts on the street [until the hawks get warmer].

 

my next step is to go hawk hp+ up front and if i still am not feeling enuf rears i may purchase these rears; http://forums.hybridz.org/showthread.php?t=135157&goto=newpost

 

we'll see how the next track event goes [mid aug at watkins glen].

 

if i go to the new rear setup, i'd hope that i'd need to reinstall the wilwood prop valve.

 

try removing your prop valve all together, you'll find that doing so will allow a bit more rear brake-at least i did.

[johnc]

IMHO, a dual MC setup is a bit much for a street car - and I make and sell one. I would try a rear pad compound with a more aggressive initial bite the the R4S. Try the Hawk Blacks. If Hawk doesn't make a pad for that caliper, call Porterfield. They will custom cut a pad for you.

 

EDIT: Your brake setup is inherently very front biased so there's only so much you can do to balance it out.

[EMWHYR0HEN]

I appreciate the advise form everyone.

 

I think i'm going to go ahead and try a different rear pad and then if that's still not enough remove the prop valve. The dual MC is going to put a dent in my wallet so I figure might at well try it first and maybe get away with a cheap fix.

 

Does anyone believe that even with a dual MC setup my bias will still be hard to balance out?

[1 tuff z]

I appreciate the advise form everyone.

 

I think i'm going to go ahead and try a different rear pad and then if that's still not enough remove the prop valve. The dual MC is going to put a dent in my wallet so I figure might at well try it first and maybe get away with a cheap fix.

 

Does anyone believe that even with a dual MC setup my bias will still be hard to balance out?

 

myron, i'd suggest removing the prop valve first. no / low cost and fairly easy to do. see how it works. if you still don't have enough then go with the pads.

[johnc]

Does anyone believe that even with a dual MC setup my bias will still be hard to balance out?

 

If the dual MC setup has an adjustable bias bar, you should be able to tune your brake bias near perfect, once you get the MC sizes correct.

[Clifton]

If 's a street car I would just drive it as is. Even if you can get another 5% out of the rears, will you ever really need it or use it, doubtful. If it was a track car I would run something larger in the rear, AZC or MM. A dual MC would be great but I would rather run a larger rear instead.

[JMortensen]

You can email tilton and they'll give you master sizes to try first.

[EMWHYR0HEN]

I've been testing the brakes all day only to be more disatisfied.

 

My front rotors get sizzling hot while I can grab my rear rotors with my bare hands.

 

On top of that I find it hard to modulate the front brakes making it really easy to lock up. Anyone else running the AZ Z car front brakes and 15/16 MC have the same impressions?

 

I think the only way to solve both problems is to go with the dual master cylinders.

 

Are there any general rules when choosing MC's? My personal prefference is a stiff pedal with a short throw.

[JMortensen]

You need to figure out the pedal ratio you are going to use, then you need to match the size of the bore to get the correct line pressure at the correct foot pressure. The stock pedal box is something like 4:1. I modded mine and got more like 4.75:1. Most aftermarket pedals are 5:1 or 6:1 if I recall correctly. If you look online you'll find spreadsheets that will help you. I tried to use one from Tom Holt, but found that it wasn't accurate when you tried to change some of the variables. Get a known good spreadsheet or just email tilton. They need to know the size of the rotors, number and size of pistons, ratio of the pedal assy, and I think that's it. You can get more intense with coefficient of friction of the pads and expected braking g's, but I think tilton can get you in the ball park without that info. I believe most of the spreadsheets base their estimates on a pedal pressure of 50 lbs, at least that's what I've seen most often.

[EMWHYR0HEN]

Thanks Jon.

 

I'm most likely going with a pedal assembly with a 6:1 ratio.

I'll post some info and pics on how i'm going to do all this.

 

Wish me luck.

 

I found this for choosing master cylinders and more info on the subject: http://www.hotrodheaven.com/tech/brakes/brakes4.htm#formula%20for%20master%20cylinder

 

 

http://http://www.hotrodheaven.com/tech/brakes/brakes3_index.htm

 

and this

 

http://www.stoptech.com/tech_info/PedalSetup-DualMaster-Guide.pdf

[EMWHYR0HEN]

I'm trying to plug in numbers to find what MC's I need.

 

Anyone know a ballpark number for a lowered 240Z's center of gravity?

 

I'm guessing 17''....?

[JMortensen]

I think your estimate is way high. In looking at what 74_5.0L_Z and tube80z had estimated their cg to be, I came up with 11-12 inches for my car. Your's might be a bit higher due to the taller wheels. Maybe 13" or 14".

 

Unfortunately actually measuring cg is a bitch and a half.

[EMWHYR0HEN]

I think your estimate is way high. In looking at what 74_5.0L_Z and tube80z had estimated their cg to be, I came up with 11-12 inches for my car. Your's might be a bit higher due to the taller wheels. Maybe 13" or 14".

 

Unfortunately actually measuring cg is a bitch and a half.

 

Thanks for the info.

 

When I plugged in 17'' for my CG I came up with 7/8 (.875) Front MC and 3/4 (.75) rear MC.

 

Now.... after I plugged in 14'' for my CG I got 15/16 (.938) front and 7/10 (.70) rear.

 

I didn't expect that much of a difference but, now i'm confused.

 

Okay, smaller bore MC's apply more force than ones with larger bores. So from that CG change I'm now recomended to have a MC with less force in the front and more in the rear. It sounds right to me if less weight is being transfered.

 

I'm running a 15/16 MC now and I think it locks to easily. Should it be entirely different with a Dual MC setup and a 15/16 front MC? Or go with a MC with less force like a 1'' bore?

[JMortensen]

Okay, smaller bore MC's apply more force than ones with larger bores. So from that CG change I'm now recomended to have a MC with less force in the front and more in the rear. It sounds right to me if less weight is being transfered.

Correct. Lower cg = less longitudinal weight transfer under braking (also less lateral when turning).

 

I'm running a 15/16 MC now and I think it locks to easily. Should it be entirely different with a Dual MC setup and a 15/16 front MC? Or go with a MC with less force like a 1'' bore?

Don't try to compare what you have now to what you'll be running with the duals. The pedal ratio is different and you won't have a booster, and the rear being a different diameter makes any comparison of when the fronts lock vs the rears useless, unless of course you thought you needed two of the same size masters...

[EMWHYR0HEN]

I started by cutting peices off the pedal box that would interfere with the balance bar and master cyclinder pushrods. I used 16 gauge steel for the master cylinder mounting plate and welded it on top of the exsisting surface on the pedal box. After drilling holes I reinforced the mounting surface with gussets and thick strips of steel.

 

I moved my MC pivot point up 1'' on the pedal to achive a pedal ratio of about 6.2. After welding the sleeve to the pedal I welded 1/4'' strips to reinforce the area on the pedal where the balance bar sleeve is located.

 

I'm using the Tilton balance bar part # 72-250

Tilton 15/16 MC

Wilwood 3/4 MC (same dimentions as the Tilton MC Just wanted to compare quality....go with Tilton.)

Tilton remote cable adjuster # 72-501

 

i'll have more pictures of the install soon.

 

Thanks again Cary for the pics.

 

 

 

 

 

 

 

 

 

[EMWHYR0HEN]

I got everything installed today and I was able to do a little testing around a few empty parking lots.

 

The good news is I'm able to lock up the rear wheels when I adjust the bias towards the rear.

 

The pedal is very firm and you have to give lots of pressure to get the wheels to lock.

 

Feels good at high speeds and when pads are up to temperature but kind of sketchy at low speeds almost feels like you can't stop fast enough.

 

I better hit the leg press.

[johnc]

When it comes time to do some track testing, put four reasonably good tires on the car. You don't want to do brake testing with crappy tires and you don't want brand new tires. In the first case, you'll not be able to test the system to its higher limits and in the second case you might horribly flat spot a new set of tires. Also, when testing, if you do flat spot a tire, your testing is probably done.