Brake bias tuning

[JMortensen]

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.

I'm surprised. When I read your master cylinder size choices and your pedal ratio I thought you were going to have a spongy soft pedal with a lot of travel.

 

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.

This definitely sounds like a pad that is too racey for the street. I had the same "holy ♥♥♥♥ it's not stopping" feeling when I tried to drive on Porterfield R4 pads to the track. After that experience I'd bed them in and then put on some street pads, and swap the pads at the track.

[EMWHYR0HEN]

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.

 

 

I was expecting this. I'm glad I bought a set of good used tires.

 

In your experience what methods worked best to find the right F/R bias?

So far ive thought of:

 

1. give the pedal lots of pressure and adjust until fronts lock slightly before rear.

2. recording F/R brake temps and adjusting bias so that fronts are 150-300 degrees higher?

3. locking up rears first, then slowly increasing front pressure or vica versa.

4. other?

[EMWHYR0HEN]

I'm surprised. When I read your master cylinder size choices and your pedal ratio I thought you were going to have a spongy soft pedal with a lot of travel.

 

 

This definitely sounds like a pad that is too racey for the street. I had the same "holy ♥♥♥♥ it's not stopping" feeling when I tried to drive on Porterfield R4 pads to the track. After that experience I'd bed them in and then put on some street pads, and swap the pads at the track.

 

I think my MC bore sizes are fairly large for a dual MC setup I think that's why I have such a hard pedal. The MC's have a max stroke of about 1'' and with the pivot moved up 1'' I have to depress the pedal about 5/8 of the way to lock them up.

 

I'm running "street pads on all fours. I'm kind of confused as I wanted something with more initial bite than what I have now but, I do have to get used to all of this so maybe it's just me.

[johnc]

What I've done is get a professional race driver to do my brake testing... I don't feel I'm consistent enough in braking to do this quickly and efficiently.

 

Set the adjustable prop valve in the middle of its adjustment range. The prop valve is the gross adjustment, the balance bar is the fine adjustment. Set the balance bar with a 50% forward bias in the car to keep from spinning and then pick two spots on the track:

 

High speed straight

Medium speed straight

 

Run the medium speed stuff first and adjust the balance bar after two braking passes. Run the high speed stuff second and adjust the balance bar after two braking passes. Run both for a couple laps and keep adjusting the balance bar until the fronts lock just before the rears. Hopefully the balance bar is near the middle of its range. If not, adjust the prop valve and try again. If, at any time, you hit the limit of the balance bar or prop valve adjustment, you might be off on your MC sizing.

 

Once things are dialed in you should never have to touch the prop valve and will use the balance bar for all subsequent adjustments.

 

For fine tuning with the balance bar, find four spots on the track

 

Medium speed corner entry - right trail brake

Low speed corner entry - right trail brake

Medium speed corner entry - left trail brake

Low speed corner entry - left trail brake

 

Using the balance bar adjust the brake balance so the car rotates well under trail braking in the above situations. You want more rotation in the low speed corners.

[JMortensen]

I think my MC bore sizes are fairly large for a dual MC setup I think that's why I have such a hard pedal. The MC's have a max stroke of about 1'' and with the pivot moved up 1'' I have to depress the pedal about 5/8 of the way to lock them up.

Yours are not that big in the scheme of things. I bought a new computer so I don't have my old emails, but I did email Tilton (after I had already bought masters) and they recommended something like a 1" and 1 1/8", and thats with a 4.75:1 pedal ratio. I guess I'm glad I already bought the masters...

 

I'm running "street pads on all fours. I'm kind of confused as I wanted something with more initial bite than what I have now but, I do have to get used to all of this so maybe it's just me.

What kind of pads are they?

 

On your brake bias testing, I'd go with #1. Another option I'm aware of is using brake pressure gauges, but you'd still need to know what pressure made the tires lock up, so #1 still makes more sense and it's a hell of a lot cheaper.

[EMWHYR0HEN]

More pics for reference.

 

 

 

 

 

[ron260z]

what would happen if you just switched the metal brake lines going into the master cylinder. put the front in the rear and rear in the front.

 

if you have a really strong front and not much rear woulden't this fix the problem.

[JMortensen]

what would happen if you just switched the metal brake lines going into the master cylinder. put the front in the rear and rear in the front.

 

if you have a really strong front and not much rear woulden't this fix the problem.

Did you actually read the whole thread?

[EMWHYR0HEN]

 

 

What kind of pads are they?

 

 

Up front i'm using Wilwood "smart pad" BP-10 which is essentialy a street pad.

 

Info: http://brakepads.wilwood.com/01-selection/bp10.html

Graph: http://brakepads.wilwood.com/02-graphs/bp10.html

 

It pretty much has a constant friction coefficient that is low.

If you look at the graph It really doesn't have any "bite".

 

I'm running Porterfield R4-S on the rears.

[johnc]

Do you want to run street pads? Is there a reason you're running different pad materials front and rear?

 

I'm asking because your pad choices are adding some variables to your brake testing/setup. For me its easier to have the same exact pads front and rear to keep pad coefficient and bite out of the balance equation.

[JMortensen]

Do you want to run street pads? Is there a reason you're running different pad materials front and rear?

 

I'm asking because your pad choices are adding some variables to your brake testing/setup. For me its easier to have the same exact pads front and rear to keep pad coefficient and bite out of the balance equation.

I think it was a previous attempt to equalize the braking. I'd get some R4S pads for the front. I don't know anything about the Wilwood pads, but I have driven the R4S on the street and there is no lack of bite at low temps.

[EMWHYR0HEN]

As Jon mentioned I was trying to balance things out with the previous setup.

 

I'm trying to find a pad with a medium to high initial bite that doesnt have such a high operating temperature. I know that's kind of rare, but I know there has to be something in between.

 

I'm actually considering on tossing the street pads because they don't provide the tourqe i'm looking for.

 

The car will be driven to and from track events and occational "spirited driving" so I don't know if it's a good idea.

 

I was thinking Hawk blues, blacks or Porterfeild r4's all around.

[JMortensen]

I'm trying to find a pad with a medium to high initial bite that doesnt have such a high operating temperature. I know that's kind of rare, but I know there has to be something in between.

That's going to be an R4S or an HPS. The blues or the R4 pads won't have any bite at all on the street.

[johnc]

I think you need to go to the various pad manufacturer sites and figure out what's a best compromise pad. The Hawk HPS+ would be a good choice.

 

Also check Carbotech. The Corvette guys love 'em - the AX6 is their autocross compound and the XP8 is their first level racing compound.

[heavy85]

I'm about to be in the same dilemma going to 240 sx rears to keep a parking brake with most likely Outlaw fronts. I'm thinking I could fix this bias issue with getting smaller caliper pistons in the front. Since I haven't ordered anything yet it's wide open. What size pistons does the AZ set-up use on the fronts so I can get a feel for sizing based on others real life experience?

 

Thanks

Cameron

[EMWHYR0HEN]

I believe they're 1.75'' pistons.

[JMortensen]

If you email Tilton with the specs they'll give you a suggestion for master sizes. I'm pretty sure AZC runs 1.75 in front and 1.38 in the rear, but that doesn't mean much of anything to you because the rotor sizes are different, etc.

[EMWHYR0HEN]

I havn't reported back here in a while after all my brake work and testing so I thought i'd give some feedback on the setup.

 

As expected the dual MC setup requires quite a bit of force on the pedal to build up pressure. It is however, very "feelable" especially when up to speed. Even after numerous 100+ MPH stops the brakes held up great with out any fade. Brake release was smooth and consistent as well. Overall, I'm glad I went with a dual MC setup to tune brake bias. With the balance bar I was able to get my smaller rears to work harder and keep up with the larger front brakes. Even If I do end up changing my setup around again (different pads, tires, rotors, calipers, or even different tracks and conditions) i'll be able to adjust the F/R bias to minimize stopping distance.

[heavy85]

If I read right your using 3/4 rear and 15/16 front. This means you reduced front to 64% of the rear (area = diameter^2). I'm thinking of using 1.38" caliper pistons instead of 1.75 which works out to be 62% less area and therefore force than the AZ set-up. So by my reasoning this should put me right in the range where you are running. Do you know if the bias bar is towards the front or rear? In other words without dual masters would I need a rear bias valve to get it nailed? Also,where did you end up with pads?

 

 

Thanks

Cameron

[JMortensen]

If I read right your using 3/4 rear and 15/16 front. This means you reduced front to 64% of the rear (area = diameter^2). I'm thinking of using 1.38" caliper pistons instead of 1.75 which works out to be 62% less area and therefore force than the AZ set-up. So by my reasoning this should put me right in the range where you are running. Do you know if the bias bar is towards the front or rear? In other words without dual masters would I need a rear bias valve to get it nailed? Also,where did you end up with pads?

The rear bias valve, if you mean an adjustable prop valve, doesn't change the bias in the same way. It has a knee in its pressure curve, so it is useful to have one so that you can get the max amount of pressure at low speeds and have it taper off at high speeds, this all being in addition to the bias bar.