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Brake bias issues

Old 10-02-15, 11:01 PM
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Brake bias issues

I have an SA with the following brake set-up:

Front: Lightweight aluminum hubs from KC Raceware, 11 inch rotors from Coleman Racing and FD calipers.

Rear: Lightweight hubs from KC Raceware, and GSL-SE calipers.

Original master cylinder and booster AND original proportioning valve.

Hawk HP pads all around

When I installed this set-up it seemed to be fairly well balanced front to rear, at the time I was on not so sticky street tires (Yokohama S-Drives).

Since then I have started tracking the car more, and have upgraded to Yokahama AD08R tires.

I am becoming aware that I may have too much rear bias for the track. The symptoms are occasional tail happiness under hard braking, especially if I jump on the brakes very suddenly, brake hop under certain situations, especially if I soften the rear shocks, (on one occasion this was sever enough to bend my rear axle housing!!) and accelerated rear tire wear.

I'm looking for suggestions for how to best deal with the situation.

I thought of adding an aftermarket proportioning valve, but my research has revealed that since my car was originally a rear drum car then it's original prop valve includes a "residual valve" to hold a small amount of pressure to the rear brakes, so I'm thinking I would like to get rid of it. Been trying to figure out a plan to do this.

I have researched and found that the FD caliper bores and the GSL-SE bores are the same, so couldn't I just install a proportioning valve from a FD and have the bias I need?

Thoughts?
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Old 10-03-15, 08:11 AM
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One other thing to look at is the stance of the car. If the front is lower than the rear it will increase weight transfer and cause similar symptoms. I had this happen on my 2nd gen E Prod car and leveling it cured the problem. Installing a manual prop valve and eliminating the factory valve would be advisable also.
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Old 10-03-15, 01:35 PM
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That is an interesting suggestion.

I recently had the car corner balanced and the shop raised the rear slightly, and now that I think of it I did notice more nosedive under hard braking.
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Old 10-04-15, 11:34 AM
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Well, the further I research, the more complicated it becomes.

I have researched and have found that it is definetely not reccomended to add an aftermarket adjustable proportioning valve downstream from the factory proportion / bypass valve. It seems that installing them in series can cause "very nasty" things to happen, so I am leaning toward substituting a factory valve in its place. I just have to pick between a later 1st gen valve from a rear disc brake car, or maybe a 2nd, or third gen valve. So I am trying to decide what the criteria should be, if it is caliper bore size and disc diameter, then the 2nd or 3rd gen valve seems to fit best,

One thing that has me confused is that all information I can find suggest that the proportion valve I have (from a drum rear brake car) SHOULD reduce the rear pressure more than one for a disc rear system, doesent make sense that I am having too much rear braking.

Maybe I should hold off on any changes until I experiment with lowering the rear ride height to what it was before...

I apologize if I am thinking out loud too much.............
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Old 10-04-15, 06:05 PM
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By changing the rotor sizes you have changed the brake bias. Why don't you just remove the OE prop valve and replace it with a Wilwood unit?

You may also need to try a less aggressive pad. That is what I had to do in my FC.

I have bigger front rotors than OE and stock rears. I ran DTC60 pads all around and had to switch to DTC30s in the back, and adjusted the prop vale until the bias was where I wanted it.

Last edited by LargeOrangeFont; 10-04-15 at 06:08 PM.
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Old 10-04-15, 07:27 PM
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since changing the ride height is the easiest thing, start there.

next step is either an adjustable prop valve, or less aggressive rear pads, or more aggressive front pads.
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Old 10-04-15, 08:37 PM
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I will adjust the ride height to see if that helps, but I am still trying to understand all the issues involved.

Why don't you just remove the OE prop valve and replace it with a Wilwood unit?
I don't want to go to the aftermarket adjustable prop valve, since to do that I would need to remove the factory valve and by doing that I would loose the bypass valve, which is a big safety factor,( it shifts the flow to the rear brakes if the front looses pressure, I think, allowing you to have at least some brakes).

By changing the rotor sizes you have changed the brake bias.
I know that changing rotor diameters will change the bias, but all the changes I made, (larger front rotor), should have shifted the bias to the front, not to the rear, and the drum brake rear proportioning valve should also cause too much bias in the front, not the rear, so I still don't quite understand why it is the other way around, and, since I don't fully understand the problem I will not change the valve yet. I will try to address the issue with j9fd3s suggestion and put more aggressive pads on the front, if the stance change does not fix the issue, that is.

Last edited by rwatson5651; 10-04-15 at 08:42 PM.
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Old 10-05-15, 06:24 PM
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The car has a dual circuit master. Both circuits are isolated within the master cylinder. In the event of a pressure loss on one end of the car, the other will still work.

I don't think messing with the stance of the car is going to fix your problem.

The residual valve is there to counteract the springs in the drum brakes, not protect you in the event of brake failure. The drum brake prop and residual valves are most if not all of your problem. You don't need any residual valve with disk brakes generally speaking.

Keep in mind the prop valve pressure output is not linear. There is a knee point in the buildup of pressure. I'm not a brake engineer, but I'm almost positive the knee point needed for disks and drums to work is different.

Last edited by LargeOrangeFont; 10-05-15 at 06:36 PM.
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Old 10-05-15, 09:54 PM
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Do the rear brakes lock up every time the car gets tail happy at corner entrance?
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Old 10-05-15, 10:39 PM
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Yes,
or put another way the only time it gets tail happy is when they lock up. The car is very well balanced, well enough that I can get it to turn in more mid corner by gently manipulating the throttle. When it is near the limit I can steer it with the throttle, the only tail happiness is when I a have to jump on the brakes too suddenly like when I'm in too hot. If I am not abrupt with the brake peddle I can still get fairly good braking without rear lockup of brake hop, but I do have to be gentle with the original application of the brake. That having been said I am still convinced the bias is off because the last track day totally wore out my rear tires, WAY more than the front.

The residual valve is there to counteract the springs in the drum brakes, not protect you in the event of brake failure. The drum brake prop and residual valves are most if not all of your problem. You don't need any residual valve with disk brakes generally speaking.
I understand that the residual valve holds pressure to the rears, but it also has a bypass valve built in to redirect the front fluid flow in case of a pressure loss on the front, maintaining braking in case of a front brake line failure, I don't want to loose that safety feature.

I agree that the knee point and slope of the valve is probably way different on a drum vs disc car. That, and wanting to keep the byass valve is what has me considering using a 2nd or 3rd gen combination valve.( All the factory valves are combination valves that include a bypass valve). The caliper bore surface areas are exactly the same as what I am running, and the rotor diameters are very close. It would have to be closer than the stock drum valve that I am currently running.

I am running a track day later this month at Atlanta Motorsport Park and will adjust the stance just to see if that helps, if it does not I will likely change the valve to a 2nd or 3rd gen and see what that does. I may also change the master to a FB unit from a rear disc car. I cannot ascertain if the drum master cylinder I am running is inappropriate for a disc / disc car. I have tried to find technical drawings of it to see if it is different or not. Cannot find one.

At this point I am open to suggestions, thanks for any help.....

Last edited by rwatson5651; 10-05-15 at 11:05 PM.
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Old 10-06-15, 05:10 PM
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You have a safety system in the dual circuit master cylinder that is in the car. If you lost front pressure you still have rear brakes. That is why the reservoir is divided.

I don't believe the later prop valves in the FC have any bleed back facility. I believe the front line is just a pass through on the valve.
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Old 10-06-15, 06:23 PM
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If you don't mind sharing, what kind of shocks and struts are you using and what are your spring rates. I ask because an issue like this can be caused by too much weight transfer at corner entry.

What I have found is that when a race car is properly sprung and has good shocks/struts you can run more rear brake line pressure than what a OE prop valve will provide.

I am not totally discounting a brake bias issue but my guess is that this isn't your only issue.
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Old 10-06-15, 08:07 PM
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I have RE- Speed coilovers on the front and the adjustable spring seats on the rear with Tokico Illuminas adjustable shocks all around. I also have the RE Speed rack conversion and the RE speed front sway bar (really big) . The spring rates are 200 on the front and 175 (or maybe even 150)? on the rear. I know this is rather soft but I choose these rates to avoid the bone jarring ride that my old stiffer springs gave me. You think this is contributing to my problem? If so, what can I do to minimize this problem short of going to higher rates?
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Old 10-06-15, 11:31 PM
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Originally Posted by rwatson5651 View Post
I have RE- Speed coilovers on the front and the adjustable spring seats on the rear with Tokico Illuminas adjustable shocks all around. I also have the RE Speed rack conversion and the RE speed front sway bar (really big) . The spring rates are 200 on the front and 175 (or maybe even 150)? on the rear. I know this is rather soft but I choose these rates to avoid the bone jarring ride that my old stiffer springs gave me. You think this is contributing to my problem? If so, what can I do to minimize this problem short of going to higher rates?
I liked the illumines when I had them on my racecar. I even have the modern version on my 2012 Mustang!


Those shocks and struts adjust rebound primarily but compression damping does seem to go up when rebound stiffness is increased. On the rear I would keep them set at 4 or 5 all the time because you don't want the rear end coming up under braking. In front I would keep them at 3 or higher depending on how the car feels.


You mentioned that the car sits higher in the rear now. As j9fd3s suggested, you might want to lower it back to where it was. This will shift the center of gravity towards the rear.


Your spring rates are pretty soft which is something that sticky tires will magnify. For comparison my IT car had 400-450lb springs on the front and 200-300lb springs on the rear. I don't think you have to go to those rates but going up 50-100lbs in the front should be doable without allot of impact on the car's ride.


Just as you want a racecar/performance car to stay flat during corning, you also don't want it to nose dive under braking.


Regarding the prop valve and brake bias with the stock master cylinder - we have a couple of E Production 1st gens in the area that still use power brakes. They seem to work fine. Both of these cars started life as either a GSL or GSL/SE so they had rear disk brakes. The point is that OE prop valve from a disk brake car may be something you need to try.
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Old 10-07-15, 07:29 AM
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Thanks for the input!

Here is my plan:

1. Lower the rear as suggested

2. Be sure to keep the rear shocks firm. I know this has had an affect because when the brake hop was the worst (and I bent the rear axle housing) I had softened them just to see how it would affect the corning balance of the car, big mistake! Your insight verifies this.

3. My pads are due to be replaced, so I am ordering some that will shift the bite more to the front, I will probably use the Hawk HPS on the rear and the HP Plus on the front. Thoughts?

4. Replace the master Cylinder and prop valve with a disk one from a FB.

5. Re-evaluate and possibly raise the front spring rate.

Thanks for your help!
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Old 10-07-15, 08:32 AM
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the above is a good plan. if hawk makes DTC pads for a 1st gen you should look into those. they are much better, although still about 10 years out of date
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Old 10-07-15, 09:20 AM
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Sounds like a good plan!
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Old 10-07-15, 10:45 AM
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Originally Posted by j9fd3s View Post
the above is a good plan. if hawk makes DTC pads for a 1st gen you should look into those. they are much better, although still about 10 years out of date
I looked at the DTC pads but was concerned that there higher heat range would make them inappropriate for the street. The stated range is 400-1600 f. I have no experience, so I was concerned about long stops when cold. I dont want to rear end grand ma .

Thoughts?
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Old 10-07-15, 12:04 PM
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Originally Posted by rwatson5651 View Post
I looked at the DTC pads but was concerned that there higher heat range would make them inappropriate for the street. The stated range is 400-1600 f. I have no experience, so I was concerned about long stops when cold. I dont want to rear end grand ma .

Thoughts?
dunno, we have a dedicated race car, so no street use. we haven't used the hp/hps stuff in 10+ years, and literally just went through the first set of DTCs so its a bit early to say. generally newer pads have fewer tradeoffs than older ones.

for example, we have run PFC pads for years, and they are almost miraculous. they modulate very well, which the drivers really like. they don't fade. pad life is really good (so far about 4x the DTC's), they are really easy on the rotors, and don't even dust much (much less than the DTC's in fact)
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Old 10-08-15, 07:24 AM
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If I can add a couple of things to the conversation. First, I used a band aid on my FC to prevent premature rear brake lock up that has served me well for years. I use Hawk Blues on the front and a standard Auto parts store rear pad. On the warm up lap, the rears heat up quickly and work immediately so you need to be a bit careful. After I start racing, the rears heat saturate pretty quickly and the fronts take over. For some reason it works extremely well on my car and gives me great modulation and predictability.

Other concern is that if you are starting to look at the DTC and their ilk, they are great pads. The only downside is their heat range and how you really need to be in it for them to work. They are not kidding when they say 400 to 1600 F. If you are below 400 degrees constantly, they will chew up brake rotors like they were made of rough granite. Be honest and take brake temp readings. Then choose a comp pad that works for your peak and regular operating temperature. It may not be what the fast guys are using.

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Old 10-08-15, 07:11 PM
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Originally Posted by j9fd3s View Post
dunno, we have a dedicated race car, so no street use. we haven't used the hp/hps stuff in 10+ years, and literally just went through the first set of DTCs so its a bit early to say. generally newer pads have fewer tradeoffs than older ones.

for example, we have run PFC pads for years, and they are almost miraculous. they modulate very well, which the drivers really like. they don't fade. pad life is really good (so far about 4x the DTC's), they are really easy on the rotors, and don't even dust much (much less than the DTC's in fact)
What compound PFC pads did you run? And what compound DTCs are you running? I'm running DTC 60/DTC 30 on my RX7. I like them and they actually work on the street without feeling like you are going to die. My understanding is DTC70s won't work on the street as you won't get them hot enough. The 60/30 setup is easy on rotors and cheap, but I go through them quickly... About twice as fast as 100 tread ware tires.

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Old 10-10-15, 11:40 AM
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Originally Posted by LargeOrangeFont View Post
What compound PFC pads did you run? And what compound DTCs are you running? I'm running DTC 60/DTC 30 on my RX7. I like them and they actually work on the street without feeling like you are going to die. My understanding is DTC70s won't work on the street as you won't get them hot enough. The 60/30 setup is easy on rotors and cheap, but I go through them quickly... About twice as fast as 100 tread ware tires.
i think the last set of PFC's was 18?

and BTW the drivers did not like the same compound all the way around, it locks the rear too fast, and its a miata. so we were running 18 in the front and whatever was handy in the back.

i didn't get to see what the DTC's were, i think 60's? i get to put the next set on in a couple weeks, so i'll look. we got one race weekend, 2 track days, and half way through the Miata's at laguna seca event*, before the DTC's were halfway through the backing plates! this is WAY shorter than the PFC's, we get 3-4 race weekends out of those**

*we had to change pads in the pits after the parade lap!
**depends on the track, laguna is hard on everything, thunderhill is really easy
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Old 10-11-15, 07:50 PM
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Progress report:

Installed new pads, Hawk HP Plus on the front, kept the HPS pads on the rear, and a new set of Ado8r tires, lowered the rear of the car about 1/2 inch and adjusted the shocks to the firmest setting.

I went for a drive through the country to bed the pads in, carefully following the instructions on the box and the results are very good.

I am very impressed with the additional bite of the Plus pads over the old HPS pads I had before.
Big difference!!!!

The old pads took way more peddle pressure, more so than I would have ever guessed.

This seems to have fixed my issue, I guess I will have to wait till the next track day to be sure as I did not want to go full race mode on the roads around where I live, even if it is way out in the country.

I still don't know how they will act when everything is good and hot, but so far, so good, the fronts are locking up first every time now. I am hoping that it is not a problem that changes as the tires begin to wear as today's experiment was done on brand new tires. We will see.
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Old 10-12-15, 11:17 AM
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Hawk HPS pads are horrible.. Low bite, mushy feel, fade easily. As you noticed HP+ are great street/light track pads. The initial bite and feel is much much better.
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Old 10-14-15, 12:00 PM
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Originally Posted by largeorangefont View Post
hawk hps pads are horrible.. Low bite, mushy feel, fade easily. As you noticed hp+ are great street/light track pads. The initial bite and feel is much much better.
qft

HPS pads cant even handle spirited street driving

Last edited by RockLobster; 10-14-15 at 01:02 PM.
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