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Discussion Starter #1
I'm not happy with the brake bias in my Elise under heavy braking during autocross events as the front ABS is triggered too easily. From what I understand this is due to the bias between the front and rear (too much going to the front of course).

In thinking about different ways to deal with this I wonder if a brake proportioning valve would work. Summit racing sells these -

Tilton Engineering 90-2003 - Tilton Brake Proportioning Valves - Overview - SummitRacing.com

It seems like this could help a good bit. Does anyone have experience with using something like this on an Elise?

Dave
 

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On mine, I don't have the ABS anymore, and having 2 wilwood prop valve (one front, one rear) like you are refering to. It is wonderfull. As you mentioned, the stock car have too much brake in the front, and the improvement is real by reducing the pressure in front.

It is quite funny that many poeples are putting bigger brake in front when already the bias is not good.
 

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i've been dealing with the same problem dodging cones with a CT AX-6 square setup. had similar issues on other cars in the past, and solved them by putting a more aggressive pad in the rear (given the limitations of stock class rules). am considering doing the same with the elise, and throwing some XP8's out back.
 

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i've been dealing with the same problem dodging cones with a CT AX-6 square setup. had similar issues on other cars in the past, and solved them by putting a more aggressive pad in the rear (given the limitations of stock class rules). am considering doing the same with the elise, and throwing some XP8's out back.
I have also put more aggressive pads on the rear. I have CL RC6E in front and CL RC6 in rear. The friction coefficient on these pads is close enough that it still leaves a heavy bias to the front. My next set will be CL RC6 front and CL RC8 rear.
 

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I'm not happy with the brake bias in my Elise under heavy braking during autocross events as the front ABS is triggered too easily.
...
It would be worse without ABS. (I know this)
You would be better off with a dual master cylinder setup, but that may be difficult to work with the ABS???

There are at least 2 solutions for dual master cylinders.
 

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dual master is more complex and you need to remove the servo assistance. By just adding reducing valves, the balance can be like perfect for 100$...

It is not the state of art solution, but work great, easy to adjust, and cheap. I putted mine one over the other one between the module ABS location and the master. Since my car is a street car, I did not put inside the car.
 

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I took a different approach-I installed a set of front calipers on the rear. Essentially, I have 50/50 brake bias at the moment. This is not optimum, but damn close. The ABS is retained and also offers a much wider variety of available brake pads. I have seen AP two pot calipers with 36mm pistons that would probably make it just right, but I need to pony up some more cash for that. I have used this setup on my dd Elise for around 6000 miles now-what an improvement!
 

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A proportioning valve should give you exactly what you need to balance the front/rear braking you desire. Of course putting on a big brake kit would help, but at $6,000.00 the proportioning valve would be cheaper.
 

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Discussion Starter #9
On mine, I don't have the ABS anymore, and having 2 wilwood prop valve (one front, one rear) like you are refering to. It is wonderfull. As you mentioned, the stock car have too much brake in the front, and the improvement is real by reducing the pressure in front.

It is quite funny that many poeples are putting bigger brake in front when already the bias is not good.
Hey,

Thanks for the info.

Just so I understand..........what is the advantage/reason for going with two prop valves? I would think one would do the same job? Am I missing something?

Thanks again,

dave
 

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Discussion Starter #10
I think the first thing I may try will be a stickier pad in the rear and if that's not enough then a prop valve hooked up to the front.

It seems pretty simple to get the prop valve hooked up - minor packaging challenges I would think but otherwise straight forward.

Is there any issue with using a prop valve and ABS? It seems unlikely but thought I should ask.

Thanks again,

Dave
 

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Changing ratio of tire widths make a difference too, brake balance feels better on my car with 205 width front, 245 width rear. Keep in mind the ABS kicks in when it determines your front wheels are in (sometimes partial) lockup. Improving front grip ratio changes the point (relative to rear wheels) where lockup occurs.
 

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Proportioning valves are usually placed on the rear channel of a non-ABS brake system to limit the pressure at the rear. They are not true linear proportioning valves, most only delay the maximum potential of the brake pressure. They do this by adding a knee to the curve of the output. THis would totally screw with the ABS btw.

What kind of suspension do you have on your Elise? What kind of tires and pressures.
If, for instance, you have adjustable suspension, having too little compression damping, or too much rebound damping can cause the front tires to skip off of bumps and will activate the ABS early.

A proportioning valve will only limit the maximum braking that you can get, it will never add more, you should figure out why you have an issue before you change something that will effect the ABS... The old cut the nose off to spite the face thing...
 

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Why 2 prop. valves? When I did my set-up, my goal was to have full control of the bias, and since the valves are cheap... But of course, you really need only the front in your case. I did that set-up 2-3 years ago. The pressure reduction required at the front is around 18%.

Last winter I installed 36mm AP racing 2 pots caliper at the rear. I did my first track session last week, and it was required to remove pressure at the rear since the car was locking the wheel at the rear, and worst, on instable section of the track with braking on S, the rear was unstable. But since I have full control of the braking bias, it is quite easy.

With my actual set-up, and if I was doing racing (I am just doing lapping), my tendency will be to adjust for dry the prop full pressure front, and find the perfect balance on the rear. On wet, I would just reduce the pressure on the front only to not play with the optimum rear pressure, and also to reduce the brake power since the car is braking too much on wet (breaking too much already on dry... I am using Raybestos ST-43 pads).

I prefer by far to adjust the bias with prop valve then playing with brake pads material. Running with the same brake pads all corner is better since all pads have the same friction and same reaction cold or hot, and the same friction at approach.

Long time ago (15 years), I tried to adjust the bias with pads, and thye result was very bad. I was running Hawk Blue in front, and greenstuff on rear. The braking after long stretch was dangerous since the blue pad have lower friction at the cold, and the green stuff is more constant. The car was suffering of too much brake after stretch, and good balance on other section of the track. Autocross is less critical then long track racing or lapping.

Honestly, my car braking is just perfect. Prop valve front and rear, Alcon disc (295x28 mm front (running the front disc for 3 years now) x 295x25 rear), Std 2 pots caliper front, 36 mm AP rear (I can supply economical brackets 250$), ST-43 pads all corner. My car have around 280 whp. Just perfect except maybe too much brake, easy to lock Hoosier R6 at 120 mph.
 

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A proportioning valve will absolutely effect ABS operation. Modern ABS systems don't use proportioning valves, but rely on caliper/piston size to achieve a base brake bias. All other "bias" would be provided by the ABS hcu through solenoid orifice sizes. The only way to get the benefit of a prop valve is to do away with the ABS. If the ABS is to be retained, the most effective way to shift the brake balance is to change the caliper/piston ratio front to rear. Actually that is the best method with or without ABS, as the prop valve becomes just a fine tuning device. I raced many years with dual masters and a balance bar, ( a system which had base balance through caliper piston and master cylinder piston size ratios), though this setup is not practical on a road car.
 

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Discussion Starter #15
Just a quick follow up -

I ended up working with the bias by using different pads front and rear. After experimenting some I ended up with EBC Yellowstuff pads in front and Bluestuff in the rear and I'm very pleased with the results. Once bedded in it became obvious that there was much braking happening in the rear that before and it feels very good at the limit. The car absolutely drops anchor now and it is very difficult to get the front ABS to trigger in the dry.

Something similar may be worth considering if you have bias issues like I did.

Dave
 

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I have 4-6 track days with CL RC8 rear pads and RC6 front pads. This has improved the rear braking. As a rough estimate, I would say that I feel about a 5% shift of bias to the rear as compared to the stock bias. I would still like considerably more rear bias as I can just barely feel the brakes start to rotate the car when trail braking.
 

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Does the front end of the car wander a bit under hard braking?

My issue is the left-front rotor has significantly more wear than the right-front. Under hard braking it pulls to the left so I have to correct it which is not ideal of course. The rotors and pads were changed at the same time, so that is not a factor. An instructor at a track day who owned an Exige told me that the proportional valve on these cars is a low quality part and most likely causing my problem. Any suggestions?
 

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re: Uneven rotor wear side-to-side

The proportioning valve deals with front-to-rear brake force distribution, not side-to-side.

There are the following factors that would make one side of he car brake and wear harder:

1. Sticking front caliper. This would be the most common cause.
2. Blockage in the brake line to the other side.
3. Pads bedded differently side-to-side. or glaze pads on one side.

The above three are common brake problems that would lead to your symptoms. Items mensioned below are possible, but very unlikely.

4. Improper alignment. Too much (has to really way to much) camber or caster on one side.
5. Too much wedight on one side of the car. Left is where you sit. The manufacturer tried to account for that, though.
6. Un-equal length lines from M/C to the caliper. Again most production cars account for this.

Are you sure that the pulling to one side is because of rotor wear? It really has to be worn to un-safe levels to make a difference. The caliper is a self-adjusting mechanism for rotor and pad wear. The pistons come out furher and further as pads and rotors wear to compensate.

Is the rest of your car and the braking system set-up properly? Frankly, I would question the wisdom of your instructor of letting what is essentially an un-safe car on the track and feeding you some kind of strange BS.

Anton


Does the front end of the car wander a bit under hard braking?

My issue is the left-front rotor has significantly more wear than the right-front. Under hard braking it pulls to the left so I have to correct it which is not ideal of course. The rotors and pads were changed at the same time, so that is not a factor. An instructor at a track day who owned an Exige told me that the proportional valve on these cars is a low quality part and most likely causing my problem. Any suggestions?
 

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A?BS and proportioning valve

A lot of ABS systems have a proportioning valve. In fact most of them do, due to the compound master cylinder.

You my not be able to see it. On the old cars in the '70's it was a separate part. The modern cars use a compound master cylinder. There is a proportioning valve built-into it. Why is it necessary?

We all know that most cars need more braking force on the front than the back. The manufacturer tries to achive an optimum braking force distribution. They use caliper, pad, disc and M/C sizing. Usually, some final adjustment is still necessary, since all other parts come in fixed sizes. The biggest problem is the, above mentioned, compound master cylinder. It is used on all production cars. It is really two master cylinders in one package. Besides providing a safety mechanism which we will not discuss, it is much cheaper to make than two master cylinders, one for front and rear. The problem is that it implies the same size M/C bore front and rear and hence same pressure in the brake line front and rear. This does not work well in most cases. Hence, a proportioning valve. Even if the manufactuerer managed to get away withought a proportioning valve through different orifices in the brake system, they act exactly as a proportioning valve! Therore, one can still be installed for fine tuning!

Most manufacturer like the front brakes to lock-up before the rears. This is safe as the car will understeer, plough straight forward instead of spinning.

For racing applications the balance should be adjusted to driver preference. Also more rear bias makes you go faster, as it will rotate the car into a corner for technique known as trail-braking. Race cars employ two master cylinders with different bores and leverage mechanism that can be adjusted. This is simple, slightly more expensive (but it is custom for a race car anyway, it is not mass produced, while the components could be) and is very adjustable.

ABS works by pulsing the pressure in the brake lines. It can work dual or compound master cylinders or proportioning valves. No problem at all.

We used to pull the proportioning valve out of the compound master cylinder (in an ABS car), install the proportioning valve in the driver's compartment in the rear brake line (from Tilton). To get better brake feel we would use a compound master from a truck (larger and had bigger fluid reservoir that did not slosh so much). It fit on the same mounting studs. This is for a production car. ABS worked great.

The problem with ABS and proportioning valve and brake balance is the ABS computer. It is a fine device that looks at all 4 wheel speed censors and applies the brakes based on it perception of slip and yaw and cornering force (it may have a g-sensor inside). Once its design parameters have been exceeded, by the braking system or the brake bias is completely out of its range, it gets confused.

Once we started getting faster and using larger non-production brakes. The ABS computer would get confused. (this was highly umpleasant as I tried to trail brake into a medium fast tight corner, it decided that the fronts were getting locked up and almost spum me, actually caused understeer. It was a strange feeling. I pulled it out next day. :) Although, there is better ABS, now, and racing ABS and ABS on faster productin cars, I think, I am so good, I do not need it anymore. :) I do feel it is of little use on dry race track. I like very aggressive trail-braking and ABS does not fit that style.

I think, more interesting in the brake bias is the rate at which the pressure rises in the front and rear brake lines during initial pedal applications. This affects how fast and how well the car settles into a corner.

Anton


A proportioning valve will absolutely effect ABS operation. Modern ABS systems don't use proportioning valves, but rely on caliper/piston size to achieve a base brake bias. All other "bias" would be provided by the ABS hcu through solenoid orifice sizes. The only way to get the benefit of a prop valve is to do away with the ABS. If the ABS is to be retained, the most effective way to shift the brake balance is to change the caliper/piston ratio front to rear. Actually that is the best method with or without ABS, as the prop valve becomes just a fine tuning device. I raced many years with dual masters and a balance bar, ( a system which had base balance through caliper piston and master cylinder piston size ratios), though this setup is not practical on a road car.
 

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Thanks for the in-depth reply!
I installed the front and rear pads and rotors
so I'll check my work. I've seen some guys applying
ant-seize to the back and sides of the metal backing plate
for fixed calipers (is that the right terminology?) in order
to ensure they slide properly.
I also installed my previous brake setup, new pads and rotors all round, but did not have uneven rotor wear.

Since the stock rotors are the same from and rear, can they be swapped like car tires get rotated if they have the same dimensions?

I understand pads have unique wear characteristics on their respective rotor (or at least I am under this impression), but could this pose a safety issue if the front rear rotors were swapped while maintaining the same pads. I believe the swept area of the Carbotech XP8 pads I have at all 4 corners cover the entire rotor face so swapping may not be an issue...thoughts?? I know these are potentially dangerous ideas, but am curious nonetheless.
 
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