is porportionating valve needed?

[vandit]

hi..yesterday we recieved our brake parts(wilwood)...and in the information brochure they mentioned that if ur master cylinders are mounted below the horizontal plane of calipers then u should use proportionating valve to retain the pressure in the line and to eliminate the double pressing of pedal.....can anyone tell me how far it's necessary....we r using two different size master cylinders for front and rear....

[vandit]

hi..yesterday we recieved our brake parts(wilwood)...and in the information brochure they mentioned that if ur master cylinders are mounted below the horizontal plane of calipers then u should use proportionating valve to retain the pressure in the line and to eliminate the double pressing of pedal.....can anyone tell me how far it's necessary....we r using two different size master cylinders for front and rear....

[Travis Garrison]

I don't think you want a proportioning valve...you should already have a bias bar correct?

If you're getting a lot of pad retraction you can use a residual valve to hold something like 1/2-2psi (they come in a couple of different pressure settings) in the brake lines at all times, which will cut down on the "take up" in the brake pedal. The down side is that your brakes could end up rubbing all the time depending on how your calipers are setup (certain o-rings pull the pistons back quite a bit, some don't)

[Homemade WRX]

go bias bar...the judges hate proportioning

[Steve Yao]

Hmmm, do the judges really hate proportioning valves or simply how it is commonly misconcepted/used?

[Ehsan]

For the residual pressure valve, I would suggest experimenting with it. Buy the extra fittings you need (typically 3/8-24 NPT with the wilwood) and then run the car without the valves and then run it with the valves. See if the driver notices any difference in the amount of force/travel needed to engage the brakes.

Though in theory they are needed, I have seen that it is not necessarily required. The small lines we run and our lightweight systems don't allow for a ton of return force at the pads so you might not need them. Also depends on your calipers as Travis mentioned.

But test it out for yourself so when a judge asks you why you have/don't have them, you can tell them you tested it and back up your choice.


EDIT Correction: As noted by Analogue below, its a 1/8 NPT, not a 3/8-24 NPT (don't even konw where I came up with that thread size. I claim residual fatigue from an all-night engine swap in Romeo).

[Analogue]

Residual valves also make the system a bit quicker to bleed. You stomp on the pedal several times to work the big bubbles out, and go back to normal practices from there. After a 3 hours bleeding fiasco at competition resulting in disassembly of a master cylinder that was thought to be busted, I'm beginning to go back to my old ways of liking residual valves.

The Wilwood part has 1/8 NPT fittings. My only gripe is that I dislike adaptors, and I dislike NPT fittings even more.

I could be mistaken, but I thought I saw a proportioning valve by the shifter of the UWA car. I suppose that is in addition to a balance bar? Since nobody can argue that that car isn't well thought out and executed, I'm sure there's a good reason for it.

Keith

[Bart Smith]

I think we need to clear up a few things. A residual line pressure valve, and a proportioning valve are 2 totally different things.

1: A residual line pressure valve is used mainly in systems with drum brakes, as the retaining springs have the capability to push all the brake fluid out of the wheel cylinders. Without the valve in the system, you would have to pump the pedal twice, to get a decent pedal feel and to remove excess travel.

2: A proportioning valve is used to limit the pressure to the rear brakes. Adjustable proportioning valves are good, but their effectiveness is more to do with normal road cars.

Proportioning valves are intended to maintain directional stability in a stopping case where a heavy brake application is likley to result in the front wheels locking.
In a racing car, where there is a need to brake while also generating considerable side forces on the tyres, a moderate brake application is likley to produce oversteer below the activation pressure of the valve. The proportioning valves design case is a hard stop in a straight line.

[Ehsan]

<BLOCKQUOTE class="ip-ubbcode-quote"><div class="ip-ubbcode-quote-title">quote:</div><div class="ip-ubbcode-quote-content">Originally posted by Analogue:


The Wilwood part has 1/8 NPT fittings. My only gripe is that I dislike adaptors, and I dislike NPT fittings even more.

Keith </div></BLOCKQUOTE>

Keith is correct, it is a 1/8" NPT fitting: http://www.wilwood.com/Products/006-MasterCylinders/010-RPV/index.asp

Sorry for the misinformation. Its been a year since I dealt with them.

[Analogue]

That's only the beginning.

Proportioning valves are designed to provide a non-linear pressure output. It has a "knee" beyond which pressure to the rears does not keep up with the pressure rise in the front.

This is because drum brakes also have a non-linear response. Ever wonder why the primary shoe lining is cut back so much more than the secondary shoe? Drum brakes apply themselves harder due to the braking torque pulling the shoe into the drum. The cut helps prevent this.

Residual valves are a different story. Typically, a 10 psi valve is used with drum brakes to prevent the problem Bart described. A 2 psi valve is used for disc brakes, as any more can cause pads to drag. I used to think of them as unnecessary clutter, but they do have some advantages with bleeding and potentially pedal feel.

Keith

[kwancho]

<BLOCKQUOTE class="ip-ubbcode-quote"><div class="ip-ubbcode-quote-title">quote:</div><div class="ip-ubbcode-quote-content">Originally posted by Analogue:
That's only the beginning.

Proportioning valves are designed to provide a non-linear pressure output. It has a "knee" beyond which pressure to the rears does not keep up with the pressure rise in the front.
...
Keith </div></BLOCKQUOTE>
Could this also be useful because of the fact that you need more rear brake under less than 100% braking? I was thinking if you have the balance bar set for your max braking situation (~70% front) and no proportioning valve, and you demand full cornering at 50% braking, the front tires will saturate much earlier than the rears.

[LU-Bolton]

Keith, you are correct in that you saw UWA's car with a proportioning valve in the pedal box. Unfortunately at design finals they were greatly criticized by design judges. I can't quite remember what the explanation was, maybe someone else remembers specifically and can comment on it.

[Nick McNaughton]

<BLOCKQUOTE class="ip-ubbcode-quote"><div class="ip-ubbcode-quote-title">quote:</div><div class="ip-ubbcode-quote-content">Originally posted by Alex Kwan:

Could this also be useful because of the fact that you need more rear brake under less than 100% braking? I was thinking if you have the balance bar set for your max braking situation (~70% front) and no proportioning valve, and you demand full cornering at 50% braking, the front tires will saturate much earlier than the rears. </div></BLOCKQUOTE>

That's exactly what we used it for. It was an experimental system, but we tried a few different valve internals and found a setup that made the car easier to drive without hurting straight line braking capacity. It still had a balance bar that was set independantly of the prop valve, but the master cylinders were sized a bit differently to the normal system.

One of the things we found is that you can set the car up for a type of corner that would usually result in a car that was difficult to drive. A lot of those bad points would go away with the prop valve system, resulting in a faster car overall. However, if the track doesn't need the tough-to-drive setup, the prop valve system isn't such a bonus.

The design review feedback wasn't as much about our application of a prop valve as much as it was general advice for people who might try to use a prop valve in place of a balance bar on a clean-sheet-design racing brake system. At the end of the day, if you've got a great brake system, a great setup and great drivers, you shouldn't have a prop valve. Our system was aimed to get around one of those factors not being so great http://fsae.com/groupee_common/emoticons/icon_smile.gif

[Homemade WRX]

<BLOCKQUOTE class="ip-ubbcode-quote"><div class="ip-ubbcode-quote-title">quote:</div><div class="ip-ubbcode-quote-content">Originally posted by SEY:
Hmmm, do the judges really hate proportioning valves or simply how it is commonly misconcepted/used? </div></BLOCKQUOTE>
touche'