Front proportioning valve?

[ViperRay]

I apologize in advance if this question has already been addressed but I couldn't find what I was looking for. I know that proportioning valves are typically placed on the line going to the rear brakes and that it allows you to decrease the pressure going to the rear brakes as needed for proper "balance". I guess I don't understand why the proportioning valve can't be placed on the line going to the front brakes to allow the rears to do a little more work in some of our vipers.
There are different "fixes" I have read for this problem including smaller calipers on the front but why can't this be done as long as there is still enough pressure in the line to lock the brakes up front? Thanks.

 

[dblankenbaker]

why can't this be done as long as there is still enough pressure in the line to lock the brakes up front? Thanks.

Because "enough pressure" to lock up your brakes at 45 mph is a fraction of what you need to lock them up at 90 mph. You will effectively have a car that goes forever when braking from triple-digits, and feels firm that last 100 feet.

 

[ViperRay]

David,
Can you put any numbers on your remark. How many PSI are necessary to lock brakes at high speeds and still have some pressure to spare? 1200?
I take it that the power booster does not make sufficient pressure then to allow even a little decrease to the front brakes? Just trying to learn something which puzzles me.

 

[Tom F&L GoR]

I posted this about a month ago and nobody argued (a rare occurence on this board.) At the risk of learning something, I'll copy it here again and ask if this logic seems correct:

"A proportioning valve limits the brake line pressure relative to further application of brake pedal. Typically, it's used on the rears to make sure you hit the oncoming barrier with the front end of the vehicle, because the occupant protection is designed for frontal impact.

An adjustable proportioning valve:

1) changes the pressure at which the proportioning begins
2) changes the rate of pressure rise decrease
3) both 1 and 2
4) and maybe you don't know if it's 1, 2, or 3

Your risk situation is if you reduce the fronts to match the rear lock-up and then find yourself in a stickier situation, meaning the fronts are sticking more than you ever expected. The weight transfer, road condition, or sticky tire will then keep the front wheel rolling instead of locking up. However, once you engage the prop valve function you've dialed into the front, and you keep pushing harder (because you don't see any flat-spotting tire smoke yet!) you are now applying a greater amount of rear brake than front brake. Depending on conditions and the prop valve, a little more front brake means a lot more rear brake and you hit the cone/guardrail/tree rear end first.

With a rear prop valve, you always have the comfort of hitting the cone/guardrail/tree front end first. "

ViperRay, if you decrease your front pressure to balance the car, then put stickier tires on the front, or have more weight transfer to the front (because you are decelerating from a different speed) then the front's will not lock and the rears will. It's not the absolute pressure, it's the balance between front and rear.

 

[ViperRay]

Thanks Tom. I was hoping you would chime in. I get all that you said. I guess I was thinking that, at least theoretically (through trial and error on a test track), you could achieve a more optimum balance by decreasing the pressure to the front calipers (just the right amount mind you) to yield a more perfect stopping situation than what is currently stock on our vipers. This would also allow us to alter the valve setting based upon our tire choice, etc. I believe there is a post recently from someone who got into trouble because of using different tires front and rear. The problem I have with fixed pressure to the calipers is that there are too many variables for this to be an optimum situation in all cases.
On the other hand, I'm not saying I'm smart enough to be able to set the valve optimally all the time either. I'm just asking if what I'm saying is theoretically feasable.

 

[Silver98]

Some years ago, many Viper experts on this board seem to be in favor of removing the famous "o-ring" in the proportional valve to increase the pressure on the rear calipers.

Is this a no-no now?

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[Tom F&L GoR]

I asked Herb Helwig (at the VCA-PVO autocross) about the "balance" of the brakes, trying to get some feedback on the conversion to 38mm or 40mm rear calipers. His answer was that they met the legal requirements... and let's change the topic. I take that as complying with the DOT safety issues of fronts locking first.

Even so, I disabled my prop valve as my first brake mod, and while it was better, it was nowhere near ideal and the fronts still lock first. Seems Dodge saved money on the dinky rear calipers, and could have saved more money by eliminating the prop valve altogether. Hence, I marched to tune of a different plumber and worked on the larger rear calipers.

StopTech had a nice writeup of brake systems on the Viper, I have a copy, but can't remember where I got it from. With full acknowlegement to them, here's what the OEM prop valve does. An adjustable one would change the pressure at which the rear brake line pressure becomes limited or the rate at which the rear brake line pressure increases, or both.

 

[NDW]

Ray,

You and I seem to be going along the same trail. I received my proportioning valve in the mail yesterday and am contemplating the same questions you have before I install it.

Newt

 

[ViperRay]

Newt,
What, if anything, are you planning to do with the rear prop valve? Whose prop valve did you buy (I put one on my 1970 Challenger from Stainless Steel Brakes which works well).
Ray

 

[slaughterj]

Some years ago, many Viper experts on this board seem to be in favor of removing the famous "o-ring" in the proportional valve to increase the pressure on the rear calipers.

Is this a no-no now?

Mine "theoretically" was removed by a Viper Tech who works with Viper Days, so I fell confident that that isn't a no-no..."theoretically"