Brake Bias valve

[eh_tony!!!]

I have the knob (not lever) styled proportioning valve (260-2220)installed to limit pressure to my rear brakes in my B car. Throughout the last race, I have been slowly dialing out rear pressure to avoid rear lockup and axle hop. During the last session of the weekend, I had a rear caliper begin to drag severely and finally ended my day with a glowing red rear rotor. Figuring I had a line problem or caliper issue, I re-built the system using all new lines from the master cylinder,new calipers, new rotors and new pads. I finally got the system bled today and noticed that again the rear calipers would not release after pumping the brake pedal. I manually released the line pressure through the bleed screw and the calipers released. I repeated the test with the same results. The third time, I adjusted the proportioning valve in (increase direction) to approx 1 turn from stop. (I turned it 8 complete turns to the increase direction, so I was pretty far out to begin with, but the manual seems to indicate that the usable range is from stop to stop)). I then repeated the test and found the rear calipers to not have stuck and the brakes seem to be operating normally.

So, My question is.. is there a limit to how far you can turn the prop valve in the "decrease" direction?? Do I have a defective valve that needs to be replaced?? Anyone ever had a similar issue??
Thoughts??

[Knestis]

I don't know from experience but your methodology sounds good, in terms of how you came to your diagnosis...

Kirk

[JohnRW]

Originally posted by eh_tony!!!:
I have the knob (not lever) styled proportioning valve (260-2220)installed to limit pressure to my rear brakes in my B car.

Therein lies the problem. The knob-style prop valves are junque. You can buy them for $30-40, but you get $5-10 worth of valve. The adjustment range, if you had a pressure-test rig to document data, is non-linear as hell and, somewhere in its 6-8 turn range, gets really sensitive. Suddenly, a quarter-turn makes a big difference, where the previous 5 turns didn't do squat. You also get lousy 'repeatability'. Do you really want to have to think "...gee, how many turns, and in what direction, do I screw this stupid little knob, now that it's raining" when you're in race traffic at 700mph ? Ummm....no.

The lever-style prop valves are a lot more linear, as Tilton etc. took the time to carve the right shape ramps onto the adjusting cam. And repeatability is as simple as "2 clicks off full soft in the dry, 3 clicks off full hard in the wet", or whatever you prefer.

[eh_tony!!!]

Ok John, I'll buy your explanation of the non-linear adjustments. I'm certainly not against spending $100 or so to get a decent valve AND one that will release..
But.. Aren't the 7 position valves the same valve body as the knob types? (looking at Wilwood (my brand) and Tilton pics, the bodies betweent he knob and level look identical) If this is the case couldn't I have the same issue on the lever type (non-bleed-back). OR, is your contecntion that I was prolly farther "out" on the adjustment than the lever type would have allowed??

[JohnRW]

The lever prop valves won't let you get into 'line-lock valve' territory (drag race term - that's how they do the smokey burn-outs in the bleach-box). I just have a wicked distrust of the knob-type valves after one was a participant in my unwitting attempt to set a 'world land speed record' for 'backwards in a VW' at Nelson Ledges about a decade ago. Why I didn't flip the car...I'll never know...

[ddewhurst]

IIRC, the Wilwood knob style proportioning valve that is in my car when turned to have minimum pressure to the rear is 57% pressure to the rear & when the valve is opened up the full ten turns I then have 100% pressure to the rears. Have not taken it apart but I would think there is nothing more than a spring loaded ball & the spring rate controls the pressure.

Have Fun
David

ps: Most of us don't need the thing in the cockpit for adjustment for sprint races. It's just another excuss why we didn't drive well.......

[MMiskoe]

Do the lever types really work either? If they are nothing more than a regulator, like your air pressure reg for your paint/impact gun, they require flow to work as all regulators have to have some flow to allow the spring & diaphram to get to equalibrium. In a brake system there is little or no flow (or shouldn't be) as the fluid is non-compressible and the amount of actual movement in the caliper is nil. The more fluid you move, the "longer" the pedal, any good system moves the pedal about none.

I've often wondered how well any of these things work after sitting through a seminar on gas regulators. They all have "droop" which is the drop in pressure from no-flow to flow, watch the needle on your compressed air reg when you pull the trigger. Is there really enough flow in the brake system to make this all happen?

Just curious if they work, maybe I have it all wrong on how they accomplish it.

[pfc]

tilton/wilwood bias valves as well as the VW factory EQ valve are miniature slave/master dual diameter hydrhaulic ratio changers. the ratio is fixed (on the order of .5-.75).
the tension adjustment controls the point at which the line function (1/1)switches to the fixed hyd ratio of the valve, the lower the tension, the earlier it occurs. if you can't limit your rear brake pressure enough, you need a higher ratio or to change the coefficient of friction of the brakes relative to frt/rear.
The tilton has the same ratio as the VW. I forget exactly what it was (have data @shop) and I expect that other valves are likely the same. I hunch FWD racecars need a somewhat higher ratio than is available. If anyone can point me to such a valve, please do. Phil Hunt


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[chuck baader]

This might be of help..the stock BMW E30 chassis has a pressure limiting valve going to the rear brakes that is factory installed. I have considered reinstalling it along with my Tilton bedcause I generally run the Tilton full "off". I wish someone could check this unit and give definitive working parameters..it just might be what all the FWD people are searching for to use in addition to the Tilton.

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Chuck Baader
SEDIV ITS #36 E30 BMW
Alabama Region Assistant Race Chair, Divisional Registrar