Bias/Proportioning Valve?

[Clatter]

Having 944 brakes on my Fastback has me wondering...

1. 944 and Fastback have different bias needs.
Mine is especially rear-heavy.. Type 4 and 915 trans.

2. Since most are setup to stop rear brakes from locking, then install into the rear circuit.
Because my car should need more rear - install the valve into the front circuit??

3. Metric fittings.. Anyone make a bias valve metric?

If you have ever run one of these things,
What did you think of it?

How about this one?
http://www.mcgillmotorsport.com/adjusta ... r-type-678

[Jadewombat]

What master cylinder are you using?

[Clatter]

Stock Type 3, unless it doesn't give good results..

[Ol'fogasaurus]

Have you looked into an adjustable proportioning valve. Not good for all situations but might be an option.

[Ol'fogasaurus]

https://www.bing.com/shop?q=adjustable+ ... 99E8DDF4D5

So many styles to look at and decide if it is right for you.

[Clatter]

So,

If my brakes are from a 944, with near 50/50 F/R weight bias,
And I'm putting them into a car that's tail-heavy,
I should put the valve into the front circuit,
Instead of the rear circuit..

Right?

[Ol'fogasaurus]

Basically, the proportioning valve slows down the amount/volume of the fluid to the brakes it is attached to but only by a shade, that speed is controlled by the internal of the valve itself.

The adjustable version allows one to "tune" the bias a bit by a knob that controls the amount/volume of fluid it allows to pass.

If you put a valve on the front brakes then their full application is slowed and the rears apply first. Putting them on the rears allow the front to apply first.

Overly simplified description but does this help?

[raygreenwood]

Basically, the proportioning valve slows down the amount/volume of the fluid to the brakes it is attached to but only by a shade, that speed is controlled by the internal of the valve itself.

The adjustable version allows one to "tune" the bias a bit by a knob that controls the amount/volume of fluid it allows to pass.

If you put a valve on the front brakes then their full application is slowed and the rears apply first. Putting them on the rears allow the front to apply first.

Overly simplified description but does this help?

This really depends on what kind of proportioning valve it is.....and proportioning is different from bias.

Some factory dedicated proportioning valves like those used on 411, 412, 914, some BMW, Audi and Mercedes ....actually do not limit volume to the brakes being proportionally regulated....until a certain pressure is reached. Then it maintains that pressure and does not allow it to rise further.

Some others...more sophisticated factory ones....can actually further reduce rear brake pressure due to body to rear axle angle to further respond to say....a more heavily loaded car.

But brake BIAS....true brake bias.....does not mess with pressure. It adjusts TIMING.....or "onset" of braking. For instance most cars have a front bias.....wherein when the pedal is pushed the front brakes start braking first even if they are running with the exact same pressure as the rear.
The pedal will move the front circuit piston X distance before the rear circuit gets any movement at all.

On aftermarket systems for dull custon cars....one might have actual seperate single circuit MC's for front and rear. The pedal moves a single pushrod that has an adjustable bias bar....tbat does not alloe contact with the other circuit until it has moved X distance.

You you can do it hyraulically....with what is eszentially a check valve that does not allow any pressure to the rear (or whichever ciruit you are delaying)....until the lead circuit reaches X amount of pressure.

The actual factory dual circuit MC's do this biasing much as you described. The front circuit which is "normally" in contact with the pushrod....will have a softer spring than the inner rear circuit piston. So it will move X distance until it generates enough spring pressure to overcome the spring pressure on the inner piston snd start it moving.
Ray

[Ol'fogasaurus]

Thanks Ray. As I said it was an overly simplified description. You did say some things I did not know though.

[raygreenwood]

Thanks Ray. As I said it was an overly simplified description. You did say some things I did not know though.

No sweat!.....I learned the difference the hard way tinkering with a,Saab....and almost rolled it!
Ray

[PhillipM]

You should never put a brake proportioning valve in a front line (apart from some very specific valves for some very weird setup - basically you'd know why you needed it, but anyway).
The vast majority of prop. valves don't change the bias at all until they get to the pressure set by the lever, or the knob, where they then start to blow off and reduce the amount that the pressure increases by compared to the input ( the documents will have the exact pressure ratio, ideally you'd pick one to match the CoG and wheelbase of your car, most people just slap one on and pray).

Putting one in the front line will mean it does nothing when you're low on grip, but when you have plenty and the weight is transferring forwards under heavy braking, suddenly it'll start reducing front pressure and you'll get more bias at the rear - just when it's going light.
At best it will suddenly feel like your brakes are giving up, at worst it'll just lock the rears and spin the car.

You should alter bias with either the caliper piston sizing/drum cylinder sizing, different compound brake pads/shoes front to rear, or - ideally - with a bias bar and twin master cylinders.
You can then use a prop. valve as well in the rear line to shuffle the brake bias forward under high grip conditions, and rearward in low grip.

I have to take issue with what Ray has said though, bias boxes do not affect the 'timing' of brake application at all, they affect the pressure balance, front to rear, the only way one could affect front to rear timing would be if your bias adjuster had a seized centre bearing or other fault. Even with single OE master cylinders with twin pistons - the pressure is transfered to the rear piston through the fluid pressure - and as anyone working with hydraulics will know, the reason we use them for brakes is because the pressure is equal across the system - not the spring, the spring is there as a backup and to ensure both pistons return past the fluid inlet port every time, regardless of hysteresis, to prevent any binding.
The front bias in road cars these days is achieved very simply with bigger pistons and discs on the front compared to the rear.
And they still run a load or prop. valve in the rear.

[FJCamper]

Hi Phillip,

I think Rays description of timing and your description of pressure amount to the same thing in application. We're one of those rare instances of using a proportioning valve on the front circuit and another on the rear. We run a '73 Super in LeMons enduros.

We mounted big 4-piston Wilwoods up front and went through 19mm, 20.6mm, and finally a 23.8 master cylinder. Using the dual valves system we were able to tune braking effort between the front Wilwoods and the rear VARGA (ATE copy) twin 40mm piston calipers.

I don't recommend this dual proportioning valve modification to anybody, as it was tedious setting it up right. But is is right, and our braking is excellent. We can even tune for different tire types. The right way to do this would be a twin master cylinder setup with a balance bar, but like most tar baby's, we got stuck more and more as we went but in the end we had a good system.

FJC

[Ol'fogasaurus]

Brake balance is hard to set at times due to conditions of the ambient air temps and moisture changes, track conditions, tires and many other things. Its kind of like tire pressure, which can change during the day or night during the surface you are riding on as there are changing temps and conditions. Having fun often means dealing with Ma Nature!

The getting of the two ends of the car balanced brake wise is important but the final "tuning them" often still needs to be done which is why the adjustable valves are often needed. I remember back in the "old days" when I was playing with street rods I was running 5.00 X 15s bias ply's in front and 8:20 X 15 bias ply's in the rear and things were very good. I had one of the rear tires go bad so I got a new pair, same brand, same size and same everything but when hitting the brakes the new rear tires would lock and slide. Since no other changes were done I assume it was just the rubber mix was different as they came from different build lots of new tires.

Someone lately was talking about right front to left rear hookups and vice versa which I have heard of being done before.

Lee

[PhillipM]

I don't think they are, the brakes all engage together, there's no timing difference bar that caused by friction/seal imbalances, which should be negligible on a decent setup.

Can I ask why would you fit a bias valve to the front and the rear?
You shift bias one way as pedal force increases and then back the other way with even more force?
That makes very little sense unless you have massive downforce imbalance (I.E Shedloads of rear and nothing up front?_ and use the front valve to pop only at pressures where you've got that downforce working?
Even then you'd have to push through the first pressure setup on application, must make it a real handful into a corner?
I know even F1 don't bother with that, they have a straight bias, even with making 2 tonne+ downforce figures.
I'd be amazed if that setup isn't doing some very weird things with your bias that could be far better with a bias box and just a rear prop/load valve.

The rare situations I was talking about - the only ones I've seen with front prop valves - were on some very weird Penske oval racers, where they had a prop valve in just the left front line, with a so a slight brush of the brakes would let the driver scrub a touch of speed, but a heavy stab would bring the nose left, to stop the car drifting up to the wall if they needed to stab the brakes to avoid someone.

Lee - the point of a prop valve is to set it so it takes out some of the variation from conditions - if you've set it right it'll adjust bias depending on how much outright grip you have (because it varies with pedal pressure) - but initial bias always needs to be set with a bias bar or the brake system sizing itself to start with - because a valve has no effect until it pops.
Left front to rear right and vice versa is a normal setup for production cars, has been for decades, because it reduces any pulling effect from brake imbalances from poor maintenance or failure

[Ol'fogasaurus]

Phil, your last point about "left to right" is interesting but I still have had the problem of pulling to one side or the other. I haven't been an auto mechanic for over 50 years now; since I mostly deal with older cars so I guess I haven't done a good job of keeping up. I'll have to do some more research on this. The last time the subject came up I was told the bias was now in the MC to brakes used to be in the distrbution block that the MC connected to; I thought it still was.

If you listen to racers talk you can get the impression that the adjustable valving is still being use but maybe it is with older cars. Most of the guys I use to hang around with are now gone.

Lee

I did a quick check and so far i have not seen any schmantics showing "left front to right rear". I will continue on looking though.

L