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..

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?
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Bus Motor Build

What’s That Noise?!?

[Danwvw]

The 944 and the 924s are about 50/50 on weight distribution but you're still going to need good front breaking, My 88 seems to lock up the back first.
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1960 Beetle And 1679cc DP W-100 & Dual Zeniths!

[Clatter]

I should add that my Fastback has a Type 4 motor and 915 trans..

So a lot more weight in the rear, unfortunately.

Because it's sitting here as a bare pan, and I'm plumbing it right now,
it would be a non-issue to install.

Check this one out;
they say you are supposed to mount them where they are easily reached by the driver..

http://www.mcgillmotorsport.com/adjustable-brake-proportion-valve-bias-adjuster-lever-type-678
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Bus Motor Build

What’s That Noise?!?

[raygreenwood]

Brake bias and brake proportioning are NOT the same thing.

Brake bias sets up when one set of brakes begins to see pressure and starts braking.

For instance.....if you had a really heavy rear end that had a tendency to push forward during enertial changes on braking...you could set up dual ciruits via two seperate master cylinders operated by one pedal. The "bias bar" accessory is a moveable clamp on the brake MC pushrod that literally starts actjvating....in this instance.....the rear brakes....before the front to give some braking to the reat first and dampen the inertial shift.

Bias valves do the same thing with spring loaded pressure valves. One valve is lower spring pressure so it starts feeding pressure to that circuit first....you coule set it for front, back or diagonal. Its about TIMING.

But....the bias valve does not adjust pressure.....just timing....of which circuit kicks in when.

Brake proportioning.....is not generally a TIMING thing. Its a pressure thing.

Ib brake proportioning.....both circuits start braking at the same time when you push the pedal......but....at a certain pre-set pressure.....one circuit.....usually the rear....stops increasing pressure....and just keeps that pre-set pressure as long as you keep braking.

This does not control enertial shift like brake bias....but it does limit pressure to prevent skidding as the enertial shift happens and pressure/weight comes off the rear wheels.

Take a look at the rear brake proportioning valve for VW 411, 412 and Porsche 914.....same one used on some BMW and Mercedes way back when....to get an understanding of the difference here.....

https://www.thesamba.com/vw/forum/viewtopic.php?t=664753&highlight=proportioning

You can buy valves that do one....the other....or some of both. Summit racing and otyers have literally hundreds for you to peruse.
Ray

[slalombuggy]

I always put them on the front of my buggies. You don't have to mount them where you can reach them but it makes adjusting them much easier than climbing under the car. I just reflare the metric lines to double flares to hook them up.

brad

[Clatter]

Thanks for the detailed explanation, Ray. Much appreciated.

While a balance bar might be right for 'settling' the suspension;
Giving some initial to the rear,
Like they teach you in motorcycle racing.
It's really lock-up that is the primary concern - where things can get hairy.


Brad, I think you hit the nail on the head;
Your real-world experience showed fronts locking too soon, right?

Interesting that ALL set-ups available for sale are for limiting the rear circuit.

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?

Probably up there right by the master..
Plug one side of the master, and use a tee after the valve?
_________________
Bus Motor Build

What’s That Noise?!?

[raygreenwood]

This is a REALLY good topic.

I agree.....too many set ups ASSUME....thst the rears as set up due to weight distribution or disc to drum ratio.....that the fronts will always prevail.

Yes.....your weight,distribution may say.....60/40 or 70/30 front to rear is the way to go....BUT.....

When you start changing things.......for instance.... on my 412.......once you do the Audi front strut modification....even though the front to rear weigjt is still identical......the front is already lower by 1"....AND....has a higher shock compression rate so it does not dive as much during braking.
In a reciprocal manner....weight does not lift from the rear wheels as much. Add to this, thicker rear sway bars and outer links.....and the proportioning valve locking up so early leaves a lot on the table as far as rear braking is concerned.

So.....add in rear discs and better pads along with the gas shock and stiffer bars in the rear....and you can add about 50-60 lbs of braking pressure to the rear end......so the actual brake bias.....really moves from about 65/35 front to rear.....to something close to 50/50.

And on some cars....you might even need to drop the front braking to allow the rear yo do a little more work.....which can reduce lifting and forward enertial shift. Some is timimg...bias.....some is pressure.... which is proportioning.

The only way to really find out is to do some test driving and adjusting. Ray

[jason]

I have 944 brakes on my bus and tried the 944 master. Took it off after first drive. Pedal was super hard. Smaller VW master made pedal softer.

[oprn]

Just think about what you are doing with putting that pressure limiting valve on the front.

As you apply the brakes the pressure front and back is equal - no problem under light braking conditions but... as you increase the peddle pressure the front braking power becomes limited but not the rear. As braking increases the weight shifts to the front and away from the rear but you are limiting the front and increasing the rear braking! Eventually the rear locks up with little weight on the rear tires and the front has most of the weight with little braking power! You have limited how quickly you can stop the car. You can still steer the front but any attempt at changing direction of the car will result in an immediate spin out!

Is that what you want?

Is that safe?

In simple terms the harder you brake and the more desperate the situation becomes the more unstable the car is.

[RailBoy]

To throw a munky wrench into this I had a street rail for years and people spoke about proportioning valves that I may need. Point, also had people say if it is not "stock", the proportioning valve that is after mkt. on a street legal car is illigal for it is not factory.... Yea, this is based on they can "Blow".. RB
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[quote="smitty24"]"HELP, I ported my own heads and now have fire coming out my ass" .[/quote]

[raygreenwood]

[Wreck]

I/ve an ajustable proportioning valve valve on my Baja split bus ,it's plumbed into the front because of the tyre radius difference , even though I have large discs brakes off a 70's era local Aussie ford/gm V8 on the rear and standard bay booster/mastercyl. front discs. Without the valve I just lock the fronts straight away , it's adjusted to still just lock the fronts first but I have considerably better braking because the rears are doing a lot of work now .

On my Ghia I've twin master cylinder set up with an adjustable bias bar . standard '69 front discs with empi style rear discs . I run a 5/8 m/cylinder on the front and 11/16 on the rear circuit . the bias bar gives a more precise adjustment in my experience , (it's what the local Formula Vee open wheeler race cars use)

[oprn]