Brake Lines and Brake Bias...

[SprintStar]

Hello All,

I've heard often that one can use a brake proportionating valve to adjust the bias between the front and rear brakes.

Is this possible with the diagonal split (X) system of the VW? The front left and rear right belong to one circuit and the front right and rear left belong to another, right?

So, do we need 2 valves?

Also, if it a good idea to re-route the brake lines to a front/rear split? How much will it cost? Is there a kit for it out there?

Many thanks!

Sprint.

[Richie]

Hi,

You're correct about the proportioning valve. But, is the brake-system of a bug x-split? I don't think so. The MC has 3 outlets. 2 are for the front wheels and the other one goes to the rear where it is split for left and right. This way it is split front and rear.
Place your proportioning valve in the line that goes to the rear, so you can change the bias going to the rearwheels.
This is mostly done in racing during wet conditions to prevent rearwheel-lockup.
I wouldn't recommend it for street use unless you can lock your proportioning valve after you found your desired setting after you fitted those huge six-pot calipers front and rear:D .
Not being able to lock your valve can cause front or rearwheel lock-up when somebody changed the setting of the proportioning valve without you knowing it.
Sorry to be so long but I felt it necessary. A proportioning valve is great to have but can be dangerous as well.

Hope to have helped (a little)
Cheers,
Richard

[SprintStar]

Originally posted by Richie
Hi,

You're correct about the proportioning valve. But, is the brake-system of a bug x-split? I don't think so. The MC has 3 outlets. 2 are for the front wheels and the other one goes to the rear where it is split for left and right. This way it is split front and rear.
Place your proportioning valve in the line that goes to the rear, so you can change the bias going to the rearwheels.
This is mostly done in racing during wet conditions to prevent rearwheel-lockup.
I wouldn't recommend it for street use unless you can lock your proportioning valve after you found your desired setting after you fitted those huge six-pot calipers front and rear:D .
Not being able to lock your valve can cause front or rearwheel lock-up when somebody changed the setting of the proportioning valve without you knowing it.
Sorry to be so long but I felt it necessary. A proportioning valve is great to have but can be dangerous as well.

Hope to have helped (a little)
Cheers,
Richard

Hello Richard,

Thanks for replying. I was under the impression that the Bug has a X circuit as I was enquiring about a late 944 master pump and was told that it's set up for front/rear and not X. It is a stepped cylinder, meaning the front and rear chambers are of different sizes. Anywhere we can check and confirm the configuration?

I was also thinking. On my 944 brake setup (I'm currently using the stock Bug master pump), I find the rears a bit lacking. Do you think it's a good diea to put the valve on the front circuit and then I'll be able to "increase" the rear bias by reducing the front? It sounds sensible as the rear calipers are smaller than the fronts.

Of coz if all the calipers are the same size, I gladly accept that the valve should be put on the rear.

Hehe... An extra question : Can I use the bias valve as a line lock? Would sure be cool if I can lock it and do burnouts. :)

Sprint.

[Richie]

Hi Sprint,

If you have a stepped MC, 19mm and 21mm it's the 944 Turbo MC.
You'll probably have a mushy feeling brakepedal and a long travel with the original bug MC.
Install the 944 Turbo MC, shorten the pushrod slightly, around 10mm if I remember correctly, and you won't need a proportioning valve.
The 944 MC has only 2 outlets and the one closest to the mountingflange is for the rear brakes. You'll have to split the line to the front brakes just like it is done in the rear.
No, you can't use this valve as a line-lock because, even when it's "closed", it still has an opening to bleed the pressure. That way you'll still have to depress the brake pedal constantely, unlike the line-lock where you'll depress the pedal activate the line-lock and release the pedal, still keeping the pressure in the calipers.
I hope it makes sense:silly:

Cheers,
Richard

[lightning bug]

If you're not getting enough out of the rears, biasing the front isn't the answer. As said above, it could be dangerous. You're problem is somewhere else. Maybe the bug M/C needs to be upgraded. However I have heard of people successfully using the bug m/c with 944 brakes. I bought the MBT M/C kit for the 944 N/A M/C. The kit comes with a non-adjustable propotioning valve that mounts to the M/C itself. They told me this will be all I need. I'm running 993 brakes by the way. Will it do the trick? We shall see.

[SprintStar]

Originally posted by lightning bug
If you're not getting enough out of the rears, biasing the front isn't the answer. As said above, it could be dangerous. You're problem is somewhere else. Maybe the bug M/C needs to be upgraded. However I have heard of people successfully using the bug m/c with 944 brakes. I bought the MBT M/C kit for the 944 N/A M/C. The kit comes with a non-adjustable propotioning valve that mounts to the M/C itself. They told me this will be all I need. I'm running 993 brakes by the way. Will it do the trick? We shall see.

I'm not trying to be stubborn. Maybe the Bug does need to be upgraded. For now, the 944 system is fine, the fronts bite real fine! But I do wish the rears have a bit more power in them...

The rears are working in the sense as in when I go bleed the brakes, I can see that they do bite and grip, just not as hard as I wish would....

Considering that the stock Bug MC is 19mm all round. And the rears are not biting enough. Does that mean I need to get a MC with a bigger rear circuit? Doesn't make much sense as in I don't think anyone sells a MC with bigger rear circuit, unless circuits are universal (one big, one small but doesn't matter which you use for front or rear)

The way I see it, the bias valve should go on the front as the rear calipers have smaller pistons to begin with and if there's not enough brake force back there, there ain't nothing we can do but look forward.... :D

Sprint.

[Superman]

You don't want to bias the front brakes. By design/physics the front does 85% of the work.

[SprintStar]

Originally posted by Superman
You don't want to bias the front brakes. By design/physics the front does 85% of the work.

That's for front-engined cars. By virtue of design, rear-engined cars can use some brake-bias in the rears to aid the stopping process....

Sprint.

[Superman]

My reference is more to physics, as in the forward motion of the car with the weight being placed on the front brakes when stopping, regardless of where the engine is mounted.

[SprintStar]

Yes, I agree. The laws dictate that wherever the engine is placed, negative acceleration will place more weight on the front axles, hence needing more brake force up front.

And I'm extending this further to say that in rear-engined cars, the weight transfer is not so extreme such that the weight distribution is more like 70/30 vs 85/15, hence the need for stronger brakes in the rears to effectively take advantage of this property.

What brakes are you using now? Same as my 944 setup? How does it feel to you? Overall is good to me, but I find the fronts a bit too strong, or should I say the rears a bit weak? Hence the question of proportionating valves. :)

Sprint.

[Superman]

My car is in thousands of pieces right now so I'm not using brakes :D

[SprintStar]

Hehe... I'm driving a '03 now and have a '02 in a million pieces too. Putting her together soon, hence I'm concerned about the brake question as I'm keen to use the 944 brakes on her too!

Sprint.