Ok, so the s2k does not use the standard proportioning valve. It is not proportioning off the front brakes at all.

The master cylinder actually controls 1 front and 1 rear (FR RL) and (FL RR) with the dual master cylinder instead of both fronts from one part of the master cylinder and both rears from the other.

So the proportioning valve is inline with both rear brake lines. It comes out of the ABS through the valve and then to both rears. So how does it operate? Is it just balancing some how?

For racing...

Is it needed? What happens without it?

 

 

(subscribing)

 

Assuming the stock system is balanced for the tires that come on the car (I think a safe assumption) it would not be a good idea to remove it. Once you get more mechanical grip (better tires, suspension, etc) you will transfer more weight to the front tires during braking. In short you need less rear brake so adding another (adjustable) prop valve inline to further reduce rear brake pressure would be ideal. If your not going to do that at least leave the stock one.

 

The first advice I will give you is leave it alone...but since I know Bill loves to tinker, that explanation will hardly suffice



The proportioning valve on our car is attached to the chassis about 10" south of the left motor mount. It's purpose is to essentially adjust from the master cylinder (post-ABS) the brake pressure bias of the rear brakes relative to the fronts.

If you chose to remove this proportioning valve, the front brake line pressure and rear brake line pressure would be equal or linear relative to the master cylinder. In the absence of a proportioning valve, you would then have to size the rear brake system to adjust the bias forward to avoid oversteer from rear brake lock-up. There are a few ways to increase the front bias by making adjustments to the rear system-by decreasing the rear rotor diameter, decreasing the mu of the rear pads, or decreasing the diameter of the rear caliper’s pistons. You could also make these changes in inverse at the front brakes for the same effect.
Additionally, you can increase front bias by decreasing weight on the front axle, increasing weight on the rear axle, lowering the CG or changing the mu of the tires.

As mentioned previously, the proportioning valve decreases the rear brake line pressure.
However it does not do this in a linear manner. The ‘knee-point’ is where the rear brake line pressure begins to be controlled in relation to the front. The pressure will taper off relative to the front at a proportional ratio beyond this knee-point in a linear fashion at at lower positive slope. On some cars, the knee-point itself is adjustable. On others, the slope of the linear relation of the front to rear proportion is adjustable. If you have a dual master with a balance bar, they are both adjustable.

So in short, there are several ways to actually adjust your brake bias without changing, removing or modifying the proportioning valve. And since there are several variables at play, it becomes a complex game of tinkering since brake feel will change under varying loads and conditions such that changing a single variable may effect the feel in a desired way under one condition and in a completely unpredictable manner under another condition.

-Asura

Bill prays to Race Gods for NO FLAT SPOTS



Adding an additional proportioning valve in series with the factory valve is a really bad idea.

 

Asura,

I don't think there can be a knee. I agree that's the way a normal proportioning valve works, but the s2k's does not balance off the front at all. It is inline on the rear lines only. It could cut pressure a bit like a normal proportioning valve, but I doubt it's doing that either. I suspect it is only balancing the rears in this particular car, but I'm not 100% sure.

My RX7 has the system as you describe it. Looking for an expert on the s2k's system to confirm the operation of the system though.

Bill

 

 

Asura Posted on May 18 2010, 07:03 PM
Is it this thing?

 

yep. Two rear lines in and two rear lines out.

Bill

FYI - I'm transplanting the entire system into an RX7, yea I'm nuts.

 

So how are you planning to do it?