4x4 + 15/16 master, not so impressed, solutions?

[EMWHYR0HEN]

So I guess it wouldnt be worth it to change over to a vented rotor, maybe i'll just rig up some vents for my track days and consider some willwoods in the future.

 

Jon, I remember reading a post where you mentioned going with the wilwoods, but peicing your own kit instead of the AZC. Mind sharing your setup?

[JMortensen]

I bought the JSK Innovations setup, which is NLA, front and rear. There is some question as to whether the rear will actually work or not. I guess I'll figure that out when I get there...

[z-ya]

On my turbo car I have the JSK front hats and caliper brackets. On the rear I have the 240SX kit from Modern Motorsports. I use the 15/16" MC on that car and the brake feel and performance is very good. I couldn't get the bias even close by just replacing the rear prop valve with and adjustable one. I ended up putting the adjustable prop valve in the front circuit, and a piece of solid line in place of the stock prop valve in the rear. I mounted the Wilwood prop valve between the MC and my MC heat shield.

 

For pads, I have the Hawk HP Plus, and have absolutely zero fade at a very brake chalenging track (NHIS). The pads are a little too aggressive for the street. I switch them to some Wilwood street pads for long trips. I have the KVR pads in the rear.

 

With this setup, you can use any DOT4 or better fluid. I've been using the Motul stuff in there for a few years now without any issues.

[Zmanco]

I use the 15/16" MC on that car and the brake feel and performance is very good. I couldn't get the bias even close by just replacing the rear prop valve with and adjustable one. I ended up putting the adjustable prop valve in the front circuit, and a piece of solid line in place of the stock prop valve in the rear.

I have a similar situation. I had thought about swapping the prop valve into the front circuit, but I remember a thread not too long ago that said this was dangerous, at least for the street. I believe the thinking was in a panic stop, you would lock the rears before the fronts, while at anything less than that, you would still have too much front bias.

 

Have you had any issues like that? And how much of an improvement in stopping was there compared to before the swap?

[JMortensen]

It is a bad idea. Here's info from Stop Tech:

2. If you have the deeply-rooted need to install your own adjustable proportioning valve, be advised that they should NEVER be installed in-line to the front brakes. The effect would be to make your vehicle rear-biased before you could say “terminal oversteer.” Front brake line pressure should always be left alone – only the rear pressures should be considered for proportioning.

http://www.stoptech.com/tech_info/wp_proportioning_valves.shtml

 

If your bias is that far off, the right answer is dual master cylinders in my opinion. Even if the result in Pete's case wasn't an uncontrollable car, the fact that the prop valve has a "knee" in its pressure gradient means that he could probably get a better bias all the way through the range were the brakes such that the prop valve was in line to the rear brakes and the bias were correct.

[z-ya]

Now that I have the prop valve in the front, I can get the proper front to rear bias. I understand if you just slap a fixed prop valve in the front, you may have too mauch bias towards the rear, which is not good in an emergency stop. I guess I don't understand why it is dangerous if you maintain the proper front to rear bias.

[Zmanco]

I took this from the Stoptech article Jon referenced earlier. This is a typical curve for a prop valve, with the knee of the curve set for~600 psi in this example. As I understand it, when you turn the knob on the prop valve, you are changing the point at which the knee occurs, but you do not change the slope of the curve after it - that is preset.

 

If the pressure to cause lockup was always the same, regardless of temperature in the brakes, speed at any given second, etc, then I could imagine making this work by adjusting the prop valve so that the fronts would lock up just after the rears. This would be a safe system, although maybe not the best performing since in this example, we still wouldn't have the optimum rear brake pressure except at the point of lock up.

 

BUT here's what I think the issue is in using a prop valve in the front: I very much doubt that the lockup pressure is always the same. I'm sure it moves around a lot based on the variables I mentioned, along with tire temp, stickiness of the track, etc. I can easily imagine there would be conditions where the "stick" of the tires was better than usual, and under these conditions it would take a higher pressure to lock the brakes. But since at this point you are on the right side of the knee on the curve, you are going to build rear pressure FASTER than the front, which means you are likely to lock your rears first under these conditions. I think that's the point Stoptech is making in their writeup.

 

Having said all that, I am tempted to try an experiment of moving the prop valve into the front circuit. If I leave it all the way open, then I should have the same situation I have today where my fronts always lock first. I wonder if turning it down JUST A LITTLE might give me some improvement but still leave enough margin that the fronts always lock first? My guess is that it might help a litte. Plus, if the rears lock first, how is that different from a traditional system with an adjustable prop valve that is just adjusted with too much bias to the rear?

 

Pete, you are the only person I've ever seen try this and am encouraged by your positive review, although still wary based on what I understand (and wrote) above. I feel conflicted so I'm going to think on this one for a while more.

[JMortensen]

Plus, if the rears lock first, how is that different from a traditional system with an adjustable prop valve that is just adjusted with too much bias to the rear?

Because on a standard system with the prop valve in the rear, the harder you step on the brakes the less work the rears do compared to the fronts. With the prop valve in the front, the harder you step on the brakes the less work they do compared to the rears. So you can tune Pete's setup to be right on at one braking level, but the harder you step on the brakes the worse it is going to get. Imagine the black line is the rear brakes and the red is the fronts. Pretty easy to see the problem now, eh?

 

 

The harder you step on the brakes the less braking you want in the rear. This is due to weight transfer. In the front you want the opposite. You have more traction in the front when the weight transfers so you can put a LOT more into the brakes without locking up a wheel. This is why you really ought to use dual masters to fix the problem. You can keep the fronts making lots of pressure all the way through, and have the rears making proportionally less.

 

The typical dual master kind of ignores the weight transfer issue though. You could even go a little small on the rear master to put "too much" pressure to the rears, and then put a traditional prop valve in the rear to give it a little knee. This would give you as close to optimal braking at lower speeds where there is not as much weight transfer, and then taper it off at higher speeds.

[z-ya]

So in essence you are saying that input pressure vs output pressure on a prop valve is non linear. It is linear up to a point, but as the input pressure increases, the output pressure does not increase at a fixed ratio to the input pressure.

 

Now I've had my car on the track many times with this setup, and so far the braking is very predictable. I've probably done at 14 or so 30 minute sessions withthe brakes the way they are. It is my street legal race car, so I only use it if my track only car is broken.

 

I'm in the planning stages of completely re-doing this car, so I may go with a Wilwood caliper in the rear with more fluid volume than the 240SX caliper. This will give me a coser to stock front to rear bias, allowing me to run the prop valve in the rear.

[JMortensen]

Exactly. What you have may be working, but even if it is working it's not optimized for any pedal pressure. At best its optimized at one point along the graph there. Did you see my post on dual master setups? Lunar240z is finishing up modifying a pedal box. Looks pretty easy to do, so that is the route I'm going.

http://forums.hybridz.org/showthread.php?t=115678