brake bias questions

[dusterbd13]

front calipers 1.77 bore size
rear calipers: 1.687
master cyl. bore: 15/16

front rotor diameter: 13 inches
rear rotor diameter: 11.1 inches

front tire height: 25 inches
Rear tire height: 27 inches

so, now that we have all the specs in play, can you help me figure out if this is a good idea? seems like ill have about neutral bias without the bias adjuster in play. is this correct? let me know what you think. im hoping to get a great braking, predicable car out of this swap.

Michael

[absintheisfun]

I am no expert, but I think that you always want less pressure on rear disc brakes--regardless of set up. The way it was explained to me, when you brake soft, the prop valve doesn't really do anything, but when you brake hard, it limits the pressure to the rear. The reason for this is that when you brake hard, the massive forward shift of weight lightens the rear and makes it very easy to lock them up.

As far as setting it up, I think that the best way is road testing it to make sure that your rear brakes don't lock up before your fronts.

[kryptik]

Road test IN A SAFE AREA by trying to get your brakes to lock up. If your rears have too much bias they will lock up first, making the rear end loose and you will feel it want to come around the front end (you may even induce a spin so be careful and make sure you are in a safe area once again). If you have not done so already, install a proportioning valve in the REAR LINE ONLY and use this to adjust rear braking power (front brakes should always have 100% available power). The last thing you want is your rears locking up before your fronts.

And yes absinthisfun rear pressure will almost always be less than front pressure.

[JRouche]

Im kinda dealing with this right now. New wilwood brakes on the front, gm discs on the back. Wilwood proportioning valve to adjust the rear brakes.

Initially I had the valve all the way open after bleeding the brakes. And for bedding in the new front brakes I closed it so all the braking would be done by the fronts. And she stops on a dime, with no front end dive.

So finally thought about including the rear brakes into the mix. The valve has many turns of the knob from full closed to fully open. I counted the turns and figured out where a 1/4%, 1/2% and full fluid flow was. I started with 1/4% open and took it on a deserted road and tried to lock up the rear. Didnt happen, so I opened it up more towards 50%, still no rear drag. Then before I could continue I got sidelined with some engine issues. Still working out the EFI. So the rear brakes are still at 50% right now.

I will get out and increase the pressure to the rear when I get a chance. Bad weather is coming, a lil light rain in So Cal.

But I will continue to open the valve till I get some dragging on the rear brakes then dial it down abit so the fronts are still the major braking force.

Its kinda hard cause the front brakes stop so well that I slow down before I can get the rears to lock up. But they should, I still have plenty of valve left, and I know the rear brakes are working. And even if I cant get them to lock up with a wide open valve that might not be such a bad thing. The only reason I want to get them to lock up is so I can see where the lock up point (most braking) for the rears is and then dial it down some so they are working to highest possible point, just short of lock up. JR

[Bryce]

If you have the wilwood prop valve full closed is really 57% open to the back.

Wilwood suggest starting with full brake to the back and dialling it down to prevent rear locking up first. My prop valve is cockpit adjustable.

[Apogee]

front calipers 1.77 bore size
rear calipers: 1.687
...

Are we to assume that you're running single-45mm [1.77"] piston front caliper? That's tiny if that is the case. If it's twin-45mm piston, that would be much more ideal IMHO. Without axle weights, center of gravity height, pad CoF, etc, we can't really calculate what you're asking, but based on the info you've provided (assuming single-45mm piston calipers), I would say you're extremely under-braked up front. If twin-45mm piston, you're at least in the ballpark depending on the rest of your car's specs.

Tobin
KORE3

[dusterbd13]

actually, i got the specs for my calipers off of rockauto. what they are are the fronts from a 2001 LS1 camaro (dual piston), mathed to 13 inch rotors from a cobra. the rears are a complete setup from an 89 300zx turbo car. 11.1 rotors.

i will be using PFC Z-compund pads front and rear, with 255 section width 240 treadwear rating tires.

i have no idea front to rear weight bias. and only a wild assumption about overall weight.

im just worried about not having enough brakes overall, or having too much at one end or the other. IE not being able to prevent rear wheel lockup, or not being albe to lockup the rears at all.
hope that helps some more.

michael

[Apogee]

You should be fine with respect to front to rear brake bias if you have an adjustable proportioning valve. Based on the components alone, you should have a slightly more front biased setup than a factory 4th-gen F-body due to the slightly smaller rear piston area and effective rotor radius. In addition to the APV, you can always choose to vary the pad compounds front to rear if you need to in order to get things balanced the way you like to get the most out of your brakes and tires.

You didn't specify whether you were planning on running manual or power brakes, and if power, the size booster you plan to run and the vacuum levels being supplied to it. That said, a 15/16" bore MC is probably not a bad choice for a power brake application using the LS1 F-body front calipers considering the F-body application used a 1" bore mated to a dual-9" booster. You didn't specify the application, although if it's the Duster, those came with dual-7.5" boosters IIRC which would provide ~44% less boost. The smaller bore MC should offset about 12% of that difference. I don't know the pedal ratios off the top of my head, so I can't compare the GM F-body with the Plymouth in that regard.

Tobin
KORE3

[dusterbd13]

yes, itll be on the duster. im only supplying about 15 in of vaccuum to the booster, and its a stock booster for a 74 duster, wigh is about 10 inches in diameter. i dont know if its dual, single, or what.

master is a brandy new 1 1/16. the pedal is still a little vague and low, but there is absolutely no air in the lines. ive pressure bled, vacuum bled, and foot bled the things. pulled about 5 quarts through it so far, so im fairly certain. ater the power brake swap, the car stops better than ever before, but i still cant lock up any one wheel on dry pavement, hence the reason for bigger brakes. my feeling is that there is no such thing as too much brakes, just not enough tire. and right now i have plenty of tire and grip.

thanks for all the help so far, tobin. any more info i can give you so i can science this thing out, let me know. im hoping to get this thing to gove you whiplash wn you hit the brakes.

[Apogee]

With the master/booster (1-1/16 & single-9.5") combination you're running, you should have a relatively high, firm pedal and probably somewhat high effort in order to achieve lockup. Lockup may not be realistically achievable depending on your pads. Did you bench bleed your master cylinder and confirm that it was fully bleed? Have you tested the booster to make sure it's functioning like it should? Your vacuum levels are really pretty good for this crowd.

Tobin
KORE3