Chevy Nova Forum banner

1 - 16 of 16 Posts

·
Premium Member
Joined
·
2,161 Posts
Discussion Starter #1
I'm working on a Nova that was converted to a 5-lug disk/drum combination, with a manual (non-power) disk/drum master cylinder. The car was not equipped with a brake proportioning valve. My understanding is that the PPV controls the rear brakes? In this application, the rear brake line was connected directly to the MC. There were no problems with braking before I began some reconstruction. My question, then, is the BPV necessary?

Thanks,

Ed
 

·
Registered
Joined
·
2,015 Posts
The prop valve also controls the front brakes also. I think your brakes needs it to be setup correctly not that you can't get away with out having one. The proportioning valve reduces the pressure to the rear brakes. Regardless of what type of brakes a car has, the rear brakes require less force than the front brakes.

The amount of brake force that can be applied to a wheel without locking it depends on the amount of weight on the wheel. More weight means more brake force can be applied. If you have ever slammed on your brakes, you know that an abrupt stop makes your car lean forward. The front gets lower and the back gets higher. This is because a lot of weight is transferred to the front of the car when you stop. Also, most cars have more weight over the front wheels to start with because that is where the engine is located.

If equal braking force were applied at all four wheels during a stop, the rear wheels would lock up before the front wheels. The proportioning valve only lets a certain portion of the pressure through to the rear wheels so that the front wheels apply more braking force. If the proportioning valve were set to 70 percent and the brake pressure were 1,000 pounds per square inch (psi) for the front brakes, the rear brakes would get 700 psi.
 

·
Registered
Joined
·
1,048 Posts
I'm by no means an expert on this subject but I believe you can get away with not having a proportioning valve as long as you have some type of in-line valve to adjust bias going to the rear.
 

·
Premium Member
Joined
·
2,497 Posts
I have disc front and drum rears. Just a master cylinder. No prop valve. No booster either. Stops with a little leg pressure but nothing I can't handle.1st gen
 

·
Super Moderator
Joined
·
12,005 Posts
I have a friend that added disc brakes to the front, then 5 years later added disc brakes to the rear. No valving and still running the single master cylinder. I have driven this vehicle and the brakes work amazingly well. While it is not the preferred way, nor a way I would do mine or a customers it does work.
 

·
Premium Member
Joined
·
31,194 Posts
It's usually not noticed until you use the brakes to their full potential. I often see it at the track, in our class, we run stock brakes and by simply changing the brake bias/proportioning, we can manipulate the car from a tight, to a loose on entry condition. Too much rear brake will spin a car under heavy braking, but will not be noticed under typical braking.
 

·
Super Moderator
Joined
·
12,005 Posts
I agree Paul and I did not try to panic stop the truck. BTW it is a 57 Chevy truck with a WS6 Trans Am front clip and a C4 Corvette rear suspension. Besides the 4 wheels discs it is dropped into the weeds and corners like it is on rails. It is so low if not careful the front bumper will hit the curb when parking.
 

·
Registered
Joined
·
2,015 Posts
AFAIK all Gm vehicles with disc/drum and disc/disc were factory equipped with valves.
Yes, prop valves were factory. Try to do a panic stop and slam on the brakes your back brakes will lock up first and could spin you right around.

I'm sure it does work well with no prop valve because your getting full psi to the back brakes which you don't want so but be careful.
 

·
Super Moderator
Joined
·
12,005 Posts
A 4 wheel disc system can be balanced without a combination valve. Caliper piston surface area differences between front and rear as well as the diameter of the rotor and tires can all be used to balance the bias of the system.

This is some information from another site that worked well for me when setting up my braking. The poster was helping another guy sort out a braking problem. The formulas and explanation he posted I found very helpful.
The pedal ratio for power brakes is around 4:1. he refers to manual so if using power brakes the ratio can be substituted.

I'm not talking about balance between m/c and calipers, but between front and rear calipers. This is way more important. Let's go through the math.

Say you applied 100 lb of force at the pedal. The pedal ratio is about 6:1 (manual brakes) which translates to 600 lb of force at the m/c.

Assuming you have a m/c with a 1-1/8" bore, that gives you 0.994 in² of piston area (= pi/4*bore²). So the 600 lb divided by that area gives 603.6 psi of fluid pressure in the brake lines (front & rear).

For the front, the caliper bore is 2-15/16" which gives 6.777 in² of piston area. Multiplying with the fluid pressure, this gives 4091 lb of force pushing the brake pads. The friction coefficient of brake pads is about 0.4 which, when multiply by the normal force, gives 1636 lb of friction force per pad. Multiply by 2 for both pads, you get 3272 lb of friction force for the caliper. The rotor radius is 5.5" and the friction force is applied at the center of the pads, so about 1" from the outer edge of the disc; or a 4.5" radius. So 3272 lb times 4.5" gives 14 724 lb.in of braking torque. The front tire has a radius of 12.7", so the brake torque divided by that radius gives a braking force at the tire contact patch of 1159 lb. That is for one wheel, so for 2 wheels you get 2318 lb of braking force from the front end.

Repeating for the rear, we get these numbers:

caliper bore: 1-11/16" --> piston area: 2.237 in²
normal force: 1350 lb
friction force (2 pads): 1080 lb
friction torque: 4860 lb.in
tire force (2 tires): 748 lb (Since there is obviously a mistake in your dimension, I assume the aspect ratio of the rear tire was 45, giving a tire radius of 13")

So, the total braking force from the front and rear ends is 2318 +748 = 3066 lb.

Front bias is 2318 / 3066 = 75.6%

With your prop valve completely close, according to the spec you provided, there would be a 57% pressure reduction on the rear line. So for 603.6 psi, that is a 344 psi reduction (= 603.6 * 0.57) or an available 259.6 psi for the rear calipers. Redoing all the calculations you get a total braking force of 322 lb for the rear (the front stays the same). Total brake force is 2318 + 322 = 2640 lb and front brake bias becomes 2318 / 2640 = 87.8%.

How much bias do you need? The front bias needed is as followed:

Front bias (%) = Front weight ratio + 100 * CG height / wheelbase * deceleration

The front weight ratio for your car must be close to 52%. CG height is about 20" (Usually the height from the ground to where the camshaft is located is a good indication) and the wheelbase is 115". The deceleration is in g's.

So when you initially start the braking process (deceleration is 0 g), The front bias needed is 52% (= 52 + 100 * 20 / 115 * 0). But as the brake force increases, deceleration raises to about 1 g (maximum braking effort). At this point, you need 52 + 100 * 20 / 115 * 1 = 69%. Unless you have an unusually high CG, with a big block (more weight on the front) and really sticky tires (higher deceleration possible), that should be the numbers you are aiming for (say 60-65% with a prop valve and around 65-70% without one).

Because, depending on how much force you put on the pedal, you should need between 52% and 69% front bias (that's what the prop valve adjusts according to the fluid pressure, hence how hard you brake).

But, because your basic system has already more bias from the beginning, you will never be able to achieve that. The results are a system that brakes (almost) only with the front end.

Even with that said, if you know someone who have the EXACT SAME set-up (including CG height, tire compound and size, etc.) and it works, ignore this, there must be an assumption wrong in my calculations.
In fact, if you assume the same friction coefficient for F & R, that value doesn't matter. I did the calculations the long way to make sure everybody was following but to get the same answer you can use this simpler equation:

Front Bias = 1 / (1 + A_r / A_f * Rd_r / Rd_f * Rt_f / Rt_r)

Where A is the piston area, Rd is the disc radius and Rt is the tire radius. _r and _f means the rear or front value.

Assuming same disc and tire radius for F & R, it simplify even further to:

Front Bias = 1 / (1 + A_r / A_f) or Front Bias = A_f / (A_f + A_r)
 

·
Registered
Joined
·
147 Posts
If you want to know if you need a prop valve don't bother getting up to speed and hitting the brakes. Get off somewhere fast and hit the brakes in a corner.....you'll know soon enough.:no::rolleyes:
That's a good test, for sure, although we don't really recommend it! :D

Here's what Wilwood has to say about Prop-valves. They kinda know what they're talking about. http://www.wilwood.com/TechTip/TechFaqs.aspx

Q: What is a proportioning valve and do I need one?

A:
A proportioning valve is a pressure reduction device. It is typically installed in the rear brake line to reduce braking efficiency and compensate for premature rear-wheel lockup; a result of incorrect front to rear brake bias. An adjustable proportioning valve permits incremental adjustments to fine tune brake bias. This ability to adjust front-rear brake bias is particularly important in race applications, as changing track conditions and vehicle dynamics usually require the brake bias be adjusted throughout the race.

Normally, you do not need to purchase a proportioning valve with a Wilwood four-wheel disc brake kit. Because Wilwood manufactures calipers with the correct piston area for each application, our kits will work with your dual-chamber stock master cylinder and stock pressure limiting valve. There is no need to modify or remove the existing pressure-limiting valve, and no additional proportioning valve is needed. A Wilwood kit will also work with your ABS control systems.

However, if you significantly change your vehicles weight and/or chassis dynamics, such as is common with muscle cars, hot rods, street machines and customs; you will likely need to remove the factory proportioning valve and install an adjustable proportioning valve when installing Wilwood brake kits. The factory valve was designed for a specific weight car, on a specific tire, with a specific suspension system, and a specific amount of brake torque at each wheel. If any of these specifications have been altered, the factory valve will not allow optimum performance of the braking system by either limiting too much pressure, or not limiting the pressure adequately. A Wilwood adjustable proportioning valve will provide easy adjustment to obtain the optimum pressure for your modified vehicle. For further clarification, contact a Wilwood Sales Technician at 805-388-1188 or email Sales/Tech Support.
 

·
Premium Member
Joined
·
2,161 Posts
Discussion Starter #16
Even thought the origination of this thread is three years old, I appreciate the responses.

Carl, the Wilwood reference is great. Thank you.
 
1 - 16 of 16 Posts
Top