Frequent Asked Question

By Dean Oshiro

There is no way we can cover all the important material on brakes and chassis in a short article. So I thought I would summarize this information by answering some of the "most frequent asked questions" we receive by email.  For a details of the material in this article please go to "The Largest Hot Rod Site on the Internet", www.hotrodsusa.com. If you have any questions please feel free to contact me at moonmr@hotrodsusa.com or phone 888-875-MOON.

Question: I have a fairly large cam and no brakes, I saw a product on shade tree mechanic about two summers ago about a electric vacuum pump. Will the electric pump work?

Answer: Having a large camshaft is common in hotrods. Most of the power boosters require 16-18 inches of vacuum to operate. The electric vacuum pump and vacuum canister cost between $350 and $450 depending on where you buy it. I feel this is like putting a Band-Aid on your problem. Your problem is you have no vacuum. What are you going to do when your motor fails or your battery is dead? We install dual master cylinders with a 3/4" master cylinder that has an output of over 1,300 p.s.i. This is the same technology that has been used for over 30 years in racing applications. No power booster is required, no proportional valve and no residual valves on disc brake applications. Also allows for additional room for your brake system.

Question: I have a hard pedal and even with a booster it takes a lot of pressure to apply the brakes.

Answer: It is one of three things: (1) Incorrect pedal ratio is the number one cause of hard pedal, (2) wrong master cylinder size; or (3) no vacuum (see above answer).

Formula for Pedal Ratio:

Pedal ratio is the ratio of leverage you brake pedal applies to the master cylinder. To determine the pedal ratio you need to measure the height of the pedal to the pivot point then divided the measurement of the pivot point to the lower arm that controls your rod to the master cylinder.

A = height of pedal
B = center to center measurement of the lower arm
C = pedal ratio
A divided by B equals C
Or example 9" divided by 1.5" equal 6 to 1 ratio.

If you apply 100 pounds of pressure to the brake pedal, 100 pounds X (6 to 1) = 600 pounds of pressure. So, if the brake pedal has been modified from its' original design the pedal ratio is effected drastically. You can now see the pedal ratio is a "multiplier" of the pressure you apply with your foot, because this is the leverage that is applied to the master cylinder.

Now, take this same formula and substitute 2" instead of 1.5" you end up with a 4.5 to 1 ratio. Multiply 4.5 times the 100 pounds of applied pressure and you get 450 pounds instead of 600 pound. That half-inch cost you 25 percent of your braking power. The same thing applies when you shorten the upper measurement.

Formula for Master Cylinder Pressure

I have been asked hundreds of times how do you determine the pressure output of the master cylinder. The following information will help you determine the proper size master cylinder:

To figure how much pressure your master cylinder is putting out:
C = pedal ratio
D = pounds of pressure apply by your foot
E = area of you master cylinder
F = pounds of pressure out of the master cylinder
C X D /(divided by) E = F

Example: If you have a 1" master cylinder the area equals 1/2" x 1/2" x 3.14 = 0.785 Square Inches. So, 100 pounds (of applied foot pressure) X 6 (pedal ratio) divided by 0.785 = 764 pounds of pressure.
If you have a 1-1/8" master cylinder, 100 psi X 6 (pedal ratio) divided by 0.9935 = 604 pounds of pressure.

Here is some info on master cylinder with "constant" of 6 to 1 pedal ratio and 100 psi being applied.
3/4" master cylinder = 1359 psi
7/8" master cylinder = 998 psi
15/16" master cylinder = 870 psi
1" master cylinder = 764 psi
1-1/8" master cylinder = 603 psi

DO NOT try to use a OEM master cylinder smaller than 1" without figuring out the volume requirement. It is like choosing between jump off a cliff or a plane, how do you want to die? Remember you can not do anything after you run out of brake fluid, but you can still press on the brake pedal harder.

Question: My 58 Vette has been completely rebuilt including the braking system. On hot days when I start out I might have3-4 inches of slack on my brake pedal until pressure becomes apparent. As the car and the day heat up I find less slack in my pedal until finally there is none and the passenger rear will start to drag.

Answer: You probably did change your master cylinder when you converted to brakes (if you have disc brakes), or you have a defective built in residual valve in the drum brake master cylinder if you still have your drum brakes. When you reapply the brake the brake fluid will not flow back into the master cylinder.

Question: Should I use Silicone brake fluid in my car?

Answer: Silicone brake fluid has a higher boiling point (around 700 degrees F.) than the ethylene glycol base fluids, but the major disadvantages is not "hygroscopic". Hygroscopic? "Altered by the absorption of moisture" What this means is since it is not a glycol based, when moisture enters the system it is not absorbed by the fluid. This results in beads of moisture moving through the brake line, collecting in the calipers. Since it is not uncommon to have temperatures in excess of 212 degrees F., this collection of moisture will boil causing steam and vapor lock, this in turn will cause system failure. Silicone (DOT 5) is also highly compressible due to aeration and foaming under normal braking conditions. If you are changing from a glycol base fluid to silicone the two types do not mix and your system should be completely purged, disassembled and dried out. When the two fluids are mixed you will get a gummy substance and it will really mess up your system. We recommend using a good DOT 3 fluid. Wilwood makes a hi-temp fluid with a minimum dry-boiling point of 570 degrees F. Dry-boiling point is measured in its virgin non-contaminated state. Wet-boiling point is the temperature a brake fluid will boil after it is fully saturated with moisture. DOT 3 fluids have a minimum wet boiling point of 284 degrees F.

Question: How do you determine pinion angle?

Answer: There has been numerous "hear says" about what the pinion angle should or should not be. Hopefully this will clear up a lot of misconceptions. One of the biggest misconceptions is the engine/pinion angle is relative to the frame. Engine/pinion angle and ride height is all relative to the ground. Most engines are set at 3 degrees, when you look at the side of the intake manifold on a carburetor engine you will notice the manifold has a wedge in it. This wedge is to allow the carburetor to sit level. The main purpose of having an engine angle is to allow for more room in the engine compartment. That is why the car manufacturers build front engine cars with the engine at an angle. Drag race cars have a 2 to 3-degree negative engine angle tipping the engine down in front. Having this negative angle allows for better weight transfer to the rear of the chassis. You can set up the engine angle at almost any degree just as long as the transmission is perfectly inline with the pinion yoke.

Question: I am setting up my suspension what is the most important thing I need to know?

Answer: Determine the ride height first. Ride Height is the height of your car when it is sitting and the suspension is at the optimum location. I can not stress how important setting ride height is on a chassis. You first need to pick out what tires you will be using in the front and in the rear. The diameter of the tire determines the ground clearance, frame angle, engine angle, shock length and correct angle to weld the front suspension. After establishing the diameter of the tires, find the front spindle location based on the manufacturer's specifications. The car manufacturers all have specifications. If you plan to install a front end kit the manufacturer should also be able to provide you with this important information. Take the radius of the tire diameter and use these measurements to set the distance from the ground. In the front measure from the center of the spindle and in the rear measure from the center of the axle. You now have "ride height" and everything should be relative to the ground. Of course the ground or suspension jig should be perfectly level, as all your measurements will be made from this origin. No chassis builder can build a chassis with optimum suspension without knowing proper "ride height".

Copyright reserved by Dean Oshiro. Reproduction without written approval is a violation of Copyright Laws. February 2001.