C4/Blazer Brakes on A-Body

[cdrod]

I've been crunching numbers to assemble a 4-wheel disc brake set-up for my 72 442 convertible project. It will be a daily driver, no track time, but I want to bring the brakes up to date. My plan is to put 12.1" C4 rotors and calipers on the front with 11.6" Blazer set-up on the rear. I'm using the factory disc/drum set-up as my benchmark to predict the improvement with the new parts. The attached pdf shows my efforts at number crunching. Can anyone check my computations, not sure I'm calculating the brake torque correctly. Here are my questions:



1. How much brake torque do I need for a street driven car running a vacuum booster?
2. The brake bias with C4 calipers up front and Blazer calipers on the rear will require a 44% reduction to the rear to get a 70/30 ratio. Is this OK?
3. This will require a dual diaphragm booster and the pedal ratio at 6.8:1 (using the manual hole on the pedal). Is this OK?

I know the C5 calipers have larger diameter pistons, but I'm afraid the rotors won't clear my rims, so I'm working with the C4 rotors.

Thanks!

Rodney

[79-TA]

I've been crunching numbers to assemble a 4-wheel disc brake set-up for my 72 442 convertible project. It will be a daily driver, no track time, but I want to bring the brakes up to date. My plan is to put 12.1" C4 rotors and calipers on the front with 11.6" Blazer set-up on the rear. I'm using the factory disc/drum set-up as my benchmark to predict the improvement with the new parts. The attached pdf shows my efforts at number crunching. Can anyone check my computations, not sure I'm calculating the brake torque correctly. Here are my questions:

1. How much brake torque do I need for a street driven car running a vacuum booster?
2. The brake bias with C4 calipers up front and Blazer calipers on the rear will require a 44% reduction to the rear to get a 70/30 ratio. Is this OK?
3. This will require a dual diaphragm booster and the pedal ratio at 6.8:1 (using the manual hole on the pedal). Is this OK?

I know the C5 calipers have larger diameter pistons, but I'm afraid the rotors won't clear my rims, so I'm working with the C4 rotors.

Thanks!

Rodney

Hello, that's a nice spreadsheet. I really appreciate how easy it is to reference the inputs and formulas.

1. Brake torque is the end goal. You need a certain minimum amount whether you have a manual or assisted system. Did you mean line pressure or pedal effort? Just looking at the value of 8888, it's not nearly enough, but more on that in a bit.

2. It's ideal to have the bias balanced hydraulically and then make small adjustments with the pressure limiting device in the rear. It sounds like your system has roughly a 60/40 split without limiting pressure to the rear. Starting closer to 65/35 would help. Aftermarket pressure limiting valves (aka knee point adjusters) aren't amazing, so the less your system needs to rely on it for balance, the better.

3. The weight of the vehicle needs to be considered. Right now, your system on the calculator looks to have enough braking force.

Your spreadsheet has an error for calculating line pressure. The output force of the booster should also be affected by the hydraulic force multiplication of the master cylinder. The booster just transmits the pedal force while adding additional force into the master cylinder push rod. The pedal force and booster force should be added together and that sum should be divided by the master cylinder bore size.

The spreadsheet is missing one last and often overlooked ingredient. Tire diameters. The diameter of the tire translates the brake torque to linear braking force that actually slows the car. If the front and rear tires are different sizes, that needs to be considered for the brake bias. Note that the calculator has an asterisk pointing out that the bias numbers are based on torque. 24.4 inches is the diameter of a common 245/45/17 tire.

The total amount of force or gain that a brake system needs can be correlated with the car's mass and expected rate of deceleration. On a level surface where the normal force of the tires is the same as the car's weight, the deceleration possible (expressed in G's) is equal to the unitless coefficient of friction between the tire and the ground. For high performance street tires, about 1 g is possible. The total braking force then needs to be equal to the car's weight in order to be able to lock up the tires. There should be more braking torque/force available for a factor of safety and in situations where there might be more normal force or grip for whatever reason. Or just for reducing pedal effort.


A large variable in calculating brake force is the coefficient of friction of the pad and rotor. Because of this, benchmarking the stock system (hopefully with a similar pad compound) is very useful. It looks like your calculations took this into account, good job!

So, make the line pressure correction and add in the tire radius step to get to brake force so you can better evaluate different options. Remember, dividing the brake torques on that spreadsheet by the tire radius will give the braking force for a single wheel/tire, assuming it has traction.


If you want to be more thorough, you can weigh your car on scales to find the actual weight balance. Then weigh it again with the car at an angle. You can then calculate the height of the center of gravity. Knowing the cg height, wheel base, and expected maximum deceleration, you can calculate how much weight your car will transfer to the front wheels under hard braking. The determined value can then be used to define the desired brake force outputs at the front and rear wheels.

[cdrod]

Brett:

Wow! Thanks for the quick reply and very thorough assessment of my spreadsheet. I was pretty confident about my most of my calculations up until the caliper force, brake torque numbers; I just didn't have a frame of reference of what "real world" results should be. So I've made the corrections you so aptly suggested, but I still have questions about my calculations. A revised copy is attached.

1. The hydraulic bias (before proportioning valves) is 58%-32% front to rear, however to achieve a 70%-30% bias at the wheel, I would need to reduce the pressure to the rear by 36%. From my research it seems most proportioning valves can reduce the pressure up to about 50% so the proportioning valve would be operating near the limits of it's design. You mentioned that knee adjusters aren't amazing. Do you think this would be a problem?

2. When calculating the clamping force at the caliper, should I divide the line pressure by 2 (split between the LH and RH calipers) or do both caliper receive the full line pressure?

3. Without measuring the CG, and assuming 1g tire traction, should the braking forces at each wheel add up to the approximate weight of the car?

Thanks for your help and critique of my work. I plan to post this up on the site for others to use once I know the calculations are correct.

Rodney

[79-TA]

Brett:

Wow! Thanks for the quick reply and very thorough assessment of my spreadsheet. I was pretty confident about my most of my calculations up until the caliper force, brake torque numbers; I just didn't have a frame of reference of what "real world" results should be. So I've made the corrections you so aptly suggested, but I still have questions about my calculations. A revised copy is attached.

1. The hydraulic bias (before proportioning valves) is 58%-32% front to rear, however to achieve a 70%-30% bias at the wheel, I would need to reduce the pressure to the rear by 36%. From my research it seems most proportioning valves can reduce the pressure up to about 50% so the proportioning valve would be operating near the limits of it's design. You mentioned that knee adjusters aren't amazing. Do you think this would be a problem?

2. When calculating the clamping force at the caliper, should I divide the line pressure by 2 (split between the LH and RH calipers) or do both caliper receive the full line pressure?

3. Without measuring the CG, and assuming 1g tire traction, should the braking forces at each wheel add up to the approximate weight of the car?

Thanks for your help and critique of my work. I plan to post this up on the site for others to use once I know the calculations are correct.

Rodney

I'll answer more thoroughly later, but here are some quick answers.

1. I don't know the weight distribution of your car, but it seems like too much rear bias. I would try to reduce the rear brake torque or increase the front and aim for at least 65:35 before limiting rear pressure. The system would work, but you won't want to risk not being able to limit the rear pressure enough. And in case anyone is wondering, multiple pressure limiting valves inline is not a safe option.

2. For calculations, all calipers and all pistons (ignoring the effects of proportioning etc for now) receive full line pressure. A similar question people ask is if they should treat an opposed piston caliper differently than a sliding caliper. In all cases, you use the area of the pistons on one side of the caliper.

3. Yes, and more. An equal amount will mean you can stop the car at 1 g. There should be a safety buffer, so the total amount of braking force should be more. If it does not add up, you will have to apply more force to the pedal, which isn't comfortable, fun, or safe. For reference, OE's comply to FMVSS 135 which mandates that tire lockup and threshold braking be possible with less than 112 pounds of pedal force.

The caveat here is that this assumes that all tires have traction and are not sliding. Adding up the values without considering the weight transfer of the vehicle can create deceptive results. For an extreme example of bad brake bias, if each of your corner groups were capable of applying 1000 pounds of braking force to the road, much of the rear might be wasted with early lockup while the fronts still might not be able to achieve threshold braking. That's why it's nice to know weight transfer. You can better understand how much normal force each tire will have to work with.

Answer 3 is assuming you're using appropriate units (ie the pound-mass and pound-force are very convenient to use while on earth. The correlation between kilograms and newtons isn't seen quite as easily.)