Help me build a manual brake system from scratch....

[FLYNAVY53]

Planning to completely gut the factory unboosted drum brake system in my 1964 Corvette. Below is my initial outline for a plan, but I welcome any and all input.

Car: 1964 Corvette Coupe - 3100ish lbs.
Use: 85% street, 15% casual autoX (I'm terrible but I have fun)
Suspension: going to Ride Tech Stg 2 - ridetech spindles in the front, ridetech offset trailing arms in the rear
Tires: 245/45/17 R compound mounted on 17x8 wheels on all 4 corners
Stock pedal ratio: 4.54:1

Lines: NiCop 3/16 and Wilwood braided stainless at the calipers
Fittings: -3/ 37 degree flare
Master Cylinder: Wilwood tandem with adj. prop. valve. Either 7/8" bore or 15/16" bore. Recommendations welcome!!
Front Calipers: Wilwood Superlite 6R - 4.04 piston area, 8.2 pad area
Front Rotors: Wilwood 12.88 inch slotted
Rear Calipers: Wilwood Superlite 4R - 1.98 piston area, 8.2 pad area
Rear Rotors: Wilwood 12.88 inch slotted

Any and all input is greatly appreciated!!

[andrewb70]

With that pedal ratio, I would go with a 7/8" bore MC...

I am also confused by this:

"Fittings: -3 with 37 degree double flare"

Andrew

[parsonsj]

Stock pedal ratio: 4.54:1

My take is that unless you make your pedal ratio at least 6.5 to 1, you are going to be unhappy with your brakes, no matter what else you do.

[Peter Mc Mahon]

The fittings are just the through frame stainless ones? Other fittings will be standard 45 degree double flare?

[FLYNAVY53]

With that pedal ratio, I would go with a 7/8" bore MC...

I am also confused by this:

"Fittings: -3 with 37 degree double flare"

Andrew

Sorry....not thinking while typing....-3 fittings with 37 degree flares

[FLYNAVY53]

My take is that unless you make your pedal ratio at least 6.5 to 1, you are going to be unhappy with your brakes, no matter what else you do.

Why is that? I realize that with a manual brake set up, you generally look for a pedal ratio between 5 and 7 to 1...that being said, the factory pedal ratio is as stated above, with the factory manual drum brakes. Pedal feel is excellent, and stopping is adequate (for drums). I'm rebuilding the system with discs in order to:

1. facilitate the suspension upgrades
2. make it a dual circuit system
3. reduce amount of maintenance associated with keeping the drums set up properly

Is there something I'm missing as far as pedal ratio and how it relates to a drum set up vice a disc set up?

Again, the input is greatly appreciated!

[parsonsj]

The question is if you can generate enough brake pressure to effectively stop the car. We'll have to get our math out to see...

A good rule of thumb is that you need to generate 800 psi at the caliper to provide effective brakes. A 6.5 pedal can generate about 45% more force than a 4.5 pedal (it's not free -- it also implies more pedal movement). But more pedal movement also allows modulation and control. That's why most manual brakes setups are in the 6 to 7 range.

Next up: how to calculate the hydraulic pressures given the sizes of the master cylinder and brake pistons.

Edit: 1200 is decidedly high end, so I pushed the designed caliper pressures to 800 psi.

[parsonsj]

Let's use 80 lbs as the force most people can exert on a brake pedal. Multiplying the 80 * 6.5 = 520 lbs. To get to 800 psi with a 6.5 pedal, you will need a master cylinder area of 520 / 800 = .65 sq. inches. That's a master cylinder bore of about 7/8".

A quick word on boosters... they provide a pedal force multiplier of 2 to 4 times depending on a lot of variables. You can see why power brakes are pretty much standard on modern cars.

Anyway, so what would it take to get to 600 psi? (my C6Z06 would provide about 650 psi before the ABS would kick in and modulate it on the front calipers). Designing for 600 nets us .867 master cylinder piston area, or a bore of about 1 1/16". 6.5 pedals work pretty well.

Let's see what kind of pressure we can generate with a 4.54 pedal and 7/8" bore: 80 * 4.54 / (.875/2 squared * 3.14) = 605 psi. That design might just work, though I'd be ready to try a 13/16" master cylinder as well. That would get you to 686 psi.

So -- you can get there with the smallest bore master cylinder you can find and that pedal ratio (which is surprising to me, I must admit!).

[dontlifttoshift]

Is there something I'm missing as far as pedal ratio and how it relates to a drum set up vice a disc set up?

Drums are self energizing so braking gets you more braking. Discs do not work that way.

Parson's math is right on the money. I don't have a C2 pedal to measure or assess the feasibility but can you cheat the pedal ratio to at least 5:1 or more?

[FLYNAVY53]

All good information and I greatly appreciate the time and effort to lay the math out for me....quite interesting actually.

Ill have to take a good look at the factory pedal and see if there is a good way to up the ratio without having to relocate the master cylinder placement on the firewall.

[FLYNAVY53]

The following quote is from the 1965 SAE paper on the new Corvette disk brake system presented to the SAE Congress in Detroit, January 1965:

"The 1964 pedal geometry is used to retain the sports car feature of a relatively low pedal height. This established a 1.1-inch master cylinder stroke for 4.98 inches of pedal travel. The resulting 4.54:1 mechanical ratio imposed a high hydraulic ratio requirement for successful manual operation."

[dontlifttoshift]

Quick goggle search shows stock 65 piston area for the front is 5.52" against the Wilwood FNSL 4.03.

The Wilwood D8-6 caliper would be a better choice than the proposed FNSL as it nearly matches the factory piston area.

For an Autox street car, I wouldn't worry about the larger diameter rotors, it won't make much difference.

[FLYNAVY53]

The only issue is that those front calipers wont work with the Ride Tech spindles. Ill need a caliper compatible with A or F body spindles as those are essentially what Ride Tech uses in their kit.

[FLYNAVY53]

Perhaps the Dynapro 6...#140-10510....that kit lists a piston area of 5.06 and pad area of 7.3. 12.19 inch rotors....bonus....its cheaper

[FLYNAVY53]

........

[dontlifttoshift]

Not for a street car, for your car and it's short pedal ratio.

The DP6 isn't a great caliper but that would probably be a better solution.

Wilwood has a FNSL 120-13228 that has 5.18 piston area. There is a Sutton thread somewhere here with all the math to convert line pressure to clamping force along with some general guidelines for manual brakes.

[FLYNAVY53]

Well, I spent my rainy Tuesday doing my homework on manual brakes since the weather was too crappy to be flying around...at least with students.

Anyway, I read through all 11 pages of the Ron Sutton sticky at the top of the forum.....excellent information, and makes everything significantly clearer. With all of that being said, I have a few questions....

Ron stated in that thread that 3500# of braking force is generally what you want for a good street/track car....he further went on to specify a 3400-3700# range.

Going with the formulas of:

Line Pressure = Force * pedal ratio / master cylinder area

and

Braking Force = Line pressure * total piston area

Assuming 80# of force as in the previous posts, even if I could achieve a pedal ratio of 7:1 (unlikely in a '64 Corvette), with a 7/8 master cylinder (smallest readily available aftermarket master cylinder), that still only gives you a line pressure of 636 psi and a resultant braking force of 3294# assuming the 5.18" piston area in the above mentioned Wilwood FSL4R (120-13227/228) calipers (assuming I can get a bracket combo that will allow me to mount them on the F-Body spindle).

So all of that being said....what am I missing? If you're going to build a solid manual brake system from scratch, where do you start? I think I've got a good handle on the math, and how pedal ratio and master cylinder bore size affect both clamping force and pedal travel.

Just as a point of reference, the stock discs on the front of the 1965 Corvette had a pedal ratio of 4.54, a master cylinder bore of 1.0 and a front caliper piston area of 5.518. Resulting in 363 psi of line pressure or 2004# of braking force. Obviously on some level we're comparing apples to oranges as one car has modern R compound tires, suspension, and 17 inch wheels, while the '65 has original suspension and 15" bias ply tires but over 1/3 less recommended braking force seems significant.

Again, would love to hear your thoughts as to what Im missing.

Thanks!!

[RSX302]

I used this link many times... http://www.jakelatham.com/radical/in...culators.shtml

On my 68 Camaro (3800lbs at the time with 13" front/12"rear brakes) I tried many different manual master/pedal ratio combos and the only one that worked half way decent to my liking was 3/4 twin masters (8:1 PR) and .65cf Race Pads. Little long in the pedal but it was good. Oh, then came the pad knock-back on a solid axle rearend due to the small mc bore. dOh

Since the race pads were noisy, dusty and chewed up the rotors with pad knock back...off they went and on with the 1 1/8" master and hydroboost. I'm now at 3600lbs since dropping the turbos with the same brakes and hydroboost. All good now.

I completely agree with the +3000ftlbs front rotor torque (my preference is +3500ftlb with big sticky tires), but that is very difficult to get with small rotors and pressure under 800psi. You'd need a massive Hulk leg.

Didn't mean to derail the manual brakes but if you set up the following manual system, I think you'd be in the lowend ballpark with your car weigh.

13" front rotors (14" preferred)
4.0sqin caliper (Wilwood Superlite 6 piston example)
8:1 PR (100lb leg force)
7/8" master

BTW, pad size is not a factor with extra braking. It only adds life of pad. Also if you go with a larger volume caliper, you need to go with a larger master as the pedal throw maybe further then you like on the same given ratio. Ultimately it will end up about the same rotor torque.

3 items that increase rotor torque, pressure, pad cf and rotor diameter.

[FLYNAVY53]

Thanks for that link....that will be very helpful!

I've essentially limited myself to 13" x 1.25" rotor because I want to stay with a 17" wheel. The 18" wheels with 30 or 35 series rubber just don't look right to me on the C2 and since this is going to be spending most of its life on the street, I'm going to stick with the 17s and a little more sidewall.

Pedal ratio is also going to be an issue. With a 12.5 inch long pedal right now, and a ratio of 4.54, theres no way to lengthen the pedal enough to increase the ratio, and moving the master cylinder up the firewall to decrease the distance between the pivot point and the MC input rod is really out of the question as well.

Are there really just not that many people running manual brakes anymore? Or is the terrible pedal ratio unique to the Corvettes?

[FLYNAVY53]

Just realized I'm an idiot and I've been using master cylinder bore size as opposed to area this whole time while calculating line pressures...

I think I may be able to come up with a combo that will work, even with the 4.54 pedal ratio....