Choosing master's bore sizes

[Motorcitydak]

I read somewhere that the master should be about 12-14% the area of the caliper's area. Rite now I have my front calipers but no rears. The front calipers are 6 pistons from a zo6 vette. I figure that I will pick up the zo6 rear calipers just for symmetry. Looking up the area of the pistons, the fronts are 7.95sq/in. Rears are 4.38sq/in. I have the 14 inch front brake rotors. For the back rotors I have a moser full floater so with the rotor adapter I have, I think the biggest rotor I can run is 12.19 so that is what I plan on using. For the pedals, I will be using a triple master brake and clutch pedal setup with adjustable bias bar. For the front master, 12-14% of that area is 0.954 to 1.113 so I guess a 1.0 master for the front. Rear comes out to between 0.525 0.6132. I believe that the smallest master is 5/8. Anyone see a problem with that?

I want to run the Tilton 604 pedals which are floor mounted and are adjustable ratio from 5.29-5.75



One other thing is that I have a viper T56 and the stock viper slave cylinder. What size master should i have for that?

[jpgolf14]

You have a nice collection of cars there. Which one do you think has the best brake feel? I would stick to the same ratio of brake gain as your favorite feeling brakes. That includes:

Caliper piston area
Rotor diameter
Master cylinder area
Pedal ratio

[Apogee]

For fixed mount calipers like the C6 Z06 PBR M-series calipers, you should only use half of the piston area, so it will be approximately 4 square inches up front and 2.2 square inches in the rear. It may not make sense intuitively, however it does if you draw the free-body diagram and analyze the forces involved.

With most race pedal setups we've done, we have used a 6:1 (or thereabouts) pedal ratio and .75" and .875" bore master cylinders for the front and rear applications respectively. I wouldn't suggest running a numerically smaller pedal ratio in a manual installation, although your pad compound will play a large part as well and should be taken into consideration. As for your math, I think you're confusing MC piston areas with MC piston diameters which are not the same thing. If you look at the front caliper, 12% of 4.00 square inches is .48 square inches, also an area. If you want to convert that to a diameter, then area = pi*r^2, so r = .391 inches or d = .782 inches.

HTH,
Tobin
KORE3

[Motorcitydak]

Thanks golf. Well the bike stops really well since its so light. I have to say out of em all, the 2002Tii is really nice. The other 2002 is a project who's time has not yet come so its just sitting. I am not sure tho what its bore sizes are but the front calipers are nice 4 pistons from the factory. Plus its 2000 pound weight makes it stop really nicely.

Tobin, you are right. That is why I wanted to post this up and have someone check over my work. Once i got that number I figured that was the size, forgot to take that then calculate out the masters cylinder area, not its bore diameter. I completely see what you are saying and will get masters of those specs. When you said those sizes that you normally use, .75 for front and .875 for rear? Just curious why the larger one for the back. Seems like the opposite would be recommended. Im not sure tho and greatly value your knowledge and input here being a person who has much more experience than myself on this

[Apogee]

Hydraulic principals are fairly simple and function on pressure which is force per unit of area, so the smaller the bore master cylinder, the higher the pressure output for a given input force. The inverse is true as well, so the larger the bore master cylinder, the lower the pressure output for a given input force. When evaluating master cylinders for a given application, you're choosing a balance between pressure and volume because the two are inversely related. As you decrease the MC bore size, you increase the pressure output but decrease the volume output proportionally. Additionally, pedal travel increases as well when you decrease the MC bore size since the volume requirements haven't changed, so you need to design for a certain volume requirement with your pedal ratio and packaging constraints since pedal travel and MC stroke length can both be limiting factors if you go too small on the MC bore size.

Google Jake Latham's brake calculator if you want to play with some of the numbers, however you really need corner weights, CoG height, pad CoF's, etc if you want to be as accurate as possible. Dual master cylinder setups with balance beams are ideally suited to race applications where the brake system is being tuned to function optimally under full-deceleration stops. When stopping a car under 1+ g of deceleration, a lot of weight transfers from the rear tires to the front up until the point of front lockup, so the reason for the larger rear MC bore is to accommodate this weight transfer and reduce the pressure to the rear calipers so that you don't induce premature lockup in the rear. Some cars like a little more rear brakes, some a little less, but .750"/.875" has workded well for us in the past with manual C5/C6 applications.

Tobin
KORE3

[Motorcitydak]

Its that kind of help and information why I love this site. Thanks a ton!!!

[Skip Fix]

"When stopping a car under 1+ g of deceleration, a lot of weight transfers from the rear tires to the front up until the point of front lockup, so the reason for the larger rear MC bore is to accommodate this weight transfer and reduce the pressure to the rear calipers so that you don't induce premature lockup in the rear."

Why a stiff front spring helps also and not just big front sway bars.

[Motorcitydak]

Indeed, nice stiff springs will be in the plan as well

I have a lot of stuff on the way from Summit rite now, those Tilton pedals, masters and remote bias adjuster. Should be here today!

[Payton King]

What pedal ratio did you get?

[Motorcitydak]

I love Summit because they are 1 state away and I get stuff from them overnight. They showed up today!


Ratio is adjustable but Ill use the 5.75:1. The clutch pedal is sticking out too far, that looks like its from the threaded rod from the master. Im not sure if it has bottomed out in that clevis or just something caught in the threads. I didn't have much time to mess with it tho. Ill get that pedal next to the brake one soon tho, I mite have to cut down the push rod to do it