Master cylinder sizing, dumb question?
Is there a formula available to calculate what size master cylinder is needed for a brake system considering vehicle weight and fluid capacity? I've done a search through the old posts and all I see is try this one or that one.... Is this a big brake trade secret? I've got a book on brake systems and it doesn't elaborate or suggest any way of choosing a suitable master cylinder either.
I've got two vehicles in the works that will be needing master cylinders soon. The first one on the list is my Son's 70 Nova with factory 11" discs on the front and 95 Explorer rear discs.
Or should I do what it seems everyone else does and just keep trying different components untill I get something that works?
Master cylinder capacity (amount of fluid it holds) is secondary to piston size. Most MC's are either 15/16", 1" or 1-1/16" diameter pistons. Which one you choose depends on if you have a booster, 2 wheel disks, 4 wheel disks, or 4 wheel drum. There are several resources on the net to help you choose, such as Stainless Steel Brakes. Master Power Brakes, and Wilwood.
The pedal ratio also has a lot to do with it, and caliper displacement.
I wouldn't count on SSBC, since they offer the same master for a range of caliper sizes and pedal ratios, assuming it will work as well for one as another. They've had many problems.
There are formulas to determine both the bore and the stroke needed, based on caliper displacement and pedal ratio. The booster adds another variable, but a master should be chosen that is not so large that one cannot still stop if the booster fails.
There were a number of Carcraft articles which delt with this. I think they are still on the web somewhere. There is too much for me to write.
There are also some thumb rules, based on caliper bore area and pedal ratio. These do not work with low-drag style calipers. (caliper bore area and caliper displacement are different)
Basically, for most power or manual disc brakes, you would choose a master cylinder with between 12-14% of the bore area (in sq"), taking care to get one with sufficient stroke, often needing more stroke with 4wl disc. 12% usually works well with a 5:1 manual pedal, and 14% works well with a 6:1 manual pedal or 3.5:1 power pedal.
With a higher pedal ratio, you can up-size the master for more fluid displacement, using the pedal lever to make up for the loss in pressure. This is often used with 4wl disc. It is also sometimes done with low-drag style calipers instead of the special step-bore master cyl.
The vehicle weight has more to do with what size wheel brakes are chosen, which then can be used to help determine the master. The 12-14% rule is fairly common from an MG to an F350.
Useful information... also to be taken with a grain of salt, like everything else you read, including this post.
I really like this quote
It really is dependent on the caliper and the pedal ratio.
The Chevelle and GTO had some bigass calipers and a high pedal ratio, just to be clear..ish.
Bore in sq", just of the circle, but only one front caliper, and only one side of it (in the case of opposed pistons).
It's just a general guide that way, and wont always get you right the first time, but if you use a little common sense of what you are using, what it used stock, etc... you will get it right or very close the first time.
It's not really technically the right way to figure it, but it is alot simpler, and usually works out.
If you check out the bore sizes of a lot of cars, you'll find the 12-14% in most with either manual 4wl disc or disc/drum, and some with power 4wldisc.
A lot of vehicles with power 4wl disc will go up to 18% on master size, and use a high pedal, combined with either hydroboost or a dual diaphragm booster. That's only a 1/8" upstep in bore diameter in most cases.
The choice of master bore and stroke has to do also with the size of the rear brakes, which normally are 50-75% the size of the front in the case of 4wldisc... usually expressed as a percentage of front and rear added together... A 70/30 4wldisc system has rear brakes which are about 1/2 the size of the front.
Front 4piston, 1 7/8" diameter (5.52sq")
Rear 4piston 1 3/8" diameter (2.9sq") 52% of front
master, manual, 1" bore (.7854sq") 14% of front
master, power, 1 1/8" bore (.994sq") 18% of front
You'll find others to be very similar. The smallest master I've found on 4wldisc came stock on the mid 70s Ford/Lincolns, and used only 13% with power brakes. It had a long stroke. Ford and Mopar tend to use smaller masters than GM in many cases. I guess that's why they often have softer brakes. Vehicles made post 1994 often have much larger master cyl bores, and discontinued the step-bore master for low-drag brakes.
piston diameter 1 7/8= 1.875
to find area use pi(rxr) or I like [.7854(dxd)] instead
(1.875x1.875)(.7854)=2.76sq" for one piston
5.52sq"= a 1 7/8 diameter 4 piston caliper.
single piston 2 5/8 diameter
5.4sq" =a single piston caliper with a 2 5/8" piston
ARI Vehicle List, brake system bore sizes:
(Do not take the sizes listed here as gospel. Some are not correct, or not accurate)
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