Question about big calipers and braking pressure.

[68 D88]

I'm new here and this is my first post. I have a question that has been bugging me for a while. Here goes.

The biggest old technology, one-piston calipers, like the JD7 option on the old Suburbans have a 3-5/32 inch diameter piston. Doing the math yields 7.82 square inches in surface area.

The big new technology, two-piston C5 calipers have 1.75 inch diameter pistons. Doing the math yields a total of 4.81 square inches in surface area.



Clearly, the JD7, one-piston calipers have more surface area, which means more braking force with the same amount of fluid pressure to the brake pad. Force = Pressure x Area. Given that, why does everyone gravitate toward the dual piston calipers (including the automotive industry)? Is it that they are easier to adapt because of the cage style bracketry? Is it because the pads are a little bigger and perhaps give a little more frictional resistance? Maybe there are better choices for pads. Any insight would be appreciated. Thanks.

[dennis68]

They are lighter which decreases unsprung weight and caliper pressure does not determine braking distance. You are far better off to run 13" or larger rotors and smaller piston calipers than to run huge piston calipers and 11" rotors.

[derekf]

Welcome aboard, 68 D88 (What's your name?).

I'm still contemplating doing a best-of-both-worlds thing... the JD7 Suburban calipers on a 13" rotor, but I'm so far away from doing brakes that I just glance at topics that relate to it.

Downsides I've heard mentioned about going that way are (at least as far as I can recall reading):

• Caliper weight = unsprung weight as Dennis mentioned
• Changing piston size changes the amount of fluid pressure/volume needed, which will affect master cylinder selection
• Master cylinder, front caliper pistons, and rear caliper pistons need to be proportional or your braking won't "feel" right.

Upsides:

• Huge piston = huge braking pressures
• Suburban calipers are what - 14$ rebuilt?
• Standard pad size = big selection
• Cast iron calipers flex less than billet aluminum

You may want to do some more research - mainly on front/rear balance and MC sizing... I haven't done enough research to be certain about anything, and some of my "facts" may be the result of a faulty memory.

Dennis, you know more than I about this, is there something fundamental that I've left out? It would seem that JD7 on 13" rotors would be a pretty cool setup.

68 D88, again, welcome aboard.

[Q ship]

caliper pressure does not determine braking distance.

It is one of the determining factors, same as rotor diameter. Increasing either will increase the torque available to stop the wheel/tire assembly.

Derek the problem with running large area calipers with large diameter rotors is putting the stopping power to the ground with street tires and correctly biasing the rears. Here is a link with more info.

[68 D88]

Thanks for the replies and the warm welcome. My name is Rob. I too have contemplated using the 13" C5 rotors with the Suburban JD7 calipers. I'm sure brake torque would be awesome on the fronts. I can see how there might be problems balancing that with the rears though. I also realize that the brackets would be more involved in building. I'm not too concerned with the unsprung weight component given this is going on a full size 1968 Delta 88. Thanks again.

[jeffandre]

I may be wrong (old and far removed from math) but I recall the area of a circle being 2(pi)(radius), or 2(diameter). Using this formula I got 9.91" area for the JD7 caliper piston and 11.0" for the C5 caliper pistons (1.75)(pi)(2 pistons). The caliper pistons on the C5 caliper also spread the total pressure along the pad, which has to help somehow, but I am not a brake expert. Plus the cooling effects, weight reduction, and pad choices for the C5 caliper make it a great choice for tight budgets IMO.

Jeff

[Q ship]

Jeff, the correct formula is A(rea)=(pi)(r squared).
For instance, a C5 caliper-40 mm bore = 1.575" diameter (40/25.4)
.7875(radius of piston)squared is .620 x pi(3.14,rounded)=1.947 sq in.
1.950 sq in x 2 pistons = 3.9 total sq in.


JD7 pistons are HUGE
3 5/32 dia, radius is 1.578, squared is 2.49 x 3.14 = 7.82 sq in.

Verified with a calculator

[Q ship]

Rob, I believe that JD7's with a 13" rotor would be very hard to balance and prone to front wheel lock up, also as you mention the bracketry would be hard to do correctly. Just my .02.

[68 D88]

Thanks much for the input. It just seemed like the best of both worlds if using the JD7 caliper and the 13 inch rotor.

[airrj1]

Rob,

I have JD7 calipers on my Chevelle. The first person that I saw run the JD7 calipers was 'BC' over at Team Chevelle. He has Cadillac rear disk brakes on his Chevelle which have a hugh 2.5" piston. And in the front he was running the 1LE tall spindle swap. Bill switched to the large JD7 calipers to better balance the the size of the front and rear caliper sizes. This improved his stopping. All is this is done with an 1 1/8" MC bore.

The problem with the JD7 calipers is there is very few rear brakes that you can run and still be well balanced in pressure. I am currently running mine with stock A-body drums in the rear, and I have Cadillac rear disks to install at some point. This setup works well but I currently have really crappy tires so I can't give a complete review.

As far as benifits, I have 12" x 1.25" front disk brakes for less than I could have updated to stock Chevelle brakes for. They will never compare with the C5 conversions, but I have been on a tight budget.

Checkout my website if you want to see what I have done. It hasn't been updated since I got the car on the road, but it is 90% complete.

[jeffandre]

Brian,
Good catch, I realized my calc error a few hours later, I mistakenly used the calc to determine circimference, not area. I still say go with C5 calipers for better pad choices and an easier time matching the rear. I installed C5 front calipers and 13" Eradispeed rotors on the rear of my 71 Camaro, the front will have 14"+ rotors and probably Alcon calipers, and I will use the proper formula this time when determining the piston sizes needed for the front calipers I swear! I may try to avoid the master cylinder stress mess by going with one of the dual balancing-bar setups out there.

Jeff

[Q ship]

I installed C5 front calipers and 13" Eradispeed rotors on the rear of my 71 Camaro,

Ok, you just got my attention!! Details, pictures, website...TELL ME MORE!!! Thanks Jeff.