braking capacity

[Jim Rosenthal]

Here's a question, or questions, that someone on this Forum would know the answer to...
How do you decide how much braking capacity a performance car needs; I know it is a function of weight, horsepower, axle ratios which imply top speed, but how do you decide it?
How do you decide which master cylinder size to use with your calipers once you've figured out how much braking capacity you need?
I'm sure there are formulas or rules-of-thumb for all this. I should mention; my car will likely have the same AP brakes that were fitted to Safir cars, and somewhere there is a master cylinder spec etc for that. But it started me thinking- how do you determine that the car is going to stop when you push the pedal? /ubbthreads/images/graemlins/confused.gif

 

[Kevin M]

The answer to this may be simpler than you think.

Is the car using 15 inch or 16 inch wheels? (I assume the original Safirs used 15”)

If it is, and if it is going to see track usage, fit the largest brakes that will fit in the wheels.

The question only becomes important with 17” or 18” wheels, where you could actually fit larger brakes than you might need.

Master cylinder size is dependant on the piston size in the calipers you select. Once you have done that, AP will have the info on the proper size master cylinder.

Just in case you are using large wheels, a quick check of cars of similar weight shows most of them using brakes of about 13” diameter. That would be a very good guess for a GT40.

 

[Gary Gibbs]

I agree with previous post. I have a program written in Matlab that does the complete analysis of a brake system. You must have matlab to run it although I think there are free programs out there that run matlab code. I will have to search to see what they are.

 

[Jim Rosenthal]

Appreciate the above. I am going to use 16" wheels if I can. The reason, as they are not original size, is the absolute dearth of street performance tires for 15" rims in sizes that will suit a GT40. There seem to be none, or so few as to indicate a trend towards none. 16" tires are still available.
I would very much like to see a program for calculating brake size, piston area, etc. It would be a good learning opportunity.

 

[James]

Jim,

You should be able to fit 12" rotors and 6 pot callipers on a 16" wheel with ease. That is what I have fitted to my 40 - AP Racing CP6000. They'll offer more than enough braking power. I got about 450HP if that helps.

Regards,

J.P

 

[Dave Wharran]

Jim
IMHO, you want as much brake as you can fit. It's better to have too much braking power so you can regulate it than to not have enough.
I fit 13" Baer rotors and 4 piston Alcon calipers in 16" wheels on the front, and 12" Baer rotors and standard Corvette PBR single piston calipers in 17" wheels rear.

Currently, master cylinder size is 5/8" dia. front and 3/4" diameter rear. The brake pedal is biased to the rear. I plan to go to a smaller sized rear MC to get more braking on the rears. Front wheels are fairly easy to lock at any speed. Pedal regulation is excellent.
These are non assisted MC's.
Pedal ratio is somewhere around 6 or 6.5:1
The brakes feel great but I don't think the car has too much braking power.

 

[Chris Duncan]

Jim,

Everything you ever wanted to know about Brake Systems

Brake Systems


and Wilwoods tech articles

Wilwood Tech Tips

The only original I've seen, a MK-V, has 11" rotors in 15" wheels. I'm using 12" vette rotors in 16" wheels with 4 pot wilwoods. Considering the vette is 3500 lbs, and the GTt40 less than 2400 lbs. it should be sufficient. I think it depends on what your going to do with the car. All out racing would merit 13" for sure but you might get by with less.

Other factors to consider are less effecient cooling when using the largest rotor that fits the wheel due to restricted air flow, of course this is balanced by less heat generated. Larger also means more unsprung weight, a factor in handling.

Another thing I've seen is rotor cracking with cooling ducts, the MKV I mentioned above experiences cracking. Apparently this happened to other original cars, and one way they alleviated the problem was by putting the duct inlets behind the radiator so that the cooling air wouldn't have as high a temp differential.

 

[Jim Rosenthal]

Thanks to all above; I am ordering the book (thanks Kalun), we have ordered the rotors from AP, and figuring out which calipers. I kind of like Dave's idea of 6 piston front calipers as the system will not be boosted. As you say, better to have too much capacity and modulate it, than not enough and run out of road and second chances.
I have not found Alcon; if anyone has a link for a dealer I'd appreciate it. Thanks.

 

[Ian Clark]

Hi Jim, Go as big as your wallet and wheels will allow, it's all wow factor. At the end of the day, or end of the panic stop for that matter, you can only stop as fast as your tires can grip.

13" rotors with six pot calipers won't stop you a foot shorter than 11" with floating twin pot calipers if your chassis and suspension geometry, brake pressure and bias, tires are not operating at their absolute peak.

You'd do well to do deflection tests on the hubs loaded and run the suspension through it's full travel while watching the contact patch. A few bucks spent reinforcing pick-up points or adjusting suspension geometry will pay greater dividends in braking efficiancy, ride and tire life than big brakes alone.

Of course if your chassis is already fully developed and proven, go back to line one! Have fun, love to see your car in the flesh

 

[Kevin M]

Baer Brakes has Alcon calipers:

http://www.baer.com/

 

[Howard Jones]

I have been looking into a brake up grade on my GTD for later this year. I will be running 17" wheels so just about anything will fit.

The setup that I am leaning towards is 12X1.25" wilwood curved vane rotors and superlight II calipers. I have gone this route because of the fairly low cost of wilwood parts as compaired to alcons and other full on race equipment. My buget must get trimmed someplace!

The above post is correct in that the rears on these cars are prone to need at lease as much brake as the front.

I have already installed a proportioning valve in the front brakes and the car seams to like about 5-10% reduction in effort to the front to work best with the granada stuff I have on the car now.

Does anybody have wilwoods on their car now? Which ones? Any comment on my thinking? Car will be mostly street with trackdays a couple times per year.

 

Dennis,

I called Rally design in UK yesterday and ordered 2 custom bells ($100)that will fit my Wilwood stuff that I bought off of Ebay. I have all the part numbers. I am doing the rears first.

 

[Dave Wharran]

Jim,
Ben O'Conner at Bear was most helpful in getting a set of brackets to mount the Alcon calipers and 13" rotors to fit my uprights and wheels. My Alcons are 4 piston on the fronts and will easily but controllably lock the 245 width tires at 100 mph. Pad squeel at low speed is the biggest problem I'm having. I'm using Baer's standard pad for these calipers. A different compound or softer pad may be the answer, but winter has curtailed most tuning/sorting.

 

Doesn't someone make a caliper with unequal size pistons with the theory being that the trailing piston requires a different size due to frictional heat differences between the leading and trailing pistons? I thought that sounded reasonable, and pretty cool.

 

[Dave Wharran]

Pat,
The Baer/Alcon 4 piston calipers have staggered sized pistons. 1.50" and 1.625" if I remember correctly, supposedly to help reduce pad taper between the leading and trailing pad edges.
Here's a couple of older picts of the calipers/rotors.

 

[Ian Clark]

Cool pics Dave!

 

[Lee]

How about these? 6 pot, unequal size pistons. i also saw their new 6 pot calipers with handbrake facility at the Autosport show. Very impressive. Anyone got any thoughts on HiSpec brakes?

HiSpec Mega Monster 6

 

[Julian West]

Hi Spec look to be very good quality, albeit the calipers are not one peice.

The handbrake mechanism is a really nice peice of work & they told me that it would be available in 4-6 weeks.

The calipers were very very lightweight, helping that all important unsprung weight reduction!

 

Good stuff!

 

[Brett James-McCall]

I know Paul Bearman has got Billet 4s for his fronts. HiSpec are just down the road from Paul and myself and am pleased that he has finally got there, as they had to delay design due to volume of work. Allegedly the best bits of Willwood and AP were looked at when he designed them. Were any prices mentioned?

Brett