Math for determining needed brake size

[johnparts]

Hey guys, I'm wondering if there is an actual math calculation for determining brake size on a car as needed or recommended by weight power etc... I have a goal weight of 3200 lbs with a hp rating in the 750 range with lightweight 20" wheels front and rear. I know most guys would say run the biggest brakes you can run inside your wheel because of the 20's however, my budget dictates more of my build then just throwing parts at it. I was looking into running a 13" vs 14" front brake system because of the nearly $1000 difference between the 2 and a 12" rear set up vs a 13" rear because again the budget calls for it. Is there a way to determine if this will be enough brake for daily/occasional track duty.

[RSX302]

Hey guys, I'm wondering if there is an actual math calculation for determining brake size on a car as needed or recommended by weight power etc... I have a goal weight of 3200 lbs with a hp rating in the 750 range with lightweight 20" wheels front and rear. I know most guys would say run the biggest brakes you can run inside your wheel because of the 20's however, my budget dictates more of my build then just throwing parts at it. I was looking into running a 13" vs 14" front brake system because of the nearly $1000 difference between the 2 and a 12" rear set up vs a 13" rear because again the budget calls for it. Is there a way to determine if this will be enough brake for daily/occasional track duty.

This is a good calculator I used to figure out pad cf vs caliper pressure on a given rotor dia.

http://www.jakelatham.com/radical/info/brake_calculators.shtml

I have 13" front with wilwood sl6's and 12" rears with 4 piston wilwoods and bp20's. Now that I'm running power assist on a 3800lbs car it stops fantastic. Not so much in manual trim. Needed high cf pads to make them work to my liking, .55-.6cf. A 3200lb car would be better on a manual system, but you may still be on the edge of needing a little more pad cf.

.4 to .5cf = street/ light race, .55cf and up = full race

[johnparts]

Thank you for that. I will be running a power assist however not sure how big of a booster I can run yet.

[Apogee]

Vacuum assist is going to require vacuum...do you have that with your HP numbers?

As for running the numbers, it's mostly a function of the kinetic energy (KE) that your brakes may potentially need to dissipate. KE = (1/2)mv^2 where m = mass and v = velocity. While mass certainly plays a roll in the equation, velocity tends to be the controlling variable given that it is squared. Regardless of how much HP you have, the level of braking capacity that your car requires is a function of the how the car is to be driven. Given 750 RWHP, pushed to the limits, you'll need every bit of brake you can afford and possibly then some if you've got the tires to handle that much power. If you only cruise it to the grocery store, then it's a different story. Calculations only get you so far, at some point you need to actually test the brakes in the envinronment they're being used, so looking at other applications given their weights and speed potentials can tell you a lot about what works and what doesn't.

Tobin
KORE3

[johnparts]

Yeah twin turbo 3rd gen hemi with a very mild cam. I will have a vacuum reserve for when I'm under boost.

[johnparts]

Vacuum assist is going to require vacuum...do you have that with your HP numbers?

As for running the numbers, it's mostly a function of the kinetic energy (KE) that your brakes may potentially need to dissipate. KE = (1/2)mv^2 where m = mass and v = velocity. While mass certainly plays a roll in the equation, velocity tends to be the controlling variable given that it is squared. Regardless of how much HP you have, the level of braking capacity that your car requires is a function of the how the car is to be driven. Given 750 RWHP, pushed to the limits, you'll need every bit of brake you can afford and possibly then some if you've got the tires to handle that much power. If you only cruise it to the grocery store, then it's a different story. Calculations only get you so far, at some point you need to actually test the brakes in the envinronment they're being used, so looking at other applications given their weights and speed potentials can tell you a lot about what works and what doesn't.

Tobin
KORE3

Shooting for mild track use. HP band will be very linear with the turbo set-up so there will be the potential of 750 RWHP when I crank the pedal but for the most part I will save my spirited driving for the track. I want this car to be very driver friendly and go long distances with it which is why if I can save money on one area and invest it somewhere else like say other safety equipment I really would like to try. I know with a 20" wheel a 13" brake is going to look silly but hey I might me able to upgrade later however I want what I need for brakes not what I want for brakes. I would run a 16"/14" combo if the budget allowed it.

[Apogee]

Anything over 14" is typically overkill IMO unless you're talking carbon/ceramic. The truth is that a lot of guys with pro-touring builds could probably get away with less brake the majority of the time which could help performance in other regards such as acceleration, unsprung weight, etc. That said, there is a balancing act in selecting brakes that will work on a multi-purpose vehicle that needs to perform well both on the street and at the track with minimal fuss in between when the time comes, and that's where the "pro-touring" form meets function IMHO. As noted above, pad compound (CoF) and temperature range will in large part determine driveability and overall performance as well.

You didn't specify which kits you were considering purchasing, only rotor diameters...are we to assume the same thickness rotors? Same calipers? Same pads? Typically you do get what you pay for in my experience, at least relatively speaking, so that $1000 difference probably accounts for those differences. If you care to share which kits you're considering, you may get more real world response from folks with similar builds, at least in terms of performance.

Tobin
KORE3

[johnparts]

Wilwood 140-9802 for the 14" kit

Wilwood 140-12275 for the 12.8" kit

Or at the Very minimum run this adapter and get the 13" Factory 03-04 Cobra brakes

http://www.ebay.com/itm/COBRA-DISC-BRAKE-conversion-for-Mustang-II-Spindles-/110839645389?pt=Vintage_Car_Truck_Parts_Accessorie s&hash=item19ce8ec4cd&vxp=mtr

[jpgolf14]

Wilwood 140-9802 for the 14" kit

Wilwood 140-12275 for the 12.8" kit

Or at the Very minimum run this adapter and get the 13" Factory 03-04 Cobra brakes

http://www.ebay.com/itm/COBRA-DISC-BRAKE-conversion-for-Mustang-II-Spindles-/110839645389?pt=Vintage_Car_Truck_Parts_Accessorie s&hash=item19ce8ec4cd&vxp=mtr

As Tobin points out, how diehard are you about the pro-touring philosophy? The hard core pro tourer would require that you should be able to drive off the highway and right onto the track. I am a big fan of that philosophy of that as well. Who wants to waste your time changing pads all the time. This philosophy requires an overkill braking setup that rejects heat fast enough to keep street pads happy on the track. You are talking ideally:

Large rotor
High vane count
Large volume pads
brake cooling

An overkill brake kits of course means more cost up front, more wheel fitment headaches, more weight and MOI to carry around. It is a balancing act.

In my opinion the front Wilwood kit you mentioned is a nice kit but would likely still require a pad swap at the track. The situation would be better if that kit used the full thickness calipers rather than the "Narrow" which reduces the pad volume over 25%. There is no reason to use the narrow caliper unless you need it for caliper face to back of wheel spoke clearance. The good news is pads are very easy to change on the Superlite family of calipers.

Correct me if I am wrong, but I believe the Cobra brakes are very similar to the C5 calipers. If so, I would be nervous about overheating the brake fluid with your kind of horsepower. In my experience those calipers don't shed heat quite a well as the big fixed calipers due to the lower aluminum volume. This situation can be helped with cooling ducts and titanium shims behind the pads.

You would definitely need a pad change for the track. Pads are much more painful to change than the Wilwoods.

From purely a hydro-mechanical sense, both are more than adequate to stop the car.

[Apogee]

FWIW, the 03-04 Mustang Cobra brakes are actually more like the 1996 C4 Corvette brakes than anything else PBR made over the years, just with 40.5mm diameter pistons instead of 38mm. The front calipers are the older pad-guided design versus the newer and better pin-guided design used on the C5 and newer GM applications, use a relatively small FMSI D412 pad spec and 13" x 1.1" [330x28mm] rotor. While the thermal mass of the rotor is more or less comparable to the base level JL9 C5/C6 Corvette at about 19 pounds, you have none of the modularity to upgrade to a larger rotor/caliper combination like you do with the C5/C6 kits.

Tobin
KORE3