According to stoptech's website, their BBK does not add any additional braking torque.

While I understand and agree with the need to be aware of brake bias, I would think the solution would be to guide customers to solutions for the rear brake, not make a front kit that is, as far as I can tell, just for looks??

The caliper is probably lighter, but the rotor is bigger, so there's no saved weight...

What am I missing here? Why would I want to add this to my car?

 

Track use. Heat capacity, longevity, cooling, and consistency.

On the OEM setup:
In a normal 20-25 min session, I would experience brake fade toward the end of the session with X10s up front or DTC70s. Pads and and rotors would last 3 to 4 track days (1 day = 4 or 5 session of 20-25 minutes). I had bring spare pads and rotors to the track. When I say that the pads/rotors last that long, that means the pads would be down to backing plates and the rotors have cracked.

StopTech BBK:
-I'm on 8 track days with a less aggressive compound. I can barely see any wear on the pad or the rotor. Others have reported getting 20-25 track days on a single set of pads and rotors. Braking is consistent all throughout the 20-25 minute sessions that I run. Pad selection is much better - the ST-40 caliper is based on the Porsche caliper used in the 964 and 993. The cost for pads is very similar to the OEM pads. Heat capacity, longevity, cooling, and consistency are in a completely different class.
-If you start running some stickier tires at the track (RE-71R, NT-01, RC-1, or better), the OEM braking setup will be the weak link in your setup. Coupled with aero, and the very common 17x9 with 255 square wheel/tire setup, and you'll need more than the OEM front setup.


For street use and canyon runs, the OEM setup is more than enough. Any sort of extended track used in a skilled driver's hands will benefit from a BBK. You'll also save money on the consumables. 3 track days on the OEM setup = replace pads + rotors = $200 + $80 = ~$280 every 3 to 4 track days. I spent over $1000 in brake consumables during one year of tracking (more than my tire budget). The BBK pays for itself in the long run.

 

My application is for the track as well. I too have taken brand new hawk pads down to metal-on-metal in one weekend. So longevity is definitely something I'm looking for!

Though I haven't cracked any rotors yet...

Thanks for the info. I assume there is enough torque to get into the ABS even with R-compound tires?

I think my disappointment with my available brake torque on track is from using HPS pads....

 

Have a read:
http://www.motoiq.com/MagazineArticl...ont-break.aspx

 

The increased mass of the Stoptech helps slow you down



But the answer for most BBK's is thermal capacity and heat shedding. The calipers and rotor diameter are an important part of the brake equation but the main explanation for your comparison would be the rotor itself. There are two (basic) components to a brake rotor, the total mass and the heat extraction capacity. The total mass relates to the total amount of braking heat you can put into them (think heat absorbency during a hard braking stop from 100-0). The heat extraction efficiency relates to how much heat you can remove through the cooling vanes or radiant heat from the rotor per second.

A good analogy is a shop sink. You've got the tap at the top dumping in water at a particular rate (this is the power of the car and the weight of the car, generating heat for the brake rotor)... the larger the tap the more water your sink will have to hold or drain before it overflows (ie rotor overheats). In this analogy the mass of the rotor is the VOLUME of the sink itself... how much water can it hold before it overflows. Lastly you've got the drain at the bottom... the efficiency of the vane design that extracts heat such as vane count, shape, straight/curved, and the airgap of the rotor for the air to move through. A small drain (poor rotor efficiency) will drain the water out slowly as you fill the sink up. If there is a mis-match in the system your sink will overflow... too much water flowing per sink capacity or drain size.

You can have a small drain and large capacity (try to absorb as much heat as you can and store it, then bleed it off slowly at a later time) but pay the penalty in heavy rotor weight. Conversely you can have a small light rotor with highly efficient heat extraction, but could be overloaded in a few heavy braking instances.

I've found the sink analogy is helpful in thinking about brakes... hopefully it made sense.

 

Absolutely right...the mass of the rotor is critical being able to absorb the amount of heat generated in an application or series of closely spaced applications where the rotor hasn't had sufficient time to cool down. Are the aftermarket rotors, net of the hub, that much heavier than the OEM rotor?

In stock car racing, rotors are available in a variety of weights and with different vane configurations to meet the needs on tracks with different braking requirements. A shorter paperclip oval with two heavy braking events on a 15 second lap would typically have a much heavier rotor than a superspeedway where there is little braking. Coleman Racing lists 48 different rotors: http://www.colemanracing.com/Brakes-...By%20ASC&c=107 . AP also has a large selection: http://www.essexparts.com/brake-discs. Note that the heavier disks are usually wider, as wide as 36mm. Weights range from less than 10lb to 20-25lb.

Cooling ducts are also part of this. On some cars there can be two or three ducts running to a duct directing the air to the center of the spindle. Some even have a fan in the duct and/or fan blades attached to hub that blow air through the wheels on to the rotors' outer face..

The rotors on BBK kits virtually always have a separate hub reducing heat transfer, and many have floating rotors allowing for expansion without distorting the hub.

The BBKs can come with calipers designed for both the greater rotor width as well as increased diameter. They are virtually all fixed piston designs which have less hysteresis and may keep the pads closer to the rotor helping on engagement. For most that is probably not that huge a difference.

One issue on all brake calipers, but especially two piece calipers is caliper rigidity. There is a lot of force being applied and that can distort the caliper reducing control. Most OEM floating calipers are heavy pieces of cast iron which tend to be very stiff. Most fixed piston calipers are two piece designs that rely on bold and bridge reinforcements for strength. Some are monoblock, usually more expensive to make and still relying on a good design for strength.

The larger effective radius provides more braking torque for a given pedal effort without displacing more fluid. Using calipers with bigger pistons also provides that effect (or a master cylinder with a small diameter), but unless done front and rear or with a master cylinder change brake balance could be an issue.

Getting the air feed away from the spindle may also improve airflow through the rotors.

Note that racing pads have a much higher coefficient of friction than street pads. For some it is over twice as high as typical street pads. For many of them that comes at the expense of low temperature performance and increased noise and rotor wear, making them less desirable or inappropriate for street use. A bigger advantage of the top racing pads is better control especially during engagement and release.

Racing tires add traction increasing the need for extra braking torque.

Net: there are lots of ways to skin the cat depending on budgets, track layout, power, aero, tires, etc. It is possible to have more than is needed, but that isn't as obvious as having less then is needed. The costs aren't high compared to some other modifications (e.g. engines) and safety is involved, so maybe a little excess in the brake system is a good thing.

My guess is most BBK kits never see a racetrack and are done for style as much as anything else.