The Truth About BBK's
 

I posted this on G35 Driver and thought it could be informative here as well...

This thread is in response to all the “if it’s not Stop-Tech, then it’s crap” posts. It seems the only requirements to be a brake expert are reading the Stop-Tech White Papers, browsing the Zeckhausen Racing website, and hating any non-Stop-Tech BBK.

Stop-Tech’s front BBK reduces stopping distance by utilizing a smaller piston area than stock. Yes, the Stop-Tech front caliper has less stopping torque than the stock caliper. Stop-Tech BBK's are the best available. But "brake balance" can be achieved by lessor brake companies.

The proper question to ask is the size of the pistons and the rotor. I’ll explain.

First, all brake kits operate on the same principals. Hydraulic fluid is pushed through a line that compresses a piston which pushes the pads on the rotor.

Second, total piston area determines the torque of any given caliper. For example, the stock caliper on a 2005 6MT has two 1.75 inch pistons (although I have also heard 1.69 inches). Do some math and the total area is 4.8 inches. This calculation uses the pistons on only one side of the caliper. So, a six piston caliper’s piston area would be based on 3 pistons. Strangely, the stock sliding calipers brake as if they had identical opposed pistons (why? Ask a physicist).

Third, the number of pistons does not affect torque. I doesn’t matter if you get 4 inches of piston area with one big piston or 9 tiny pistons.

Fourth, rotor diameter effects brake torque. For example, if the only modification you did to your stock brakes was increase the front rotors to 14 inches then front brake bias would increase 2% and stopping distance would be longer.

However, if the only modification was to swap the 12.1 inch rear rotor for a 13 inch you would reduce front bias 2% and stopping distance would be shorter. That rear rotor upgrade is starting to look pretty good now...

Fifth, our cars come from the factory overly front brake biased (74% front/26% rear per my calculations). Although this increases stopping distance, a front biased vehicle is easier to control.

Therefore, reducing front brake bias will balance the car and decrease stopping times. However, shift the bias too far and the rears may lock up before the front. VERY DANGEROUS!

Stop-Tech has determined that magic amount of brake bias to magically balance a 350z/G35. I would love to know the piston area of the Stop-Tech front BBK.

So you want to know the effectiveness of your specific BBK. Get a ruler and measure the piston areas and rotor diameter.

Then use a brake bias calculator like the one at http://dsr.racer.net/brake_bias.htm. Make sure you set the front and rear master cylinder ratios to 1. If the ratio is the same as stock, then your brake distance will be unchanged. If the ratio is more front biased you will take longer to stop and unnecessarily burden your front brakes.

If you are going front only, make sure the total piston area (and front brake bias) is slightly less than stock. A general rule of thumb is no more than 5%. Also, search the forums and call the distributor to determine if there are any problems with that specific BBK locking the rears.

For example, I have Wilwood 6 piston calipers on 13 inch rotors. The total piston area is 4.06 or .74 inches less than stock resulting in 72%/28% bias or 2% more rear bias.

A Wilwood 6 piston superlight BBK with a 13 inch rotor will stop in a shorter distance than stock. (However, If I measured the front pistons wrong and they are actually 1.69 inches then the stopping distance will be the same.)

Of course different BBK's will have better heat dissipating characteristics, pedal modulation, etc... but that is for another post.

The piston sizes for the SL6 calipers are 1.62/1.12/1.12 . They have a typo on their website for the SL6 Narrow-body lug-mount calipers.

Good info! I encourage anyone who gets a chance to sit in or drive a car with a BBK. Brakes are often times an essential mod that goes overlooked.

good write up

+1 thanks for the info.

wonder what the stock Brembos would come out to?

very informative without being exhausting to read... thanks!

Wow I learned something... I have been looking a the Wilwood front and rear setup.

The type of pad used would also make a diference in stopping distance aswell, right?

No... braking distance is dictated by the grip of the tires on the first stop, then by the temperature limit of the pads/fluid over repeated stops.

To some extent, pads that don't have much bite with sticky rubber will have increase stopping distance. Pads with great bite and crummy tires will do the same.

Then you get into fade and thats another ballgame.

A couple of additional thoughts.

First, making too large a change for the better can result in some unpleasant results. Because a street car uses only one MC the fit of wildly small pistons on the front for the sake of elevated rear performance can come back to bite. The reason is that static bias and dynamic bias do not work totally hand in hand.

As you reduce front area you generally require greater pressures to achieve the same rotor torque. This is the "firm pedal" that some simply equate to better brakes. (not really, it's just fluid balance thing) The kicker however is that if you need greater pressure to make the fronts work you also get greater pressure to the rears. Cool huh! Not so fast...the problem is that rear brakes are metered by a proportioning valve. Meaning if the line pressure in is greater...the line pressure out is lower. Too much of a good thing.

Keep in mind you can swing bias upwards of 10% with a pad change alone. Here when you need more front brake for the track due to greater weight shift you can move from BP10 to Poly B and not need to do any more.

Jakes calculator is well laid out but often for more dedicated twin cylinder set ups on race cars. If you want to cut it down a bit and keep it simpler you can use my base one.
http://www.tceperformanceproducts.co...alculator.html
Don't worry it's accurate. I helped Jake with his layout.

Yes, but you need a bias calculator that includes coefficient of friction such as http://www.tceperformanceproducts.co...alculator.html. Once you feel comfortable with the calculator you can plug different numbers in.

The one problem is finding a reliable measure of coefficient of friction. It is my understanding that each company uses its own method to calculate CF.

One more thing about my Wilwood 6SL's. I absolutely hated the BP-10 "Smart Pads". They were dirty, noisy, and dangerous in the rain.

I replaced them with Porterfield RS-4's which work much better.

Decent OP.

The problem is that 99% of people who buy BBK's, buy them for a look. Not a function. It's fcuking pathetic if you ask me, and it dilutes the marketplace for that kind of part.

Similar things can be said for suspension parts.

That pad change won't have a direct effect necessarily on the stopping distance but rather tune the ends of the car with more or less rotor torque. The tires being the same point of road contact.

The bigger issue with pads are their use function with regard to temps. With a low Cf pad you can stop "race car like" with more pressure- just like oem parts on the track; "they were great for the first few laps, then...." . But with the proper track pad you can use less pressure, achieve the same tq and do it repeatedly without fade.

Well there are those that mod for show/track/and appearance, doesnt mean that people who mod for anyother reason than racing are "pathetic". :icon22:

What I would really love to see is thread devoted to posting the piston diameters of the different BBK's.

Does any one have a pic of what a twin mc setup looks like in a z? Also is this how they are able to ajust brake bias in the car, or can this be done with just a proportional valve.

A twin set up in a street car is both extremely messy and rather pointless. Figure on gutting your dash to put in a reverse hung set up. Probably not going to happen.

You cannot truly "adjust" brake bias with a prop valve. All they do is reduce rear line pressure based upon a predetermined knee point and to the percentage you want removed from that point forward with more pressure.

The more pressure in, the less pressure out. How much those to split is determined by your turning the knob. A twin MC set up offers true bais adjustment that's constant as its based upon the bores in use and the position of the pivot ball in the pedal. It simply allows you to select the amont of percentage you want and keeps it there regardless of psi.

You would need some pretty expensive testing equipment to rei-engineer the front/rear brake bias back into the system.