The ATS/CTS caliper Big Brake Kit swap has a lot of traction, although seldom to never does anyone appear to make a note of, nor attempt to fix, the front bias problem that results from simply choosing a caliper that fits vs. choosing a package designed for the car. So I decided to make a good attempt at calculating the change in bias for a slew of the popular kits available for this car so that people could understand their options better. I’m not a brake engineer and am 100% open to constructive criticism and ways to improve the accuracy of this article.




It’s not this pretty anymore....

Why a BBK?



+10 hard parking points for looks greater thermal capacity to reduce/eliminate brake fade - which I’m not going to believe is a challenge for you unless you track the car possibly improved (shorter) stopping distance, if a more optimal front/rear brake bias ratio is set vs. stock. the opposite is also true if the balance is worse than stock.


Useful inputs to this article:

TCE Performance Brake Bias Calculator



StopTech White Paper on Brake Bias



This data is not intended to be perfect, but is intended to be useful. All numbers should be taken as rough. The only numbers varied from the defaults will be for front and rear piston size and rotor diameter. I believe a more accurate representation of each kit would actually use the effective diameter (where center of pistons squeeze?) of the rotor as well as the radial pad height. I’m having trouble tracking down the rear piston size so using 1.5" plucked off someone else’s thread. All inputs in inches produce the following results.


Brake Bias Calculator Inputs

The results are in!


It should probably appear as no surprise that the kit with the least amount of brake engineering in it would result in the greatest push forward in front bias. In all likelihood this also means the front tires are doing too much work, the rears too little, and optimal braking has been compromised; thermal capacity is up, yeah... but so is braking distance. One needs not thermal capacity on the street, nor do I think my typical #trackdaybros want less than optimal performance. The ATS caliper swap may be the cheapest BBK, but you’re paying to brake worse. Ew!


The following chart is from StopTech is for a 2001 S4 and not a BRZ, but is useful in understanding bias changes; what spurned this article originally was people not knowing or shrugging off the ATS swap bias change. Bias change on an S4 from front (left) to right (rear):


Expressed in terms of F:R ratio along the top, we see stopping distance increase with too much front or rear bias. I believe the pattern exhibited here to be what is likely common in how most cars are designed; OEM bias being on the cusp of a balanced setup and skewed towards the front. Per CSG Mike on FT86club, OEM bias intentionally is a bit rearward to execute stability control functionality. Note a ratio of 2.6 to 2.9 above - a “small” change of +0.3 - has significant changes on this car. We don’t have a chart like the above for the 86, but the values from our calculations are below.


I have two friends with the Wilwood front and rear kit; one ran with the provided front pads (Wilwood BP10) in the front but originally Carbotech XP10 in the rear. He said he was locking up the rear even under partial braking; this would seem to jive with the rearward shift above and much higher coefficient of friction for those rear pads.

Fixing the Front Bias Problem

Step 1: all the people claiming there is none can recognize there is a problem. A proportioning valve can be installed to adjust F:R bias; it seems like it should be a mandatory component of the ATS kit however it is missing in the required components, suggested components, and instructions for this kit here. FRS BRZ 86 CTS Swap. I am not under the impression it is all that easily to calibrate and it would seem to take a lot of adjustments and trial and error in a parking lot with some chalk to see what kind of performance you could achieve. Nice to have an OEM or other BBK friend to chalk up a benchmark.


Alternatives include trying to find just the right higher friction pad in the rear, which seems odd, or installing yet larger components on the rear, which per some calculations seems to typically result in less front bias, not optimal bias.

Kits That Do Bias Right

One of the original articles is by StopTech, and I’ve calculated this on the BRZ and WRX both now; they have 50 piston combos on the ST40 and it seems like they run the math and stick in the right configuration. Moving towards more rear bias seems to range from little difference to optimal - my guess is AP Racing did the math, then a lot of testing, and came up with their choices. Let’s look at some individual kits. I’ve additionally included RR Racing below, with great kits but bias calculated separately from above.


User swift996 on FT86 had this setup. I’m updating this article as I learn more and the thermal capacity of this kit seems questionable. The OEM BRZ rotors I believe are 294x24 mm. The rotors in this kit are 305x20.5 mm; surprisingly thin. The bias is great, but is this a problem?


Do you track? If no, these are fine. Still a great and very affordable option.

If yes, you may find these inadequate due to your skill level, braking style, power level, and tracks you visit. Swift had an Innovative supercharged BRZ and in some initial posts everything sounded great.

I ran a track day at Mid-Ohio yesterday. I had 7 or 8 20 minute sessions. Not that Mid-Ohio has a ton of heavy braking zones (with the chicane configuration) but the brakes performed great. No fade and very consistent.



Still holding up great had 4 sessions today at Summit Point - Shenandoah Circuit. I might try some different pads. I rode in a workers car who had AP Racing brakes and I was a little jealous, those things are the real deal. Part of it might be sitting in the passenger seat, but I was impressed.




However he ultimately did swap out to some other brakes after VIR.

Well I did have some issues but not sure the root cause.



- Brake Fluid wasn’t completely flushed (so a mix of RBF 600 and the OEM stuff)

- Rotors were wearing heavily from the BP-30 race pads (compound is probably from a meteor)

- VIR has some long straights [...] I think these might be a little limited with forced induction on a large track without brake coolers. I had no issue at Mid-Ohio or Summit Point but carrying speed of 135-140mph and slowing to about 40mph is a lot.



I had an initial 4 pot kit with really thin rotors, good for autocross/street use but failed when I was at VIR. I did have the supercharger on but still, I think they would have failed under stock power. They did hold up fine at Summit Point on stock power but not as big of braking zones there. Bottom line, if you can drive at an intermediate level on track, that kit isn’t sufficient.




Is this kit for you? Maybe. It uses a larger caliper (DynaPro) but only requires 15" wheels to clear, it appears. Off track, you’re all good. On track? Depends a lot on your skills and your tracks. It’s still a very valid option and winner for the 86 platform, although I fail to see how it would actually be a great fit for a WRX weighing 400 lbs more. If you are in this market, also highly consider the RR Racing options below, which I do think are superior at this price point in terms of performance.

I have this kit, and I also have some data. I consider myself an intermediate+ HPDE driver. I mostly go to Gingerman, Blackhawk, and my favorito Autobahn. I am near Road America although have not been on it with the BRZ; it is a much faster track than these. I’m sure the BRZ would hit 125 stock on a straight there, if not easily more, and mine is not stock.


My fastest straight may be 105. I’m on e85 with a header and tune. My fastest apex 70. I have CarboTech XP8 pads front and rear.

Eh this stuff was a lot runnier than I expected. Yes I do paint like a child.


Using Tempil paint on my rotors, they are peaking between 750 and 1000 degrees F. Pads are probably 10% cooler. Using Alcon caliper strips, my calipers are peaking between 370 and 410 F. This is also a proxy for fluid temp which probaly runs a little cooler. These are well within the operating temperature range of both my pads and fluid. It is likely the Wilwood 4R would be sufficient for me.

I recently supervised/held my beer while a buddy replaced friction rings on his Wilwood 6R. I have quite a few more track days than him, so I’m not sure why his needed to be done already. However they were much cheaper than mine, $300 for the pair. Through a buddy for a deal, mine are going to hit $450 for a pair. Mine do come with all new screws and clips hardware though for the 2-piece rotors, which the Wilwoods did not; all components were reused. My CT pads appear to be 16mm thick of pad material and 23mm including backing plate, or 0.63" and 0.91" respectively.


This kit is also the only one of the individually broken out kits that has dust boots, of interest in salty/dusty areas or year round use. Mine are a bit cooked and brittle though. In the other systems care should be taken to clean the pistons nicely before pushing them back in.

My 17x8 RPF1s fit over these, no spacer, and have the weights mounted on the outside of the wheels. 17x9 have straighter spokes and would not fit. I also have no interest in running spacers. All systems have a bit different sized calipers and fitment tolerance to be checked with your own planned set up.



The AP Sprint kit also is affordable and well engineered. It doesn’t come with pads by default - although many of us probably eschew the provided pads in other kits for our own choices. I believe they are 0.8" on this kit including the backing plate.



This is a very popular and well engineered kit, with a lot of happy fast track guys running it. Very thick rotors - 299x32mm - and I think a bit better clearance than the StopTechs. I’m not sure if the discs come with new mounting hardware but appear to be around $300 a pair for new friction rings as well.


This is the only number where the rear was toggled as well - if I have it wrong, the deviation is off. If I have it right, it’s quite a large swing - I wonder how that affects ultimate performance on our car. If my OEM piston size on the rear is off this is probably largely exaggerated. Getting some of the finer details right would probably show less of a drastic change as well, for example using the correct pad radial height and rotor effective diameter. However this is the best information I’ve got at the moment. Is the swing truly this large? I don’t know. The kit does have a lot of rear brake though. The RR Racing kits push it back with rears too, I’ll comment more on that under them below.


This kit uses narrow calipers, which allow for much greater wheel choice than many other kits. I do not know hat this means in terms of caliper or fluid heat though; one would suspect possibly higher peaks. I have no data. The pad appears to be a little thinner as well - 0.65" on the narrow caliper vs. 0.80" on their other calipers - this is total thickness including the backing plate. The latter size and other options using Wilwood calipers are available through RR Racing packages.


RR Racing has a lot of nice options. I am listing them out separately due to having some bias information from them directly, so we’ll ignore an apples/oranges comparison with the numbers I calculated above and give them their own area.


RR Racing has the same piston area on all their 86 offerings. Each kit also has the same 12.8" rotor. They offer a Sport Performance kit, then a Stage I through IV kit. Same piston area, same rotor = same bias. As shown in the chart above, OEM Front/Rear match bias with RRR Front/OEM Rear. To the right we see options with rear brake upgrades which shift the bias more towards the rear, a pretty good amount.


I asked about the advantages of the bias towards the rear; it was noted that it can become a big advantage when running cars that have added rear downforce, such as the Element Tuning time attack car. RR noted even without ABS, they haven’t induced lockup on a 1.7 F:R ratio with the sport performance rear. This setup also takes more heat load off the front and distributes it to the back.

Do you even downforce bro?


The Sport Performance kit uses a caliper that I believe is larger, so potentially more wheel clearance issues, than the Wilwood WRX 4R kit. Outside of that though, it looks like it is hands down the cheapest kit to buy and run. It’s cheap to get, the rotors are way beefier than the WRX 4R, and you’re replacing them for probably the same or less cost than a 4R friction ring and certainly less effort. Whether you want more clearance and/or bling, the Stage I and Stage II options are far better choices than installing the Wilwood 6R alone, as they do not shift the bias. Want to reduce weight and increase thermal capacity? Go for Stage III or Stage IV, again better than doing 6R alone.


These guys look to have a great entry product, and a great way to add front brake without upsetting bias, and choosing to take on more rear once you have the appetite for it. With the Wilwoods IMHO one should not be installing the 6R without the 4R. I have two friends with that kit, who do in fact have it done both front and rear.

All The Rest...

I would not bother with the Z32, Wilwood 6R only, nor ATS swap on this particular car myself if starting from scratch. There’s already a lot of options out there that are much simpler to install, affordable to obtain, and maintain or improve OEM bias. Also let’s be quite clear on this: no kit actually engineered for the car is moving the bias forward even one smidge.