brake bias for cadillac caliper swap and other BBKs
 

wrote my first article on kinja comparing some of these kits and their bias changes with my best effort info:

http://sickmint79.kinja.com/ats-and-...390.1472560687

Rememeber, the stock system intentionally has a "little extra" rear, intentionally, so that VSC can function.

I wish more people thought their decisions through like this.

so between the two wilwood options (front only, no rear), 4 piston is better?

a buddy sent me this page after he saw the article: http://www.ft86club.com/forums/showthread.php?t=32594

wilwood had this to say: http://www.ft86club.com/forums/showp...6&postcount=18

although i don't find that comment very satisfactory.

per http://forums.nasioc.com/forums/show....php?t=1265631
the BRZ front rotors are 294x24 or 295x25.5

per http://www.ft86speedfactory.com/wilw...g#.V8uadPkrKUn
the 4R WRX rotors appear to be 305x20.5. i will let someone like CSG Mike comment on that, as others noted that seems rather thin. so i suppose there is a real question of how much improvement this kit actually gives you in thermal capacity, for the BRZ or WRX. (certainly more for us being 400 lbs lighter though.)

OP initial track impressions -
http://www.ft86club.com/forums/showp...0&postcount=71
http://www.ft86club.com/forums/showp...8&postcount=72

it would be cool to have some track temps.

some later impressions -
http://www.ft86club.com/forums/showp...9&postcount=76

those are simply not speeds i am running locally.

he appeared to eventually go to the 6 pot with a larger WRX rotor with OEM 86 still in the rear. i didn't see anything about a prop valve so it would seem this would have shifted bias forward a lot.

http://www.ft86club.com/forums/showthread.php?t=63636

I would like to mention that RR Racing also has a 6 piston Wilwood BBK (stage 1 & 2) which they claim to have tested and refined for proper brake bias on our platform. they're reasonably priced as well for $1000 a set. they do state to run more aggressive pads in the rear though if you're only running the front BBK. it seems to be a good option for the price to performance ratio if you're in the BBK market. however, for the majority of us, just upgraded pads and fluid will do the trick unless you want that bling factor lol.

^^^

I've got the RR Racing stage 2 BBK with BP20 pads. When using PMU HC+800 pads in the rear, there seemed to be a lot of front bias. I could hear/feel the front tires lock up early in braking zones. Switching the rears to Carbotech XP10 pads seems to have returned the brake bias to normal. Also appears to have taken a significant load off the front brakes overall (less wear, smell, etc.) I definitely think the kit works better with a more aggressive pad in the rear.

That said, theoretically... I wonder if you want brake bias to be pushed slightly forward on sticky track tires. Under hard braking there should be greater weight transfer to the front (relative to a stock tire for example).

i see lots of option at RR
http://www.rr-racing.com/Front-Big-B...gt86fbk001.htm

although i don't see anything about caliper piston sizing. it would be cool to add them if i can collect the info.

chat with them directly. updated! a lot of really nice options from them!

Great article. Glad more people are thinking about bias. When I did the calculation for stock a while back I got 65/35 assuming equal friction pads front and rear. I put on Raybestos ST45 front ST47 rear early this year which should have put bias closer to 60/40. I've been very happy with them. Only 2 track days at Barber and 2 AXs so far, but I have lots of confidence with the brakes. I haven't felt anything that makes me think I went too far rearward with the bias. One reason I did this was lots of people talking about the front brakes overheating. No hint of overheating and changing pedal feel at the track days at an intermediate pace. Hope to do a track night at AMP later this month. The brakes feel great on drives in the mountains. There's great modulation. I don't seem to have any problem getting just the amount of force I want when carrying the brakes into a corner. When simulating a panic stop, the fronts still get ABS activation with no hint that the rears are. I may have felt a hint of tail wag a couple different times when on the brakes in a slight turn on a road that had some loose sand/gravel. I had read of guys getting great pad life from Raybestos ST43 pads. Hoping that these are the same, because they aren't cheap. I'd get them again in a heartbeat though. I'm actually interested in trying out the cheaper Wilwood WRX 4 pots for the weight savings. I bet they'd work well for me with proper pads front and rear.

does anyone actually have the piston size from the RR kit or is it still just taking their word for it?

I hear the WRX 4 pots move the BIAS to the rear as wheel, does that make them a better option?

When I checked into it (Wilwood WRX 4 pot kit) a few months ago I calculated that it moved the bias forward about 2%.

Thanks

Sounds like it would be ok if you balance the pads a little. My other option was STi front's only. I think the stock rears are quite capable.

RR IS reasonably priced. And their bias seems no that bad off as some of retrofits. BUT - if even vendor says that for proper use one needs to use different pads front-rear, i translate it "we didn't care about getting right bias, so use this hack to workaround" putting big red sign against considering them. I get if such hack is used for retrofits, i get it if used for non-standard aero / staggered wheel & tire setup .. but NOT if for standard set marketed for most cars, without clearly stating if it's for some specific setups.

They use the standard Wilwood caliper piston sizing.
6 pot, 1.38 inches first piston, 1.12 inches 2nd and 3rd pistons.
4 pot, 1.25 inches on the 4 pot sport kit.

Rotor sizing is published on their website but it's just a standard STI rotor.

There is some context to this statement which I think is important. RR Racing seems to be of the opinion that our cars benefit from additional rear brake bias even when stock. They have recommended people with stock cars to have more aggressive pads in the rear. That is part of the reason why their rear kits send the bias so far backwards. While I don't think having additional rear bias will hurt, I don't really agree with the statement due to the overwhelming body of information and experience that says the OEM bias is pretty much optimal and honestly, I think it was just a marketing grab to get people to buy the rear kits.

Regarding the price point, it's clear to me that using standardised parts is how they achieved the price point they have. Unfortunately, this does mean that things aren't perfect and their front kits DO have a small forward bias shift when used with OEM rears. @JRitt ran the numbers in another thread that I can't be bothered to find right now where he saw around 3% bias shift forward on the 6 pot kits and 5% bias shift forward on the 4 pot kit. He was of the opinion that it was a bit too far forward for their customers who track at the highest levels of HPDE and motorsport but I think it would be perfectly adequate for the remaining 9/10 customers.

I mistakenly read this as the Wilwood 4 pot kit for the WRX which was discussed earlier. Haven't checked on the OEM Subaru 4 pots. Previous post from me refers to Wilwood.

One note, and something I've not seen mentioned on this forum (though it's mentioned in the Stoptech white paper linked in the OP's article)....

If you run very high grip tires, then you can potentially generate more forward weight transfer than on OEM tires. Essentially this means that the "ideal" brake bias for a car on hoosiers is not the same as it is for a car on OEM tires (or in the rain).

This is not to say that you should increase your front brake bias from OEM when moving to sticky tires. The OEM brake bias has other design considerations beyond just achieving the shortest stopping distance possible and the ABS system complicates things.

But a massive shift rearward can suck just as much as a big frontward shift. Small changes in bias can shorten stopping distances slightly and help a driver feel more comfortable with the car, but it's important to think about the complete picture.

A rearward shift can also be helpful if you're running aero that's actually effective.

- Andrew

i'm actually confused on the RR. on the one hand they provided that excel bar chart, and noted the friction was the same front and rear. i don't know how any inputs to it were made or measured at all though.

on the other, i got the piston sizes in an e-mail - and it's the standard wilwood 6-piston front - which using the calculator appears to push it almost as forward (12.88 wilwood rotor vs. 12.8 RR) - even if i try to make a better effort at comparing something like effective rotor diameters, the shift forward still ends up being a 4% increase vs. oem instead of 5% just using outer diameter.

so it is hard for me to conclude the bar chart is valid and the net change vs. stock is zero. thoughts?

1.62 1.12 1.12

and indeed the same caliper.

dark's experience http://www.ft86club.com/forums/showp...97&postcount=6 would seem to jive with it actually moving the bias forward as well. i am having a tough time accepting the bar chart now.

stoptech pushes it back a little, the sprint more... i'm under the impression when you go as far back as wilwood or RR front and rear together that you may/should have a fair amount of aero on the car?

a problem with my article/knowledge is that i don't really know where things are truly problematic... i do know that between 1000 and longer braking distances and 1500 and oem or shorter, that i'll happily spend the extra 500, even though i'm not doing anything on the track that would impress anyone. at least my car is more capable of it!

These calculations are sound and necessary for sanity check. However, a few % difference should not be a major factor to evaluate a kit. The actual bias will never be the same.

- front / rear pad and rotor temperature difference, thus friction difference, throughout a session or even one braking zone.
- RPM and gear dependent engine brake on rear.

These two factors alone can easily shift over 10%. Moreover, it is hard to say one bias is better than other. Optimum bias changes throughout braking phases. In the initial phase, I want very strong front bias as rear unloads (Wf >>> Wr). In the middle phase, I want mild front bias as rear settles(Wf > Wr). Towards the end phase, I want strong front bias (Wf >> Wr).

Thanks I worked out the maths WRX 4 pot will move the BIAS a little towards the rear, might actually be a very good combination with stock rears

correct. OEM WRX 4 pots are the same caliper as the Z32

w I thought the article raised a lot of good questions. I don't have the bandwidth to go through all of the numbers again, but it's great to see people thinking these issues through! Slapping the cheapest kit available on the car always appears to be a reasonable option in the short-term, particularly to someone who is working with incomplete information (which frankly, is most people since we can't all be experts on everything). Since you're going to own your car for a number of years however, it makes a whole lot of sense to carefully choose where you'll invest a couple grand of your hard-earned money.

One of the OP's quote in the article really made me think about the idea that someone may not know what they're really getting until they have it:

We (Essex/AP Racing) stand by our choices/options for FT86 brake applications. We did the math. We did our testing. We found that unless you have some fairly drastic modifications on your car (huge rear downforce), any significant bias shift either frontwards OR rearwards is inappropriate and will hurt your overall brake performance. That could come in the form of longer stopping distances, shorter pad and disc life on one or both ends of the car, or pad or fluid fade. Unfortunately some of those effects are hard to recognize if you have no baseline for comparison. That means you may be leaving a lot on the table and not even knowing it.

Our Essex Sprint and Endurance Kits are now running on many hundreds of BRZ's, FRS's, and GT86's all over the globe. Our incidence of problems or issues is incredibly low. Most of our customers tell us that our brake kit was the single best modification they've done for track use, and that it has saved them tremendously on their lap times, consumable costs, and wrenching time. Many of those customers have also had their kits on their car for four years now, and are still on the first set of discs! Finally, keep in mind that ALL of those customers are running OEM rear brakes with upgraded pads and lines...NOT a complete rear brake kit.

So was it worth a few hundred bucks more initially to get the best front brake system available? We think so, unless you're just looking for those extra 'hard parking points' mentioned in the article which come with a rear BBK. I saw a great quote the other day that I think is extremely applicable to the brake kit market. "Buy the best and you'll only cry once.":thumbsup:

I also questioned that bar chart and asked RR to publish all of the relevant numbers on their components for calculating bias somewhere fairly early on in their big thread about their brake kits. I was subsequently crucified for posting in their thread, told I had poor forum etiquette, told to go look up the information myself, etc. We subsequently ran the calculations using the standard Wilwood components, and determined that the bias numbers we found are definitely not optimized for the majority of our customers (mildly modified cars attending autoX, track days, HPDE, time trials, etc.).

I have the RR kit, I will post what happens to the car once I install matching set of pads front and rear... BP20 + Ferodo DS2500 felt ok, but the BP20 have 0 bite and I was used to running Raybestos ST43 on the OEM calipers... granted, I did have major Front bias on my car running ST43 front and DS2500 rears, and got used to running the car that way.

BP 20 + ST45 rear would cause the rear to lock up under braking, so not a good combination either, (had bought the pads to go with the ST43 in the OEM setup, but bought the kit shortly after and never bought ST45 for the RR kit)

I just ordered Ferodo 2500 for the kit to match my rear (and stop the excessive squeling from the BP20s) And will report after I try the setup at the track day.

What tire spec are you running? Would be interested to hear back on the results.

215-45-17 Hankook RS-3 on OEM wheels.

I have the RR 4pot Sport kit and I'm running it with BP10 in front and EBC Yellowstuff in OEM rears right now and I have been very impressed with this combination for daily and intermediate track use. Knowing what I know about the bias of the SP kit now, this makes sense because the BP10s have ~.45 peak friction coefficient and the yellows have ~.55 which I'd guess sets my bias close to OEM.

I do have a set of Yellows for the front kit which I had trouble fitting when I first got them. It could have just been because I was tired at the time but I think they may need a tiny bit of grinding to fit. At this point, I'm just keeping them around as spares for when the BP10s wear out.

You have to check what size are the pistons... as the 4 pot come in different piston sizes raging from twin 1.12 inch pistons all the way to 1.88 inch pistons... Your kit could have a different bias depending on this.

i need to update the article with new information on RR. i think the one thing i don't currently know is the piston size on that sport 4 pot. it is one of these superlite 4's i believe:
http://www.wilwood.com/calipers/Cali...ite%20Internal

I was pretty committed to getting the RR stage 2 BBK, but then I started second guessing myself with the AP BBK. >.>

I guess I am about 25% on the aesthetic side and 75% on the performance side. the 25% goes to the RR racing with both front and rear BBK while the 75% goes to the performance of the AP racing BBK while it's only the front BBK and stock rears

ryoma: there should be also some % for less often replaceable wearables with increased thickness pads & disks. From those 75% mentioned i guess you also track car .. then less money spent on wearables becomes important enough imho.

i believe this car is polymatrix b, front and rear, front and rear wilwood kit, rotors with 10+ track days on them. non-trivially faster driver

https://scontent.ford1-1.fna.fbcdn.n...98340277_o.jpg

this one is polymatrix b front, oem calipers rear, i believe xp10 rear, rotors replaced 2 weeks ago; after 6 sessions, 2 wet. some angles look better for the slots but overall the wear on these looks surprisingly quick.

https://scontent.ford1-1.fna.fbcdn.n...92194037_o.jpg

How are they even worn down that fast :confused0068:

So, today I took the car to the track, ran Ferodo DS2500 front and rear, I feel the braking is very neutral, close to OEM, but I decided to lend my car to an instructor who bought an FRS that came with STI front and rear calipers and drove it like that for half a year.

He confirmed that the RR-Racing kit is well balanced compared to how front biased the STI brembo felt on his car... He now has AP racing Endurance kit, but hasnt had the chance to test them.

updated this - https://www.pointmeby.com/2016/09/24...z-frs-gt86-86/

Love the "about the author"!

I noticed you posted that RR Racing SP kit uses 38mm pistons. I recall that I had measured my pistons with a crappy ruler when I got the kit and found the same value and posted that in a thread somewhere. Did you get that number from my post or elsewhere? I'm just curious because if the number came from another source, then that is totally cool that there are 2 sources for that value and the SP kit is actually really well matched with the OEM rears.

no, i used your number. the calc essex used got it from a wilwood caliper but i wasn't clear how they knew which config? and those are the only 2 data points i know for the kit. so next time you are fishing around in there let me know if you find a better measurement tool! ;p

Cool, I'll spend a few dollars on a proper tool so I can get an accurate measurement. I'm going to be swapping pads and doing final partial bleed in preparation for the winter season in the next few weeks so I can measure it then. I'll post my findings here when it's done.