Getting my head around BBKs

[TommyC 33]

I've been trying to understand how brake bias and pedal feel/travel are affected by big brake kits.

I'm really stuck on the fact that 4pot calipers must end up needing a greater volume of fluid to be 'moved' by the master cylinder than the standard ones. So I mean the distance the pedal travels under braking must surely increase with a 4pot?!

I've read on other posts that the standard calipers have 2 pistons slightly larger than 40mm diameter. This gives 10,250mm2 (square millimetres) of piston area. For a 4pot caliper to have approximately the same piston area (and therefore achieve the same pedal travel) the pistons would need to be about 28mm diameter.

Most 4pot calipers I've seen offer different piston sizes, but none this small.

Does this mean you need to upgrade the master cylinder at the same time as fitting 4pots?

Has anyone with a BBK noticed greater pedal travel after fitting?

I like to understand the engineering/mechanics behind stuff and if anyone can explain I'd be grateful 🙂

[Church 209]

You shouldn't lump all of aftermarket BBKs together. Some vendors care more about keeping brake bias stock with careful part selection & testing it both on special brake test rigs & also on skidpads, some less, just quickly throw together parts from some part bin, make mounting adapter to fit it to specific car and call it done, some try to keep bias for front only BBK if used with OE rears, some may require to pair with their rear BBKs too, some may intentionally shift bias some way because they "think" that it will be better or for cars with specific aero mods that change grip bias and may benefit from brake bias change aswell. And then there are some users that don't care about bias and brake performance at all, if they only can put eg. brembo branded brake calipers for cheapest budget possible, even if retrofitting from completely different car, eg. cadillac sts, evo or sti. Also some race setups may have installed brake bias adjuster in mind.

Hence i'd probably look more to big names/big vendors with proven reputation in field BBK products and look up description for those and inquiring extra nfo from vendor via email, if needed. GT86 tS & BRZ PP brembos, TRD brembos, APR formula BBK, Willwood kit for our cars, APR Essex sprint & endurance kits and such.

There are several variables that may affect bias. Not just caliper slave cylinder areas, but also rotor diameter, brake piston distance from wheel center, brake pad shape & area. I'd be careful with trying to "calculate" bias by using just some of variables, if you don't know the rest.

P.S. depending on your needs (eg. if driving reasonably short track sessions, not having forced induction and very grippy tires), it's possible that you may get by with stock (imho rather good for OE) brakes, just by changing pads/brake fluid and maybe adding brake air ducts. At very least this should much improve different wheel fitment/brake caliper clearance vs aftermarket BBK options. Should also be much easier on initial investment required and be much better for daily driving in corrosive winter-salt scenarios. (Some BBKs sometimes may let save on wearables in long term, if one tracks a lot).

[Lauren 2259]

I have AP four pots. I haven't changed the rears. There is no difference really in the brake bias that I have noticed. Pedal travel is not increased. You've probably read about the poverty spec front calipers that are with the smaller front discs. This isn't an issue in the UK unless you have a Primo. 

So there is no need to change the master cylinder, you can just buy the AP four pots with 330mm discs, fit them and  be done. Reyland offer the best deal on this really at £1500 for the kit. 

[Lowe 191]

Can't say I've heard it happening with the 86 platform, but on some cars it is possible if fitting a badly set up BBK to drain the master cylinder which can be fun! These are all pertinent questions though, and ultimately you need to talk to the manufacturers to understand their goals when designing the kits. I doubt most tuners and retailers will know if there's any significant bias shifts or if the kit is designed to work alongside stock rears or not. Like you say there's so many variables to take into consideration that unless you've got either some cast iron data or a bias valve and a private road for testing/set up, I'm not 100% sure it's worth the outlay for most people. The stock brakes aren't that bad really.

[shiftspark 41]

I think @Ade had some calculations on brake bias etc on his project thread.

[TommyC 33]

11 hours ago, Church said:

You shouldn't lump all of aftermarket BBKs together. Some vendors care more about keeping brake bias stock with careful part selection & testing it both on special brake test rigs & also on skidpads, some less, just quickly throw together parts from some part bin, make mounting adapter to fit it to specific car and call it done, some try to keep bias for front only BBK if used with OE rears, some may require to pair with their rear BBKs too, some may intentionally shift bias some way because they "think" that it will be better or for cars with specific aero mods that change grip bias and may benefit from brake bias change aswell. And then there are some users that don't care about bias and brake performance at all, if they only can put eg. brembo branded brake calipers for cheapest budget possible, even if retrofitting from completely different car, eg. cadillac sts, evo or sti. Also some race setups may have installed brake bias adjuster in mind.

Hence i'd probably look more to big names/big vendors with proven reputation in field BBK products and look up description for those and inquiring extra nfo from vendor via email, if needed. GT86 tS & BRZ PP brembos, TRD brembos, APR formula BBK, Willwood kit for our cars, APR Essex sprint & endurance kits and such.

There are several variables that may affect bias. Not just caliper slave cylinder areas, but also rotor diameter, brake piston distance from wheel center, brake pad shape & area. I'd be careful with trying to "calculate" bias by using just some of variables, if you don't know the rest.

P.S. depending on your needs (eg. if driving reasonably short track sessions, not having forced induction and very grippy tires), it's possible that you may get by with stock (imho rather good for OE) brakes, just by changing pads/brake fluid and maybe adding brake air ducts. At very least this should much improve different wheel fitment/brake caliper clearance vs aftermarket BBK options. Should also be much easier on initial investment required and be much better for daily driving in corrosive winter-salt scenarios. (Some BBKs sometimes may let save on wearables in long term, if one tracks a lot).

These are also my concerns Church. I'm rather impressed with the standard brakes but I just don't like the fact they're covered in surface corrosion 😕

My thoughts were to take them off to clean them up and paint them, but then if I'm going to the hassle of that I might as well go for something that looks far nicer and performs even better! (But then there's the price 😲).

3 hours ago, Lauren said:

I have AP four pots. I haven't changed the rears. There is no difference really in the brake bias that I have noticed. Pedal travel is not increased. You've probably read about the poverty spec front calipers that are with the smaller front discs. This isn't an issue in the UK unless you have a Primo. 

So there is no need to change the master cylinder, you can just buy the AP four pots with 330mm discs, fit them and  be done. Reyland offer the best deal on this really at £1500 for the kit. 

I'll have a look 🙂 Are these bells and rotors? (I would hope so at the price).

[nerdstrike 186]

I'm not convinced it follows that more piston area must give more pedal travel. The pads are already skimming the disks, so any movement is limited to compression of the pads, deflection of the disk, and squidge in the brake lines. If the brake lines have seen a lot of miles, I can see how the oblig. replacement braided lines with the new calipers might undo some of the squidge you'd grown accustomed to, resulting in similar feel despite greater displacement.

It also depends on the channels inside the pistons - it wouldn't surprise me if the modest BBKs were deliberately keeping the volume close-ish to OEM to avoid having to fit new master cylinders. A bigger evenly compressed pad should give you more braking with less pressure after all. Then there's mass transfer to consider with the brake bias...  goodness me it's complex if you want it to be!

[Lauren 2259]

7 hours ago, TommyC said:

These are also my concerns Church. I'm rather impressed with the standard brakes but I just don't like the fact they're covered in surface corrosion 😕

My thoughts were to take them off to clean them up and paint them, but then if I'm going to the hassle of that I might as well go for something that looks far nicer and performs even better! (But then there's the price 😲).

I'll have a look 🙂 Are these bells and rotors? (I would hope so at the price).

Yeah of course. That's for non-floating discs (which seem overkill for me), but you can pay more for floating discs and even more for AP discs. It's a great deal £1500 for everything you need. Mine have been on nearly a year now. They've been amazing. The stock brakes are fine for the road, don't bother changing them if that's all your doing with them. I do quite a lot of track work as well as competing in the sprint series. I've found the stock brakes with aftermarket pads do tend to fade away if you push them hard. The AP's on the other hand are pretty much endless and more importantly over anything else they keep their consistency. I do agree the stock calipers are horrifically ugly, though I care not about stuff like that really. It was far more about performance for me. 

Mine:

IMG_5057 by Lauren Blighton, on Flickr

[surrey86 352]

Another advantage is the BBK will be lighter.

You wouldn't believe it unless you fitted yourself but the larger calipers are a fraction of the weight of the OEM stuff, light enough to lift with your little finger! Must be saving over a kilo a wheel.

[will300 812]

There is some good information about brake bias here: https://www.pointmeby.com/2016/09/24/which-big-brake-kit-bbk-should-you-get-for-the-brz-frs-gt86-86/ (these is based on calipers available in US)

[TommyC 33]

15 hours ago, will300 said:

There is some good information about brake bias here: https://www.pointmeby.com/2016/09/24/which-big-brake-kit-bbk-should-you-get-for-the-brz-frs-gt86-86/ (these is based on calipers available in US)

Yeah, that's well written and reasons out the folly of pushing bias forward from standard. I'd never really considered that throwing it forward like some badly thought out kits do, or chucking on 4pots that 'fit', can actually INCREASE stopping distances 😬😲

The more I explore this the more I feel things are best left alone brakes-wise unless there is a real need for improvement (track use etc).

That said, I still want something better looking behind the wheels! I think I'll have to save up and buy proper stuff front and rear. Best get on a few track days so I have some justification 😆😆😆

[Church 209]

Just better looking? Consider spray painting calipers. (of course using special paint that can stand better high temps). I guess, should cover 80% of that goal, for very cheap.

[TommyC 33]

1 hour ago, Church said:

Just better looking? Consider spray painting calipers. (of course using special paint that can stand better high temps). I guess, should cover 80% of that goal, for very cheap.

Yeah, that'll be the direction for now I think 🙂

But not some of these nasty things!! 🤢

https://m.ebay.co.uk/itm/4x-Black-3D-Style-Brake-Caliper-Covers-Universal-Car-Front-Rear-Kits-10-5-UK-/112656908220?ul_ref=http%3A%2F%2Frover.ebay.com%2Frover%2F1%2F710-53481-19255-0%2F1%3Futm_source%3Dtwenga%26utm_param%3DeyJlcyI6MCwicyI6OTcyMDIzNywiY2kiOiIwMzRlYjZhZTgzMTMyMDgwOTJiYWJmMGMyYzRkMzI3NiIsImkiOiIzMzE2MjA2MDE3ODczNzAyNjMyIiwidHMiOjE1NDA1Njk1MjEsInYiOjMsInNvIjoxNTAwLCJjIjo2MzcxN30%3D%26utm_campaign%3Dtwenga%26utm_medium%3Dcpc%26ff3%3D4%26pub%3D5574631662%26toolid%3D10001%26campid%3D5338353462%26customid%3D034eb6ae8313208092babf0c2c4d3276%26mpre%3Dhttps%3A%2F%2Fwww.ebay.co.uk%2Fitm%2F4x-Black-3D-Style-Brake-Caliper-Covers-Universal-Car-Front-Rear-Kits-10-5-UK-%2F112656908220%26srcrot%3D710-53481-19255-0%26rvr_id%3D1717831764500%26rvr_ts%3Db11962571660a86689a72c83ffff33bc&_mwBanner=1&_rdt=1&ul_noapp=true