Brake bias calculations for BBK
 

Hi guys, I'm curious to find out how the brembo upgrade will affect brake bias percentage (at least on paper) compared to stock brake system. I've found this handy calculator which lets you input your current and modded setup so that you can see any change in bias percentage: http://www.main.experiencetherave.co.../brakemath.xls

Using that calculator, and with the standard BRZ/FRS setup as a reference point, I was able to determine that by upgrading to the STI brembo setup all round (F + R) would result in a forward bias increase of 16%.

Is that something that you'd consider acceptable?

Anyway, my 86 is the "lower spec" model which has a slightly different factory setup (smaller brakes than the BRZ/FRS), and I'm having difficulty calculating specifically for my car....as there's no data available for my rear brakes. For instance, my brake size is:
Front: 277 x 24mm dual piston (ventilated) - from the NA Impreza
Rear: 286 x 10mm single piston (solid) - no data available?

The nearest size is the 290 rear rotor from the BRZ/FRS, and if I selected that as my original rear brake, the calculator tells me that by going full brembo's all round my brake bias will shift forward by 25%!!! Holy cow that's alot don't you think?

So perhaps I'll need to run OEM pads for the front brembo's, and a street performance pad for the rear brembo's. What do you guys think?

Pads are cheap.

Put the brembos on the front.

Run whateverpad you need to at the back.

^what he said

I've already purchased the rear brembo's so would it still be safe to use them? I don't mind replacing pads more often...

I've said it in the other thread a couple of times, but I would not run the rear Brembos period. That's just me...I'd choose something else.

- Andy

What about the Subaru 4/2 pots? I thought I remembered you or someone saying they were better than the Brembos as far as bias goes.

Any chance you could comment on the oversized 4 pot kit I posted up?

http://www.ft86club.com/forums/showthread.php?p=1773632

I'm also curious about what kind of bias you think would be appropriate for this car.

I forget what the bias works out to when using the 2 pot Subaru rear, but it's better. The front subaru 4 pot is really only an upgrade in brake feel, weight (i think), and looks. I'd do it if I found a decent deal.

As for that oversized 4 pot kit, just the hassle of custom rotors and pads makes me say "meh."

I really like the stock bias or close to it. And I like to use the same pad compound all around to keep things simple too.

EDIT: On the bias change, the rear FHI 2 pots shift bias forward. Front FHI 4 pots have no effect on bias.

- Andy

Thanks. Yeah I think I recall seeing you saying that somewhere else. But I'm also curious as to what makes the bias so "un-balanced" when full brembo's are fitted to this platform? Maybe I'm being this too simplistic but how come they work well on the WRX but not on this platform?

It's because they were designed for a different car. A 2006 STi has a curb weight of 3351 lbs and a weight distribution of 59/41, which means 1977 of those lbs are on the front axle. Compared to a BRZ that weighs 2750 lbs with a weight distribution of 53/47, or 1457 lbs on the front axle. All that is static weight.

Then you get into weight transfer. The STi is a big car, comparatively. So it's got more inertia when moving than a BRZ. I wasn't able to find any info on its center of gravity, but I have to assume that it's higher than a BRZ's, which is only 18 inches. So when you step on the brakes, there's going to be a lot more weight applied to the front axle and the front brakes than there would be on a BRZ. The taut suspension helps control weight transfer, but there will still be some.

STi Brembos are designed to control the amount of weight presented by the weight transfer of the larger, heavier vehicle, which means that the fronts will be very overpowered when applied to a smaller, lighter vehicle that doesn't put as much weight on the front when it brakes. For the rears, they're designed to manage the weight that's left over from the fronts. Because the two vehicles don't transfer the same percentage of weight forward, it will upset the bias of the smaller vehicle.

I'm sure I'm wrong on some details and I've grossly oversimplified the entire process, but the concept is there. RCE or CSG will correct me if they see fit.

They work in this application as they do on the WRX...it's not different, it's just not talked about very often because people want to feel that their four corner Brembo setup is the shit.

The rear Brembo setup has smaller pistons than the rear Subaru/FHI 2-pots which in combination with other factors causes reduced rear brake torque. The upgrade you will feel the most with always be the front. They do ~70% of the braking in front engine cars.

What you can do and I know plenty of WRX/STI folks who do this as a solution: run a more aggressive rear compound. You want to balance that out as too aggressive in the rear will encourage wiggling and instability under braking. Using Legacy777's spreadsheet that you linked, using a low cF bad up front and a standard street performance pad with higher cF actually gives you a pretty negligible change in overall balance - slightly rearward.

Thanks that does make a lot of sense....but then I wonder then how the BRZ TS feels on hard braking since it comes with brembo's? I'd have to assume that Subaru would have taken all this into consideration before slapping them on the BRZ....no? As far as anyone can tell the calipers are identical to the STI, and they made no mention of changes to MC and related stuff (not that I know of anyway). Perhaps they accounted for this in their suspension tune...

I'm sure the BRZ TS system isn't a carbon copy of the WRX STi system. They would have had to make changes somewhere to make a more effective bias.

That's interesting, I didn't know that.

Based on the spreadsheet I linked earlier, the rear FHI 2-pots has the same piston size as well as rotor diameter, so overall rear bias remains unchanged compared to stock BRZ/FRS. But you do gain a lighter stiffer caliper with 2 opposed pistons....:iono:

Hm. The 2 pots have a larger effective disc diameter, compared to yours anyway. For the 290mm rear folks (US/Cad/high-spec) it's aesthetic - which is why I haven't done it yet.

Hmmm somehow I think the extra piston does not mean double the piston area as such....something to do with total clamping force being similar even though your got more piston)....because if that was correct then we wouldn't be having the issue of brembo rear having less piston area versus stock (coz brembo rear caliper has 2 smaller pistons instead of 1 larger in the stock caliper). Anyway I might be totally wrong here :bonk:

I was told the below when I asked a similar question:
From this thread.

Yes, you only need piston area on one side with opposed piston calipers. And also yes, there is a very slight bias shift rearward with the subaru wrx 4 pot front.


EDIT: I'm wrong on the bias change, the rear FHI 2 pots shift bias forward. Front FHI 4 pots have no effect on bias.

- Andy

So is there really a benefit to going to a 4 pot front caliper vs. staying with the OEM 2 pot sliding caliper?

Aside from looking cool? :D

For the subaru 4 pot front it's just the coolness, a slight change in weight, and a slight change in feel.

The brembos get you a much bigger heat sink, but more of a shift in bias.

- Andy

The FHI 4 pots have smaller piston area than the stock 2 pot sliders, using the same effective diameter - hence they move bias rearward.

Yeah I was editing my post when you quoted that.

Switching to brembos is going from a 295mm disc to a 326mm disc, plus adding 6mm more width to the rotor and god knows what to the piston area. I wasn't able to access Fizz's spreadsheet.

Just switching to the 326mm disc should multiply the applied torque by 1.1 because of the extra distance from the point of rotation to the point of applied pressure. I wonder if our calipers would fit over a 30mm wide rotor?

No. Legacy GT fronts however, will. Those are paired with a 316x30mm OEM rotor.

Curses, there goes that idea. Thanks for the tip about the LGT calipers.

The OEM rear calipers should work. Ours is 290x18, a 2004 STi's is 316x20. Just need to make a bracket for the caliper.

You will need the full assembly from the front of the LGT, including dogbones. I don't recall where the bleeder is, but you might have to swap the calipers right to left to get the bleeders in the right place. Both pistons should be the same size in the caliper, so no concern with flipping them side to side.

This.

Also @gramicci101 you could do this: http://www.knsbrakes.com/c/caliper-items/2279

Nissan Z32 adapter bracket, Nissan Z32 caliper, LGT rotor.

Interesting, thank you.

Z32 calipers are dirt cheap as remans too. You might even be able to find aluminum ones.

I've been considering a slightly larger BK (e.g. Tarox B32 should save about 10 lbs in total).
Example:
http://www.factory-solutions.com/cms...r/DSCI0432.JPG

However, even with slicks I'd actually prefer more of a rear bias (and not more front bias).
Has anybody installed pads with a higher friction coefficient in the rear in order to work around this and if so what option did you choose?