Brake bias calculations for BBK

[Fizz]

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?

[diss7]

Pads are cheap.

Put the brembos on the front.

Run whateverpad you need to at the back.

[Roadcone]

^what he said

[Fizz]

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...

[Racecomp Engineering]

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

[Cuchullain]

Originally Posted by Racecomp Engineering

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.

[Racecomp Engineering]

Originally Posted by Cuchullain

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

[Fizz]

Originally Posted by Racecomp Engineering

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

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?

[gramicci101]

Originally Posted by Fizz

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.

[OrbitalEllipses]

Originally Posted by Fizz

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?

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.

[Fizz]

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...

[gramicci101]

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.

[gramicci101]

Originally Posted by OrbitalEllipses

The rear Brembo setup has smaller pistons than the rear Subaru/FHI 2-pots

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

[Fizz]

Originally Posted by gramicci101

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....