E89 BBK calculator

[B21]

A few people expressed concern about brake bias as a result of twiddling with the front and rear discs..

So I've done a calculator for the E89 with the current used and known discs, calipers and pads to look at how different combos work out.

I've assumed all pads have the same coefficent of friction, which for street pads is not unreasonable..

As you can see, both what BMW did on various E89s and what I'm suggesting for BBK upgrades pretty well match..

For those that want to understand brake bias I refer to .. https://youtu.be/yeuLeDlTqIs?si=mfF8rk6m7nCfbWWS

If you want to noodle here's a copy

https://www.dropbox.com/scl/fi/rt7ceuw6 ... l74bi&dl=0

[RobbiZ4]

Cool!

[B21]

Cool!

High praise indeed

[RobbiZ4]

Well, I have to compare that with my 380 front brake first. Then I will extend my praise.

[B21]

So I had made to errors in my speadsheet..

I was using diameter rather than radius..but since I'd used it consistently it don't affect the ratios etc..also on al ot of investigation I confirmed the Mustang caliper uses 3 sizes of piston.. 34/36/38..I used 34 as an average..I reworked it with 36 and also did a seperate fresh calc using each size and addon them up..same result

As to master cylinder size / issues I'll cross that bridge when I get to it..AFAIK people who have used the M2/M4 brembo caliper haven't reported any issues..but the increase in net piston size means more grip on the front..so definitely need the bigger disc / caliper on the back to avoid going too far out of bias..bottom line shows what happens with stock 35is rear brake..of course E89 has EBD anyway..

I'm putting stainless lines all round which should help amelerioate any give in the system..revised spreadsheet here..

[axelleveau]

Thanks for sharing all that info B21. On the topic of master cylinder, there is something that keeps bugging me when talking about front to rear bias and focusing on discs sizes: Isn't the bias supposed to be mostly a master cylinder thing and disc size but not a piston size/amount? My rational with this is that the MC is what displaces the fluid volumes. If there is more piston area then the pressure increases less for the same fluid volume displaced. So what would drive front and rear pad force is always and only the MC volumes. Coupled with discs diameter this then sets the front and rear braking torques. Never had the time or tools to dig into this more though I "could" setup proper simulations for that...

[Ed Doe]

You could probably replace that calculator with the flowchart:

1. Do you want a bbk for aesthetics? If yes then crack on the rest means fuckall to you. If no then continue.
Do you track the car? If yes continue, if no, you don't need a bbk
2. Have you done more than 5 trackdays in the car, and received some tuition. If yes continue, if no, do more than 5 trackdays and some tuition and come back to this question
3. Have you upgraded your pads with fresh fluid, stock discs and calipers? If yes continue, if no, try upgraded pads and regular brake fluid flushes, and come back to this question
4. Have you suffered actual brake fade? If no, revert to point 2 and re-run. If yes, look at brake cooling
5. If you've legitimately made it this far then you need a bbk.

[B21]

Thanks for sharing all that info B21. On the topic of master cylinder, there is something that keeps bugging me when talking about front to rear bias and focusing on discs sizes: Isn't the bias supposed to be mostly a master cylinder thing and disc size but not a piston size/amount? My rational with this is that the MC is what displaces the fluid volumes. If there is more piston area then the pressure increases less for the same fluid volume displaced. So what would drive front and rear pad force is always and only the MC volumes. Coupled with discs diameter this then sets the front and rear braking torques. Never had the time or tools to dig into this more though I "could" setup proper simulations for that...

So in old fashioned cars pre E85 …there was a brake bias or proportioning valve ..in principle a simple high pressure resistance to fluid flow that would limit the amount of pressure that the MC that would go to the rear caliper pistons…in race circles you can buy a version that can be manually set from with the car..the idea being that this would be adjusted such that the rears would never lock up before the fronts..

BMW changed this with the arrival of DSC/ABS/EPS etc,,

A new concept was introduced as it came for free as part of computerisation of braking..

EBV: Electronic brake force distribution

EBV is a component of ABS and controls the brake force distribution between the front and rear wheels.

Advantage: Regardless of the load state of the vehicle, the best possible braking distance is achieved while driving stability is maintained.

Modern vehicles have relatively large brakes on the rear axle to shorten braking distances. To prevent the rear wheels from being overbraked in certain driving situations, EBV permanently monitors slip. EBV controls rear axle slip in dependence of the front axle.

So…providing the front / rear brake forces remain roughly in proportion then the DSC unit should be able to cope…of course most of us have no idea where those boundaries are…but..

To look at EBV and how well it copes..on an E89 type vehicle when the brakes are pressed hard for a sustained period of time then weight transfer moves from the rear to the front ..as the COG is above the height of a line drawn through the axles there will be a pitch forward movement rotating around the COG..


So on an E89 with normally a roughly 50/50 weight the rear brakes need to able to exert enough force to drive the rear brakes into abs mode at say 900kgs…but as weight transfer takes place around 20-30% of that mass will ‘transfer’ to the front ..so now the rears need to only to manage with 650kgs on the rear to reach abs but the fronts that only needed 900kg need 1150kgs of force to drive the fronts into abs..

So brake bias has to manage with a ratio of 1:1 through to 1150/650 = around 1.3 more front brake..

With a fixed proportioning valve tricky to get it right..even more so with say a saloon car with on one day just a driver then 4 pax n luggage the next.

EBD sorts all that out and clearly has a set of parameters to cope with a range of circumstance…we don’t know the limits..

So my logic was to try and ensure that the front / rear braking forces remain within a limit trod by previous designs..so if it works for the factory combos it should work for the non factory combos providing you keep within these ratios…..any way that’s my theory..

[B21]

You could probably replace that calculator with the flowchart:

1. Do you want a bbk for aesthetics? If yes then crack on the rest means fuckall to you. If no then continue.
Do you track the car? If yes continue, if no, you don't need a bbk
2. Have you done more than 5 trackdays in the car, and received some tuition. If yes continue, if no, do more than 5 trackdays and some tuition and come back to this question
3. Have you upgraded your pads with fresh fluid, stock discs and calipers? If yes continue, if no, try upgraded pads and regular brake fluid flushes, and come back to this question
4. Have you suffered actual brake fade? If no, revert to point 2 and re-run. If yes, look at brake cooling
5. If you've legitimately made it this far then you need a bbk.

1 yes
2 no
3 yes
4 yes
5 3 out of 4 ain’t bad to paraphrase Meatloaf…

The corollary argument is as follows taking your logic

For a 18i E89

1 no
2 no
3 no
4 no
5 no

so for a 28i/30i/35i/35is why does the manufacturer fit bigger brakes ?

As none of those variables will change ?

As certification tests are done at 125 mph or less the 18i brakes are clearly adequate to meet the 35is certification requirements..discuss.