[upshift_downshift]

Hi all, looking for some info on the pre-lci E90 335i stock braking system:
1. Brake piston diameters, front and rear
2. Brake pedal ratio
3. Brake booster assist ratio
4. Master cylinder bore diameter

Some stretch info (not expecting anyone to know this):
5. Front and rear caliper stiffness, as in deflection per pound-force

[Justin Daniels]

1. Front: 57mm (ref) , Rear: 44mm (ref)
2. Can confirm tonight, probably around 4:1
3. If you can give me a vacuum reading of the booster line I can give you the assist. My research came up with an 8" and 8.99" diaphragm (ref 8.0" - ref 8.99"). 8.99" shows dual, 8.0" doesn't specify. I can pull mine apart at some point to find out if you really care that much.
4. Master Cylinder bore: 1.0" (ref)

[Justin Daniels]

I can confirm that E90 brake booster (PN: 34336779721) is a tandem 8" diaphragm.
This is from a 2006 325i (VIN: WBAVB13536KX41685).

Not sure how the math works out for a tandem booster; but if you get me the vacuum reading I can probably figure it out.

Also, totally forgot to measure the brake pedal; next time I'm in the shop I'll do it.

[Justin Daniels]

Originally Posted by upshift_downshift

Some stretch info (not expecting anyone to know this):
5. Front and rear caliper stiffness, as in deflection per pound-force

What exactly are you trying to figure out here?

Deflection in the same plane as the rotor in the direction of rotation during braking? Lateral deflection with any side loading?

They are floating calipers front/rear, so do you mean the calipers alone? Or the caliper assembly including the bracket?

I could probably rig something up with a dial indicator, shop crane, crane scale, and a lever. Just need to know what you're after... and, out of curiosity, why?

[upshift_downshift]

Originally Posted by Justin Daniels

What exactly are you trying to figure out here?

Deflection in the same plane as the rotor in the direction of rotation during braking? Lateral deflection with any side loading?

They are floating calipers front/rear, so do you mean the calipers alone? Or the caliper assembly including the bracket?

I could probably rig something up with a dial indicator, shop crane, crane scale, and a lever. Just need to know what you're after... and, out of curiosity, why?

Holy crap you are the man!

I'm putting together a calculator for the brake balance and pedal feel in Excel. Want to compare brake kits to OEM with real data. I did the same spreadsheet designing brakes for Formula SAE.

I mean deflection as in the spreading of the caliper outboard, axially from the rotor. This deflection causes fluid displacement (and therefore pedal sponginess), along with piston seal retraction (I am assuming 0.5 mm for this as rule of thumb). Pad and brake line deflection play a small role but it's pretty negligible.

I'm thinking the best test would be to relate brake pressure to the overall width of the caliper body.

Unfortunately I won't be able to get the vacuum reading any time soon, but I believe it's the same vacuum that the turbos get. Should be about 7.5 psi of vacuum, or 15 in. Hg

[Justin Daniels]

Originally Posted by upshift_downshift

Holy crap you are the man!

I'm putting together a calculator for the brake balance and pedal feel in Excel. Want to compare brake kits to OEM with real data. I did the same spreadsheet designing brakes for Formula SAE.

I mean deflection as in the spreading of the caliper outboard, axially from the rotor. This deflection causes fluid displacement (and therefore pedal sponginess), along with piston seal retraction (I am assuming 0.5 mm for this as rule of thumb). Pad and brake line deflection play a small role but it's pretty negligible.

I'm thinking the best test would be to relate brake pressure to the overall width of the caliper body.

Unfortunately I won't be able to get the vacuum reading any time soon, but I believe it's the same vacuum that the turbos get. Should be about 7.5 psi of vacuum, or 15 in. Hg

I've actually got a spreadsheet I'm working on to calculate overall increase to brake torque between the OEM 335i setup and my DIY BBK; using the same sheet to ensure MC sizing is correct so I don't have to He-Man the brakes for a 30-minute session. So we may have some common interests. It's tough because of the DSC/DXC module gets involved in the proportioning in a major way. Will also be calculating CofG position delta between my stock 328xi to the race prepped 325i.

I'm also probably going to end up using my DD 328xi to get some of the brake data. I have some 3000psi 1/8NPT transducers kicking around; may make a bleeder adapter and check to see actual fluid pressure at the caliper. Should be able to calculate the booster gain from this number (with a given pedal force) as well.

I understand the appeal of knowing exactly what the brake system is doing; but I'd consider these variables to be so similar from caliper to caliper that they may as well be constant and just left out of any differential analysis. Alternatively, collecting the data for all options would basically require you to purchase all options in order to make the comparison.

Pad compound/area, piston area, master cylinder sizing, pedal ratio, booster assist (if any) will likely affect pedal feel more than anything as minor as a few thou of axial deflection ("spread") of the caliper. I remember FSAE being all about that stuff, but the meat in the seat in a 3,000lb tin-top is not likely to notice the difference.

[upshift_downshift]

Originally Posted by Justin Daniels

I've actually got a spreadsheet I'm working on to calculate overall increase to brake torque between the OEM 335i setup and my DIY BBK; using the same sheet to ensure MC sizing is correct so I don't have to He-Man the brakes for a 30-minute session. So we may have some common interests. It's tough because of the DSC/DXC module gets involved in the proportioning in a major way. Will also be calculating CofG position delta between my stock 328xi to the race prepped 325i.

I'm also probably going to end up using my DD 328xi to get some of the brake data. I have some 3000psi 1/8NPT transducers kicking around; may make a bleeder adapter and check to see actual fluid pressure at the caliper. Should be able to calculate the booster gain from this number (with a given pedal force) as well.

I understand the appeal of knowing exactly what the brake system is doing; but I'd consider these variables to be so similar from caliper to caliper that they may as well be constant and just left out of any differential analysis. Alternatively, collecting the data for all options would basically require you to purchase all options in order to make the comparison.

Pad compound/area, piston area, master cylinder sizing, pedal ratio, booster assist (if any) will likely affect pedal feel more than anything as minor as a few thou of axial deflection ("spread") of the caliper. I remember FSAE being all about that stuff, but the meat in the seat in a 3,000lb tin-top is not likely to notice the difference.

Those things affect pedal feel because they affect the total displacement due to caliper piston movement. Piston movement after contact with the rotor is only due to compliance in the system, which mainly comes from the caliper spreading open. In other words, piston area, MC bore, pedal ratio etc. affect the sensitivity to caliper stiffness. If you had an infinitely stiff caliper (and brake lines/pads/pistons), your brake pedal would be like pressing on a rock, not matter what your bore diameters are.

[Justin Daniels]

I understand that, I'm not disagreeing with you; I'm just saying that I'd be surprised if the difference was noticeable, to a driver on a track, between the various aftermarket solutions. I'm not trying to trash talk any data gathering; I fully support the idea! I just wonder if the juice is worth the squeeze in this case since the numbers you need are not readily available.

I'm happy to rig up some test with a dial indicator and pressure transducer to get you the numbers you need; I'll be able to help with stock LCI 328xi calipers, Brembo 18Z calipers, and potentially pre-LCI 325i calipers if I want to dig them out of the trash. Unfortunately no 335i unless you know someone who can loan me one front and one rear caliper.

Also, for the booster, my rough math was 7.8" diaphragm diameter and 1.5" center piston bore in the diaphragm plates. I'll get better measurements at some point but that should put you in the ballpark.

[upshift_downshift]

Originally Posted by Justin Daniels

I understand that, I'm not disagreeing with you; I'm just saying that I'd be surprised if the difference was noticeable, to a driver on a track, between the various aftermarket solutions. I'm not trying to trash talk any data gathering; I fully support the idea! I just wonder if the juice is worth the squeeze in this case since the numbers you need are not readily available.

I'm happy to rig up some test with a dial indicator and pressure transducer to get you the numbers you need; I'll be able to help with stock LCI 328xi calipers, Brembo 18Z calipers, and potentially pre-LCI 325i calipers if I want to dig them out of the trash. Unfortunately no 335i unless you know someone who can loan me one front and one rear caliper.

Also, for the booster, my rough math was 7.8" diaphragm diameter and 1.5" center piston bore in the diaphragm plates. I'll get better measurements at some point but that should put you in the ballpark.

No worries, I understand the logic there. I just wanted to try and get as many pieces of the puzzle as possible. If you have the time and desire to rig up a test, then go for it, would be interesting to see what you find. Otherwise I wouldn't worry too much about it. Like you said, I would have to buy a kit anyways to test it for stiffness.

[Justin Daniels]

Originally Posted by upshift_downshift

No worries, I understand the logic there. I just wanted to try and get as many pieces of the puzzle as possible. If you have the time and desire to rig up a test, then go for it, would be interesting to see what you find. Otherwise I wouldn't worry too much about it. Like you said, I would have to buy a kit anyways to test it for stiffness.

I won't make it a priority, but I'll get around to it at some point. I'll run a dial indicator at the centre of the piston, or mean piston radius, to best represent deflection in the region you're most concerned about.

Cheers.

[Justin Daniels]

2006 E90 325i Pedal Ratio: 3.75:1

Brake Booster diaphragm plates are 200mm with a 60mm hole in the middle for the piston assembly. Which means both plates have a combined surface area of 88.6 square inches; which at 20 inHg vacuum (guesstimated) provides ~870lb of assist.

Stock 325i brake master cylinder has a 25.4mm piston for the front and a 22.2mm piston for the rear; meaning total assisted line pressure should be around 1560psi for the front and 2080psi for the rear, assuming a 100lb leg force and the DSC/ABS module doesn't have any funky biasing when it's not active.

Hope this helps.

[Volasko]

All this engineering talk is getting me hot. Don't stop.