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      03-27-2019, 01:13 PM   #1
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My $1500 DIY BBK

BLUF: Porsche Cayenne calipers on an E90, using M5 and X3 rotors, to fit under a 17x9.5 ET35 Apex ARC-8. As of 27 Mar 19 this is a work in progress/project. If you came here for final results, check in later.

I've got a 2006 E90 325i I am turning into a track car (lapping/time attack, maybe some wheel-to-wheel). I don't like the idea of throwing money at problems that can be better solved with patience and creativity, so I decided to look into building my own big brake kit.

Calipers
I locally sourced a set of Porsche Cayenne calipers (2006 with 18Z front) for cheap and figured they would be a good starting point. Compared to the stock 335i caliper the front piston area is basically on par with the StopTech ST60 (~7-8% increase), though pad selection isn't as good. Rear piston area drops off 13% compared to the 335i caliper, but only 4.5% compared to the 325i caliper. Since this is going to be a track car I can get around most of this issue with master cylinder sizing and brake balance adjustment, however I may swap these out for a different rear brake that is closer to the ST40 in overall piston area (which is roughly the same as the 335i rear).

Only modification I am hoping to do to these is moving the cross-over tube to the other end of the caliper since these will need to be run on the opposite side of the hub compared to the Porsche fitment. I know p0lar has milled these for radial mount but I am hoping to not have to resort to that; maybe I'll hit a roadblock and won't have a choice though, who knows.

Rotors
Finding reasonably priced rotors was a pain in the ass; I spent hours on Brembo Europe's online catalog (life saver) trying to find what would work. Initially I was going to use the Porsche rotors, re-drill them to a 5x120 pattern, and run a hub-centric spacer to make up the hub register difference; since rotors are fairly inexpensive for the Cayenne, surprisingly. Instead, after a few hours of research, I decided to go with E39 M5 front rotors for the front of the E90 and F25 X3 front rotors for the rear of the E90. These were as close to the Cayenne rotor diameter/width I could find while still having a BMW bolt pattern/hub register.

The M5 rotor should sit about 3mm more inward than the 335i rotor due to the increase in rotor height. Pad swept area covers all but 10mm of friction surface (so what, I'll have a lip, rotors will probably crack/warp before they ever wear that much anyway). The M5 rotor is 2mm narrower than the Cayenne rotor, so I will be running 1mm shims on the back of the pad; this will also help with keeping heat away from the pistons/seals/fluid. Some people use titanium backing plates for weight savings, but stainless steel is cheaper and a better insulator of heat. F25 rotors in the rear are the same height as the 335i rear, so clearance shouldn't be an issue. Pad swept area should cover most of the friction surface.

Going to likely run Centric blanks from RockAuto or Zimmerman blanks from FCP Euro (lifetime replacement). Slotted are nice and all, but it's going to be (for not) a fair-weather car so I'm not too worried about wet rotors. Less likelihood of cracking as well. Plus they are cheaper.

Pads
One downside to this setup is that motorsport pad selection isn't all that great on the front calipers (because it's for a truck, basically). Carbotech are well reviewed and fairly priced, so I'll probably go with them. I can get Pagid RS29 for the fronts if I want to spend another $240CAD for the pair over the Carbotech XP10.

Whole Package
Having components that all work on paper is one thing; but putting it all together is another story. Ideally, I don't want to machine the caliper much/at all. I don't want to go to a radial mount, I don't want to mill the face of the mount ears, etc. I don't like running custom parts on a racecar because that means you need a bunch of custom spares... I'd much rather run off-the-shelf parts in case something breaks.

I was worried about my self-imposed wheel size limitations. Keeping costs down was the goal for this project, so I wanted to be running 17" wheels and tires. As such, this whole package must fit underneath an 17x95.5 ET35 Apex ARC-8. Thankfully, this wheel has some of the best barrel clearance I've ever seen on an aftermarket wheel! Quick calculations and template making leads me to believe that this will work. From my math, the 18z caliper in the front over an M5 rotor will not stick out any more than the ST60 does (Apex has confirmed that the ST60 355mm kit fits under these, so I used that kit as my fitment benchmark) so spoke clearance should be good. Barrel clearance looks good too after some cardboard aided design (see pics).

Next step is to buy some Centric/Zimmerman blanks and start designing a bracket that will locate the caliper. I'll keep everyone in the loop!

Thanks for reading.

TLDR

Front Brakes
  • 2006 Porsche Cayenne front calipers (Brembo 18Z) for a 350x34mm rotor
    • Piston sizes: 30mm, 34mm, 38mm
    • Piston area: 8.522inē
    • Difference in piston area from 335i: +7.73%
    • StopTech ST60 difference in piston area from 335i: +7.48%
  • E39 M5 front rotors (345x32mm)
    • Centric Blanks: 125.34063
    • StopTech Slotted: 126.34063SR/126.34062SL
  • Carbotech XP10: CT1014-XP10
Rear Brakes
  • 2006 Porsche Cayenne rear calipers (Brambo) for a 330x28mm rotor
    • Piston sizes: 28mm, 30mm
    • Piston area: 4.100inē
    • Difference in piston area from 335i: -13.02%
    • StopTech ST40 difference in piston area from 335i: +0.21%
  • F25 X3 Front Rotors (328x28mm)
    • These actually have the correct 75mm hub bore for the rear E90 brakes
    • Centric Blanks: 125.34134
    • StopTech Slotted: 126.34134SR/126.34134SL
  • Carbotech XP8: CT978-XP8
Misc. Info
  • No milling of caliper or upright; not converting to radial mount
  • Probably not running dust shields (racecar)
  • No e-brake (racecar)
  • Dual master cylinders, no booster/ABS (racecar)
  • Must clear Apex ARC-8 17x9.5 ET35 barrel and spokes
  • May swap rear calipers for something that doesn't drop piston area so much
  • 1.0mm stainless steel shims for front brake pads
  • Under $2000CAD ($1500USD) front and rear
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Last edited by Justin Daniels; 03-28-2019 at 02:58 PM..
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      03-27-2019, 08:01 PM   #2
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      03-28-2019, 09:08 AM   #3
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If the new rears are smaller why wouldn't you keep the stock rears? Or am I missing something.
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      03-28-2019, 10:07 AM   #4
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Quote:
Originally Posted by weehe126 View Post
If the new rears are smaller why wouldn't you keep the stock rears? Or am I missing something.
My apologies, in my post I was comparing to the 335i hardware because that was the cheapest OEM upgrade for my 325i. Though 13% less piston area than the 335i caliper, the rear Cayenne caliper has only 5% less piston area than the 325/328i caliper.

For the cost of upgrading the rear to 335i calipers and rotors, the Brembo units from the Porsche seemed to make more sense. The slight decrease in piston area can be compensated for with an appropriately sized master cylinder; with a .7" front MC and a .625" rear the brake balance should be pretty close to stock and pedal force should increase by around 10%.

Two pistons per pad spreads the applied load better and allows for a slight increase in cooling. The increased pad surface area also improves brake torque and the increase to the overall rigidity of the caliper itself will help. Another important factor in the decision was the ease of pad changes, pulling a pin and swapping pads is a lot easier that removing the caliper. The fixed caliper will also require less maintenance and has less moving parts (no slider pins to worry about). Oh, I also had the vehicle in front of me so it was easier to snag all four calipers!

That's my logic at least.

EDIT: It should be noted that the even the StopTech kit ($2500 USD) for the rear of the E9x (ST40 caliper) area offer almost zero increase to piston area over the 335i brakes (assuming a 28mm and 34mm pistons, from my research). The benefit from the kit comes from increasing the mean pad height and the other points noted above.

Last edited by Justin Daniels; 03-28-2019 at 10:16 AM..
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      03-28-2019, 03:55 PM   #5
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Who is p0lar? You can keep them lug mount and mount them on the inner side of the plate adapter, because although caliper will clear the rim barrel it won't clear the spokes if mount outwards. Due to size even with regular OEM pads braking would be sufficient for e90 325i. Just make sure you mount them correctly. I have done this on VW and had to flip calipers upside down and move the cross brake line.
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      03-28-2019, 09:45 PM   #6
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Quote:
Originally Posted by feuer View Post
Who is p0lar? You can keep them lug mount and mount them on the inner side of the plate adapter, because although caliper will clear the rim barrel it won't clear the spokes if mount outwards. Due to size even with regular OEM pads braking would be sufficient for e90 325i. Just make sure you mount them correctly. I have done this on VW and had to flip calipers upside down and move the cross brake line.
Some guy in the BMW community who has modified the 18Z for radial mount; came across his name in my searches for brake information.

I'll be able to figure out fitment when I get the rotor in, I think the mounts on the caliper will try to occupy the same space as the mount ears on the upright; so I may just clock the caliper a bit. Ultimately my rotor choice will dictate the inboard/outboard position; fancy bracketry will have to solve for the rest!

I'll definitely be doing some trial and error with compounds; with such a big increase in brake mass and pad surface area, combined with a fairly big drop in weight and lowering of the CofG height, I may be going overkill with the XP10s. I'd rather run a more rotor-friendly compound if I can get away with it. We'll see.

Thanks for the feedback.
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      03-28-2019, 10:11 PM   #7
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They sell these already modified for radial mounting. Cayenne Turbo, known as banana calipers are radial. They come on the Panamera too. Link p0lar thread. I would like to see.
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      03-29-2019, 07:46 AM   #8
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Quote:
Originally Posted by feuer View Post
They sell these already modified for radial mounting. Cayenne Turbo, known as banana calipers are radial. They come on the Panamera too. Link p0lar thread. I would like to see.
Yeah I saw them from http://www.goingsuperfast.com/Brake.html pre-milled. But I didn't want to be paying for someone else to rebuild/paint/mill a caliper when I could do that all myself if needed.

As for the banana calipers, they are for a bigger rotor than I was interested on running (368mm) and require removal of the caliper for pad swaps, which I wasn't too keen on. Though I believe they actually had slightly better pad selection than the 18Z. Pedal feel would have been a bit messed up with a 20% increase in piston area over the 325i.

I also got the Cayenne S calipers for pretty cheap, $450CAD for the set.

Here are some "p0lar" threads that came up in my search. It looks like he may have only been doing the brackets for the E46, but he certainly does mill the 18Z caliper. I didn't go down that rabbit hole since I wasn't interested in modifying the caliper if I could avoid it.

http://www.m3forum.net/m3forum/showthread.php?t=546400
http://www.m3forum.net/m3forum/showthread.php?t=510079
http://www.m3forum.net/m3forum/showthread.php?t=412927
http://www.m3forum.net/m3forum/member.php?u=46297
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      03-30-2019, 08:52 AM   #9
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Couldn't find in neither of those threads how the bracket looks like. Anyhow, the mounting tabs on e46 and e90 are the same, just hubs are with approximately 20mm lesser offset on the e46. Meaning you can take e90 caliper install it on e46, put 20mm spacer between hub and rotor and all will lineup. So if p0lar has brackets for e46, m5 rotor and Cayenne calipers it will be of a great help to you.
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      04-01-2019, 08:20 AM   #10
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Quote:
Originally Posted by feuer View Post
Couldn't find in neither of those threads how the bracket looks like. Anyhow, the mounting tabs on e46 and e90 are the same, just hubs are with approximately 20mm lesser offset on the e46. Meaning you can take e90 caliper install it on e46, put 20mm spacer between hub and rotor and all will lineup. So if p0lar has brackets for e46, m5 rotor and Cayenne calipers it will be of a great help to you.
I don't know what rotor his brackets are to suit, and they would require the caliper being milled, so I'm not going to go down that route (unless it ends up being the only way forward, then I may contact him).

Thanks for the advice/interest.

Cheers
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      04-07-2019, 10:37 PM   #11
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2008 335i E90  [4.00]
Brake balance, brake balance, brake balance....
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      04-08-2019, 06:35 AM   #12
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Quote:
Originally Posted by vasillalov View Post
Brake balance, brake balance, brake balance....
The ability to select a proper sized master for the rear (at the expense of pedal feel) should put me close to the stock balance. Any fine adjustments after that point will be done with the balance bar in the pedal set.

If it's really out of whack I'll have to source a different rear caliper.

I should note that the 4.5% decrease, compared to the 325i hardware, in piston area for the rear on the Cayenne calipers is still less than the Wilwood Aero6 rear (8.55% drop) for the E9x; I'd be curious to see how the balance is perceived on those, especially since they are designed for the stock master cylinder.

Thanks for the input!
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      04-08-2019, 08:12 PM   #13
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Finally got around to measuring the brake pedal and booster diaphragm plates.

According to my rough measurements:
  • Brake Booster Diaphragm Plate Diameter: 7.87 in
  • Brake Booster Piston Assembly Bore: 2.36 in
  • Total Brake Booster Diaphragm Surface Area: 88.625 inē
  • Pedal Ratio: 3.75:1
  • Brake Master Cylinder Bore (front): 1.0 in
  • Brake Master Cylinder Bore (rear): 0.874 in
This means that the booster adds about 870 lb of assist to whatever you're stomping on the pedal at. Assuming 100lb leg force, effective force into the booster is 375 lb. Force out of the booster onto the master cylinder piston is ~ 1245lb. This means front lines pressure will be ~1586 psi and rear line pressure will be ~2076 psi.

If I run build the pedal set for a 6:1 ratio, use a 0.75 in master cylinder for the front circuit and a 0.625 in for the rear circuit, it will take an additional 17lb of leg force to get the front line pressure to where it was stock (~1589 psi). The rear line pressure, at this leg force, will be increased by ~10% to 2288 psi.

Comparing the Cayenne calipers to the 325i, at these new line pressures resulting from a 117lb leg force, there would be an increase in clamping force in the front of roughly 8% and and increase of roughly 5.2% in the rear.

All that to say; I think the balance delta shouldn't be noticeable to the meat in the seat; and if it is, it can be dialed out with bias adjustment.

EDIT: on the stock master cylinder + booster (assuming DSC/ABS module doesn't have any built-in bias), this same setup would result in a 7.7% increase in clamping force in the front and 4.5% decrease in clamping force in the rear. This would definitely upset the balance, for sure.

Last edited by Justin Daniels; 04-08-2019 at 08:18 PM..
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      04-13-2019, 12:17 AM   #14
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If anything, you want more rear bias to the rear in the E90, not less. One of the best feelings in confidence the car can give to the driver is the feeling that of a slight rear squat on initial bite before the front brakes take over the bulk of the braking force. Think of it like the landing of a jet. Rear touches first then the front "slams" in.

That is a fine balance that is best achieved through proper sizing of caliper pistons (as you pointed out). It can further be fine tuned by pad selection as well.
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      04-13-2019, 12:20 AM   #15
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Quote:
Originally Posted by Justin Daniels View Post
Finally got around to measuring the brake pedal and booster diaphragm plates.

According to my rough measurements:
  • Brake Booster Diaphragm Plate Diameter: 7.87 in
  • Brake Booster Piston Assembly Bore: 2.36 in
  • Total Brake Booster Diaphragm Surface Area: 88.625 inē
  • Pedal Ratio: 3.75:1
  • Brake Master Cylinder Bore (front): 1.0 in
  • Brake Master Cylinder Bore (rear): 0.874 in
This means that the booster adds about 870 lb of assist to whatever you're stomping on the pedal at. Assuming 100lb leg force, effective force into the booster is 375 lb. Force out of the booster onto the master cylinder piston is ~ 1245lb. This means front lines pressure will be ~1586 psi and rear line pressure will be ~2076 psi.

If I run build the pedal set for a 6:1 ratio, use a 0.75 in master cylinder for the front circuit and a 0.625 in for the rear circuit, it will take an additional 17lb of leg force to get the front line pressure to where it was stock (~1589 psi). The rear line pressure, at this leg force, will be increased by ~10% to 2288 psi.

Comparing the Cayenne calipers to the 325i, at these new line pressures resulting from a 117lb leg force, there would be an increase in clamping force in the front of roughly 8% and and increase of roughly 5.2% in the rear.

All that to say; I think the balance delta shouldn't be noticeable to the meat in the seat; and if it is, it can be dialed out with bias adjustment.

EDIT: on the stock master cylinder + booster (assuming DSC/ABS module doesn't have any built-in bias), this same setup would result in a 7.7% increase in clamping force in the front and 4.5% decrease in clamping force in the rear. This would definitely upset the balance, for sure.

I am not going to check the math here, seems legit. However, you NEVER slam the brake pedal full force. Not on the road, not on autox, not on track. Are you assuming linear function across the pedal travel?

EDIT:
Final words of wisdom, if I may: you would definitely want to spend some time reprogramming your ESP module. You will reap bigger benefits on the track there than from BBK.
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      04-14-2019, 10:23 AM   #16
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Quote:
Originally Posted by vasillalov View Post
If anything, you want more rear bias to the rear in the E90, not less. One of the best feelings in confidence the car can give to the driver is the feeling that of a slight rear squat on initial bite before the front brakes take over the bulk of the braking force. Think of it like the landing of a jet. Rear touches first then the front "slams" in.

That is a fine balance that is best achieved through proper sizing of caliper pistons (as you pointed out). It can further be fine tuned by pad selection as well.
More rear bias compared to stock, right?
I agree, though I think a lot of it comes down to driver preference as well.

Right, final brake torque is what I really care about; pad radius and compound will factor into that as much as piston area. But I agree, it's a lot easier to start with the right piston area and fine tune with bias/compounds. I won't be suffering with a super long pedal that way.

We'll see how it all feels once it goes together!

I'm going to search for people who have run the Wilwood BBK; their rear caliper drops piston area more than the Cayenne one does. Would be interesting to get feedback from the guys using it.

Quote:
Originally Posted by vasillalov View Post
I am not going to check the math here, seems legit. However, you NEVER slam the brake pedal full force. Not on the road, not on autox, not on track. Are you assuming linear function across the pedal travel?

EDIT:
Final words of wisdom, if I may: you would definitely want to spend some time reprogramming your ESP module. You will reap bigger benefits on the track there than from BBK.
Oh I know, I think I've only ever "stood on the brakes" on the track once when a car ended up crossed up in front of me into the braking zone after a straight. I was just using 100lb leg force as a basis of comparison between the two systems. I assume a linear pressure increase with a linear increase in leg force, seeing as all of the equations are linear functions. Is this not a safe assumption? I haven't really dug into the books yet so any advice is welcome! My math, so far, was to determine master cylinder sizing to compensate for the drop in rear piston area.

I won't be running any ESP/DSC. I'll be running manual brakes using independent master cylinders.

I appreciate the input. Thanks!

Last edited by Justin Daniels; 04-15-2019 at 06:56 AM..
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      04-15-2019, 08:08 AM   #17
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Running the numbers for brake torque, keeping pad coefficient of friction the same for comparison sake, I came up with the following front torque bias figures:
  • 325i Calipers (300/300 rotors, stock system): 31.8%
  • Cayenne Calipers (345/328 rotors, manual brakes, neutral balance): 44.6%
  • StopTech Calipers (355/345 rotors, stock system): 42.2%
I think the Cayenne setup is close enough to be a good starting point for "dialing in" with balance bar adjustment and pad compound. I'll just need to decide whether I want a stiffer pedal or longer pedal.

EDIT: 135i front rotors are 338x26, the extra 5mm in effective pad radius would result in a 39.6% front overall torque bias. Downside is that they have a 79mm centre bore and would need a hub-centric ring to fit the 75mm hub in the rear.

Last edited by Justin Daniels; 04-15-2019 at 11:36 AM..
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      04-15-2019, 11:52 AM   #18
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Any updates on the adapters?
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      04-15-2019, 12:11 PM   #19
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Quote:
Originally Posted by feuer View Post
Any updates on the adapters?
I'll be making my own.

I'm 99% committed to the M5 rotor in the front so I've got an offshore cheap rotor coming for R&D.
For the rear, I'm now torn between x3 front or 335i front rotor for the rear. Rotor centreline will be 1mm inward vs the x3 rotor and the caliper will sit 5mm further out from the hub and I'll need a 2mm hub-centric ring to centre the rotor on the hub. I need to make a decision on this before I get any bracket designed.

EDIT: attached the dimensions I'm working with, for those interested.
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File Type: pdf E90-Rotor-Comparison.pdf (311.7 KB, 19 views)

Last edited by Justin Daniels; 04-15-2019 at 12:18 PM..
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      04-15-2019, 03:26 PM   #20
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Quote:
Originally Posted by Justin Daniels View Post
I'll be making my own.

I'm 99% committed to the M5 rotor in the front so I've got an offshore cheap rotor coming for R&D.
For the rear, I'm now torn between x3 front or 335i front rotor for the rear. Rotor centreline will be 1mm inward vs the x3 rotor and the caliper will sit 5mm further out from the hub and I'll need a 2mm hub-centric ring to centre the rotor on the hub. I need to make a decision on this before I get any bracket designed.

EDIT: attached the dimensions I'm working with, for those interested.
I would personally skip the e39 m5 rotors. Less rotor offset will hinder wheel options. Also, hub is different on those too. So it won't be hub centered. But is your choice.
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      04-15-2019, 06:07 PM   #21
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Quote:
Originally Posted by feuer View Post
I would personally skip the e39 m5 rotors. Less rotor offset will hinder wheel options. Also, hub is different on those too. So it won't be hub centered. But is your choice.
According to dimensions from Brembo, the E39 M5 will sit outboard only 0.1mm more and has the same hub bore diameter (79mm). Is it the taper on the hub, where the rotor rests, that's different?

I'm not at home right now but will double check the drawings when I am.

Cheers.
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      04-15-2019, 08:44 PM   #22
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Quote:
Originally Posted by Justin Daniels View Post
According to dimensions from Brembo, the E39 M5 will sit outboard only 0.1mm more and has the same hub bore diameter (79mm). Is it the taper on the hub, where the rotor rests, that's different?

I'm not at home right now but will double check the drawings when I am.

Cheers.
Non m E39 hub is 74.1 vs E90 72.56. Rims aren't interchangeable. That means the rotors aren't either. I'm not sure about e39 M but I assume is the same as non m e39.

You need to look at the hat. Is 50mm for e90 vs 45mm for e39. Don't just look at the center line of the blank.
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