Downsizing Rear Calipers from F30 to fit 17" Wheels

[tftimbn]

Hi all

I'm looking to downsize my rear calipers to F30 calipers so I can run a 17" wheel in the rear. I've had this confirmed by many, but no one can confirm what calipers to run.

I see the 340i M-Sport (same calipers as F80) can run 17" wheels, so why exactly can't I run these? I'm aware the rear rotors are smaller, but downsizing the rotor doesn't make the caliper go in more. Is it that the hub has the caliper spaced farther away from the hub?

Does this mean my only options are to use 330i base calipers with a 330i rotor?

I'm looking for a cheaper alternative than running the vargas setup.

Thank you!

[M3SQRD]

The caliper mounting locations on the hub are different so the same caliper can be used on different rotor OD but the same pad radial depth. Most aftermarket custom caliper adapter brackets are designed to move the caliper more outboard so a larger diameter rotor can be used. I’m not sure if anyone makes a caliper bracket that moves the caliper inboard. Also, the rear brake rotor has an integral barrel for the drum parking brake which may/may not be the same diameter for the f8x and f3x.

The oem blue two-piston rear caliper should be identical between the f8x and f3x. Like I said above, a caliper adapter bracket that moves the caliper inward to work with the f3x 330i smaller rotor OD. I’d use realoem.com to verify whether the drum parking brakes have the same barrel/drum and shoe diameters.

[tftimbn]

Originally Posted by M3SQRD

The caliper mounting locations on the hub are different so the same caliper can be used on different rotor OD but the same pad radial depth. Most aftermarket custom caliper adapter brackets are designed to move the caliper more outboard so a larger diameter rotor can be used. I’m not sure if anyone makes a caliper bracket that moves the caliper inboard. Also, the rear brake rotor has an integral barrel for the drum parking brake which may/may not be the same diameter for the f8x and f3x.

The oem blue two-piston rear caliper should be identical between the f8x and f3x. Like I said above, a caliper adapter bracket that moves the caliper inward to work with the f3x 330i smaller rotor OD. I’d use realoem.com to verify whether the drum parking brakes have the same barrel/drum and shoe diameters.

So, from what I've found, 330x20 rotors are the key. These are found on 335/435 F chassis, which have a 185mm e-brake diameter. The F80/F82 also has a 185mm e-brake diameter. I ended up having 330x20 rear rotors on my F10 535i, but those have an electronic parking break. I took the rotor off regardless to verify fitment, and it did fit.

So, I'm looking at 330x20 (actually 330x24 from EBC) spec'd from a 2016 335i. I feel like the answer in this case is to also use calipers and pads from this same car, but you have me thinking about the outward spacing on the hub. If I bolted these calipers to replace my M caliper, wouldn't the pad sit farther away from the rotor then? Does this make sense? Perhaps the solution would be to use 340mm rotors from the 340 m sport?

Trying to find this stuff out without buying and having to return parts.


EDIT:

I identified F30 330 M-Sport, 335i w/o M-Sport, 340 w/o M-Sport all share the same rear 330x20 rotors, calipers, and pads. Autozone P/N's #73046 (rotor) #19-B7217 (R caliper) #
19-B7216 (L caliper) #DG1610 (Pads). So any of these above applications are good for the parts search. However I am still concerned of caliper/pad alignment on the rotor if they are indeed spaced farther from the hub.

I have a buddy who's last 7 of the VIN on realoem.com pointed to the 185mm rear ebrake, which means he should have 330x20 rotors and an M-Sport brake package. I'll see if he will let me borrow a brake setup for test fitting.

[M3SQRD]

Originally Posted by tftimbn

So, from what I've found, 330x20 rotors are the key. These are found on 335/435 F chassis, which have a 185mm e-brake diameter. The F80/F82 also has a 185mm e-brake diameter. I ended up having 330x20 rear rotors on my F10 535i, but those have an electronic parking break. I took the rotor off regardless to verify fitment, and it did fit.

So, I'm looking at 330x20 (actually 330x24 from EBC) spec'd from a 2016 335i. I feel like the answer in this case is to also use calipers and pads from this same car, but you have me thinking about the outward spacing on the hub. If I bolted these calipers to replace my M caliper, wouldn't the pad sit farther away from the rotor then? Does this make sense? Perhaps the solution would be to use 340mm rotors from the 340 m sport?

Trying to find this stuff out without buying and having to return parts.

The f8x rear is 370 mm x 24 mm. Your 330 mm x 24 mm would need a caliper that’s moved inboard by 20 mm. If you directly mounted the 330 mm rotor with the blue 2-p caliper in the stock f8x caliper location then 20 mm of the rotor would not be used. The pad radial depth is 63 mm so only 43 mm of the rotor from the outer radius will be used (doing this will likely result in pads with severe tapered wear because the piston’s area is no longer compressed against a pad fully supported by the rotor face).

You need someone like Seems Legit Garage to make a caliper mounting bracket that shifts the caliper 20 mm closer to the center of the hub. Whether you go with 330 mm or 340 mm OD rotors, you have the same problem relocating the caliper closer to the hub by 20 mm and 15 mm, respectively.

Doing this is going to shift the brake bias forward by 7-8% to 69% forward bias (assuming same pad CoF is used on the front and rear) which is significant. Dynamic brake bias shifts braking farther forward. For the f8x wheelbase, track widths, weight, etc. will see another dynamic 9-10% forward shift so that would make your proposed setup to have 79% of the braking done by the front brakes.

[tftimbn]

Originally Posted by M3SQRD

If you directly mounted the 330 mm rotor with the blue 2-p caliper in the stock f8x caliper location then 20 mm of the rotor would not be used.

In this case, I'm looking to mount the caliper that matches those rotors. I will not be using the F8x 2 pot calipers. My hypothesis here is that the smaller caliper will correct this difference for 100% pad contact. I would consider the vargas setup, I'm just trying to avoid spending $1700+ at the moment.

Just stumbled across this too

https://f80.bimmerpost.com/forums/sh....php?t=2022801

Any thoughts on that?

[M3SQRD]

Originally Posted by tftimbn

In this case, I'm looking to mount the caliper that matches those rotors. I will not be using the F8x 2 pot calipers. My hypothesis here is that the smaller caliper will correct this difference for 100% pad contact. I would consider the vargas setup, I'm just trying to avoid spending $1700+ at the moment.

Just stumbled across this too

https://f80.bimmerpost.com/forums/sh....php?t=2022801

Any thoughts on that?

I believe the BMW 2-p calipers use the same radial depth pad and caliper mounting hole spacing. Also, I think the caliper for the smaller OD rotors use only a 40 mm piston diameter, not 44 mm. The 40 mm piston diameter has a 17% difference (smaller) piston area than the 44 mm piston diameter. This reduction in rear piston area will shift the brake bias even farther forward. It’s getting close to the point where the rear brakes aren’t going to be doing much braking at all. Regardless, the key is having a properly designed caliper adapter bracket to relocate the f8x caliper mounting points to the f3x caliper (which may/may not be different from the f8x mounting points and pad radial depth) and the smaller 330 mm brake rotor OD.

300 mm e46 (?) 328i brake rotors and a single-piston sliding caliper from an e46 M3? Good luck finding pads that will fit the caliper and use the full radial depth of the rotor.

[tftimbn]

So I did go ahead and try out E46 rotors with F30 300mm pads and calipers, here is the pad depth. I reached out via PM to the guy who suggested that if that’s what he ran before I bolt these up.

View post on imgur.com

View post on imgur.com

Although looking at the VS kit, it looks like theres full pad depth. So I’m not sure what rotors are being used here. I might go grab some rotors from
autozone for testing and just return if anything.

View post on imgur.com

[M3SQRD]

Originally Posted by tftimbn

So I did go ahead and try out E46 rotors with F30 300mm pads and calipers, here is the pad depth. I reached out via PM to the guy who suggested that if that’s what he ran before I bolt these up.

Although looking at the VS kit, it looks like theres full pad depth. So I’m not sure what rotors are being used here. I might go grab some rotors from
autozone for testing and just return if anything.

That’s terrible coverage. If you have the FMSI number of F30 rear pads, you can compare the dimensions of other BMW pads and see if there’s an identical pad except for the radial depth. What is the rotor radial depth (difference between OR and IR of the disc face)? You know the OR is 165 mm. The radial depth can be determined from the e46 pad geometry. Which e46 is this rotor from?

The rotors used in the other kit appear to be M-drive (alum mounting hat with steel pins cast into the alum hat and iron disc) rotor machined down to a smaller OD. You can clearly see the steel pins and only two rows of venting holes in the rotor face. Stock M-drive rotors have three rows of venting holes.

[tftimbn]

Originally Posted by M3SQRD

That’s terrible coverage. If you have the FMSI number of F30 rear pads, you can compare the dimensions of other BMW pads and see if there’s an identical pad except for the radial depth. What is the rotor radial depth (difference between OR and IR of the disc face)? You know the OR is 165 mm. The radial depth can be determined from the e46 pad geometry. Which e46 is this rotor from?

The rotors used in the other kit appear to be M-drive (alum mounting hat with steel pins cast into the alum hat and iron disc) rotor machined down to a smaller OD. You can clearly see the steel pins and only two rows of venting holes in the rotor face. Stock M-drive rotors have three rows of venting holes.

Interesting now that you mention that, the outer ring of holes is REALLY close to the edge of the rotor. Hmm.

I ordered rear rotors for a 2003 M3.

My buddy with the 330 came by today, I verified he has 300mm rotors which means he does not have m sport. Coincidentally, he needs pads and rotors on his car so I’m going to use his garbage ones for fitment testing anyway.

I will also take measurements of the rotor and get back tomorrow.