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      07-31-2017, 10:51 AM   #2
The HACK
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Drives: 2006 MZ4C, 2021 Tesla Model 3
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I can't tell you what you'll need, but I can tell you where the hurdles are.

1. Size of Brake Master Cylinder: If the size of the master brake cylinder isn't a near perfect match, you will be chasing a soft or stiff pedal AND lack of performance from the swap. If this hurdle can't be cleared, no amount of "hacking" you do to the system will make one work with the other. This is going to determine, no DICTATE the ratio of master cylinder to slave (piston size on the calipers) size. If this doesn't work, forget it.

2. Size of slave (Piston Surface Area) cylinders: See number 1. IF the master cylinder size match, which most of the time on a BMW, it does between certain trims (i.e. master cylinder size is the same despite different part numbers between E36 M3, E46 M3, Z3 M, and Z4 M, but not between the M and the non-M variant...i.e. the E36 325, 328, and E46 323, 325, 328, and 330 all use the same size master cylinder, but not the same as the M3/Z3M/Z4M master cylinder), then you have to see if the slave (caliper pistons) match as well. If it doesn't, then things become complicated, but not impossible. If the size of brake caliper piston area is within 10% of each other, and if the new caliper piston area is SMALLER than the OE size, you can still POTENTIALLY have a working system. All it does is effectively shift brake bias one way or another by the percentage difference between the two piston areas. This can be remedied either by using a more aggressive pad on the end that the bias shifted from, or going to a larger diameter rotor.

On a side note, the E46 M3 has 2 different brake parts, one for the run of the mill M3, and the other for the ZCP (competition package) equipped M3. The competition package equipped M3 has larger rear caliper piston, but also larger front rotor to effectively equalize the shift in rear brake bias due to the change in piston diameter. I believe the master cylinder area remained the same between the two trim levels. The Z4 M runs the same setup as the E46 M3 ZCP, but speaking with a few brake industry veterans, most production car tend to have too much brake bias toward the front brake, because, well, the front does the most work regardless of how the car's laid out. A better, more performance oriented brake system would want to spread that load a little bit more evenly. A lot of aftermarket high performance brake suppliers make kits that shift the hydraulic fluid brake bias on our BMWs to the rear by 5-10% on purpose.

3. Caliper radial and axial mount location: Once you've determined that the master brake cylinder size is the same, and the caliper piston area to be equivalent AND keeps hydraulic fluid bias within a small range, the "fun" part begins. Because you're swapping from a sliding caliper design to a fixed caliper design, first you must figure out if the caliper's radial mount accommodates the size of rotor you wish to run. For example, most calipers have a range of overall radius/diameter of rotors they can accommodate, the 4 piston RacingBrake caliper I run up front works for a rotor as small as 340mm to a 365mm rotor. Then you MUST determine HOW to make the central axis of the caliper (the centerline between the retracted pistons) align perfectly with the axial centerline of the rotor.

This means, first having to determine how the caliper is mounted to the kingpin. If it's an axial mount, meaning the bolt holding the caliper to the kingpin is parallel to the axle, your options, based on the determination above (axial and radial clearance) would be to have a spacer bracket to move the caliper in a way that it's centered axially and have the pad sweeping area match the outer diameter of the rotor. HOWEVER. If the caliper's mount creates an axial center closer to the car than the rotor's axial center, this won't work. You will have to shave off parts of the fixed caliper's mount to accommodate, which weakens the structure rigidity of the caliper. Or if the rotor is significantly smaller than what the caliper is originally designed for, this won't work either. Or if the rotor is significantly larger than what the caliper is designed for, it won't work. If the caliper you're using is radially mounted, then it gives you some flexibility because you can build a bracket that mounts axially to the kingpin, then have the caliper radially mounted to the bracket. The bracket can be milled to a specific thickness and height to make sure the caliper is mounted axially centered to the rotor and radially sweeps all the way to the outer edge of the rotor (the top of the pad meets the top of the rotor). With a radially mounted caliper, as long as the rotor falls within the range that the caliper is designed for, you can machine/make a bracket to fit it.

Again, as long as the first two parameters fit (master and caliper piston sizes match within a range).

4. Pad sweep area: Typically, a multi-piston fixed caliper have a bunch of smaller pistons vs. a sliding, single (or double) piston setup. The result is pads for multi-piston fixed calipers are usually a lot shorter and longer, while single or double piston sliding calipers have pads that are much taller or more square shaped. IF you're using your stock rotors and just swapping over the calipers, you have to look at the actual sweep area of the new pads and see if it covers about the same general surface area as your OE setup, as well as the height of the pad to see if it cover the same sweep. Because the size of the sweep surface also determines frictional force, therefore the same piston (or roughly the same) surface applied to different sweep area will alter the brake bias calculation. If the new surface area is much larger (or smaller) you can always use different pad compounds to re-adjust the brake bias.

The other factor is mostly aesthetics. If your new pad surface area is roughly the same, but with the mult-piston setup the pad is longer but shorter, it will leave a portion of your OE rotor uncovered and since that part will never get scrubbed by the pads, it will be covered by rust very quickly.

5. Rotor selection: Thus far we've only assumed that you're only swapping the calipers, not the rotors. Once you take rotors into consideration the equation becomes complicated AGAIN. You have to figure out where the rotor's axial center is, how the rotor's hub depth and size affect other various mounting points, the dust shield, the location of the steering knuckle and tie-rods, and other suspension hardware. Even assuming that a BMW rotor SHOULD fit a BMW hub (5x120mm w/ a 72.6mm hub), there's no guarantee that the rotor won't interfere with one or more of the components listed above.

Now, having said all that. Typically, fixed calipers are designed in a way to fit a specific subset or parameters, and variation in master cylinder sizes usually, again, fall within a specific range or quanta given the same manufacturer (i.e. see 3 series example above). So a lot of times, people have good luck fitting, say, a Porsche Boxster Brembo caliper to a BMW E30 M3. It's not to say that the M4 caliper to Z4 M would be a monumental task. But I certainly wouldn't assume that it'll be as easy as just building a custom bracket to fit either. The least I would do, is to figure out how the parameters fit within 1-3 above before I would ask "how."

And in all reality, unless you're getting the M4 calipers on the dirt cheap, the entire process of research and fabrication to make it fit may end up costing you more time, money, and headaches than to just buy, say, a RacingBrake (although they don't sell just the calipers alone, even though there are simple ways such as shim kits to make it work) or a StopTECH or any of the reputable brake vendor's kit that offers better performance for less cost.
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