Wheels, brakes, and coilovers - Pt. 2 Brakes

F30 Brake Clearance

The obvious issue with designing a wheel is fitment.  If I'm going to achieve Objective 2 (creating a product) it needs to be capable of fitting more than just my own car.  Once I locked in the overall wheel and tire size to 18x9 +30 with 255/35-18 tires, a specific meaty tire combo that fits all corners without wheel spacers, I needed to concern myself with brake caliper clearance.

Fortunately, Brembo is great about supplying just enough data to CAD up some clearance volumes.  (See here)  There are a lot of BMWs, the Volvo S60R, and plenty of other cars that come with stock Brembos so the data is available.  The caliper model was not available but all of the rotor options were.  That provides the stock offset from the spindle or rotor hat face to the rotor centerline.  Head on over to any caliper supplier with dimensioned drawings and you have yourself a clearance volume.  For the F30, I did that for Stoptech, Dinan, and Wilwood and I'll be able to physically check clearance on my car.

Now I've got enough information to select my rim halves.  With the proper model setup...

... I can just click a drop down list and switch my rim half lengths to see what clears all of the calipers.  I wanted the longest outside rim half I could fit because for a given overall wheel width that would mean a shorter inner rim half.  The shorter the inner rim half is, the stiffer the whole wheel assembly will be up to a point.

Objective 2 - Key Result 3

(Other product opportunities)

Seeing all of the fitment issues that can come up, I saw an opportunity to make a new product I've been wanting to make for a long time.  Fully floating brake rotors.  With my own design I could change the rotor centerline position and give myself more room for larger calipers or a longer outer rim half.  It could also be an inexpensive performance oriented option over sourcing larger BMW components from other models.  The one item missing I'll have to physically measure myself to determine rotor offset is of course the upright, you can't go too far.  Nevertheless, I also know how expensive BBK's are and there aren't a lot of options for the F30.

I'm guessing right now I'll need to come in under $1800 retail for a front kit to be successful.  I don't think it needs to be an all out 20"-wheel-required 10 piston caliper.  Find a good front/rear balance, increase thermal mass and torque as much as possible, full floating lightweight rotor hat, quality but not overpriced American made replacement parts with a racing pedigree.  Sounds nice.

Wilwood is without a doubt cheaper than pretty much every brand out there.  Calipers come from forgings with radial mounts, pistons have dust boot options, brake pad compounds are expansive enough, and replacement rotors are relatively cheap.  I think this brand can help make this happen.

Below is a space claim sweep of the Wilwood Forged Narrow Superlite 6 piston caliper (light greyish), stock Brembo 370mm rotor (dark grey), and the initial PdV 355mm floating rotor assembly (blue).  Not pictured is the 340mm rotor on my F30 which has the same rotor centerline position.  Since its a square setup, the same process occurs on the same wheel with the 4 piston caliper for the rear.

It seems sourcing the floating rotor assembly hardware from AP Racing or Brembo isn't easy.  I am going to start out with my own design for now and keep looking.


F30 Caliper Piston and Rotor Sizing

I need to actually build a brake system calculator before I freeze anything but I started documenting whats out there.  For starters I'm going to target the M performance in terms of bias and what I'll call a front/rear torque ratio which is 62.3%/37.7% and roughly a ratio of 1.072 which is just a comparison of front and rear pad radius from axle centerline.

A similar setup from Wilwood results in the following:

Front

  • 6 piston sizes in mm (41.15/28.45/28.45)
  • Total pressure area of 5202.1 mm^2 compared to 5026.5mm^2
  • 355x32 rotor

Rear

  • 4 piston sizes in mm (31.75/31.75)
  • Total pressure area of 3168.5mm^2 compared to 3041.1mm^2
  • 332x32 rotor (ideally I will find a thinner one)
That results in a 62.1%/37.9% bias and 1.069 torque ratio.  There is a roughly 3-4% increase in piston volume which doesn't sound like it'll be an issue for the master cylinder.  More research and calcs are required as well as a look into what brake bias people prefer for actual racing conditions and good tires.

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