fe1rx AP Racing BBK Designed and Installed

[fe1rx]

The short story is that I have built a front BBK for my 135i using AP Racing Pro 5000 R calipers and 355 mm x 32 mm (14” x 1.25”) full-floating rotor rings.







The long story follows …

About a year ago I was approached to design a custom AP Racing brake installation for a WRX STi (rotor hats and mounting brackets). I realized that the calipers and rotors being used at the front of that project were well suited to the 135i in terms of fit and bias so I took the opportunity to design and build rotor hats and brackets for the 135i while I was at it. I have now purchased the bits from Essex Parts to complete this installation.

DESIGN GOALS

I approached the design process with these goals:

1) Track capable braking system when using track pads front and rear (HAWK DTC-60 or similar).
2) Compatible with street pads front and rear for street use (HAWK HPS or similar).
3) Front pad changes without removal of calipers.
4) Reduced compliance by means of stiffer calipers and reduced knockback by use of two-piece full floating rotors.
5) Compatible with ABS by negligible change in PV characteristics.
6) Zero to small increase in front brake bias when using the same pad compound front and rear.
7) Compatible with OE wheels (Style 261 7.5” x 18 ET 49) without spacers.
8) Compatible with Apex ARC 8 wheels (8.5 x 18 ET 45) without spacers.
9) Retain radiant heat shielding of ball joints and steering rod end joints and improved cooling, without resorting to flexible ducting.
10) Brake hose must incorporate correct bulkhead fitting and rubber grommets.
11) Retain OE rear brakes.

PERFORMANCE GOALS

1) Improved pad and rotor life on track.
2) Reduced front rotor temperatures on track.
3) Elimination of piston wear (by use of stainless steel pistons)
4) Elimination of piston boot overheating (by elimination of boots)
5) Reduced piston knockback – better pedal feel

CALIPERS

AP Racing Pro 5000 R calipers are true racing calipers with an anodized rather than painted finish, no dust boots on the pistons and anti-knockback springs behind each piston. They are forged two-piece units and are truly works of art. Importantly, AP provides design drawings and technical data, and, through Essex Parts in North America, they provide very good technical support.

The Pro 5000 R caliper family are intended for track use primarily (because of the lack of dust boots). For all-season street use AP Racing also make a variety of painted calipers with dust boots, which would be a better choice for all-season street use.

Neither AP Racing (the UK based manufacturer) nor Essex Parts (the US based distributor) make an off-the-shelf BBK for the 135i. Their E92 M3 and 1M kits fit 19 inch wheels and fit a different steering knuckle geometry so are unsuitable. They also use larger front rotors that would increase front brake bias excessively.


ROTORS

I chose Essex Parts’ J-hook 355 mm x 32 (14” x 1.25”) full-floating rotor rings. These connect to the rotor hat by means of bobbins which permit radial expansion of the hot rotor, and also lateral play to reduce piston knockback due to flexing of the wheel hub.



I am using their AKB anti-rattle springs on every other fastener. These prevent the rotors from rattling at low speed, but they also negate some potential benefit of the lateral float provided by the bobbins. If I get any sign of knockback, I will remove these springs to see what change that makes.



BRAKE HOSES

The caliper uses a banjo bolt interface so custom brake lines were required. I have documented my quest for suitable braided brake lines elsewhere on this forum. I found Techa-Fit very easy to deal with and I like their product so I approached them to produce custom lines to my drawing. These lines incorporate the correct bulkhead fitting and grommets and fit exactly as intended.



BRAKE PADS

I have had good luck with Hawk DTC 60 track pads in the 135i, and these are available for both the OE rear caliper and AP front caliper. I am going to give Hawk HPS a try as a street pad. Although the OE front and AP pads are comparable in geometry, the AP pads use a 6-mm thick backing plate vs the 5-mm thick OE plate, and the AP pads provide 1 mm more friction material thickness.

BRAKE FLUID

I am sticking with Castrol SRF (replaced once a year) as a brake fluid.

ROTOR HATS

Rotor hat design is typical and the detail design was aided greatly by the information that AP Racing made available. The hats were made in a CNC vertical machining center from 6061T6 aluminum and were hard anodized.











CALIPER BRACKETS

Caliper brackets were also made from 6061T6 aluminum with hard anodizing.



WEIGHTS

This installation results in a significant weight saving:

Rotor Assembly – OE 21.4 lbs / AP 17.6 lbs
Pads – OE 3 lbs / AP 3 lbs
OE Caliper (with integral bracket) – 12 lbs
AP Caliper with bracket and hardware – 7.2 lbs
Net weight saving per front corner – 8.7 lbs

DUST SHIELDS

Essex strongly advises against running a rotor dust shield for track use because it reduces the ability of the rotor to radiate heat away. Heat radiated into the ball joints is not good though, so I made shields to provide protection to the ball joints.



BRAKE DUCTS

I am attempting to run without brake duct hoses, opting instead for fabricating scoops that direct air into the center of the hub. The scoops extend inboard of the inside edge of the tires as far as practical – the constraint being interference with the brake hose bracket at full lock. Having installed steering stops allows me to extend the scoops inboard a bit more than would otherwise be possible.

I went through many cardboard iterations before settling on the final design, constructed of 0.080” 6061T6 aluminum sheet, TIG welded.











BRAKE BIAS

Brake bias is affected by several factors:

- Effective radius of the rotor
- Caliper piston area
- System pressure (if different front to rear)
- Friction coefficient of pads (if different front to rear)

The relevant factors are summarized in the following image. As you can see, pad sizes are comparable with OE, but the AP pads are slightly thicker and have thicker backing plates.



The AP rotors provide dramatically better cooling with large air passages and directional vanes.



Crunching the numbers for the OE brakes reveals a front brake bias of 65%. This increases to 67% with the AP installation. This assumes equal friction coefficient front and rear, and equal hydraulic pressure. The bias can be altered further by using different brake pad compounds front and rear, and will be altered dynamically when threshold braking by the vehicle’s electronic brake force distribution software which reduces rear hydraulic pressure. In any case, a slightly increased front brake bias is acceptable when running track tires, because their increased grip will result in more forward weight transfer under braking.

I will get a chance to track test this installation in a couple of weeks and will report back after that.

[$iriu$black]

I was just trying to understand what you wrote about your Ohlins coilover install as i have a set myself that I am looking to install and here you are with yet another brilliant project that you just accomplished. Well done yet again sir.

... teach me your ways.

[Traf]

Dayum dayum dayum, you are awesome dude.
Seriously build me one lol !

ps: typo in the first line, you said 335mm instead of 355.

[houtan]

Seriously awesome and impressive. Thanks for sharing. Is this something you would be willing to make available to others?

[deesea]

This sounds so good, and looks amazing.

Curious what the negative effects to running a caliper with no dust boots yield for daily street driven applications - why is it bad to run a car without one?

Also....how much did you spend on the front BBK in CAD if you don't mind me asking.

[Traf]

Are you planning on doing something for the rear also ?

[Fume]

Thanks for sharing! Love reading about your thought process and the methodical approach you take for modifying your car.

[xBlueStreakx]

Beautiful.

[asbrr]

Lovely, I unfortunately chose the wrong type of engineering many years ago. This type of work would be so much more satisfying to me.

Well done sir!

[Stewy_604]

Slick man, great write up and a sick brake setup!

[fe1rx]

Originally Posted by houtan

Seriously awesome and impressive. Thanks for sharing. Is this something you would be willing to make available to others?

Thanks! Let's see how it works first ...

[fe1rx]

Originally Posted by deesea

This sounds so good, and looks amazing.

AP does make beautiful stuff and the Pro5000R series calipers are particularly attractive.

Originally Posted by deesea

Curious what the negative effects to running a caliper with no dust boots yield for daily street driven applications - why is it bad to run a car without one?

A caliper without dust boots is going to require more care and maintenance. OE opposed-piston calipers with dust boots are virtually maintenance free by comparison if only street driven.

Originally Posted by deesea

Also....how much did you spend on the front BBK in CAD if you don't mind me asking.

Let's just say that a rational person would buy a StopTech Trophy BBK for serious track use, and they would save money in the process. It takes substantial time and effort to design and build a prototype installation.

https://www.essexparts.com/brake-cal...ont-six-piston

plus

https://www.essexparts.com/brake-dis...lacement-rings

plus hoses

plus hardware

plus raw materials

plus hard anodizing

plus design time

plus machining time

plus (pads, fluid, installation time ...)

[fe1rx]

Originally Posted by Traf

Are you planning on doing something for the rear also ?

No. I find the OE rear brakes to be adequate.

[ShocknAwe]

Always an interesting read. Thanks for posting!

[fe1rx]

I have now got the car on the road, but will put a couple of track days behind me before I report back in detail. For the moment, I will say that the pedal is noticeably firmer even when driving on the street.

I thought I would share a few more details about the design process:

CHECKING THE WHEEL FIT

Before committing to the rotor hat geometry, I wanted to check if both my OE wheels (Style 261 7.5” x 18 ET 49) and Apex ARC 8 wheels (8.5 x 18 ET 45) would fit without spacers. Selecting a rotor diameter fixes the radial position of the caliper, but the depth of the rotor hat is a variable that can be adjusted to provide more or less spoke clearance. Also, by thickening the rotor hat mounting pad a spacer could effectively be built into the hat if necessary.

I had a CAD model of the ARC 8 wheels that I had created for other purposes so checking the fit on the ARC 8 was an easy digital process.



Checking the fit on the OE wheel required building a template. I printed the digital model at 1:1 scale, laminated this to a piece of aluminum sheet and used the trimmed template to confirm fit on both wheels.





The template check confirmed that my selected rotor hat geometry would clear both wheel spokes. The increased barrel clearance on the ARC 8 wheels reveals their thinner barrel construction and shows why these wheels are so much lighter than OE.

With all the parts now built, the following images show the fit of the final product.









CHECKING THE DUCT FIT

Interference between the ducts and the brake hydraulic hoses could be catastrophic so I wanted to take no chances here. To confirm positive clearance at all ranges of steering and suspension travel I removed the spring to permit the suspension to be fully cycled. I have maintained a comfortable 15 mm clearance between these two parts.



THE BRAKE BIAS MATH

The brake torque generated by a brake caliper is proportional to:

Brake Factor = Wheel Diameter x Brake Pressure x Friction Coefficient x Effective Radius x Piston Area

Considering each term in turn:

1) Wheel Diameter: This is the lever arm for the tire. As the overall diameter of the wheels front and back is identical, this term can be dropped.
2) Brake Pressure: while some vehicles have hydraulic proportioning valves that alter reduce the rear brake hydraulic pressure as a function of system pressure, our vehicles do not. Front and rear pressures are the same, except under extreme conditions when ABS / Electronic Brake Force Distribution reduce the rear pressure as a function of wheel slippage. Accordingly, front and rear pressure are the same when calculating the basic brake bias and this term can be dropped.
3) Friction Coefficient: I intend to run the same brake pad compound front and rear, so the friction coefficient can be assumed to be the same front and rear. Hence this factor can also be dropped when calculating the basic brake bias.
4) Effective Radius (Re): This is the lever arm for the brake pad and is distance from the centre of the wheel to the center of the brake caliper piston (or the average radius for a multi-piston caliper). AP publishes this value for their caliper, and I have estimated it based on measurements for the OE calipers.
5) Piston Area (A): This is the piston area (or sum of all piston areas for a multi-piston caliper) on one side of the caliper.

This gives a simplified Brake Factor formula:

BF = Effective Radius x Piston Area

And if we define the Front Brake Factor as BFf and the Rear Brake Factor as BFr, the Front Brake Bias is:

Front Bias = (BFf / (BFf + BFr)) x 100%

OE Front Brake: (338 mm OD Rotor, Pistons 28.0 mm, 32.0 mm, 36mm)

Ref (OE) = 141 mm
Af (OE) = 2430 mm^2
BFf (OE) = 342678

AP Front Brake: (356 mm OD Rotor, Pistons 27.0 mm, 31.8 mm, 38.1 mm)

Ref (AP) = 149 mm
Af (AP) = 2504 mm^2
BFf (AP) = 373809

OE Rear Brake: (324 mm OD, Piston 42.0 mm)

Rer = 135 mm
Ar = 1385 mm^2
BFr = 187035

Therefore:

OE Front Bias = (342678 / (342678 + 187035)) x 100% = 65%

AP Front Bias = (373809 / (373809 + 187035)) x 100% = 67%

As I have mentioned previously I am happy with the slight increase in front brake bias as this is appropriate for use with stickier tires.

[asbrr]

Excellent work as always. Sort of a side question, how does the master cylinder get "sized" for the braking system? I understand it's related to the total volume of fluid being pushed through the system, which in turn amongst other things is related to piston size. So basically, at what point should one consider upsizing the master cylinder when upgrading brakes?

[fe1rx]

Originally Posted by asbrr

Excellent work as always. Sort of a side question, how does the master cylinder get "sized" for the braking system? I understand it's related to the total volume of fluid being pushed through the system, which in turn amongst other things is related to piston size. So basically, at what point should one consider upsizing the master cylinder when upgrading brakes?

Thanks. The simple answer would be "when the pedal starts to feel too long", but that answer is probably ignoring the actual cause and applying a bandaid solution.

Adding total caliper piston area to the total brake system will mean more fluid will be needed to move he pads a given distance. Compliance in the calipers will have the same effect. So will knock back, brake shims, crumbling piston noses, flexing piston bottoms, compressible pad compounds, warped backing plates, rubber flex hoses, flexing in the firewall mounting of the master cylinder, etc.

BBK manufacturers (should) size their calipers to work with the existing rear calipers and master cylinders, so a BBK per se should not be a reason to change the master cylinder.

The only time the master cylinder should need upsizing is to achieve a firmer pedal at the expense of some mechanical advantage. In some cases it works well (I did it on my RX-7 and was very happy with the result), but the better solution would be to address the fundamental cause of excessive compliance, in one or many places, first.

In my case with this AP BBK, the small increase in total piston area is a trivial 2% which should correspondingly lengthen the pedal, but the pedal feels firmer and higher because the caliper (structurally optimized), pistons (domed bottoms, anti-knockback springs, no composite nose), pads (thicker backing plates, no shims) and rotors (full floating) are all designed for improved stiffness.

If you are at the DDT on Friday or Saturday this week say hello and I will let you give them a try.

[asbrr]

Originally Posted by fe1rx

....snip...

but the pedal feels firmer and higher because the caliper (structurally optimized), pistons (domed bottoms, anti-knockback springs, no composite nose), pads (thicker backing plates, no shims) and rotors (full floating) are all designed for improved stiffness.

If you are at the DDT on Friday or Saturday this week say hello and I will let you give them a try.

Makes a lot of sense, thanks for the explanation. I felt a corresponding improvement in pedal feel going from stock 128i calipers to 135i front calipers, I can imagine the improvements when doing a proper kit.

You're coming to DDT? Excellent, I will be there this Saturday for lapping...albeit the weather is supposed to be horrible. Looking forward to meeting you and checking the car out, will definitely say hello!

[Z16A]

Hi fe1rx, how are the brakes holding up? My buddy has the same calipers on his Z06 with 315 tires all around. The stopping power is amazing and they handle the heat so well..

Did you decide to make any kits for us poor guys who don't have mills of our own?

[MightyMouseTech]

Beautiful work as always.

I use the same pad combination on my winter/track car.

HPS are great on the street, and the DTC are amazing on track.

Love that all Hawk pad compounds are compatible. No rotor swap required when hitting the track.

[Joao Basso]

Hello fe1rx,

Great report here, thanks.
I want to do some brake ducts like yours. I need in fact to do something to cool the brakes as my OEM 135i brakes and road pads are on the limit of temperature on track usage with only 240 bhp and road tyres. I have burned pads and seals once.
Maybe my E87 ride height is higher then yours (so I may need to adjust the design) but I was wondering if you are willing (and kind) to send me the flat pattern dxf file (or BMP) of your brake ducts as a starting point?

Thanks in advance.
Joao