Project 5.0 Mustang (The White One) Making it Stop with Essex Parts AP Radi-CAL Brake System

by Mike Kojima

Our Project Mustang came stock with pretty decent brakes. From the factory our Ford features 4 piston Brembo calipers with 355x32mm one piece rotors. However, we still wanted to upgrade our brakes as our much wider tires, 20" wheels, upgraded suspension and engine would stress the brakes a lot more. Our Mustang also weighs a punishing 3700lbs which really cooks the brakes. The Ford factory has put larger brakes on some of the higher end SN197 Mustangs so we figured we should follow suit as well.

When researching high-performance brake options we were drawn to the AP brake kit made by Essex Parts. Essex Parts manufactures their brake kit using AP calipers and rotors.  What made their kit stand out in an industry full of good Mustang big brake kits is their use of the innovative, advanced AP Radi-CAL brake caliper. 

Read more about project Mustang!


The Radi-CAL caliper was born in 2007 with a new concept in caliper design.  Prior to this brake calipers were blocky affairs with the best of them designed to be stiff for best pedal feel and the greatest efficiency. 

With the advent of CAD and FEA, the best calipers started to get pared down to save weight while maintaining stiffness. Calipers were made with forgings and innovations like Monoblock construction and elaborate bridges which enabled designers to further trim the bodies without sacrificing stiffness.   

What made the Radi-CAL caliper different from previously designed calipers is that it was designed to be optimized for dynamic loads, the first caliper to be designed with this in mind. When the brakes are applied the caliper distorts into a parallelogram and the Radi-CAL was designed using FEA with this in mind.

The result is a very unusual looking caliper that is over 30% stiffer and about 30% lighter than previous designs. The open design of the Radi-CAL with maximum material removal and flying buttresses means that that caliper receives quite a bit of cooling airflow.  It also means that the caliper is low profile allowing it to fit behind smaller wheels with tight spoke clearance.


Our kit uses the Radi-CAL Pro 5000R caliper.  You can see the unique asymmetrical low profile shape in this picture.  You can see the aggressive CNC milling to pare every unnecessary gram of aluminum away.

The caliper is made from a two-piece forging. The intricate bridge and flying buttresses left after the milling process improve stiffness greatly while eliminating excess weight. Cooling airflow can reach the interior of the caliper as well.

The caliper also features internal fluid passages and a corrosion and heat resistant hard anodized finish. The internal fluid passages lower the external profile and cannot be damaged by a hasty tire change in the pits. The hard anodizing won't burn off or change colors with hard track use.


The Radi-CAL Pro 5000R caliper uses domed back ventilated stainless steel pistons. Stainless steel has poor thermal conductivity, 50% less than regular steel which reduces heat infiltration to the caliper and brake fluid. The domed back improve the piston stiffness which gives better brake feel. The vents in the face of the piston help cool the brake backing plate and allow less contact area for conductive heat transfer. 

The caliper has staggered piston diameters to reduce brake pad taper wear and the piston sizing is selected to provide perfect hydraulic proportioning with the stock Mustang hydraulic system. This is critical with today's cars equipped with ABS, electronic brake proportioning and stability control. 

The caliper also has stainless steel abutments to prevent the brake pad backing plate from wearing and digging into the caliper's aluminum body which can causes sticking, uneven pad wear, and poor pad life. 

Finally, the Radi-CAL Pro 5000R caliper has anti-knockback springs. These are springs under the piston that push the piston back to the fully outward position when the brake pedal is released. The anti-knockback springs prevent a sudden low pedal caused by flex in the front wheels bearing system or upright. This is a common problem on fixed piston calipers on production cars modified for racing with wider R-Compound tires.

The increased load causes the hub, bearing and or upright to flex, this knocks the pistons into the caliper.  When the brakes are applied, the pistons must push back out of their bores which causes a sudden low pedal!  This can be very frightening for the driver. Anti-knockback springs keep the pistons fully outward and the brake pedal firm even if they are pushed in by flex.


The Radi-CAL Pro 5000R caliper is radial mounted so this adapter piece must be used to fit the caliper to the Mustang upright. It is CNC machined from billet aluminum and uses NAS studs for the caliper mounting points. The adaptor is cored out to minimize weight and for brake duct clearance (Essex Parts sells duct kits). The adapter is hard anodized for corrosion resistance just like the caliper.
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Tuesday, October 25, 2016 4:46 AM
That is one serious brake setup, makes me wish I could buy the kit for my st185... My current setup are brembos from a CTS-V but this seem like a really nice upgrade!
Tuesday, October 25, 2016 10:56 AM
God AP Racing makes nice stuff. I love all of the details in their parts. The only thing they're missing is PFC's 2 piece rotor that uses essentially a big circlip instead of any hardware. No hardware with dozens of washers and spring clips, no torquing, no rattling, lighter weight. http://www.performancefriction.com/motorsports/v3-disc.aspx

I'm curious to hear how the anti knockback springs are for day to day driving. I drive my track car with race pads in it all summer and the slightest bit of a sticking caliper can cause a cyclic squeak as the pads rub against the rotor. I'm imagining having spring loaded pistons means that race pads constantly rub and could potentially always be squealing.

Are you having issues with knockback on your car? It seems like something you'd only want if you're having a problem. My WRX was awful for knockback on the track so it would've been nice to have anti knocback springs for that car, but my E36 M3 is fine.
Tuesday, October 25, 2016 11:21 AM
You mention the chassis code is SN197, its actually S197.

Also, I'm curious why brake ducts weren't tackled before a costly brake update was undertaken. Lots of track-used S197's find awesome results ducting the 4-piston Brembo setup, which resolves most, if not all, brake fade issues.
Tuesday, October 25, 2016 11:27 AM
Those PF snap ring rotor setups are sweet! Why hasn't that been done before?
Mike Kojima
Mike Kojimalink
Tuesday, October 25, 2016 11:34 AM
The N denotes its a production chassis. As far as why did we upgrade the brakes? Because we can and we can get away with less aggressive pads with larger brakes.
Mike Kojima
Mike Kojimalink
Tuesday, October 25, 2016 11:36 AM
I think almost all production based cars get knockback to some degree once they start making grip.
Dan DeRosia
Dan DeRosialink
Tuesday, October 25, 2016 1:26 PM
I'd say that I wish the Pro 5000 R calipers were available in the size I want, but it's probably better for my pocketbook that they're not.
Tuesday, October 25, 2016 2:14 PM
^I'm pretty sure they have a 4 piston version of the caliper...
Mike Kojima
Mike Kojimalink
Tuesday, October 25, 2016 7:56 PM
They do!
Sunday, October 30, 2016 7:00 PM
That is a wicked caliper. Hopefully we'll see more calipers like this in the future.

I didn't see any CNC milling on that caliper though (except for the bolt holes). It all looks to be one beautiful forging.
"You can see the aggressive CNC milling to pare every unnecessary gram of aluminum away."
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