Parking Brake

I purchased the option parking brake kit from Superlite. Although it provides all of the key components the builder still needs to fabricate a custom bracket for the brake lever and route the cables to the calipers on the rear wheels. While this isn’t complicated, is a pain in the ass. My biggest issue was trying to figure out where to locate the lever in the cramped cockpit. This is why many builders replace the lever with a E-Stopp electric brake kit. Essentially it’s a linear actuator with an Electronic Control Unit (ECU) that utilizes a button to toggle between no tension (brake off ) and 600 pounds of tension (brake on). The unit is used in many hot rods and SL-Cs. Most SL-C builders mount the unit in the side pods. The issue with this location is that you have to take the body off to service it.

I spent a lot of time thinking about where to place the E-Stopp and one of Stephen’s forum posts provided the answer. He mounted the unit in the recessed floor which affords easy access for maintenance. I was already planning on creating a closeout panel that wrapped around the chair to create a flat floor. I guess I’ll just be able to smuggle less contraband now LOL Stephen also fabricated a channel to protect the cables… brilliant!

So this was the plan until I spoke with Allan and Bob. Allan has successfully used the e-Stopp in multiple cars, but he had two cars in which he couldn’t reliably get the parking brake to hold. He called the manufacturer to see if he could increase the 600 pound tension and they indicated that they’d have to completely redesign the unit. Bob also had similar issues and the car let lose while on a trailer. He spent a fair amount of time trying to get it to work reliably to no avail. While many people have had success with the e-Stopp, two very talented builders have had issues with it while the car was in the build stage.

Back to the drawing board. I then discovered motor-on-caliper parking brakes. Instead of using a cable to actuate the caliper these units use an ECU to drive motors that are directly attached to the calipers. Apparently TRW has shipped over 60 million units and Brembo and other manufacturers have integrated the motor directly into the hydraulic caliper. This reduces unsprung weight by removing the need for a separate caliper and bracket. It’s my understanding that some units implement a true emergency brake by using accelerometers to ensure that the car doesn’t spin when the brake is applied.

The TRW looked like an ideal solution. I tried to buy an aftermarket version or at least get documentation on the OEM versions to no avail. So to the junk yard I went. It’s easy to pull the calipers and ECU, but I couldn’t figure out how to activate them. I tried using a CAN bus sniffer on a running car to figure which CAN bus messages where use to activate and deactivate the brake. My conclusion was that that the e-brake ECU was deeply integrated into the OEM’s system and that it wasn’t going to be as easy as finding two messages and broadcasting them via my MoTeC system. There’s likely a OEM implementation out there that might be that simple, but god knows how long it would take to find.

So after over a year of intermittent researching and tinkering I ordered an E-Stopp. On the day it arrived I decided to do one final search before opening the box and installing it following Stephen’s method…. AND I found this… HiSpec Motorsport Spot Electric Parking Brake (EPB) Really?

It’s a standalone solution that includes two calipers and an ECU unit! To prevent accidental engagement/disengagement you need to hold the button down for two seconds. The harness enables the addition of a backlit button or LED to indicate when the brake is engaged. My understanding is that it has adjustable tension and that it’s capable of holding a mid-size truck (will get specs from Hispec). At £640.00 (~$835.00) there an value given that the Superlite option costs $999 and an E-Stopp costs $479.00.

I ordered the unit which takes 28 days to manufacturer plus shipping from the UK.

The unit seems to be well built and I was happy to see a quality assurance tag (see picture below) on the harness indicating when and by whom it was tested. The top of the ECU housing appears to be anodized cast aluminum with the backside potted with epoxy to encapsulate the electronics. The included button has an LED to indicate if the brake is engaged or not. It’s useful if you want to hide it or if you’re going with a race car interior, but I’m likely going to upgrade it.

The following photos compare the Superlite caliper (left) to the HiSpec caliper (right).

I expected the overall HiSpec system to be lighter, but I expected the calipers to be heavier due to the integral motor. I’m thrilled that the HiSpec solution has lighter calipers which results in lower unsprung weight. The following table lays out the weight for three different parking brake scenarios.

Part UnSprung Hispec Superlite Superlite & E-Stopp
Calipers Y 5.7 6.5 6.5
Caliper Brackets Y 1.0 1.0 1.0
ECU & Wiring Harness N 1.4 n/a 0.5
Handle N n/a 1.1 n/a
Handle Brackets N n/a 3.0 n/a
Cables N n/a 3.5 3.5
Actuator N n/a n/a 4.8
Total Unsprung Weight - 6.7 7.5 7.5
Total Weight - 8.1 15.1 16.3

There are three features to prevent accidental engagement/disengagement:

  • The button must be held for two seconds

  • An optional wheel speed input to prevent activation when moving

  • An optional ignition input to prevent deactivation

I spent a while trying to figure out how to mount the calipers. In particular I couldn’t figure out what the snap spring was for… at some point Bob pointed out to me that it’s a floating caliper design. Duh, that explains a lot! The caliper can slide (i.e. float) 0.35” towards/away from the rotor. I figured it would would be best to place it in the middle. While mocking the bracket I was constantly measuring and re-centering the caliper. I then figured out it would be easy to 3D print two temporary spacers to keep the caliper centered.

The rotors are 28 mm wide (1.1”) and the space between the pads is 1.2”. To ensure that the pads were perfectly spaced on the rotor I tried using some metal shims, but they weren’t the exact width that I wanted and they kept falling to the floor. So guess what? I 3D printed two brake pad spacers the prefect width and with a right angle to keep them from falling. The openings in the brake pad spacers aren’t for style points. They significantly reduce print time and the amount of material used. This is the exact opposite of CNC machining in which this would increase machining time.

Since the hydraulic brake caliper is at 3 o’clock I placed the parking brake caliper at 9 o’clock. While mocking the bracket I figured out that it’s critical to locate it in exactly the 9 o’clock position. Otherwise I would will get different fitment due to the curvature of the rotor and the shape of the upright (it’s sloped -15 degrees from vertical). I tried to do everything on the car, but after a couple of misses I decided to pull the upright… I hate removing ball joints. I think I’m going to put some anti-seize on them when I reassemble!

Rear left upright, the line scribed in the blue dye is the 9 o’clock position (when looking from the other side)

The Superlite bracket requires you to drill holes in the the side of the upright which has machined pockets behind it (i.e., the recesses above and below the 9 o’clock position in the picture above). This area is 0.3” thick which is only 0.95x the diameter of a M10 bolt which is well under the 2x rule of thumb for thread depth in aluminum. Given that the parking bracket doesn’t see much stress and that any stress puts one screw in compression and the other in tension this isn’t an issue. That said, it doesn’t feel right to put the screws there. In any event, the HiSpec caliper results in a bracket that places the screws towards the center of the upright. After careful measuring, it appears that the holes can be placed in the web between the machined pockets on both sides (see picture below). I would like to confirm the 0.934” dimension with Superlite before drilling!

Horizontal cross sections of the Superlite and new brackets

After a bunch of iterations I wound up with the following bracket. This could be made from a 3/8” angle aluminum, but the Superlite bracket is machined from billet. If we’re going to machine it, we might as well give it some pockets to make it a little more like the upright!

To hold the mock bracket in place and ensure that it was parallel to the upright’s edge I used a piece of 1/4” scrap aluminum (yeah, that’s not bending). I wanted to position the bracket inboard of the upright’s chamfer so I used some scrap 1/8” aluminum to make a spacer and lined everything up using a centerline groove I designed into the mock bracket.

In the picture below you will note that the rotor’s OD is parallel with the edge of the parking brake’s pad. This leaves approximately 1/8” between the rotor’s OD and the caliper’s ID. You will also note that the caliper isn’t exactly centered on the caliper pin (or whatever it’s called).

The following picture shows an outline of the bracket and where and how deep the threaded holes will go to achieve 2x screw diameter (i.e., 20 mm).

Next step is to wire it up and test it. Apparently you need both calipers plugged into the ECU for it work.