Power Brakes: Part 1
Several owners that I know with finished SL-Cs lament not having power brakes. I asked Allan, who’s built 28 SL-Cs and 5 GT-Rs, and he indicated that brake pedal effort is the number one complaint by far. I don’t have power brakes in my cobra, but the SL-C is heavier, more powerful and with active aero, it’s capable of quicker deceleration. I’m also not a spring chicken and my right knee bothers me more than it used to.
Power brakes typically use engine vacuum to operate on a diaphragm. Whomever figured this out back in 1927 was brilliant because vacuum is a side effect of a combustion engine so it’s a “free” power source. Vaccuum-based power brakes have been implemented by at least one SL-C builder (Joel), but the diaphragm takes up a lot of room and it won’t fit inside of the footbox. If I were to place it outside of the footbox, as Joel did, it would collide with my radiator outlet duct.
Electric vehicles (EVs) have power brakes, but no combustion engine and therefore no engine-generated vacuum. So, what do they do? Apparently, early model S Tesla’s used an electric pump to create a vacuum which was a poor solution because the pump ran constantly which was a drain on the battery. Apparently, several years prior the Tesla hack, the Toyota Forerunner employed an electromechanical brake booster, so Elon wasn’t the innovator in this area.
Fortunately, Bosch manufacturers an electro-mechanical brake booster which is used by many OEM EVs. In addition to removing the dependance of vacuum acting on a large diaphragm, the pedal feel can be adjusted through the configuration of braking characteristic curves. This allows the iBooster to be used across models or support different driving modes within a model (see diagram below). While having different braking modes is interesting, just being able to finetune one mode is extremely useful. The only way to change brake pressure on a SL-C is to swap the front and/or rear master cylinders which is both messy and time consuming. The SL-C’s tight footbox makes this a bit of a nightmare and master cylinder increments are at least 1/16,” so changes aren’t granular. While I haven’t found anyone that’s hacked the CAN bus to change the default brake characteristic curves, other builders have figured out the CAN bus messages to brake by wire and determine the master cylinder’s position.
There are two primary versions of the iBooster; GEN1 and GEN2. I bought a used version of each on eBay to compare. I liked GEN1 vs. GEN2 for the following reasons:
- It was a bit more compact in the intended orientation.
- It has a nice cast aluminum body with several machined surfaces whereas the GEN2 is all stamped steel and seems shoddy by comparison. I’m sure this is my perception rather than an issue with the design. GEN1 feels like the engineers were focused on a high-quality innovative solution and GEN2 was engineered to hit a price point.
- There is more information on how to hack the CAN bus on GEN1 than GEN2 which makes sense because it’s been out longer. While CAN bus integration isn’t required, it’s something that I’d like to do in the future.
GEN1 (left) and GEN2 (right); the reservoirs vary widely amongst the OEMs
GEN1 iBooster donor cars include;
- Audi A3 e-Tron
- Chevrolet Bolt
- Chevrolet Malibu
- Honda CR-V (MY 2018, 2019)
- Jaguar i-Pace (MY 2019, 2020)
- Porsche Panamera (MY 2017+)
- Tesla Model S (MY 2015+ with autopilot)
- Tesla Model X (MY 2015+)
- Volkswagen Passat hybrid
- Volkswagen e-Golf and Volkswagen e-UP
I didn’t want to pay the Tesla tax, so I purchased a GEN1 from a Honda. The mounting flange, master cylinder and reservoir are easily removed and appear to vary amongst different OEMs. The master cylinder has pressure ports cast on both sides with only one side being tapped. I assume this is accommodate fitment for different cars including left vs right-hand-drive within a model. In addition, I read somewhere that the Honda version was machined differently to accommodate a different master cylinder.
Enterprising builders have utilized the iBooster in homebuilt EVs, restomods, hotrods, etc. They figured out that iBooster can be operated in a fail-safe mode without any CAN bus connections. The ECU only needs four wires (two constant +12v power, ground and ignition) and four wires that connect to the position senser to the ECU. There are multiple wiring kits available (e.g.,
EVcreate,
Tulay’s Wire Works,
SGH Innovations) or you could cut an OEM harness.
Given that all of this has already been figured out, installing an iBooster is straightforward. However, the SL-C presents several challenges:
- The footbox and nose are tight.
- The pedal box sits on the floor which means that iBooster needs to be properly tilted to align it with the brake pedal.
- The pedal box uses a balance bar to connect to the front and rear master cylinders and the iBooster has only one master cylinder.
I considered mounting the iBooster in the nose, but the extended footbox projects too far for it to be mounted longitudinally (a standard footbox would probably result in a simple install). I also considered using a bellcrank and mounting it transversally, but that seemed like a hack. In the end, I designed a bracket that locates the iBooster as low and close to the pedal box as possible inside the footbox. I read somewhere that pedal force should be as concentric as possible with the input rod and that it must not diverge more than 3 degrees. My bracket tilts the iBooster’s nose down 15.75 degrees from the vertical to perfectly align, as far as I can discern, the center of the balance bar with the iBooster’s input rod. Any changes to the iBooster’s location would change the angle.
Laser cut 1/8” 4130. There are 36 tabs and slots. All of the corners are relived and there is only five thousands clearance between the slot edges and the tab edges (i.e., the tabs are 0.125” thick and the slot is 0.135” wide). The only edge that I filed was the bottom of the angled plate. This was by design to keep the weld gap small. If I had CNC machined the plate that wouldn’t have been required, but laser cutting is much cheaper. I assembled the four sides, wiggled them into the bottom plate, added some clamps and welded. As intended, the part pretty much self jigged which ensured that iBooster is at the correct angle.
Front view, bracket welded and primed with a rattle can
Rear view. Note the two nuts that are welded to the bottom rear edge. The left vertical plate is larger than the right to mount the GM accelerator which most SL-Cs utilize.
Many builders add a plate under the pedal box to prevent the floor from deflecting. If you look carefully at the pedal box, you realize that it was carefully optimized to reduce weight. There are only five relatively small contact patches with the aluminum floor (see diagram below). The four mounting holes are centered around the brake pedal. Note that the rear bosses are slightly larger than the front bosses and that there is only a small rectangular contact patch in the upper left to counteract force applied by the clutch pedal. In addition, the mounting bolts are only 2.225” apart in the direction of the brake pedal force (i.e., front to back). For these reasons, the bracket extends under the pedal box and provides two additional bolts in the rear of the bracket. When combined with the large truss supporting the iBooster, the bracket significantly stiffens the floor while only raising the pedal box by 1/8.”
Mounting holes and floor contact patches
The next step was to modify the pedal box to convert the dual cylinder configuration to a single cylinder. I used the mill to remove the top of the front and rear master cylinder brackets. This in no way reduces the strength or integrity of the pedal box base. The cast aluminum is high-quality, there wasn’t even a hint of an air pocket and it felt like I was machining billet.
The red cap points to the area where the upper portion of the brake master cylinder supports and studs were machined off. Note that the clutch mount in the background is intact. The lower studs are bolted to the bracket.
The bracket mounts to the front and rear brake cylinder lower studs. This allows the entire assembly to be installed and removed as a single assembly. The bracket clears the ECU and clutch master cylinder by about 1/16.”
The next steps are to:
- Replace the balance bar with a custom adapter. I’ll know at that point if I left enough space between the pedal box and iBooster to achieve full master cylinder travel.
- Temporarily wire the ECU and the position sensor.
- Temporarily plumb the master cylinder to pressure gauages.
- Determine if the master cylinder has enough volume to support the Brembo GT calipers.
- Adapt the reservoir inlets to AN fittings.
If anyone has information on iBooster master cylinder volumes or how to set the brake characteristic curves, I’d love to hear about it.