In a previous post, I discussed my plan to implement power brakes with a GEN1 Bosch iBooster, I fabricated a bracket to mount it and I machined the pedal box.
The next step was to connect the brake pedal to the iBooster. Since the iBooster has only one input rod the front/rear brake bias will be adjusted via a hydraulic proportioning valve rather than the balance bar that came with the pedal box. I removed the balance bar and was surprised at how small the mechanism was. I would have thought that more contact area would be required, especially given that the pedals are cast aluminum, but the Tilton guys know what they’re doing.
Stock balance bar (left) and custom balance bar replacement (right). Note that the bolt heads are drilled for a safety wire.
I wanted to replace the balance bar with something that was unquestionably strong and had a precision fit. I’m pretty sure that 6061 aluminum would have been strong enough, but when dealing with brakes it makes sense to be conservative. I considered using steel, but it’s heavy and it would rust inside the pedal. McMaster offers tight-tolerance (0.0005" to 0.0000") high-strength 7075 aluminum rod. While it doesn’t like to be bent, it’s incredibly strong which is why it’s used in aerospace for structural applications.
Using the lathe, I machined the rod a few thousands wider than the pedal and drilled and tapped 5/16"-24 holes in the ends. While there should be minimal lateral forces, I laser cut retention discs from 0.060" stainless steel to restrict lateral movement. The 5/16"-24 rod ends are held in place by grade 8 washers and hex bolts with drilled heads so that they can be safety wired.
I removed the clevis from the iBooster’s input rod to keep everything compact and to reduce the number of connections that might introduce play into the braking motion. Note that some OEMs utilize an extension rather than a clevis. I then fabricated an adapter from 1/2” x 3/4” 4140 bar, 1/2” OD 0.058” wall 4130 tube and 5/16”-24 4130 tube ends. The center was tapped for the input shaft’s M10 x 1.5 mm thread and I bored holes 80% of the way into bar to receive the tubes. It would have been less work to fillet weld the tubes to the bar, but the bored holes help jig the tubes during welding and they result in a stronger assembly.
Parts ready for welding; adapter bar (left), tubes (middle) and tube ends (right)
The adapter connects the two rod ends bolted to the brake pedal insert to the iBooster’s input rod. The tube ends haven’t been tacked yet and the input rod’s bellow has been temporarily removed.
While the base of the tubes were jigged, I didn’t jig the ends of tubes and one of them warped inwards during welding, so I’ll need to remake this part. I’m worried about having enough master cylinder travel so the iBooster is spaced 1/4” off of the bracket, the tubes were left long and the tube ends are only lightly tacked. Without any fluid in the system, I pressed the brake pedal until it bound on the adapter bar. This resulted in about 5-1/4” pedal travel, as measured from the middle of the pedal, and about 15/16” of master cylinder travel. From what I can tell, the master cylinder has a 1” bore which would mean that I shouldn’t need much travel. However, I think that the single input rod drives two internal master cylinders (the reservoir has two feeds) and I have no idea how that changes things.
The master cylinder has two M12 x 1.0 mm pressure ports, so I purchased a set. D’oh! The seat is deeply recessed and the hex nut bottomed before the flare seated. No one seems to spec how far the threads extend which required looking at a lot of pictures to find one that would work. Fortunately, the second time was the charm and K-Motor Performance had the solution.
M12 x 1.0 mm to -3 AN fittings; note how much longer the M12 threads (the ones pointing down) are on the right fitting offered by K-Motor Performance
There are lot of unknowns regarding master cylinder travel, how the internal master cylinders are sized, the size of caliper pistons (although I could figure that one out). So, the next step is to plumb the iBooster into the car and see what happens. I’ll measure the pressure at the front and rear calipers with a gauge both with and without the iBooster powered up. That should give me a good idea if I have enough pedal travel, what the amount of boost is and if the master cylinder size provides enough volume for my brakes. I have the Brembo GT upgrade, so I’ll need more volume than the stock brakes.