10 Aug 2008

I may be backing away from using push-rod suspension; “simple, cheap, and easy” might win out. The more suspension motion that’s needed, the more volume is consumed in the chassis for the rocker-arm, and with 6″ suspension travel, the arm gets too large, never mind the weight. If the shocks are located down low, they cook in radiator exhaust air and crowd the master cylinders. Moving them up out of the air duct, raising them so that they’re visible to look cool is poor form due to raising the combined 14 lb weight so high. And, placing them parallel the centerline of the car means increased side-loading on the rocker assembly due to the push-rod changing angles. A rocker setup weighs more, costs more, and takes longer to build. Eh, I think I want to go simple on this car.

Push-rod setups do look really sweet, but the reason they’re used on F1 cars has nothing to do with looks; it’s to minimize the shock’s aero drag – a situation we need not worry about. (F1 cars also have suspension motion on the order of 1″, not 6″ like we have.) The one valid argument is that rockers can greatly improve installation ratio, where the shock can be made to move 1″ when the wheel moves 1″. With a traditional outboard arrangement, the shock may move much less than the wheel so stiffer springs have to be used. Since the shocks don’t move much, there can be issues with stiction of the shock valving. However, this isn’t a problem here because of how much suspension travel we have.

After moving the parts around in CAD, it’s not bad. The installation ratio ends up about 0.6, so the spring rate has to be about 2.8 times as much (installation ratio squared.) It means that while the wheel moves 6″, the spring moves about 3.6″. A 1″-thick bump stop brings total stroke to 4.6″, and since it’s good to have some overhead, the same shocks will work fine. Checking the effective wheel rate shows a gradual rising-rate, just as desired.

In other news, I took the transaxle and Wavetrac LSD to a shop to have the LSD installed. I don’t know how many of you’ve seen the movie, “The Fast and The Furious”, but this place and the customers were right out of the movie. That said, the mechanic was amazing to watch. He had the tranny apart in five minutes and it’s good he checked it because there were several things wrong. First, one of the shift-rod ball-detents was broken. Second, transaxles have a magnet in the sump to trap metal debris. The previous builder apparently didn’t know how to reinstall it and guessed wrong. When we opened it up it had attached itself to the final drive gear! Good thing it hadn’t been run. Anyhow, he completely disassembled the gear clusters, closely inspecting each gear and syncro. They all looked good except for third gear which had sharp burrs on it from overzealous shifting. He said reusing the parts could cause it to jump out of gear – no thanks. I left it to be repaired properly.