Daily Archives: April 22, 2018

Still thinking about the Sprint-T and Happy Earth Day

I was walking and thinking about the Sprint-T. The main thing I was thinking about was torque control of the rear axle and how to do it without spending any more money than I already have. The current plan and existing hardware for locating the rear axle and also mounting the springs and shocks uses a part called a “swing arm” that I have shown a picture of in the past.
These are MY swing arms
Well, I can slightly modify one piece to run a steel link with rod ends that runs from a bracket on the rear axle to a bracket on the nose of the swing arm. This would triangulate the swing arm and make it a rigid link that would prevent the axle from rotating under power and because it’s offset to the right also plant the wheel that lifts when power is applied. Basically what it amounts to is welding a pair of plates to the nose of the arm with a hole to run the mounting bolt through, then running the previously-mentioned link from a bracket under the axle to the nose of the swing arm. Now I couldn’t do this for the 3000 pound car this part is intended for, but for ~1500 pounds and 300 or so horsepower it should be adequate. And if it isn’t the part that would bend or break is not very expensive and is easy to replace, that being the rod end on the front. So I guess I would need to keep a pair of those rod ends and a pair of generic vice grip pliers to remove the broken bits from the swing arm for a roadside repair. But I should be good because the Speedway catalog torque arms for this size vehicle use the 5/8” rod end while this part uses the 3/4” rod end which is up to 30% stronger. And another benefit of controlling the rear axle rotation like this is the ability to almost instantly change the pinion angle from street to race and back again. It’s like a turnbuckle to control the pinion angle.

And with a torque arm this short I would need to run brake floaters to prevent wheel hop under heavy braking. The floater decouples the brake torque reaction from the rear axle and sends it directly to the frame via another radius link, making the rear frame area visually cluttered but dynamically simple. Because the Sprint-T has such limited wheel travel short links from the brake floater to the rear hoop of the roll bar will work in spite of there only being 5″ from the center of the axle to the roll hoop. Then instead of using spring rates and shock absorber settings to control the brake reaction at the possible detriment to handling and grip while turning I can leave those settings for hustling through the corners. And these links will be under very little load, because the torque reaction under braking would be forward from the axle, while the traction reaction from the axle would be backwards. So lightweight aluminum links would work fine between the floater and the frame.

And while I realize my little project isn’t as earth-friendly as riding a bicycle, it’s still an order of magnitude better than anything on this list . So, have a happy Earth Day anyway.