Tag Archives: doomsday devices I haven’t made

Still stuck on imagi-building that A-MOD car

I have a mind that must chew on problems to solve, not social problems, physically solvable problems like bicycles or race cars. And the one that’s occupying my mind is the A-MOD car. Like all good engineering problems it has a simple premise: Make the quickest car possible on an autocross course within these constraints; 72″ minimum wheelbase, 42″ minimum tread, 900 pounds minimum weight with driver and full fluids, and 10″ minimum wheel diameter. There are safety rules common to every car in the series, and because having a really stiff suspension is a huge advantage you have to have sprung suspension, not that actual springs are required, there are cars that are riding around on actual hockey pucks between the suspension and the frame, but something that isn’t a solid piece of metal against another non-moving piece of metal.

Anywho, working through those minor strictures I have evolved literally thousands of cars with uncountable variations of “suspension” over the last few weeks. But what I keep coming back to is a single-pivot rear suspension with a solid axle and a single disk brake halting the axle. It’s simple, strong, and fairly lightweight. And relatively easy to adapt to a chain drive. Another thing is it works extremely well with a common trope in cars this size, mounting the engine on the rear suspension. Basically this is done to simplify the drivetrain and shorten the chain on chain drives. It’s really bad for unsprung weight so I’m only doing the designs as due diligence to make sure I’m covering all the bases, and because researching this adds to the knowledge base for other designers, and what I learned to pass on to other designers is the unsprung weight is pretty close to zero if the Center of Mass of the engine is mounted right at the pivot point of the suspension which is physically impossible unless the rear swing arm is mounted in such a way as to create a virtual pivot point in or near the CG of the engine, which kinda negates the purpose of mounting the engine on the swingarm in the first place, simplifying the rear suspension and drivetrain.

Another suspension I have been looking at is the classic 4 link and panhard rod with the forward pivot of the 4 link even with the front drive sprocket of the chain drive and the rear pivot right on the centerline of the rear axle so the chain and the suspension swing through very close to the same arc and making having the same chain tension through the suspension travel simple.

Another thing I have been looking at is mounting the rear part of the chain drive solidly to the rest of the frame and using CV axles to run the power to the wheels as shown in a book I bought, How to Build Motorcycle-Engined Race Cars by Tony Pashley. and in this video by Vasily Builds where he modifies his swingarm buggy into IRS. Way more complex to build but less unsprung weight than almost anything else, but the 4 link is lighter overall, allowing weight to be added where it will do the most good, making the final decision a tossup or a painstaking virtual build of both to see which would be better. I have the time but not the software to do the virtual build, and the 4 link is both cheaper and easier. And since I have more time than money, cheaper is the way to go on this one.

But I would still rather be building the Sprint-T. This is just something to keep my mind occupied so I don’t build a doomsday device and just destroy everything. Speaking of which, I can’t find my yellowcake, I have the lemon and the cinnamon, but not the yellowcake. (Mad scientist in-joke, you don’t have to laugh if you don’t get it, but if you don’t get it and want to get it google “Nigerian yellowcake” for the source of the joke.)