One thing I have been thinking about is the rear suspension. Well two things actually. The first thing was it saves less than $40 for me to use the steel swingarm/spring mount in the Mid-Bucket suspension, and it seriously complicates the rear frame. The second thing was that putting the rear spring that far out on the suspension complicates the roll stiffness bias. Putting enough spring to give the right rate for a 2 wheel bump over the end of the axle just makes the roll rate too high for the light front end even with the front springs almost on the kingpins. Seriously I’m going to mount a spindle with the steering arm on the axle and put the side of a pencil against the steering arm with the point against the axle and turn the spindle back and forth and watch the mark on the axle. The point that is furthest away from the kingpin boss is the closest point of the lower spring mount on the axle. That will give me the most front roll stiffness I can get with just the springs. It still won’t be enough to balance the car with the springs mounted on the trailing arm or to the rear upright. So I either have to use a front anti-roll bar, or I have to mount the rear springs closer to the centerline.
Mounting the rear springs closer to the centerline means making the de Dion truss strong enough to handle vertical forces. I have been thinking about that, and if my sums are right putting a truss of 1″ square tubing with 0.5″ round verticals and diagonals on top of the 0.5″ round tube truss would support the rear of the car with adequate safety margin. “Adequate” in this context means no damage if I hit a few pot holes or slide off the track during a time attack run, it doesn’t mean taking the car off-roading in the desert.
The other thing I have been thinking about is how light I could get a front-engine bucket like the Sprint-T. By adapting an A-904 Torqueflight to a Pentastar V6 with a steel 9″ Ford rear axle I could get 1450 pounds without even trying hard. Start swapping out an all-steel 9″ for an aluminum and magnesium quick change in the rear axle and carbon fiber in the driveshaft and we are looking at a 1400 pound bucket with 305 HP on 87 octane regular without changing anything in the engine or calibration. Get serious about making the frame lighter with aluminum instead of steel and a space frame instead of a ladder and maybe 1375 pounds. About 4.5 pounds per HP. That’s like a 750 HP Corvette or Viper running on Regular gas. As in car*√-1, imaginary. Even with the frame from the Speedway kit we are talking a hot rod that could give anything short of a Hellcat or Z06 a run for its money 0-60 MPH, on 87 octane regular gas. There are 2 ways to make a fast car, 1) lots of power and 2) no weight. 1) is fast in a straight line, 2) is fast everywhere. I think I like 2) better.
Incidentally, did I ever say how I figured out that I needed to use different size tires and how different they had to be? The book Chassis Engineering has a graph of grip over load for a typical race or passenger car tire. The problem here is it’s a graph for just one size of tire, and has to be scaled for a different size of tire. The way I did it was to leave the shape of the graph pretty much alone for the big tire and then scale the grip by dividing the load and grip by the ratio of the squares of the widths between the tires. F’rinstance the street and street tire autocross will use 195/50 fronts and 245/40 rear tires so the graph was scaled down by a factor of 1.58. This is not perfect, but it is close enough that I can come close to getting the balance right with just the springs and a light anti-roll bar to make up the difference of not having the perfect spring rate. For street and highway driving I will need slight understeer for stability, but for the autocross I need a slight oversteer for better turn-in and rotation through the middle of the corner. I’ll be able to do this with just a swap of shocks and springs which I have to do anyway because I need a lower ride height for autocross than for the street. Theoretically I could do that without changing springs and shocks but that would require carrying around a slew of scales and platforms and adjusting before and after the competition, or I could just do that once and pull those shocks off and put them in a case with labels as to which one goes on which spot on the car. Instead of spending time adjusting and re-adjusting the spring seats to lower or raise the ride height and move the weight around to balance the car in turns, I do it once at a test track and then just swap the coilover shock/spring units at the race from street to race and back to street in just a few minutes instead of the hour or so (if I get lucky) each way adjusting the spring perches.
And I have wandered around the verbal landscape long enough, time to wrap it up and do something else.
Opus the Unkillable
Witch on a Bicycle 