And one of my favorite subjects is how to make the Sprint-T lighter and safer, which was not caused by seeing Ryan Newman’s Daytona crash. Actually it was brought on by wanting to make the frame fail in such a way that the engine didn’t try to join me in the driver’s compartment in a frontal collision. Second consideration was using as much 0.060″ wall tubing as possible. I say “as possible” because while some of the 0.120″ wall tubing is required by safety rules, some of it is required because of the stress risers created by the 0.120″ wall tubing.
And once again I wish I could show you what I see in my mind, when I’m thinking about the Sprint-T. The rest of the time I don’t want you to see what’s in my mind, that’s like being on the wrong side of the eyewall of a hurricane full of garbage. But seriously, I wish I could show and not tell about the frame for the Sprint-T.
OK basically The Rules require the hoops and diagonals and upper parts of the roll cage to be 0.120″ wall, but to balance things out so the cage holds together that means the bottom frame rail has to be 0.120″ as well at least between the rear and the front hoops. The fun(?) part is deciding how far back the 0.120″ wall has to go, at least to the rear hoop but behind that do I want 0.120″ all the way to the rear bulkhead/bumper, or do I want that to be a crumple zone up to the rear hoop? Going back to The Rules at least one of the diagonals must be a single length of 0.120″ wall tubing the same diameter as the hoops (1.5″), but do I want/need more than one diagonal, and if I want/need to have two diagonals do both of them need to be 0.120″ or can the lower stressed one be 0.060″? Add into the mix that I can also have a rear hoop and a left and right hoop, and run a diagonal from the front crossbar to the rear corners of the left and right hoops and get a much stiffer but slightly heavier frame.
But we were approaching the point of diminishing returns for frame stiffness given the suspension design of “stick” axles front and rear. I mean the main point of torsionally stiff frames is to keep the front and rear wheels at the best camber angles and also to balance the roll rates so the weight transfer between the front and rear outside tires can be tuned for desired handling behaviour. Well stick axles don’t change camber angles unless the inside tire is lifted out of the plane parallel to the ground which likewise limits the weight transfer ratio between the outside tires. And that one didn’t come out completely right, as the inside wheels can also go over a bump and remain in contact with the ground and not be lifted by trying to transfer more weight than exists on the inside tire by either excessive roll angles or by roll centers that are too high and transferring weight without compressing or extending the springs. This is the mechanism that allows changing the handling by raising and lowering the roll center on one end. The closer the roll center is to a line running through the center of gravity of the car the less control the springs and anti-roll bar exert over the weight transfer and also the less the car will roll over on the suspension in a turn. Get the roll center higher than the center of gravity and the car will try to roll opposite the cornering force and pick up the inside tire. This is why standard kit T-buckets are no good for autocross and Solo Racing because they have such high rear roll centers to compensate for the “normal” size difference between the front and rear tires on the street.
Anyway, back to the frame. Running left, right, and rear hoops will give me four uprights on the rear so lots of crush resistance where the majority of the driver sits, which is good. But also less tie-in at the top of the rear hoop which is ungood (not actually bad becauuuse there are other ways to tie-in and brace the top of the rear hoop). And running left, rear, and right hoops means an extra hoop of 0.120″ wall plus the crossover bar over the front of the cockpit and the verticals under the crossover and the diagonals from the verticals to the rear hoop have to be 0.120″ where front and rear is just the two hoops plus the crossover bars between the two hoops and whatever gussets they get attached with and the diagonals that are a single piece of tubing between the hoops. Like I said, compromises and where weight can be saved.
Also, while I’m trying to do this for the 1:1 car I’m also trying to figure out how to build the 1:25 Mini Sprint-T, which is where this mess got started because of the upper rail running from the front bulkhead spring mount to the front hoop.