Tag Archives: Space Frame

Ooops.

I made a statement two posts ago about revamping the frame design adding 30 pounds to the car. That was an error based on the fact that I added or relocated 8 frame braces during the redesign so I figured the weight for all 8 changes. Then last post I stated the empty car weight was 1450 pounds which was accurate based on calculating the length of every frame member and multiplying that by the weight per foot of tubing for that size tube.

There are 2 different tubes used in the Sprint-T. Most of the frame is made from 1.5″ X 0.060″ wall round DOM tubing, except the rollover structure which is 1.5″ X 0.120″ for rules compliance. Now because of the design there is room for interpretation as to where the frame ends and the rollover structure begins, but I’m taking the interpretation that any diagonal braces that touch the rear hoop inclusive are rollover, and everything else is frame. There are a lot of tubes in the frame, the front and rear hoops are the longest at roughly 16 feet apiece as they wrap around the body, and then the upper and lower frame rails at roughly 100″ each for a total of 400″ or just under 34 feet total. Then there are the front suspension mount, the internal diagonals for the rear hoop, the bracing inside the top halo, and the rest of the diagonals that connect everything together to make one solid structure. That comes to another 40 or so feet in mixed sizes.

So, all told the Sprint-T frame carries a few more pounds than the Speedway kit frame, but is orders of magnitude more stiff, both in bending and especially in torsion. The Speedway frame is 2 hunks of 1.5″ X 3″ X 0.120″ wall rectangular tubing with round tube crossmembers to hang the engine and transmission from. Adequate in bending loads under all circumstances, but lacking in torsion to be charitable. It is an upgrade from the Model T frame or any OE Ford frame used back in the day, but that is damning with faint praise. It was designed as a cruising or freeway frame, not for racing and especially not for racing around cones set up in a parking lot. It was not made for installing a rear roll hoop that meets SCCA safety rules for one thing and as an open car (no roof structure) it would be required to have a full cage, and there is no place to mount the front hoop either. Now there is a place to mount a drag race style cage, but not an autocross legal cage. This is annoying because using the kit frame would make the build so much easier. And probably cheaper, too. There would be work involved with relocating the suspension pickups to prevent the tire-lifting behavior of the stock rear axle and the resulting power oversteer in tight corners even with radical bigs-‘n’-littles tire sizes (I saw a YouTube video of a bucket getting sideways off a corner with 14″ wide rear tires and 3″ wide fronts).

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Moving the driver forward completely changes the frame design

Moving me forward of the original firewall really changes the frame design. And I’m not just talking about swapping the seat and gas tank around, either. The original frame could be considered an exoskeleton as almost every part was outside of the body to some degree in order to increase torsional stiffness. Well the new frame is much smaller, only 27″ wide to fit the firewall on the body, up to the point the engine and drive bits mount which will have to be wider because of the transverse mounting.

Basically the part of the frame forward of the firewall are going to be like a ’50s Indy roadster. This is still going to be hella stout and light, but the frame members are going to be different than the Sprint-T derived frame. This frame is going to be designed for a mid engine from the get-go instead of trying to cram a mid mounted engine in a front engine frame. The other major change is all of the frame is going to be round tube, most 1.5″ diameter and most 0.060″ wall thickness except for the bits around me that have to be 0.120 by SCCA rules. This will lighten the frame by a few pounds, but nothing spectacular. What will do the most to lighten the frame is tucking everything inside the body and doing the engine and transmission mounts first instead of making them fit what was already there. Also the Sprint-T was designed around a roll cage that won’t be there because the driver isn’t where the roll cage is designed to protect.

I can make this frame as stiff and light as the Sprint-T, it will just take a bit more brain work to do so. Instead of spreading the frame as far apart as possible I’ll have to concentrate on using internal structure for stiffness. It can be done, and I’m the guy who’s crazy enough to do it. Incidentally, the changes move the fuel tank right over the center of gravity of the car so the balance doesn’t change at all between full and no gas. The Princes of Serendip are victorious again! (Look it up)

On other fronts I need to get that brake hone this evening so my walk will be extended to the auto parts store. And tomorrow’s walk will be by the pharmacy to pick up more not-sad pills for my brain. Tonight I’ll be walking about 3 miles to get some bonus check-ins on my mobile game. I haven’t been able to walk as a workout for 2 days now and I need to stretch out a bit.

More later.