Daily Archives: November 12, 2018

The bandaids are off

Well the full extent of the damages are now visible as I am between bandages. I think the thumb will be OK going without another bandage, but the index finger needs antibiotic and another bandage. The index finger got really close to losing the chunk off the tip, but I think it will heal without the big ugly scar.

The index finger in all its gory glory.
Very close to losing the tip of my finger.

The thumb injury can barely be seen a day later, but it bled like I severed a major artery when I did it.
'Tis but a scratch.

Construction on the Sprint-T will have to wait until the index finger heals over so I don’t get bad stuff into my bloodstream through the break in the skin, but I have been thinking about order of construction for the frame.

First thing I need to make are the roll hoops because they dictate where everything else in the frame has to go. And I have been thinking about how to make two perfect hoops. The raw stock is a thermoplastic, meaning it can be formed easily when warmed to something under its melting point but above room temperature, a candle or similar is a good heat source for bending styrene plastic. I just need to use a round thing to bend around that has the same radius as the anvil of the tube bender, in scale, and a flat surface to make the second bend against to get all three legs in the same plane, easy-peasy. So, put the anvil of the mini bender on a board or something and put marks on the board for a 90° bend and where to put the leg of the first bend to get the distance between legs on the hoop the right distance apart. Put the top of the hoop on the mark for the beginning of the 90° bend with the leg from the first bend on the mark for how far apart the legs should be and make the second bend and cut the legs to length after bending.

The next thing would be to cut two pieces of raw stock to a scale 52″ for the main rails between the hoops, so the bracing inside the hoops will just clear the body when everything is together. And since this build is going to be fitted with a bellypan, the bellypan would be a useful place to use as a jig for getting all the bits and pieces for that level of the frame together. So cutting the bellypan to size, gluing the lower frame rails to it and the roll hoops to both seems to be the next logical order of operations. After that the crossmembers that run between the legs of the roll hoops seems to be next. Measuring the height of the top of the bellhousing for the horizontal brace across the front hoop and cutting stock the same width as the crossmembers would be the next step as would placing that member at the right height. Doing the same thing for the harness bar on the rear hoop would be next.

At some point I would need to bend the top rails for the frame and put them in, this would probably be a good time. Also for the diagonal from the top of the rear hoop to the bottom of the front hoop. The other diagonal is going to be more tricky because of the pickup point for the front of the swingarm coming from the rear axle. I need to see where the front of the swingarm is in relation to the diagonal to see if I have to make a jungle gym just for getting that point solidly located in space, or if I can just kink the diagonal a bit to catch it, then run another diagonal from the pickup point to the bottom of the front hoop to triangulate it and to the top of the rear hoop to preserve the triangulation of the front to rear diagonal.

And I have no idea where the top mount for the front coilover needs to be to clear the front tire at full lock and full bump, like when I hit a curb on a tight corner with the inside front tire. That would be an ungood thing to happen in a run, but to have the tire hit the frame would make bad plusungood, if not doubleplusungood. And yes, that was Orwellian New Speak used to describe an engineering scenario. I respect the classics. I mention this because I lack the tools to determine on my computer where the diagonal from the bottom of the front hoop has to be to clear the tire at full bump and full lock, without making the bracket on the axle any higher than it has to be for the coilover to clear the steering. Sure working from the side view I could just make the diagonal just tangent to the tire at full bump and call it “good enough”, but that would make the axle just a tiny bit heavier than it has to be, which is all unsprung weight. And my views on unsprung weight are only slightly less negative than my views on drunk drivers.

I can calculate the tangent based on diameter at the tangent point at full lock where the inner edge of the tire crosses the plane of the diagonal between the bottom corner of the front hoop and the top of the coilover, without needing to use 3D modeling but there are still width effects I need to account for and dynamic effects like axle flex and frame flex. Now frame flex is going to be measured in hundredths of an inch on the full-scale car, but I can’t get an accurate estimate of the load to determine axle flex beyond the load needed to get the spring compressed to full travel. At that load the axle flex is going to be in the same range as the frame flex, 0.01″ or less but the shock load against the bump stop could get another 0.02″ on top of that and times the moment arm that could get the tire as much as 0.08″ higher and closer to the diagonal, which would mean moving the top mount of the coilover even higher to clear the tire at full bump and full lock. Or I could just make the diagonal clear the straight ahead tangent at full bump and let the difference in height at full lock provide the dynamic clearance.

And if a picture is worth a thousand words I just posted more than 3000 words counting the pictures.

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