Tag Archives: over thinking a problem

Thinking about the Sprint-T again

Not a big think, because it’s a small but important thing. What I was thinking about was what to do about the top of the frame?

Just in case you missed it, there will be a structural bellypan welded to everything on the bottom of the frame for aero and structural purposes. The problem is that leaves an open space to catch water and debris that will lead to rust spots as the frame ages. Possible solutions include moving the pan to the top of the bottom frame members which has the advantage of making the body mount easier, but which requires mounting the engine 1.5″(38.1mm) higher and also leaves a nasty lower area as far as aero is concerned. Plus that just moves the rust out area someplace harder to inspect.

Anywho, what I was thinking about was using some of the HDPE plastic I have for the fenders and hood to cover the exposed gaps in the frame. Then as I was thinking about the bellypan on top of the frame it occurred to me that it would be much easier to make access panels from HDPE than from steel or aluminum, and I wouldn’t need to paint it. And if I did the HDPE on the bottom I could just hotknife the hatches from the HDPE and be able to reuse the cut piece as the hatch because a hotknife has a very small kerf. For the Mini Sprint-T it wouldn’t make any difference because I would do the same thing for either one, glue a piece of 0.01″ styrene to the frame between the tubes. Visually it wouldn’t be any different on the model, a flat surface outside the body is a flat surface outside the body. Flat is flat, model or 1:1 scale. The main difference is if I put the solid bellypan above the rails I would paint it to match the body color of Omaha Orange aka Schoolbus Yellow, and the HDPE is a different kind of yellow. On the bottom the difference would be the steel pan would be black and the HDPE would be the same yellow as the top, because it’s cheaper that way. Or I could go with the black HDPE because it ain’t that much more expensive, and I’m really not quite that big a tightwad. Or getting back to cheaper, the flanges and bits to mount the HDPE plastic might be enough to make the part of the frame that goes under the body smooth might be enough to not need to weld the pan to the frame for stiffness, and I could just go with HDPE top and bottom and save a few ounces. Not to mention HDPE is cheaper than steel for the moment because of the tariffs. We make HDPE here in the US so no tariffs.

Well, this is the second post today, and there was a lot of stuff in the first post what with all the pictures, so I’m going to bliss out to my trance mix and get some meditation in.

I think I’m having low grade allergies

I saw on the news that we are getting inundated by the smoke from the fires in the northwest US and it was triggering allergies across the Metromess. That partially explains the eye irritation and minor pains from old injuries and especially the problems getting to sleep at night. When I’m having an allergy attack my eyes get gooped up and irritated and I can’t sleep. And if I’m getting like this from thousands of miles downwind, what are the locals suffering through?

Still thinking about the Sprint-T because what else am I going to do while I stare at the walls trying to get tired enough to sleep? What has been running through my mind is the front diaphragm, trying to get it both light and stiff. The front diaphragm is a triangle but has long spans of tubing that can flex under load, plus there are some places that pick up loads in the middle of a tube (major structural no-no) that need to be braced to prevent flexing. So, thinking about where to place the braces without adding any more material than absolutely necessary, leads to several options. First, the frame rails meet at the dead center of the diaphragm top and bottom which would cause the top tube to flex a bunch without a brace, as the bottom frame rails meet at a point with the side pieces of the diaphragm that is already braced by the natural design. So the natural move on this one is to run a single tube from the lower frame rails to the upper frame rails where they meet each other and the top tube of the inverted triangular structure, but that still leaves a lot of unsupported tube in compression between the upper spring mount and the frame rails, that will wander all over the front of the car if it isn’t contained by bracing. So, bracing then. I forgot exactly what the change in stiffness is as bracing is added, but it’s nothing like the change increased by moving the frame rails further apart which in expressed by d4. Googling the expression shows there is no direct formula for all thin structures in compression but several depending on how the member is loaded. In the simple case the stiffness increases linearly with the number of braces assuming the load is centered on the structural member, which is not the case for the top tube of the front diaphragm. I downloaded a free engineering text because I clearly need some refreshing on the subject. Anyway, for this structure stiffness increases linearly until a limit is reached where additional bracing does not change stiffness if I’m reading correctly. And this is another reason why I have difficulty falling asleep at night, because I could hit that theoretical maximum with a simple shear web if the radiator wasn’t right behind the front diaphragm and needed the empty space for airflow. I had forgotten how much I enjoyed structures during intro to civil engineering in college. It’s like a ridiculously long word problem in algebra. And going for the big prize of strongest structure with the least weight when there are several answers that are close…

Ad I’m having troubles seeing the keyboard and screen again so it’s time to end this and rest for a while.