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.

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A minor breakthrough

Still thinking about making the TGS2 lighter and simpler to build and I had a minor revelation. If I lower the 3″ tube until it is on the axle centerline that will reduce the bending moment enough to not need any bracing other than what is provided by the double-shear spring mounts (that’s a plate on either side of the coilover heim with the mounting bolt going through both plates) welded to the top. That unloads the top radius rod on the 4 link allowing for higher loads from restraining the brake rotation etc.

Someone asked me IRL how I know there is going to be so much weight in the back of the car. That one is simple: The engine and trans combined weigh 620 pounds with the CG about 10″ in front of the rear axle centerline on a car with a 100″ wheelbase that will weigh about 1600 pounds. I’ll let you do the math yourself, after I set the equation up for you. The arm for the drivetrain is 90″ and the weight is 620. The arm for the rest of the car is going to be 40″ if we are incredibly lucky, probably real close to 50″ split the difference and call it 45″ and the weight is 980. Moment is weight times arm. Add the moments together and divide by weight. That is the total arm of the car, between 63 and 65.5 and since we used the front axle as zero datum and we have a 100″ wheelbase that makes the rear percentage equal to the momentarm, I.E. between 63 and 65.5 percent. I learned this one when I was taught weights and balances for flying. Yes, I used to be a pilot before the wreck. Sioux this one was for you 😀 Math IRL for your students. Oh and the reason I don’t know the exact arm for the rest of the car is there are a bunch of parts I don’t know the exact weight for nor where they will go on the car when it’s finished. So I had to use a SWAG for the arm of the car without engine.

Excuse me I zoned out for a moment listening to an old piece from the ’70s on YTM; Mike Oldfield’s “Tubular Bells (Pt.I)”. They had just gotten to a part of the piece I call “The Procession of the Instruments” that starts about 17:00 into the piece, and continues to the end of Side One of the LP. This was one of my first experiences listening to polyrhythmic music, which my mind finds very relaxing. Anyway, Mike says the name of the instrument and they bring it up in the right channel then mix it over to the center front then over to the left and back but still up enough to hear if you focus on the sound. I didn’t know this at the time but polyrhythmic music has a calming effect on people with ADHD and PTSD. At the time there wasn’t any such thing as PTSD by that name, it was “Shell Shock” and didn’t happen to kids (that they knew), but I had been diagnosed with “something” tied to my high IQ, that we now know as ADHD. Anywho, when I listen to polyrhythmic music I zone a bit and get real calm.

Back to the car, I’m really feeling torn between totally enclosing the roof and windows and saving weight. Putting a roof and windows on it will make the car look more complete, but will need A/C in the summer. Or I could figure out some other way to keep dew and rain off the instruments and save about 50 pounds, most of it off the back of the car, by leaving the A/C in the donor vehicle and cooling the car by leaving the windows off. That would save even more weight but lower the gas mileage on the Interstate a bit. Basically all I really need is something to cover the cluster and seat when I’m not driving. I could make a snap-on cover like old hot rods and sports cars used in the ’40s and ’50s, called a tonneau. These days a tonneau is basically the thing that goes over the pickup bed when it’s empty, but back then it kept the interior clean and dry when the roof was down or if there wasn’t any roof at all. I bet I could make something with the HDPE sheet and some Velcro™ and get the same effect and roll it up inside the car when I was driving.

And I think I have meandered across the screen enough for today.