21 Sep 2008

I’m debating the use of round versus square tubing. Many builders prefer square because they say it’s easier to cut. On the other hand, round tubing weighs 21.5% less, is equally strong in all directions, and looks nice. There’s something about a round-tube chassis that looks… what, smooth, more refined somehow. I’ve used both types and don’t feel strongly one way or the other. I do have a tubing notcher for doing round tubing so maybe I’m biased. Many builders say square is easy to cut with a hacksaw but I don’t think they’ve cut many two or 3D junctions. The worst is a square tube that meets up with two other tubes with none of them at 90 degrees. It forms a tricky three-dimensional cut with each side of the tube having its own angle. Yes, single plane cuts are easy though I read that even two-dimensional cuts drive some builders crazy.

However, I realize the car’s supposed to be easy to build and that means having easy-to-follow drawings. Showing drawings of round tubes with curvy ends isn’t going to be very helpful; about all I can do is show the overall length. I can “unwrap” the tubes so paper templates can be made, wrapped around the tube, a line scribed, the tube cut, and presto… Only – it doesn’t quite work out that way; square tubing isn’t really square, the corners are radiused by varying amounts. Also, tubes have finite wall thickness, something that “unwrap” drawings can’t take into account. The problem is that the inside of the tube might need a fairly different length than the outside at the same point on the edge. Some tube drawings could take a lot of fiddling to make them work right. (OTOH, many of the tubes have to be round due to being part of the rollcage. So, while a tubing notcher isn’t required, there’s not getting around dealing with round tubing.)

Also, as Gibbs notes in his book, having tube drawings with infinitely accurate dimensions is very misleading due to welding heat warping the chassis, cut variations, and very slight mistakes accumulating through the chassis such that by the time the builder gets to the other end, the lengths are way off. Regardless, drawings of some sort are needed. I’ll probably make drawings for square tubes where there’s a choice and let builders decide whether they follow the drawings or use lighter round tubing instead… I know which my car will use. (I should add that using square tubing off-axis, and unwrapping tubes in general, aren’t something that SketchUp’s particularly good at and takes a long time for each tube).

In other news, the engine tray won’t be removable; Turns out the drivetrain can come out by removing only one tube. It saves a few tubes and does away with half a dozen bolted connections, a dozen screw connections, and all the threaded inserts. The one removable tube will be the diagonal above the drivetrain and will have rod-ends to make sure it fits in spite of build tolerances and heat distortion.

While the drawings may look a lot like the last ones they’re more polished and… focused. The engine bay is fairly complete, missing only a couple tubes on the lower panel and engine mounts (the rear one is in place.) I’m working my way forward, making final tweaks and working on tube intersections so individual tube drawings can be made. The upright cylinders at the back are the shocks while the large horizontal cylinder is the muffler – with 3″ inlet and outlet 😉

The last picture shows the diffuser. Kimini is very stable as speed – I did something right – so Midlana is getting the same treatment with a smooth undertray in addition to side-exiting radiator ducting. As an open car there’ll be more turbulence but it’s worth a try and a future iteration might be a hard top!

I’ve got some good leads on engine builders and Kimini’s sale helps that happen; I just have to decide when. The bent roll cage tubes will be the first tubes fabricated, followed by the bottom rails. After that it pretty much depends which end of the chassis is built up first. Starting at the front gives plenty of time for the engine to be done by the time I get to the engine compartment.

And finally, I’m selling two of the shocks, QA-1 DDR7855. They’re too long for the rear suspension, except these exact units are used at the front, so consider it the first two components for your future car! Brand new, never used, still in the box, $470 shipped anywhere in the lower-48. Contact me if interested.