26 Oct 2008

Things are changing – again. After a buddy reviewed the chassis he asked, “Why didn’t you use simi-trailing arms for the rear suspension like on Kimini? It’s a nice compact layout and allows long links.”  Um…

It’s a combination of things: the forward links can’t go too far outboard because they hit the wheels; they can’t go too far inboard because they hit the drivetrain; even in-between they hit the intercooler on one side and the alternator on the other; I wasn’t sure, due to the simi-trailing nature of the arms, if the camber and toe curves would be polite. Perhaps it was the workload of getting the chassis into the computer that kept me from spending the time on it. Or, perhaps I just got tired of dumping so much time into such a small part of the chassis with little to show for it. Well now that the chassis is (more or less) done it’s being redone – again.

The trailing links can be made parallel to chassis centerline if they’re attached part way along the lateral links. It’s not the best engineering decision though because of the bending loads imparted into the lateral links – unless they’re made really strong. This might be an acceptable place for engineering compromise because it solves a number of packaging problems all at one time: it greatly decreases the amount of “stuff” behind the drivetrain; the lateral links can be made as long as I want; because it’s then true trailing arms (parallel to chassis CL), there’s no toe-steer going on; it also frees up more space for the muffler, an necessary component that easy to forget. The shocks/springs can package fairly straightfoward, but again, only if they’re mounted slightly inboard of the lower pickups. Since the trailing links pick up from this area of the lateral links already, it’s probably okay – if it’s strong enough. I’ll run the numbers, but since Kimini had to use 1.25″ tubing for the lower tube – without the spring load – it may go as large as 1.5″ OD square tubing, due to the separate large vertical and longitudinal loads. Overkill? We’ll see what the numbers say.

So if it sounds like I’m talking myself into redoing it again, well, yes. Better this way than to build it from steel and then become unhappy with it. I wasn’t real happy about it before, and my buddy’s comments pushed me over the brink of redoing it.

In other news, I think I found a bug in Sketchup. Recall I’d been bitching about how sometimes tubes don’t “snap” to where they’re expected to. I just caught it red-handed; it was easy to see this time because it was in a clear area of the drawing. Clicking on a node I wanted to connect to, it looked like it connected fine, but it was very slightly off. Zooming way in showed that it was snapping to a point in thin air, ignoring the valid node right near it. Huh? Selecting the area confirmed that there’s nothing there and yet it wanted to connect. Sigh, going to have to keep a close eye on that.