7 Dec 2008

The final chassis is progressing though the round tube drawings bother me. As mentioned before, it’s not clear the best way to convey how to cut the ends – or even how much to try and make the drawings precise. The overall length is good, and the angles of the end cuts, but I’m not so sure about index angles, which I think will be rather useless to a typical garage builder. If a drawing specifies that the miters are offset at each end by, say, 27.3 degrees, how is a typical builder going to make use of that information, really?

Curious about build accuracy, I asked FSAE teams how they make use of laser-cut tubes – just what does someone do once they have perfectly cut tubes? The theory is that with them accurately cut, the chassis becomes “self-jigging.” Teams confirmed that while having the tubes pre-cut is a tremendous time-saver, that’s all it saves. They said a builder cannot just weld laser-cut tubes together and expect the chassis to be square; it’s just not going to happen without a big heavy fixture to combat weld distortion. The reason I mention this is that just because tubes are accurately cut has little to do with them fitting due to tolerances. What good are tube drawings with 0.01″ resolution when the tubes are cut and installed into a chassis with a realistic accuracy of >0.125″?

If someone hands me accurate chassis drawings, what would I do with them? I’d make sure a few major tubes are accurately placed and not sweat the rest. About 90% of chassis tube placement just don’t matter much; the various minor tubes just keep the big ones in place and the inboard suspension points where they’re supposed to be. All the tubes in between, eh. That’s how I intend on building the chassis myself, using the drawings to double-check overall dimensions, but other than gross lengths and angles, they’re a guide only. That’s all they can ever be due to weld distortion making accurate drawings fairly pointless.

This isn’t anything new. The Gibb’s book (“Build your own sports car”) supplies complete drawings of the chassis accurate to 1mm (0.040″). Yet right up front he warns: ” Precise dimensions are provided for all components, but in practice allowance must be made for welded joints and distortion. It is advisable to cut tubes – and check and trim as necessary – as the chassis is built up, rather than cutting all tubes before work starts.” So there it is from another source, having drawings with high resolution aren’t of much use other than a reference.