BIG MOD EDIT:
Table of Contents, Pictures with analysis:
Book frame with mods and results
Table of Contents, Models:
Book frame in Grape by Andrew Moore
Frames with cages in Grape by Andrew Moore
Early version of Car9 in Grape by Marcus Barrow
Kurtis Kraft 500 frame in Grape by Andrew Moore
Original first post below:
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After re-reading cymtriks' document, and reading Wesley Linton's thesis for the first time, both found here under kitcaranalysis and final thesis respectively:
http://locost7.info/mirror/chassis.php I'm wondering why there aren't many builds, if any that incorporate their ideas.
In cymtriks doc, he shows how to achieve 2683 ftlbs/deg of twist and even cuts the weight of the chassis by 7 lbs with these mods. Overall it only adds two tubes to the cut list. He states that the chassis in book form has 1180 ftlbs/deg and weighs 181 lbs.
In Linton's thesis he is modifying a Luego chassis. He goes from 1352 Nm/deg (997 ftlbs/deg) at 120 Kg (264 lbs) to 6030 Nm/deg (4447 ftlbs/deg) at 127 Kg (280 lbs). He even goes on to analyze a simple roll cage and returns with 9152 Nm/deg (6750 ftlbs/deg) at 170 Kg (374 lbs).
Of course all of these figures are with an FEA program, so the real world will be slightly off, but the idea the same. In Linton's he even states that actual baseline measurement of a real Leugo chassis was 1330 Nm/deg, and that his FEA program made the chassis heavier than needed as the intersecting parts of tubes were not removed.
I'm not entirely sure why the difference in chassis weight is there, but it may be that cymtriks wasn't including all the panels of the chassis, or something. The other good thing about both of these is that they give numbers for each change made, so if you only can do one or two things, you can see what kind of affect it will have.
Anyway, I just think that given the amount of time and effort these two have put into their writings, and I'm sure there are others I've missed. It's a little disappointing that most people aren't actually using half their ideas. Sure they don't provide you with a cut list, but isn't that half the fun?

EDIT: This isn't aimed at anyone in particular, just a general thread.