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Author: Subject: Fwd Trike
rowlocks

posted on 29/8/11 at 01:26 PM Reply With Quote
Yeah a slight increase in stiffness is all I would be after. The only reason it was really floppy at first is because the joints werent very well made. And the main chassis rails were the same size as all the other pieces (I had lots of firewood that size). In a real ladder chassis the main rails would be significantly bigger, to be able to take twisting and bending loads.

Making stuff out of wood first is good I think because its weaker and you can twist it and feel which joints get stressed and feel where the triangulation needs to go. If you build it out of steel first, its so strong that you might not be able to tell that a joint is more stressed in comparison with other joints.

Actually it might be difficult to distribute the loads around the edge of the glass in the same way as a monocoque chassis can, even if you cut out the surround. Point loads might not be good for glass, lol.

[Edited on 29/8/11 by rowlocks]

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JF

posted on 29/8/11 at 07:57 PM Reply With Quote
Jeez Sam lighten up a little. I never said that you should just throw some bits and bobs together and take it for a spin on the public road.

And yes manufacturers use safety factors, and yes I know what they are, but my point was that even though they do use a safety factor there are still problems once in a while. When you really know what you do calculations can help a lot in design. It's when you don't know how to do the calculations and try anyway that the big failures happen. Because the outcome might indicate you can do with a lot less then you actually need. That's why I don't really advise going the calculus route for the amateur home builder.

I really do think it's saver to go by your gut feel and try it out in a stretch of farmland or something similar. You might find some weak spots, it might break, it might be fine. If it does seem sound and you go on to make it road worthy, get msva etc. Then still I'd check the whole chassis over on a regular basis. And I'd paint it a light colour that shows cracks at an early stage.

And I do believe that IVA is a good thing. And yes they may be over enthusiastic, and going over the top on some bits. And yes it's a shame it costs so much. But it's good for every home builder to have somebody else give your car a look over. If you've worked on it for so long you'll miss many (critical) safety concerns. But IVA doesn't catch everything. I could build a chassis completely by the locost book. But use far thinner walled box section. The IVA inspector wouldn't know. But it might fall apart on the very first speed bump. Hence the Aussies demand a stress test. Might be a hassle, but again I think it's a good thing.

And please dear Sam, I'm not stupid, and when I do get round to building my own design. It will probably be over engineered, and end up weighing more because I prefer it on the safe side and won't calculate if I can make that tube a little thinner. But I'd design it using common sense.

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Sam_68

posted on 30/8/11 at 09:18 PM Reply With Quote
quote:
Originally posted by JF

And please dear Sam, I'm not stupid, and when I do get round to building my own design. It will probably be over engineered, and end up weighing more because I prefer it on the safe side and won't calculate if I can make that tube a little thinner. But I'd design it using common sense.

Well, dear JF, I guess we'll just have to differ.

You don't need calculus to work out the stiffness of a spaceframe; it's very basic maths and the principles are well known and readily available. If you're mathematically illiterate, there are freely available computer programs that can do it for you.

Gut feel is fine if you have a good instinctive understanding of structures and load paths, but judging by a lot of amateur-built spaceframes, very few people do.

At least the OP has gone down the route of mocking-up a model in timber, but that model has identified a flaw; common sense says you address the problem before progressing to a build, not rely on a sheet of glass to add the missing stiffness and hope for the best.

Stupid is as stupid does.

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