A lot of chassis I know have box section on the floor of the chassis and tube everywhere else. I know it makes it easyer to blot on a flat floor, but is it done for cheapness or is box better at some twisting motions than tube ?
I had intended to use solely 38mm Tube but am now not sure. Help ?

[Edited on 18/5/04 by F1 Mini]

This has been discussed before and i cant be bothered to type it out again....

See this thread:

http://www.locostbuilders.co.uk/viewthread.php?tid=13091

Cheers,
David

round is stronger

Do we have to go through this again??

Square is stronger in compression, tension, bending and torsion (torsion only up to a point!).

Bending is simple to understand, think of it as a double I beam. Go into second moments of area if you like...but you will find its stronger...

Compression and tension due to the higher cross section...

Torsion, now this applies up to a limit. For small forces square deforms less than round. However for larger forces square fails at the stress points, ie at the corners. This is all down to the polar second moment of area (square is smaller, so resists bending better) For a chassis the stresses are not high enough to worry about this failure!

The loads in a space frame are compressive and tensile the majority of the time. So a square tube chassis will be stronger. Although it will also be heavier than one made of the equivalent round tube....hence why racers chassis are usually round tube...

Cheers
David

Remember that round is stiffer pound for pound, 1" box has more cross sectional area than 1" round, this is the reason its stronger, but, it weighs more.

If you were not constrained by material sizes, but, ordered it by cross sectional area, then things would be different.

Yes, spot on, hence the last sentence in my post above

And I'm always talking size for size... i.e. 25mm box compared to 25mm round of the same wall thicknesses...

If you bring the cross sectional areas up to the same (by either upping the diameter of the round or by making the wall thickness bigger). Then there is no difference in the tensile behaviour, square is still stiffer/stronger in bending. But the round becomes stronger in torsion for all loads...

Cheers,
David

Ah that being the answer I now see why they use square and round in the design.

Here's an F40 Chassis made to original design spec's.

[Edited on 18/5/04 by F1 Mini]