Anyone know how much of an advantage round section tube chassis have over traditional square section? I only ask as I'm thinking of building one but does the extra expense and effort of usng tube actually yield some noticeably positive results or is it not worth the bother?? Cheers Oli.

for the same weight, round is stiffer. There are two ways of doing this, same size but greater wall thickness, same wall thickness but bigger size. Bigger diameter is better.

If 25mm x 1.5 square is more than strong enough then 25mm x 1.5 round is lighter. I'm told it is easier to weld than 25mm x 1 square.

round is easier to bend, if you have a tube bender. this will save you some welding.

round can be harder to fit panels / brackets to.

Really I always thought that round tube was much harder to weld....hmmm I shall have to have a play with the welder me thinks. So really you can either have the same weight but stiffer or same stiffness (oo-er) but lighter. So when all these companies go on about the tube being so much stiffer and stronger and lighter etc etc they're bending the truth a bit! Are Mcscorely (don't know how to spell it!) 's plans considered the ones to go for now days? I'd also like to use an Avon type rear end using a sierra or freelander diff and drive shafts on double wishbone rear suspension. Does such a design exist at the moment? If not anyone fancy knocking me up one ???? Any good sites on how to design/build chassis that are worth a read? Cheers Oli.

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Originally posted by oliwb
I always thought that round tube was much harder to weld.

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As above 1" round is lighter than 1" box although not as strong if you try and bend. It is however stronger in twist if tha makes sense.

I went for a hybrid, box for the bottom of the frame as it makes fixing the floor easy, tube everywhere else.

Tube is harder to weld, only because your torch has to be in 360 different positions as you follow the seam, square is just 4.

Regards Mark

At the donnington show one trader went to great lengths to explain to be the big difference between round and square and that is that you can bend round quite a bit which means less welds and more continuous lengths. This means a stronger chasis. Of course you could beef up the square section but thats another matter.

For square section bending it can distort the walls. Look at square section steel on some cheapo chairs where the inside face on a bend dips in.

A rough estimate. If you use Ø25x1,5 instead of 25x25x1,5 you will save about 10-15kg on a book frame. Depending on what equipment you have, it will cost you a few hundred to maybe a thousand hours of build time. For a serious, competitive, lightweight build it's worth it, if you planing to use a pinto and go to a track day now and then, no.

would the round tube not be a more pain to cut as it would cutting to a curve to match the adjoining piece.?

If you are buildinga simple square structure then square tube is easier to shape the ends.

However if you start playing with compound angles then round is easier to profie the ends.

All true space frames should have the tubes in compression and or tension so the twisting argument should not hold any weight, but in reality it does.

I would go for a square base and round everywhere else.

John

if you want to maintain the same bending strength as 25mm square tube, use 30mm dia round tube - you would still save some weight, just not quite as much.

[Edited on 25/9/06 by silex]

Oli what are you up to? Are you after building another car?

Caber