Spaceframe joints with dissimilar tube sizes

I believe this is one of the more misunderstood and overhyped aspects of hobbyist chassis design. Yes, round is slightly stiffer than square per unit weight. However, square is stiffer for a given tubing size. There's the grand morsel of theory for everyone to beat to death, anyway.

But the real issue is not in theory but in practice. In order to use the chassis frame for anything, you will need to mount suspension, drivetrain, body, fuel tank, etc. and there's the advantage of square/rectangular tubing.

I believe this is one of the more misunderstood and overhyped aspects of hobbyist chassis design. Yes, round is slightly stiffer than square per unit weight. However, square is stiffer for a given tubing size. There's the grand morsel of theory for everyone to beat to death, anyway.

But the real issue is not in theory but in practice. In order to use the chassis frame for anything, you will need to mount suspension, drivetrain, body, fuel tank, etc. and there's the advantage of square/rectangular tubing.

Let's be honest here: the real reason enthusiast builders use round instead of square is that in their minds, round looks racy and square looks agricultural. If they backed up a few steps and took a more balanced and objective approach, they'd be more likely to choose square tubing. I like the car pictured here. It represents sound, practical, intelligent design.

100% agree (again)

Major practical disadvantage of square vs round is that you cant bend it which probably means it's crash performance is better.

I believe this is one of the more misunderstood and overhyped aspects of hobbyist chassis design......

but there doesn't seem to be the mania for perfect triangulation there once was. I wonder what Costin and Phipps would make of all this?

100% agree (again)

Major practical disadvantage of square vs round is that you cant bend it which probably means it's crash performance is better.

Edited by cheapracer, 07 January 2012 - 13:10.

No, round in fact is quite easy to bend, easier than square even.

The exact point I was as trying to make - once you give up theoretical stiffness for practicality then apply the practicality logic throughout the chassis. That is why I think switching to round tubes in this part of the chassis might be the best overall solution.

Yep that's what I meant! Didn't think it was possible to bend square tube without cutting and welding?

Edited by Magoo, 07 January 2012 - 13:43.

Yep that's what I meant! Didn't think it was possible to bend square tube without cutting and welding?

Bent square tube is very common, some will even be sitting on chairs or at tables reading this with bent legs (the chairs and the tables, not their legs!).

Most 3 wheel pickups here have large section square bent chassis, wondrous thing is steel - even Chinese steel ...

http://www.alibaba.c.../showimage.html

ok what I should have said was 'Didn't think it was possible to bend square tube without cutting and welding in the average home builders garage without buying specialist equipment or ending up with unsightly wrinkles on the inside radius'!!

I wasn't disagreeing so much as expanding upon your remarks.

Oh, except for the bit about American construction. That was a bit harsh. Acre for acre, the English have built as much useless and dangerous junk as anyone, triangulated as it may have been.

I believe this is one of the more misunderstood and overhyped aspects of hobbyist chassis design. Yes, round is slightly stiffer than square per unit weight. However, square is stiffer for a given tubing size. There's the grand morsel of theory for everyone to beat to death, anyway.

But the real issue is not in theory but in practice. In order to use the chassis frame for anything, you will need to mount suspension, drivetrain, body, fuel tank, etc. and there's the advantage of square/rectangular tubing.

Let's be honest here: the real reason enthusiast builders use round instead of square is that in their minds, round looks racy and square looks agricultural. If they backed up a few steps and took a more balanced and objective approach, they'd be more likely to choose square tubing. I like the car pictured here. It represents sound, practical, intelligent design.

ok what I should have said was 'Didn't think it was possible to bend square tube without cutting and welding in the average home builders garage without buying specialist equipment or ending up with unsightly wrinkles on the inside radius'!!

Edited by cheapracer, 08 January 2012 - 07:33.

Edited by NeilR, 08 January 2012 - 23:11.

Glad nobody told me.

I make my own square bending dies from scrap steel laying around and use a common hand pumped hydraulic pipe bender the same as I use for the round tube.

The trick is in the sizing of a piece of rod you lay inside the die's radius to push the inside of the box in concave as it bends, get that right and you get nice bends.

Like this guy's effort, very similar to my mine, which is also made from scrap (notice the rod laid on the centre of the radius). His is totally hand lever based but mine come out nicer than his probably because I found early on the dies needed to be bolted together or else you can't remove the box tube from the die so I presume he has too much clearance so he can get his box tube out of the die easily but the finish suffers ....



Because of the concave on the inside radius, these bends are very strong but of course can never look as good as round tube.

I did not fully understand your method but i still think its worth mentioning that you can use sand to bend pipes too. usually with the aid of heat but with hydraulics im guessing you wil not need to.



The big gain with this form of forming is that you don`t get wrinkles in the inner bend, at least not when you use heat. so its probably, structurally stronger.

Edited by Tony Matthews, 08 January 2012 - 11:20.

I did not fully understand your method but i still think its worth mentioning that you can use sand to bend pipes too. usually with the aid of heat but with hydraulics im guessing you wil not need to.

Sounds like a neat technique - however the concavity must reduce the stiffness not increase the strength?

Edited by cheapracer, 08 January 2012 - 13:14.

You can use low-melting point alloys, too. I've known it to be used in bending large-diameter tubing for furniture, where appearance is vital. Fill the tube with molten alloy, let it cool, bend the tube, heat it gently and the alloy falls out - on to your shoes...

The square tubing that I've seen bent has a rather nice smooth concave trough on the inside radius.

Edited as 'allots' doesn't seem to be a word...

Edited again to say I didn't see cheapie's post...

You can use low-melting point alloys, too.

saw a "how it's made" episode on discovery about the manufacture of trumpets or some similar instrument... they filled the brass tubes with water, cooled it so that it would transform into ice, and then bend them..

I think "allots" is a proper word.

I think "allots" is a proper word.

well, I am currently giving headache to people with tube benders, having only one tube of 40x1.5 mm 4130 left, and I need a roll hoop bent..

Possibly slide it up the guts of some 45mm PVC pipe?

Why not 38mm x 2.5 mild DOM just for the rollbar?

This is a hazy memory but I think plumbers used to have long coil springs they wound down into round central heating tubes.
Then they bent the tube , either by hand if it was just copper , or in a pipe bender. the bend was a really nice quality and afterwards they unwound the coil spring to get it out of the curve.

I guess the snag is you can only reach so far before the friction between the spring and the inner tube wall overcomes the tendency of the coil spring to lose diameter as you wind it down the tube.

I do know they used it in preference to the sand method.

Edited by Tony Matthews, 08 January 2012 - 18:50.

Glad nobody told me.

I make my own square bending dies from scrap steel laying around and use a common hand pumped hydraulic pipe bender the same as I use for the round tube.

The trick is in the sizing of a piece of rod you lay inside the die's radius to push the inside of the box in concave as it bends, get that right and you get nice bends.

Like this guy's effort, very similar to my mine, which is also made from scrap (notice the rod laid on the centre of the radius). His is totally hand lever based but mine come out nicer than his probably because I found early on the dies needed to be bolted together or else you can't remove the box tube from the die so I presume he has too much clearance so he can get his box tube out of the die easily but the finish suffers ....



Because of the concave on the inside radius, these bends are very strong but of course can never look as good as round tube.

gone to considerable effort to get the rear shocks upright.

the black tubular chassis, rear view. Rear shocks are mounted on outlying tetrahedrons.