Hi,
I know this has been covered before, but can't find any precise information so:
Can anyone forsee any problems if I build my chassis (for a blade engine) and replace all the follwing bars (from the book) with the corresponding round instead of square (25 and 19mm):

G1, G2, S, T, P, g, h, k, c, d, j, RU1, RU2, Z, Y, W1, W2, V.

Not bothered about all the fishmouths I will have to cut, or the welding, just worried about strength issues.

And for anyone that bothers to look them all up - thanks.

Pat...

I replaced most of my round tubes with square!

How come you want round?

Cheers,
James

The main 2 not compressed in the length are G1 and G2 so I'd keep them 2 square. I think the tunnel ones may have an issue - bending strength should be greater for square over round in the direction that the side lie. At funny angles, I'm unsure.

When the length of the bar is in compression/tention (W1 and W2 for example) I can't see an issue.

Is it not the case that in general round tube of a given diameter is not as strong as square section of an equivalent size?

My simplistic view is that the amount of material in a round bar cross section (circumference) is equal to PI x Dia. = approx 80mm for 1" round tube versus 4 x Side length = 101.6mm for 1" square tube.

Does that equate to round bar needing to be 4/PI times the length of the side of square tubing to be of equal "strength"? If so then something like 32mm round tubing should be used or alternatively 25.4mm with a thicker wall.

By the way, I also replaced all the round bits on my chassis with square with the exception of the rear luggage area.


Cheers,
Craig.

I have also heard that for the equivalent tube size/weight square section is stronger.

Ned.

A lot of pain for very little gain, the problem with working with round tube is not so much cutting the fish mouths but cutting them in the correct angular relationship to each other.

For a spacefame of equivalent stiffness a frame made of round tube with be lighter.

You really have define "stronger"....

size for size square wins all comparisons, but it's heavier.

weight for weight for then tensile and compressive strength is equal, but round wins on buckling strength.

However, "strength" is rarely the main issue unless we have a perfect spaceframe, stiffness is really what we should be comparing.

I used quite a lot of round on my chassis. You could say I only used square where panels and brackets are going to mount. This ends up being most main tubes, so most of my triangulation is in round, although there are plenty of exceptions of course. I also used round on the top of the sills and tranny tunnel, as I think it will look a lot better once the panels and trim are on having rounded edges there.

Liam

quote:

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I only used square where panels and brackets are going to mount

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when using tubing under bending conditions, you need to move up to 30mm diameter tubing (assuming same material thickness) to attain the same second moment of area.

Cross sectional area of 25mm x 25mm RHS with a 1.6mm wall is equal to 150mm square.

Cross sectional area of 25mm diameter tube with a 1.6mm wall is equal to 118mm square

Cross sectional area of 30mm diameter tube with a 1.6 mm wall is equal to 143mm square


Of course 1 inch box will be stronger than 1 inch round, as it was nearly half as much more material in it.

the question is how does 1 inch box compare to 30mm round tube?
The round tube would still be lighter than the box.

[Edited on 9/3/05 by clbarclay]

Under bending and torsion, the 30mm tube comes out on top in both cases, although not by much at all, thats why I suggested the 30mm tube of same wall thickness.

In direct tension the tube would loose out to the square, but again not by much. Although saying that, there is not really enough tension in any of the members to make that a worry, plus the welds would almost certainly fail first.

So there's no reason why I can't make the tranny tunnel in 30mm round?
This would also give a nicer edge on the top to fold ali round.
Pat...

Seems there is no structural reason not to although welding round tube isn't as easy (I have found anyway) compared to square.

If it is smooth lines you are after, have you considered just bending 16swg sheet into a trannie tunnel? You will find posts by Cymtriks where he recommended this approach as it is stronger and lighter than using tubing of any kind. I know 16swg isn't "easy" to bend but I'm sure you could manage it, even if you had to do it in a couple of sections. If you do follow this approach it is probably worthwhile welding in a hoop or two from 5mm flat bar to contain the propshaft if it fails.

Cheers,
Craig.

Though about using a sheet, but will make the pipework and handbrake a little more tricky.
May be best and easiest to just do the top two transmission tunnel bars in 30mm round.
Pat...

quote:

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Originally posted by Avoneer
Though about using a sheet, but will make the pipework and handbrake a little more tricky.
May be best and easiest to just do the top two transmission tunnel bars in 30mm round.
Pat...

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Yep, doing the BEC thing so will leave the Driver side of the tranny tunnel as per the book and have the passenger one parallel and a lot narrower than the CEC equivalent - will give the passenger loads of leg room then.
Cheers,
Pat...

Wouldn't more driver leg room allow for bigger pedels (less chance of catching 2 pedles by mistake). In a standard chassis the driver has more leg room for pedls, but the passanger room is not too bad.

I would go for an increse of room on both sides.

Yep, but the bike prop will be in the way as it runs right down the drivers side of the tunnel and not down the centre like on a CEC.
Pat...