Through reading some of the other topics here I think I'll go with round tubing, but I have no experience with welding. Can anyone reccommend a fish mouthing tool and what would the best kind of welding that I should learn?

For round tubing, look for a small app called tubemiter.exe

Input the data, print the sheet out, arap it around the tube and cut to the line.

Welding ?

Mig and tig

Don't be tempted by gasless unless you absolutely must.

If this will be the first chassis you have built , then you're setting off on a very steep learning curve - mig is the easiest to learn , but round tube is much harder to cut right and weld .
Good luck

This is what you need for the welding

http://www.mig-welding.co.uk/

John

hacksaw and decent file and vice IMHO

I have got plenty of tools which could do this, an assortment of grinders, plasma cutter piller drill etc so have tried. But in the end just cut accurately then file in a fishmouth.

Regards Mark

I wouldn't dream of a round tubing chassis without a holesaw based tube notcher (or better)....waaaay too much work without...and they aren't that expensive.

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Originally posted by mr henderson
This is what you need for the welding

http://www.mig-welding.co.uk/

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You'll want something like this...




They come in either cheap Chinese or more expensive versions, the cheap one I have has stood up more than well enough.

IF you can accurately measure the required angle, then there'll be next to no filing required to fit.

If you're doing a car with non-exposed tubes though I'd go with square myself.

but there is something WEIRDLY satisfying about neatly fitting fish mouth that square tube can't match.

Yeah I figured getting myself on a course, which I admit I haven't even looked into yet but at least I know what I want to learn now :-D

I've not got a real design at the mo but I wanted to use round tubing for the strength (don't want to orphan my baby by bending it round a tree!) so to hell with the steep learning curve. I was going to build a trailer anyway so there's my welding practice :-)

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Originally posted by Ninehigh I wanted to use round tubing for the strength

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quote:

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Originally posted by Doug68

quote:

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Originally posted by Ninehigh I wanted to use round tubing for the strength

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Size for size assuming the same material, Circular tube is weaker in tension and compression than Square tube.

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Originally posted by Alan B
And lighter by the same factor....

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However, this discussion has a context, and that is spaceframe car chassis.

Now that there are various people on this thread who have informed opinions, I would like to know why so many skilled chassis designers choose round tube

John

Doug, I have a mechanical engineering degree, so I'm familiar with all that, but thanks anyway.

You are of course correct about buckling, but the usual arguement is to use the round tubes weight advantage and increase it's diameter to boost it's I value.

Typically increasing the round diameter by 25% will make up for the loss of buckling strength and still be marginally lighter.

There are cases for both round and square...it just depends how you present your arguement....I use both round and square.

Hi Alan,

I hope I didn't cause offense I was just trying to bring out some facts and hopefully cause a few people to get curious enough to go properly research the subject. Rather than having it stuck in their head that X is better than Y.

Can you confirm I got the maths right then?

I've been working for so long implementing software I think my Eng brain has gone soft occasionally.

Doug,

No problem at all......no offence taken whatsoever..

The only minor point is that both compressive and tensile strength are directly related the cross-sectional area and not I value.

However, the chances of a compressive load causing a pure compressive failure in this scenario are very, very slim indeed, almost certainly a compressive load would cause buckling failure, and consequently the I value does then come into it. So as far as it really matters you are correct that I is important.

I completely agree that the X better than Y arguement is quite complex.

Ok, which one would be less likely to bend, and therefore crush me, when some spanner decides they haven't seen me and rams into the side?

On a size for size basis it would be square.....on a weight for weight basis, optimising the OD it would be round.

However, and it's a big however, neither will give you significant side impact protection. If the car is not restricted at the opposite side and the impact is fairly widespread and even then more than likely you'll just get shunted sideways.

So many variables and unknowns really.....
Is side impact protection a major design consideration for you?

I'm concerned about safety in general, I'm a fine driver so I'm not worried about ploughing head first into something I'm more bothered about someone hitting me and the whole frame just bending or folding and doing me in. Like my girlfriend keeps saying I'm "not insured yet!"

Ninehigh,

I too have jsut started my build, and am starting my Square tube chassis today.
Personally, i feel that round tube looks better, and offers better strength / weight ratio's, however, you really wont notice the difference in the type of build that were embarking upon.
Keep it simple is my advice. You could worry about the odd gram here or there, but square tube is good enough for a self build.

If youre worried about what other people are going to do on the road, then scrap it, and spend the money on a season rail ticket or bus pass. There is only so far you can go to be safe, and to be honest, this type of car is quite a way from that if involved in an accident. The trick would be to be "Motorcycle" vigilant when driving on the road, and save the thrashing around for the track.

By the way, youll probably blow your current speed record to bits in ths car if it gets finished. Mine is 185 on a bike, and my car wont get anywhere near that i think.

Incidentally, im going for TIG but have a MIG as well.
Tig is very tidy, but probably only once you know the basics on how to weld.
MIG is not so tidy, but is good for filling holes, getting onto hard to reach spots or bodywork repairs. I restored a mini with mine, but it is a nightmare on clean sheet.

ARMOTO sells a pre-cut chassis metal pack (Square!) which ive bought and start today. He delivers really quick too. Ive yet to look and see that all parts are there, and of the correct dimensions, bt he does say that if anythings missing, it would be replaced the next day. The cuts and service was so good, to be honest im not that worried if there is anything missing. Im confident itll get sorted.

Yeah good point, mind you I feel more uncomfortable on the train with all those people so close to me...

I'm gonna have to look for a welding course

I would buy your welder (a good quality one) and visit your local engineering workshop to raid their scrap metal bin. Tell them your working on a college project or something, and they might even give you a load of 25mm tube!!

Get the welder and practice, practice, practice. Read as much as you can on the subject, which is all i did. Its quite daunting at first, but all of a sudden, it will click, and youll have A1 welds 85% of the time. For the other 25%, just grind them back, and try again.

Im about to upload some pictures of the bottom part of my chassis, completed in about 30 minutes. Preparation is key. It helps to work from a datum, and on a flat surface. For this, ive set up a work platform from 1" ply on stilts and drawn the book chassis dimensions onto it AS A GUIDE only. Start at the square piece in the passenger compartment, and work forward. The back end will go on last.

Some may disagree, and you might think about it, and prefer to do it a different way. The thing is, there is no set way to build, (though there are some chicken & egg stages). Its whichever way you feel comfortable. Youll learn quickly by mistakes!! I still am!

Well I'm going to have to make a trailer to transport it until it's road legae, so there's some good practice :-)

Oh, and if you get on a train, just keep scratching your head and slapping the back of your hand as if swatting something then occasionally say: "Got you ya little B'stard". Theyll soon move!!

I heard the best way to keep the seat next to you free is not to pile your stuff on it, but when someone approaches pat the seat and smile :-D

Thats good fun on a late night home on the tube!

Just to add, mine is TIG welded, square lower frame, round tube upper and diagonals, makes it much easier to rivit and glue the floor.

I'd go for square tube - easier to cut, and for a given diameter of tube more weld length to hold it together!

I thought the main reason 'proper' track cars use round tubes is the much better availability of high tensile grades of steel seamless tube in round compared to square. (Seamless tube is supposed to be better as welded tube can allegedly split along the weld line - also heat effect of welding process can weaken tube - but for your first chassis I wouldnt worry about any of that!)

Regards
Hugh

Just keep it simple. Use square. For the size and weight of the car, it wont make any difference for 'normal' use.
As hugh says, its easier to weld too!!