Hey all

I've been wondering, why are all the downtubes square? I can see no reason why they all need to be square, but I'm no expert. I want to design my own buggy and would like to use a round downtube. To the experts out there: Are there any reasons to prefer a square over round downtube?

Leech

Leech,
Simple anwers...

Less weight for more strength with square tube over round.
Ease of manufacturing - Not an issue with home-built.
Ease of attachment - Again a marketing decision; take-a-part is critical for sales, not important for the home-builder.

Two sciences: The Science of structural steel Design and Engineering, and the Science of Marketing.

Which qualities and features are important to YOU?


Safen Up! Buggy On!

"Often wrong... Never in doubt"

the coreylama

down to strength and workabilty its easyer to work with a sqaure tube then a circuliar tube

Are you sure about the strength thing, I can see ease of manufacturing, assembly, but I always thought that a round tube was stronger than a square one of the same wall strength, maybe the folds help, but keep in mind, half the down tube failures are twists which boxes are terrible at compared to a tube.

Msyelf I used 2" stainless schedule 40 pipe for the down tube, should never bend, also helped get the weight ballanced the way I like it. With a little more weight in the nose, I get pulled nice and sideways most of the time, not the back or front washing out on me.

Pablo here is right about strenght tube win over squere.That is why all bike frames are made from tubs.As far as weight it can weight the same but again tube win in strenght.That is why our back axles and siderails are made out of tubing.However downtubs are squere for marketing reasons and it is stonger in that point .That way You can take it apart without compromising strenght of connection.I have learn all about it in my five year technical school.There is lots of design and science behind building right buggy so that is why it is better to buy one.Those guys spend lots of time to get it right And I want to Thank for it.
Cheers

there is twist in the downtube that needs to be held solid which the shoulders of a square tube achieves nicely,while the bolts that hold it together are secondary, with a round tube only the bolts prevent the twist.

you can overcome some of the twist with an offset down tube. the force changes from twist to a bending action.

cheers
Jim

Thanx for all the replies. I did some calculations on the round tube and bolts to connect it. I think a round tube will be able to withstand all the forces in the tube, the bending as well as the torsion. Moddeled the buggy in solid works to see what it would look like. And the bent round tube looks really nice!!

Round does look nice, put some good time into making sure it'll bolt on to the frame properly without being loose or rattling, mine is welded solid, then again, so is the rear axle, One of the main reason's you'll always see the buggy hanging out the rear hatch of my car. No rattles though.

Yeah, you are absolutely correct about the rattling. I need the buggy to be bolted together though cause I drive an old Mazda 323. No space for a buggy in a 323.

Put it on top

Out the hatch, I drive a VW Rabbit, fold down the rear seat, open hatch, cram front wheel in, bungy to car, bungee hatch closed, off you go.

I also made the 6hr drive to seaside with the buggy bungeed to a ghetto roof rack.

get a trailer....and yes the round tube is far stronger than box section

Trailered the stuff this time, way nicer, then when on the beach, you can unhook the kite trailer and send someone in for coffee runs without your kites going bye bye

Looked it up. Here's the deal. Round tubes are far stronger than square tubes when it comes to bending. However, square tubing offers superior internal torsional resistance (i.e. twist), which is the main force exserted on a buggy's downtube.

sooooooo is it work me trying a round down tube do you think or is it not work it

Square tubes twist less. Simple as that. When you turn the wheel is at an angle and this exerts a large rotational force on to the down tube. A round tube would twist straight away but the square has alot more resistance.

I'd love to see you twist a section of stainless steel schedule 40 pipe though, probaby not going to happen. Down side to round tubing though would be to make a drop swan neck out of it, probably be pretty hard to manage.