# Square tubing?



## nogod (May 30, 2009)

Is there a reason we are using round tubing? wouldnt square work just as good?


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## 18bikes (Jan 15, 2007)

have a google for some of the older Pace's, they used externally butted square tubing

matt


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## dbohemian (Mar 25, 2007)

nogod said:


> Is there a reason we are using round tubing? wouldnt square work just as good?


If you have the same envelope (lets say 1.250") square will weigh more than round. Also, in the past it was easier to make round butted tubing.

Dave Bohm
Bohemian Bicycles


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## thefuzzbl (Jul 5, 2006)

i use square tubing and rectangular tubing all the time on cargo bikes. its cheap, weight usually isnt a factor, and its hella strong.


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## nogod (May 30, 2009)

ok i can see that if you look at the two side by side. the square has a larger circumference, so to say. i wonder though if you you used a smaller square if that would be any weaker?
anyone know of any sites that have data on structural strength of tubing?


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## highdelll (Oct 3, 2008)

circles are stronger than squares


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## FruitaGuy (Jun 11, 2009)

I'm sure there are lot's of places you can get data on stress risers and strength results on various cylindrical tubing, but you would have to know how to ask the question I think. It's true that round tubing has traditionally been the builders choice for a few reasons. It's made in huge quantities and in lot's of different alloys and it's very easy to work with. However, builders for decades have been changing the shapes of tube for various areas. Square tubing however, is not a good deal, unless it's for things like fuzzbl is doing, where it sounds like light-weight, cyclical durability and ride quality are not a top priority. Cylindrical tubing does something that square tubing cannot..it distributes cycle stress over a greater surface area and generally does not have pinpoint stress areas. If you look at the history of high-strength engineering, you will very rarely see squares. On shining example of where this failed is the De Havilland Comet jetliner...the first commercially made airliner. The first generation had square windows and the plane's fuselage kept breaking apart mid-air. Investigations finally showed that the corners of the windows had especially high stress areas and led to incredibly fast crack propagation across the body of the plane, so the windows were switched to a round design. Anyway...the point is that in a high-stress applications, square is not better than round.


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## nogod (May 30, 2009)

ok i see your point. from looking at failed square tubing and failed round tubing ive seen in numerous places. the square buckles at the corner and splits out. where round tubing dimples then when a corner is made that then splits. so cut to the chase remove the corner

thanx for explaining this concept to me guys


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## highdelll (Oct 3, 2008)

nogod said:


> ok what physics make a circle stronger then a square?


it's got to do with the way stress can be driven around smoothly - like a whirlpool in a circular pool vs a square pool.
With a square, the forces back up and load at any of the 'kinks' or corners.

How do ya like my physics explanation - cool huh?


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## nogod (May 30, 2009)

highdelll said:


> it's got to do with the way stress can be driven around smoothly - like a whirlpool in a circular pool vs a square pool.
> With a square, the forces back up and load at any of the 'kinks' or corners.
> 
> How do ya like my physics explanation - cool huh?


lol very cool thanx :thumbsup:


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## Jerk_Chicken (Oct 13, 2005)

Supposedly, companies used square tubing at the HT to get more welding area and more overlap for a stronger joint, then profiled it to round , as it wasn't needed to be square elsewhere. Some have tubing that is square at both ends, but round in the center sections.


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## RoyDean (Jul 2, 2007)

highdelll said:


> circles are stronger than squares


It depends on how you load them. There are plenty of applications where square cross sections (or even more dramatic shapes, such as I beams) better handle the load in question.

To say a circular cross section will always be "stronger" than a square is simply not true.


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## FruitaGuy (Jun 11, 2009)

Jerk_Chicken said:


> Supposedly, companies used square tubing at the HT to get more welding area and more overlap for a stronger joint, then profiled it to round , as it wasn't needed to be square elsewhere. Some have tubing that is square at both ends, but round in the center sections.


That actually does not make any sense at all. If you take a round tube, and then make the end square...you still have the same surface area, unless you stretch the tubing, which creates a whole new problem. Then, you have a square to round transition point, which will only increase stress in that area even more. That's good marketing...not good construction.


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## Jerk_Chicken (Oct 13, 2005)

FruitaGuy said:


> That actually does not make any sense at all. If you take a round tube, and then make the end square...you still have the same surface area, unless you stretch the tubing, which creates a whole new problem. Then, you have a square to round transition point, which will only increase stress in that area even more. That's good marketing...not good construction.


If you're welding a top and downtube together at a headtube, maybe those two tubes can have more overlap, and also the dimension of the headtube can be less dependent on the round, taller combo of the two.

Just speculating.


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## FruitaGuy (Jun 11, 2009)

Jerk_Chicken said:


> If you're welding a top and downtube together at a headtube, maybe those two tubes can have more overlap, and also the dimension of the headtube can be less dependent on the round, taller combo of the two.
> 
> Just speculating.


Ok...got it, I see what your saying there. Just like my frame, the top tube and downtube are welded together for a few inches. It's true that on some frame sizes, headtube dimension can be important.


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## highdelll (Oct 3, 2008)

RoyDean said:


> It depends on how you load them. There are plenty of applications where square cross sections (or even more dramatic shapes, such as I beams) better handle the load in question.
> 
> To say a circular cross section will always be "stronger" than a square is simply not true.


that's true...
I was talking in general and about tubing.
I suppose it depends on the 'type' of loading too, and exactly where the stresses are applied.


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## nogod (May 30, 2009)

either way it seems most tubing are structurally strong enough for most mtb setups. except maybe that 80 foot drop.

what ive seen in 99% of frame failures, its the joints that fail. so i might try a square tube design. but probably reinforce the weaker design flaws or use round or oval tube in certain areas. and yes this will be my first real bike build. i do have a lot of experience with metal fabrication. but like everyone i cant possible know everything lol. hell i might just think i know something. what draws me to building my own frame is two things: the distaste to spend money i dont have for a bike that i will probably modify. and i have short legs and long body, making it hard to find a frame to fit me and be strong. im 6'2" 275 (no im not fat lol small gut though maybe i should slow down on those black & tan beers) my in seem is
only 29"


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## herbn (Sep 17, 2005)

In metal square tubing is the strongest for a given area ,round is the best strength to weight in that area. I wonder about carbon though, especially where tubes meet.Carbon fibers are best if they're straighter,hence the huge 1.5 lower headsets.There are fibers that run straight up and down the fork legs and they hardly have to bend as they transition into the steerer. So big rectangle tubes kind of make sense in carbon,especially since the stuff ,i think efficiency of a round tube is secondary.


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## MaxxCutts8 (Nov 7, 2008)

Not sure but I think its kind of the same idea as an egg. If you squeeze an egg its hard to break but if you take some thing with a thin layer such as an egg shell and put it into a square form then its strength won't be as much. I really have no scientific base for this but this is what I always thought


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## pvd (Jan 4, 2006)

It's like everything else in bike design: it depends. It's really important how the stresses and forces are going through the tube.

I love square tube. I just rarely see a place to use it on a hardtail frame.


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## nogod (May 30, 2009)

MaxxCutts8 said:


> Not sure but I think its kind of the same idea as an egg. If you squeeze an egg its hard to break but if you take some thing with a thin layer such as an egg shell and put it into a square form then its strength won't be as much. I really have no scientific base for this but this is what I always thought


 same concept as a concave dam. the force is going along the structure out towards the sides, rather then straight through down stream. a straight dam would never hold, without making it really thick. i grew up in oregon by the clackamas river we have a few dams.


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## highdelll (Oct 3, 2008)

well, dam it!


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## herbn (Sep 17, 2005)

I thought it was pretty good as chainstays,on my ol klien,


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## Smokebikes (Feb 2, 2008)

I once built a frame (hard tail) with all square tubes (4130 from AircraftSpruce) except the ST, BB, and HT........it was *very* flexy and did not handle well at all.


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## nogod (May 30, 2009)

Smokebikes said:


> I once built a frame (hard tail) with all square tubes (4130 from AircraftSpruce) except the ST, BB, and HT........it was *very* flexy and did not handle well at all.


how thick were the walls?


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## shiggy (Dec 19, 1998)

nogod said:


> how thick were the walls?


Well, the largest square 4130 tubing Aircraft Spruce has on their site is 1x1" (0.035, 0.049, 0.065).

Note that as per the AS charts the 1x1x0.035 weighs more than round 1.5x0.035 tubing.


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## FruitaGuy (Jun 11, 2009)

shiggy said:


> Well, the largest square 4130 tubing Aircraft Spruce has on their site is 1x1" (0.035, 0.049, 0.065).
> 
> Note that as per the AS charts the 1x1x0.035 weighs more than round 1.5x0.035 tubing.


Oh wow..yes, 1x1" is pretty small, I can imagine that it would have been rather flexy, especially considering the "standard" steel tube diameters are 1 1/8" for down and seat tubes.


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## nogod (May 30, 2009)

lol it looks like i should do my math


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## highdelll (Oct 3, 2008)

I also think spruce is better for large water-planes...


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## vulture (Jan 13, 2004)

*Square seat stays*

I have built a couple hundred frames using square tubing for the seatstays, some were cut and welded wishbone style and some were "S" bend continuous. The only failures were beginner fabricator error. As far as ride there were no noticeable differences and the look was distinctive. I have not done it in a while because I did not want to be pigeon-holed but I will do them again if anyone asks. Lots of experts, engineers, physicists etc gave me their opinions and occasionally contradicted one another. One bit of info I got was that for the same dimensions square was stiffer than round in bend loading, not torsion. I used 1/2 by 1/2 .035 and set them on the diamond angle. The rotation made the stays fill the space as if they were 5/8 round. Some folks love the look and some folks hate it. I personally like it and have not seen anyone else do it that way.


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## nogod (May 30, 2009)

any pics? ok i founds pics on your site damn pretty drop outs


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## highdelll (Oct 3, 2008)

^^^nice


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## restlessrider (Nov 29, 2007)

... and have not seen anyone else do it that way.[/QUOTE]

Is that an invitation to plagarize your design?
By the way I think you are in Bend and I go to Bend often for work -
I'd love to see your shop sometime.
Rich in Ashland


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## nogod (May 30, 2009)

highdelll said:


> I also think spruce is better for large water-planes...


lol yea the spruce goose is awesome even if it sucked


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## shiggy (Dec 19, 1998)

vulture said:


> I have built a couple hundred frames using square tubing for the seatstays, some were cut and welded wishbone style and some were "S" bend continuous. The only failures were beginner fabricator error. As far as ride there were no noticeable differences and the look was distinctive. I have not done it in a while because I did not want to be pigeon-holed but I will do them again if anyone asks. Lots of experts, engineers, physicists etc gave me their opinions and occasionally contradicted one another. One bit of info I got was that for the same dimensions square was stiffer than round in bend loading, not torsion. I used 1/2 by 1/2 .035 and set them on the diamond angle. The rotation made the stays fill the space as if they were 5/8 round. Some folks love the look and some folks hate it. I personally like it and have not seen anyone else do it that way.


Vulture-built


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## karpiel666 (Jan 7, 2005)

Banshee scream has rectangular chain stays, seat stays and partially rectangular tubes and is pretty much the stongest bike EVAR!!!


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## vulture (Jan 13, 2004)

*come visit*

Rich, call me anytime you are coming to BENDOR. Phone # is on my blog, (link below) I did not invent the bicycle or square tubes, if you want to build some square stay rigs I say build them, just make sure you are having fun.


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## RoyDean (Jul 2, 2007)

shiggy said:


> Note that as per the AS charts the 1x1x0.035 weighs more than round 1.5x0.035 tubing.


Somebody may want to verify my math, but I think this is wrong. I get a cross sectional area of 0.135 sqin for the 1" sq tubing and 0.161 sqin for the 1.5" round.

[edit] square tubing calc done assuming perfect corners... rounded corners decrease cross sectional area


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## shiggy (Dec 19, 1998)

RoyDean said:


> Somebody may want to verify my math, but I think this is wrong. I get a cross sectional area of 0.135 sqin for the 1" sq tubing and 0.161 sqin for the 1.5" round.
> 
> [edit] square tubing calc done assuming perfect corners... rounded corners decrease cross sectional area


Square tube does have rounded corners and IME the corners are usually thicker than the wall at the center of the sides. A round tube has a (more) consistent wall thickness.

SA's charts are pretty consistent that the square/rectangular tubing is heavier than round.

It is pretty amazing how much weight difference there can be with a slight change in wall thickness. I externally butted a 280mm 1.375, 0.065 tube. Center wall brought down to ~0.040 for ~80mm with ~30mm tapers on each end. Weight loss was ~30g.


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## nogod (May 30, 2009)

shiggy said:


> Vulture-built


 i would have to be in the crowd that likes the look


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## sund (Jul 13, 2009)

First time poster here, but I love reading the frame builders forum as a budding mechanical engineer.

I ran some quick numbers on the way square and round tubing respond to bending stress. Here's what I found:

Given that both kinds of tubing have the same outside dimension and wall thickness (i.e. a 2 in. x 2 in. x .125 wall square tube and a 2 in. dia x .125 wall round tube), a square tube will see *58.9%* as much bending stress as the round tube. However, as a trade-off, the square tube will weigh *27.3%* more.

This calculation assumes a force applied perpendicular to the square surface. Things would be different in the case of the diamond orientation that was shown above (and looks sweet BTW).

Of course, torsion is a different story, and as was mentioned above, in square shapes, stress tends to 'pool up' in the corners, causing fractures and the like, where as round shapes do not have this problem.

Hope this helps!


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## nogod (May 30, 2009)

sund said:


> First time poster here, but I love reading the frame builders forum as a budding mechanical engineer.
> 
> I ran some quick numbers on the way square and round tubing respond to bending stress. Here's what I found:
> 
> ...


very helpful and outstanding first post. thank you!:thumbsup:


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## leadphinger (Sep 18, 2007)

sund said:


> First time poster here, but I love reading the frame builders forum as a budding mechanical engineer.
> 
> Hope this helps!


My first post was a dirty limerick...

Welcome! Keep up the techno-speaky, mathematically supported comments and you are liable to become a resource.


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## scuppy (Nov 10, 2007)

I wouldn't mind playing around with square tubing, but it is 5 times as expensive as round, plus my tube bender doesn't do square so good.


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## Jayem (Jul 16, 2005)

karpiel666 said:


> Banshee scream has rectangular chain stays, seat stays and partially rectangular tubes and is pretty much the stongest bike EVAR!!!


Nope, it was a poorly designed bike where they just added gussets when stuff started to crack. These bikes broke, and not because the riders were going "too big", it was because they didn't design it well in the first place. They made it "super beefy", but there's more to designing a bike for strength than just adding material. The stress risers always prevail.


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