Siu Blue Wind

Just out of curiosity....


Which is stronger - the square tube frames that companies like Specialized and Kona are using or the traditional round tubing? I always thought round was structurally stronger but it seems newer designs are leaning more towards square.

DannoXYZ 

It depends upon the direction of the load. Strength-to-weight in lateral bending is good with round. But you get much better rigidity/stiffness torsionally with square-tubing of the same diameter at a slight cost in weight. The equations governing thin-wall tubing is as follows:

Maximum Torsional Stress = T/2*t*Ar where:
T= Torque
t = wall thickness
Ar = cross-sectional area enclosed by the centerline of the tube

Ar for a circular section would be pi*r^2
Ar for a square section would be b^2 (b is the length of the side)

TexasGuy

scuse me sir but why couldnt you put diagrams to go with this too

eubi

It depends on how it is expected to be loaded.

The bending stress in a beam is given by:

Stress = M*y/Ixx

Where M is the bending Moment
y is the distance from the neutral axis
Ixx is the second moment of inertia of the beam along the xx bending axis

The resistance to bending stress in a beam is determined by what engineers call the second moment of inertia of the beam. When you bend a beam, the stress is highest as you move from the bending axis of the section. The reason for an "I" beam to be shaped the way it is is that it has a lot of material at the top and bottom to better resist the bending stresses. This way it is strong but light.

Square tubing can be made to simulate the stress distribution you would see in an "I" beam. Thicker material at the top and bottom, with thinner material on the sides. You have light weight, and material where you need it.

The down side to this wonderful I beam is if it is subjected to the same bending load in another direction, it will be highly stressed. Now the second moment of inertia will be Iyy. From the equation, if Iyy is less than Ixx, you can see the stress will be higher.

On the other hand, you can put a bending load on a tube from any direction and the stress will be the same. Ixx = Iyy.

I know this is probably clear as mud. We really need some diagrams and manipulatives to illustrate it better. Try this:

Take a yard stick and bend it across its long side. Hard, huh? Now bend it across its short side. Much easier, right? The amount of material is the same, but depending on you you orient it, it can be made to resist bending better.

Now take a piece of PVC pipe. It will bend the same no matter how you bend it.

Let's not even talk about when happens when you twist a square or rectangular tube!

Retro Grouch 

I don't know but I'll bet that the square tube designs are cheaper to build. Everytime that I learn about some new great idea for a bike frame, it turns out that it's easier and cheaper to build.

DannoXYZ 

I'd like to see bike tubes that go from triangular to round to square to deal with the different stresses at different ends of each tube. And the chainstays actually do need to be ovalized laterally rather than vertically....

Siu Blue Wind

By cheaper do you mean because of the design or less material involved?

Siu Blue Wind

So are you saying that a DJ bike would be better with square and maybe an XC would be better with round? Are there any square road bikes? Or (that sounded funny) any road bikes with square tubing, despite weight?

eubi

I don't think so...what is a DJ and XC bike?

Sorry, I'm just a clueless commuter on a Cannondale.

I would tend to want to build a bike with round to oval tubing. When you add the corners things get strange in torsion. I can think of no benefit to using square tubing on a bike.

Also, when it comes to manufacturing, round is the easiest to produce.

Those square hole drills are expensive!

DannoXYZ 

Yeah, I really like how the Serottas have tubing that changes diameter and are ovalized in different directions. The seattube gets fatter as it goes down to the BB. Both it and the downtube are ovalized laterally to resist sideways forces from pedaling. And the downtube is ovalized vertically where it meets the headtube. Pretty neat.

Ritehsedad

If you're talking about the tube cross section, generally speaking a circle is going to be stronger than square of the same (or even greater) cross-sectional area. I believe that a lot of the square or diamond shape tube cross sections are intended to be more aerodynamic.