Choosing Tubing Size

[UnfinishedProjects]

I would like to get a better understanding of why certain tubing dimensions are chosen and the basis behind it.

Let's take a custom motorcycle frame for instance, using hollow DOM tubing. I know if I were to build it, I want to satisfy 3 criteria: lightweight, strong and good looking. 2 are based on concrete data and 1 is opinion based.

I want to base this on "area moment of inertia", which will basically determine how stiff a member is. If you increase the diameter, the member gains stiffness, reducing deflection. That's a good thing. Another thing I notice is increasing the diameter of tubing has a more of an effect on the stiffness than increasing the wall thickness.

With that said, why aren't more frames (motorcycle, car, truck, etc) made from larger diameter, thinner walled tubing?

Here's an example. Let's compare 3 pieces of DOM tubing. All are 36" in length and will have a 100 lbs of force applied.

A. 1.125" Diameter, 0.156" wall weighs 4.84 lbs, deflects 0.91" and sees a bending stress of 35,413 psi

B. 1.25" Diameter, 0.120" wall weighs 4.34 lbs, deflects 0.75" and sees a bending stress of 32,722 psi

C. 1.5" Diameter, 0.095" wall weighs 4.27 lbs, deflects only 0.5" and sees a bending stress of 25,976 psi

Now I know there are a ton of other considerations, but I feel this is a very simple test that shows good data on tubing dimensions. As diameter increased, even though wall thickness decreased, deflection went down, weight of the tubing member went down, and in my opinion looks were improved.

I like the look of large diameter tubing, some don't; totally understandable. Especially if you are building an old school bike where you want to use smaller diameter tubing to create a more accurate look.

What is everyone else's opinion?

Do you like the benefits of larger diameter tubing? Stiffness increases, but how about running internal wiring/tubing? More room to run wires and hoses.

Sometimes due to space requirements, large tubing cannot be used. I also don't think tubing diameter should be increased so much that the frame looks ridiculous. You wouldn't want a chopper frame based on all 4" tubing.

What do think is the minimal wall tubing to use. I know 0.12" wall is good to go in most applications, especially bike and car chassis. How about going with 0.095" tubing? Penetration would be alot easier to achieve.

Any input would be awesome as I really want to start fabbing frames one day.

-Kev

[UnfinishedProjects]

Nobody has any input?

[Gadget]

I can't comment since I know nothing about it but maybe Stan will when he comes online.

[brucer]

arent most motorcycle chassis made of chromoly?

larger diameter thinner wall tubing will be stronger because of the surface area of the tube..

[Brink]

my background is offroad. Rockracing, rockcrawling, etc etc.....take it for what it's worth. Not motorcycles but I work with tubing every day.


1. you gain strength much faster with increasing OD compared to increasing wall thickness.

2. weight adds up fast.
We build the "structure" out of 1.75" .120 wall DOM. (type of material makes a big difference also, ERW, DOM, Chromo in that order) Anything that will not contact a rock in a worst case senario gets smaller tube. 1.5" or 1.25" .095ish.

3. most of the factors involved are how force will be applied to the tube. For example take your example above. If that force is applied perpendicular to the tube in the center of it's length it will fold the tube. If it is applied to the end of the tube pressing down the length it can withstand much much more.

lots of variables, lots of factors when it comes to how much tube is enough tube.


Like I said though...not your exact application. But we do build them to roll a dozen times down a rocky mtn and be flipped back on their wheels to keep going.

[minnfatz]

Brink hit on probably one of the most important parts of tubing selection. If you hit a tube with something solid perpendicular to the centerline, the thinner wall tubing will collapse much easier. Take a paper towel roll and hold it at both ends and try to flex it and it is pretty sturdy, but crease it even a little perpendicular to the centerline and it will fold easily. Another fun one is to put an empty aluminum soda can on the ground sitting upright, then carefully stand on it with one foot. The can will hold your weight, unless you are a fat bastard like me, then have someone finger flick or lightly tap the side of the can and it will collapse instantly. I can't remember all of my physics, but it really depends on how the stress is applied. If I were to build a bike from scratch, I would go with larger diameter tubing. I think it looks cooler. Look at what Bourget and even Orange County Choppers puts out. They run fairly large O.D. tubing.

[WookieWelding]

it also depends on the weight of the chassis to the forces put on the tube ex.. a light chassis can use 1 1/2 .120 wall tube now a heavy chassis would be 1 3/4 or 2 inch tube

[junes]

on your examples i have a few questions:

is this a canterlieverd beam or supported on both ends? What type of scenario are you using
Next where is the load being applied on the beam? At the end? Middle??

[JolietJames]

Wow thread ressurection! What the heck happened to Brink on here anyway? He has one of the best build threads on a popular 4X4 site I've ever read and was happy to see him on here. He must be busy because I haven't seen him post on here once in recent months. Too bad, I'd like to see what he's working on!
BTW if You let Brink or any of us off roaders build a bike it would look like the Cat bike those sellout Dbags over at OCC built for Caterpillar but with a cooler paint job.

[brucer]

also on a street bike, if your building something to ride and cruise around on like a custom harley... you want weight, it makes for a better ride.. think cadillac vs. yugo