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strength of thin-walled shaped steel tubes?(8 posts)

strength of thin-walled shaped steel tubes?kenyee
Nov 15, 2001 3:31 PM
I've been noticing odd comments about how thin-walled steel tubes (like the ovalized ones on the Lemon Zurich or the Cervelo Prodigy) would be easier to damage. One of these comments was in the Lemond Zurich reviews area.

Does anyone have a link to any web sites that prove or debunk this in more detail? This technique of not doing perfectly cylindrical tubes seems to be getting more common (you see it in Serottas, Airbornes, etc.), so it doesn't make sense for it to be weaker (though it does seem intuitively obvious :-)...
re: thin wallsSteveS
Nov 15, 2001 3:39 PM
I don't have any direct experience with this problem, but I did read on another forum where someone complained how easily their Columbus Ultra Foco tubing dinged,dented. It is not ovalizing or shaping that would make a frame more likely to be dentable, as there have been shaped tubes for a long time now. The problem would most likely pop up as steel is made thinner and thinner in order to be lighter.

You might go to Anvil cycles website and post a question to Don Ferris (builder) as to whether this is true or not.
re: thin wallsjacques
Nov 15, 2001 4:24 PM
SteveS is exactly right. In a quest for lightness, tubes have gotten very, very thin in the non-stressed areas. I put a huge dent into a DeRosa top tube once by tapping it with my Look plastic cleat, half-way between the head tube and the seat tube - which is one of those paper-thin, non-stressed areas. You can keep riding the frame, but only if you can stand the constant questioning from the peleton to the tune of "hey man, how'd you put that ugly ding in there?"
Common Sensegrzy
Nov 15, 2001 4:22 PM
Thinner walls ar easier to dent b/c there's less material. Shape has an influence, but it's minor if you're rough on your toys. Round tubes are one of the most efficient designs from a *general* strength to weight perspective - all things being equal. Shaped tubes have a practical use in terms of aerodynamics and dealing with specific load applications. It also is heavily hyped by the marketing guys. All the aerodynamics in the world isn't of much use if you have skinny legs.

If you realy want all the info take a Statics course that should be offered in any respectable engineering core program - that's my background. You'll get all into the cross sectional moments of inertia (Ixx, Iyy, Izz) - something the bike hucksters fail to provide - and will learn all about bending and the deflection of beams. Something that most small frame builders never studied.

Serotta doesn't do much in terms of aero shaped tubes - butted, tapered and bent yes, but not much of the "aero" ploy. Try not to lump Serotta in the same bin as Airborne, they're pretty much complete opposites.
Why do skinny legs matter?kenyee
Nov 15, 2001 6:58 PM
I don't remember much of my engineering core courses after having done software for so 10 yrs :-)

A lot of them do wind tunnel tests (I know Cervelo definitely does) which is why they have a good reputation as a TT company. They never mention skinny legs though..

Airborne claims "ovalized" tubes. Serotta claims "tapered". I know they're different in build philosophy (to say the least ;-), but I wanted to emphasize that companies seem to be moving away from generic perfectly cylindrical tubes of old...
Beer can effectKerry Irons
Nov 15, 2001 5:06 PM
The general rule is that if a steel tube wall is less than 1/50th the tube diameter, it is susceptible to easy denting. Obviously this is not a sharp cliff (thicker than 1/50th = bullet proof, less than 1/50th and you can dent it with a pea shooter). But if you do the numbers in your head, you can see that a 0.5mm wall thickness (common if you want lightweight steel), then a tube greater than 25mm (1") diameter will dent easily. And virtually all tubes are greater than 25mm these days, many much greater than 25mm. This explains why steel tubes were minimum 0.6mm wall thickness for many, many years. People didn't accept dented tubes when their only choice was steel. With the advent of aluminum oversize tubes (possible because of aluminum's lower density giving it thicker walls for a given weight) and the ability of Ti alloys to give lighter frames, lightweight steel has now pushed the envelope into the beer can region.
Where is 1/50 rule from?kenyee
Nov 16, 2001 7:51 AM
I've also noticed that companies don't seem to post their minimum tube wall thickness in their specs...
Bending and Tensile strengthDAC
Nov 16, 2001 4:55 AM
As an engineer, I would just like to point out the theory behing thin wall tubing. When you have x amount of metal, and wish to make a tube that maximizes bending strength, a wide tube is stronger than a narrow tube is. This is generally true also for tensile strength, but when you make the wall of the tube too thin, it will tend to buckle under compression. Depending on the type of loads for a specific tube, there will be an optimum diameter, not considering things such as corrosion and damage.