Frame 3 is to be a sport touring/commuter for my wife. I'm trying to decide what tubing to use, and am a bit concerned about tube durability because my wife isn't as careful with her bike as I am.

For a given diameter and wall thickness, would something like Columbus SL be any more durable than, say, Columbus Cromor?

Thanks,

Pete

Actually Cromor might be the more durable of the two, as it has thicker walls, IIRC.

SP
Bend, OR

Well, let's say we were comparing SP (alas, no longer available) to Cromor...they are the same wall thicknesses. Are there any durability differences between the alloys?

As far as I understand, durability = fatigue resistance? I guess that stiffness influences the fatigue resistance a bit. At any rate, tubes rarely fatigue (except perhaps at the butt transition), the joint does, in the heat affected zone. So the brazer's skill is the factor I'd say. But also, I think modern high-strength alloys are a step above cromo or SL, with better grain in the HAZ, and thus better fatigue resistance.

As to cromor vs. SL, they are both pretty much 4130 steel. SL might have a bit more cold work and thus higher strength?

"concerned about tube durability because my wife isn't as careful with her bike as I am."

what are we talking dents?

I think I have proven that Spirit for Lugs is pretty durable. I had less than a month on it when I rode it through two giant potholes at speed. No damage.

Dents, potholes, running into ****, that sort of thing.

Oh for dents... then thick walls or heat treated (i.e hard) tubing perhaps. I'd look into hooded style or socketed dropouts as well, they are a bit stronger I think. But good wheels & wide tires would come first.

Tensile strength values are most commonly pulicized for tubing but the specification that you should be comparing for dent resistance is the yield strength. Probably the best reference source I find for SL tubing composition is at; https://desperadocycles.com/The_Lowdo...ing_Part_4.pdf

This arcticle is almost 30 years old so odviously does not discuss any of the newer tube sets and does not directly discuss cromor.
SL and cromor are both 4130 specification "chrome-moly" tubing so will have very, very similar yield strength properties. Biggest difference is that it appears that columbus cromor was seamed tubing while SL was seamless but metalurgy is probably very, very similar.

The newer metalurgy and heat treated steel tube set formulations do have higher yield strength but that extra strength is most typically exploited to make a thinner, lighter tube of comparable strength rather than a stronger tube of the same thicknes. If you can find yield strength values for the newer tube sets you can make a comparison though there are many other variables such a how the strength of the steel is changed by brazing or welding heat. Also note that increased yield strength is often achieved by also making the metal less ductile (more brittle). It will be stronger but the failure mode is that it is more likely to crack than to bend once the limits are exceeded.

The above is the same discussion if you were talking True Temper Ox versus S3, S3 has been made thinner by heat treating to gain strength. Methodolgy of jpoining has as much to do with overall strength as possibly tubing selection. What are we talking bike tossing and such; generations of wild eyed Italian racers have been abusing tubing sets with little impact, death, or mayhem.
Perhaps some gentle rider education is in order.

Well, it took about half a century of neglect for the chainstay on this Legnano to rust all the way through (Columbus or Falk tubing):



It still hasn't completely failed...

...and it looked worse then that. Picture a hotdog in the microwave for too long. I just taped the ends back down.

High-strength thin-wall large diameter steel is not necessarily a recipe for dent resistance. In a private conversation, Georgena Terry warned me off of buying a FastWoman (several years back) for my wife. With the thin tubesets she then used, she was seeing more frequent occurrence of beer-canning with those frames. Still a great racing frame for her team, however, just don't dent it or squeeze it. We aren't that careful.

I really wonder what failure modes the OP needs to prevent, if it's not just ALL of them.

Well, after assurances from the group over at VelocipedeSalon as well as here (road fan notwithstanding), I ended up going with Spirit for Lugs. Parts are en route, design is just about finalized, and construction should begin before the end of the month.

Pete

Well now that the serious steps have been taken, I feel free to recomend Tig welded straight wall .035" tubing, at least for the main frame. I wonder what the weight difference would be. About 30 inches of .3mm tubing weight added, but no lugs, it wouldn't be all that huge. Anyway, sounds like your bike will be a lot more cherishable. Maybe she just needs something so nice she takes care of it...