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Large dia steel frame tubes: good, bad???(6 posts)

Large dia steel frame tubes: good, bad???RedPat
Dec 29, 2003 9:37 AM
What do you all think of steel frames with thinwall large diameter tubes? How does it affect the ride? Are they worth the additional expense (from a custom builder)? I'd like to hear your thoughts on this. I'm planning on getting a custom frame in the next year and I like the look of larger diameter tubing. I also want a light bike (I weigh 150 lbs) but fairly rigid, more a climbers bike than anything else. Thanks!!
Some thoughts...Nessism
Dec 29, 2003 4:35 PM
The stiffness of a tube will increase exponentially with increase in diameter. Because of this, people looking for a stiff frame should look for large diameter tubes. The stiffness will also increase with tube thickness, but to a lesser degree.

One nice thing about going with oversized thin-walled tubing over standard diameter thicker tubing is that for a given tube stiffness, the oversized tube will be lighter. There is a downside to going too thin though; they will dent more easily than thick tubes.

So the question is: how stiff, how light, and how durable do you want? A good builder should be able to juggle the factors to give you what you want.

I'm in the process of specing a frame right now using super-oversized tubes; 1-1/4" top and seat tube, 1-3/8" down tube, 17 mm seat stays and 24 mm ovalized chain stays. I'm using Foco MTB tubing for most of the frame, and because of the thin-wall thickness, I don't think it will be overly ridged (hopefully). I'm actually thinking of using a slightly thicker Zona down tube for more stiffness. I weigh 165 lbs. and like stiffness in the bottom bracket. I don't think a stiff frame makes one faster, but it feels good to me - thus my tubing choices.

Good luck.

Ed
I don't think that the larger diam steel tubes are only about abill
Dec 29, 2003 4:37 PM
aesthetics -- the increased diameter serves the same function as on alu frames. The design permits a stiffer ride on a lighter tubeset. If the tubes were of the same diameter but of a lighter wall, than the tube would be lighter but flimsier. Others know more about this stuff and all the parameters, and I'm undoubtedly confusing concepts, but I think that's all you really need to know.
Even so, as has been said many times on this board, it's about the design, and generalities go out the window pretty fast. If x tube is big but is shaped here, it's going to ride differently from the same wall thickness and same diameter but butted or shaped there. Some of it is almost entirely theoretical, but most of it's true enough and accounts for the variations you see in ride.
And then there are some that think that the differences are almost entirely from the fork, and that's a whole 'nother can of worms.
Denting is the issueKerry Irons
Dec 29, 2003 6:08 PM
Back in the day, the saying used to be that the tube wall had to be greater than 1/50th the tube diameter to make the tube resistant to "beer canning" on impact. Obviously, this is just a rule of thumb, and presumably better steel alloys can push this limit (I don't know which physical property would have to improve, nor how to relate that property to the wall thickness/tube diameter relationship). This is the advantage of lower density materials (Ti or Al) in that they can have a thicker wall for any given tube diameter and still maintain the same weight.
Yes, but modern Al is more prone than steelNessism
Dec 29, 2003 8:02 PM
Steel is not the only material with denting issues. Thin walled aluminum tubesets are even more prone to denting than steel.

A tubes resistance to denting is dependent on its thickness, yield strength, and (I think) it's hardness.
To look at a materials denting resistance, we must look at these characteristics.

For example (thickness is for center butt on top tube):

- Columbus XLR8R- 0.6 mm thickness, Rs = 510 N/mm2
- Columbus Starship- 0.7 mm, Rs = 380 N/mm2
- Columbus Ultra Foco- 0.38 mm, Rs = 1100 N/mm2 strength

While Ultra Foco is the thinnest, it's also the strongest, so in the end, it's more dent resistant than the aluminum tubesets listed.

Ed
Yes, but modern Al is very thinKerry Irons
Dec 30, 2003 4:39 PM
I agree, most "modern" Al frames are striving for superlight, and so are even more dent prone than the thin walled steels. Given that Al density is rouughly 1/3 of steel, Al tubes could be 3X the thickness of steel and have the frame be the same weight. I don't know how dent resistant such an Al frame would be, but you're right about Al tubes in the 0.6-0.7 mm wall thickness - VERY "dentable." Back in the day, the standard (and pretty dent resistant) steel tube was 0.6 mm, and now there are Al tubes of that thickness. You don't have to be a rocket scientist to realize how much more fragile the new Al tubes are.