big steel frames: butted vs straight gauge
I'm 6'7" 225lb, and looking at 68cm (27in) used bikes. When I find stats on some of them, the smaller sizes are butted, but the largest size is single gauge. I'm a little confused, I thought butting made the frame stronger? Why go straight gauge for the biggest frame? Or is it that they are taking material away from the center of the tube, not adding material to the junctions. Which is better for a big steel frame?
Last edited by tawlly; 07-09-11 at 11:10 AM.
Butting doesn't make a tube stronger, it makes it lighter for the same given strength. For example, if two frames, A & B, are equally strong, the one with butted tubes will be lighter. The problem with taking that information and applying it to real world examples is that you probably won't be able to determine whether the two frames are equally strong in the first place, without being a metallurgical engineer, or having one as your best friend. If you can find some manufacturer-provided info about the frames' load-bearing capacities (and you deem it credible) and they're comparable, go for the butted version.
It would be easy to make a decision about this sort of thing if we weren't Clydes, who are forced to play on the extreme edges of frame structure. In the fly-weight world, it's generally considered to be true that straight-gauge tubing (or heaven forbid, pipe) is not as high-quality as butted or double butted tubing. But as soon as you start dealing with 250-400 pound loads, those assumptions go out the window.
Would you say that a butted chromoly tube in a big size is likely to be more "flexy" than straight gauge chromo, all else being equal? I guess I'm also wondering if the biggest vintage frames are single gauge because of cost, or for added strength?
I don't know enough about structural capacities of tubing to say. If the ends of the butted tubing are the same wall thickness as the straight-gauge tubing, it seems to be common sense that there might be a little more flex in the butted frame. But then again there might not, since most of the stresses take place at the joints, which is where the wall thicknesses of the two frames are the same, and not in the middle of the tube's span (which is why butting and double butting work in the first place). Sorry I can't be more help.
tawlly, The larger the frame, the more flexable it is when compared to a smaller frame of the same material. Butted tubing is good for length wise strength, not as good for axial strength. At some point the engineers have decided that butted tubing isn't practical for a specific tubing length of a particular frame material. It can also be a decision made by the accounting dept. to not spend the extra cash on a frame size in little demand. Either case maybe true, but I choose the former.