TiHabanero

What affect does the length of tubing butts have on the overall lateral stiffness and vertical flexibility of the main triangle? Is the center section wall thickness more of a factor? If butts are too short does the frame have tracking issues such as high speed wobbles or unable to carve corners tightly? Is it better to have DT short butts at the BB shell and not the HT end? Seat tube butts longer at the HT than the ST end?

Andrew R Stewart 

Since the wall thickness has less effect on torsional stiffness the tube diameter I suspect the length of the thin section will be more about weight then flexibility. I seem to remember a similar thread on some forum many years ago with the pros saying much the same. What I do strongly suspect is that the length of certain butts can effect the frame's willingness to buckle on frontal impacts. There are many incidents of frames bending at the butt transition. I'll be interested in reading what those who are smarter then I have to say. Andy

unterhausen

butting was invented because joining methods reduce the strength at the joints, particularly wrt fatigue life. All those effects you list would take a lot wimpier frame than can be built with commonly available bike tubes.

Andrew R Stewart 

To follow up- Some tubes do have different thin section lengths (for the same diameter and wall) then others. If your frame is short enough you could spec a long center section tube and end up with minimal butts. The concerns are that the HAZ might travel down the tube further then the butt section is long. That's a no no. If you're using lugs this is more likely then if you're TIGing. (IIRC when Sachs brought out his tube sets he speced longer end butts with his lugged construction in mind).

Placing braze ons that see some forces on the thin center section is also to be avoided if possible, like DT shift lever bosses. Or ft der mounts. There are plenty of cases where cracks have developed at these locations.

The usual reasons to seek out this kind of goal (longest possible thin wall center sections) is for low weight. The longer you ride, the longer you make frames the more you'll understand frame weight is vastly over rated as a ride quality factor. Use tube diameter as your first level of frame flexibility/stiffness. Andy

calstar 

+1

TiHabanero

Got it. Tube diameter is the dominant factor in frame stiffness. Tube butts and center sections have minimal impact when compared to tube diameter.

Nessism

The moment of inertia relates to the stiffness. Here is a nifty calculator you can play with to compare different configurations of tube diameter and wall thickness...https://www.engineersedge.com/calcul...re_case_12.htm
Basically, the stiffness of a 1.0mm thickness tube is more than double that of a .4mm tube (as one would think) so thickness is a major factor in overall stiffness. So if the butt lengths are longer that means that more of the tube is stiffer and that does affect the frame overall. Mind you frames don't deflect much in the vertical direction but I can say from experience that a frame like ELOS does ride smoother than SLX and ELOS is (obviously) oversized.

Steelman54 

This is spot on!

TiHabanero

Nessism, that is a neat tool. Not sure how to interpret the info, however I so see how changing the wall thickness shows the moment of inertia change, and that it does make a difference. Not sure how it translate to real world application, but it is good to know that the differences in "stiffness"can be striking.