Tube butted section length?

[Mr. Hub]

Hello first of all,

I'm Stefan from Germany and I'm pretty new on that forum. I started framebuilding a couple months ago. After riding old/new road bikes and some track bikes, which I build up by my self. Here is my humble blog by the way.

Well, I'm asking myself how long the butted sections of the tubes should ideally drop out? For a strong longlasting construction. The columbus tubing charts are recommending generally 2 3/4'' till 3 1/8 inches for the tt, dt sections. But how about the seat tube? I have on my seat tube a butted length of round 6''. Because the seat tube/ bottom bracket area is a pretty claimed spot, especially while pedalling and on track frame intended. Can I decrease the length a bit when I constructing smaller frames? Because of the length-rigidity ratio?

Hope some one can help me out.

Regards,
Stefan

[Andrew R Stewart]

Stefan- Most companies or their distributors provide the butting profiles that they aim for. Most of the time the actual starts and ends of the butts are very close to the claimed dimensions. But sometimes things "go wrong" so measuring is the best policy to insure you cut the tube where it makes the most sense. You can make a fancy Butt Gage or just clean out the insides of the tubes and by looking down the insides look for the change in reflective nature of the tube walls. A rule or spoke can be inserted down the insides and used to indicate the change of wall thickness. Using a caliper to check the wall thickness at the tube's ends, weighing the tube and balancing the tube at it center point will serve to corroborate the butt profile.

As to how long a thick section you should try to maintain, it depends. Welding by a skilled guy allows less total heat to be put into the tube so the butt length could be less. Some tubes are profiled for this. A lugged or filleted joint by a slower guy might see a lot more heat. So a longer butt (and a thicker wall as well) might be better here. In an ideal world the thick section might extend past the lug tip (as example) by an inch or two. Maybe two or three inches beyond a fillet joint.

But real life doesn't always fit this. Being a shorter guy and married to a short woman I find that i need to cut the TT or DT a bit less then ideally. The current S&S coupled frame I building pushes this even further. This is why good skills and practice/experience is a must when building for others. Andy.

[cfrisia]

have you seen this?

Bob Brown Cycles: analysis

looks like the biggest HAZ is extending to about 0.5"/13mm from the joint.
This does not apply for larger fillets, however - but it gives you a qualitative idea - depending on you construction method - e.g., that the HAZ is bigger with lugs but less critical.
Another indication are TIG-specific tubesets, where the butts are considerably shorter than in tubesets for lugged construction.

regards from cologne

[veryredbike]

A while back a well known builder responded to my post on this... I THINK it was Dazza. Not entirely sure.

They told me that they used 40mm past the lug tip for downtubes and 20mm past the lug tip for top tubes. I tried to find the post, but couldn't. Sorry, so vague, but I figure it pro[vides a bit of an extra datapoint.

[Mr. Hub]

Interesting, rather exploratory! Thank you!

Regards from Lower Saxony