Here’s a quick 60 second guide to bicycle butted tubing.
Excluding carbon bikes the weakest point on any bicycle frame is where the various bits of tubing are joined, either by lugs or TIG/MIG welding. The thicker the tubing is at the point of the weld or lug, the stronger the frame will be. But if the tubing was of the same thickness across it’s length it would be too heavy.
The solution is butted tubing which decreases the thickness or gauge of the tubing in the center whilst retaining it’s thickness at the ends, where the lugs or welds are.
Bicycle Butted Tubing
The process of butting is expensive but enables a frame to be strong and light. It’s made stronger by having a large welding face at the ends. But, it’s also made lighter by having a slim gauge in the middle where the extra strength is not required.
There you have it. Bicycle butted tubing is a great bicycle technology! For more information check out this page by Reynolds.