Hey all,

I'm pretty tall, and even though I love the look of shorter head tubes, I'm forever destined to ride big frames with tall head tubes.

I'm interested in how much added flex is added with a taller head tube. Specifically, I want to know if different head tube materials drawn to the same thicknesses would make a frame intolerably stiff/flexy. I would love to have a stainless steel head tube, but I don't know how stainless compares to 4130 in terms of flex.

Assuming they are drawn to the same thicknesses, would a 9cm SS head tube give more or less flex than 4130?

Sorry to ask so many metallurgical questions lately. Just trying to talk myself out of some of my crazier ideas.

All steels from 1010 carbon steel to ultra high strength stainless like Reynolds 953 have virtually the same "stiffness" (E= ~200GPa), so a head tube of any steel alloy with similar length, diameter and wall thickness will be equally stiff.

Thanks, that is great to know. I'm assuming this is true of stays and fork blades too. It seems like this is a good alternative to chromed socks on a bike, at least if working with SS is work the extra trouble.

Yes; it's true of all the tubes in a frame and fork. The higher strength alloys - because of their higher ultimate tensile strength and yield strength - can be drawn with thinner walls than lower strength alloys like plain carbon steel, and thus can be lighter because there's less material. The thinner walls, though, will make the tube flexier. Going to larger diameter tubing like oversize (OS) or double oversize (XL) restores stiffness lost due to thinner wall thickness while still saving weight.

Thank you. I was aware of all of the above concerning other steels (4130, Hi-Ten, Reynolds 531, etc). I wasn't sure with Stainless though, thinking it might be more metallurgically different. Very good to know.

I'll add to this in that being the largest diameter (and for that matter the thickest walled usually) tube (don't forget that it's also the shortest tube) head tube flex is more in the mind of the marketing department then in real life. The head tube is the stiffest tube on a bike.

Now if you're good with making claims about possible secondary aspects of having a large diameter head tube (like a large diameter steerer, down and top tubes) and calling their resulting contribution to a stiff frame because the head tube is stiff, fine. Andy.

I'll ask two more related questions then:

I've heard it's very unforgiving to braze SS lugs to steel tubes. Is this true the other way around (4130 lugs to SS tubes)?

For a 1" steer tube, the standard head tube OD seems to be 31.7mm which would make the ID 29.5mm with 1.1mm wall thickness. Would it be okay to take a thicker tube (32mm, or 1.25 in) and have it reamed out to 29.5 ID? The walls would be slightly thicker. I'm struggling to find SS head tubes in stock right now.

To allay anyone's fears, this is all hypothetical from me. I'm entertaining the idea of replacing a cracked head tube on a frame, and I know using SS is downright stupid for a first-timer.

I agree that using SS on structural aspects of a frame without plenty of practice is foolish. There's nothing wrong in theory to use a thicker wall tube then usual for a head tube. This is sometimes done when fillet brazing to help reduce the distortion from the heat. But many would say there is a practical limit somewhere. A 1.25 tube is really about 31.75mm, only .05mm more then the 31.7 you mention, it's commonly referenced as 31.8 and often components that fit around one are labeled as 32mm. So take published "sizes" with a grain of salt.

SS isn't the easiest to machine so after the build head tube reaming will be harder on your tooling, shop grade cutters won't last long without care. SS is more difficult to braze, then 4130. It doesn't like bronze filler and the heat take up is also different. So when joining SS and non SS, especially with differing masses, the temp control is critical. SS's need to form an oxide is very strong and one's flux only goes so far.

I would strongly suggest that you don't worry about sourcing a SS head tube (or three) right now and instead just get some generic SS tubes and start torching them. Then do some destruction testing (very important with the steering end of a bike) before you get any real bike tubing.

BTW For a bare tube to really look good it needs a lot of finishing. Polishing brazed SS is a lot more involved then one thinks at first. Especially if your brazing needs any (as in ANY) filing first. This is another aspect that you should find out with a practice or four piece. Andy.