Reasonable minimum distances above TT and below DT (on HT)?

[hans736]

I would appreciate some feedback on how how small of a distance is reasonable on the head tube above the top tube and below the down tube? In the following drawing these distances are marked with blue circles and through the dark red dimensions: https://ibb.co/FsC1cKc

It's a titanium frame, 48mm OD head tube, zs44 headset.

Context: I want to maximize the space between the top tube and the down tube because I would like to put a hydration bladder there which wouldn't fit properly if the space is too small. On the other side I guess that welding too close to the bottom and the top of the head tube could warp the head tube where the headset bearings sit and thereby increase the probability for high-speed shimmy. This warping risk is what I thought is the limiting factor to making these distances smaller. True?
I am not welding the frame myself. A manufacturer will weld according to my specifications. I could just ask this manufacturer what is reasonable which I will do of course. But they may have an incentive to just say something with an unreasonably high margin of error (to guarantee no warping) because there is no downside for them to a high margin of error (i.e. they don't care about my water bladder goals) but a potentially large downside if they do mess up the head tube and have to throw away the tubes they have already welded together. Similarly: It will probably also be quicker and easier for them to manufacture the frame with larger distances above the top tube and below the down tube.

Have you tested out in practice what the (reasonable) limits (smallest distance) are here?

[shand]

I think in situations like this you have to go with what your builder says. What everyone else thinks is irrelevant. For reference, I shoot for 12mm because I think it looks right. I think 10mm would work too. But if someone came to me and said can you make that distance smaller because some random people on the internet said you should/could then I'd probably walk away from that work.

Having said all that, are you sure you can't get that room somewhere else? Obviously you have a zero stack headset already but switch stems and increase the headtube length?

[hans736]

I think in situations like this you have to go with what your builder says. What everyone else thinks is irrelevant. For reference, I shoot for 12mm because I think it looks right. I think 10mm would work too. But if someone came to me and said can you make that distance smaller because some random people on the internet said you should/could then I'd probably walk away from that work.

Having said all that, are you sure you can't get that room somewhere else? Obviously you have a zero stack headset already but switch stems and increase the headtube length?

Hi Steven,

I agree with your point in general about having to rely on the manufacturer. But it's still helpful to get a reference like the one you provided (12mm). I have seen this distance range between 6mm and 15mm. So I was wondering what an appropriate minimum is.

I can't get this space from elsewhere: the frame is based on bike fitting measurements and I designed it around a Vecnum suspension stem that has a 3° upward angle which makes my head tube smaller and the fork height also cannot be decreased for tire clearance reasons.

[TonyP]

Would it be feasible to cut the head tube down after the frame is welded up?

[Andrew R Stewart]

I have used 10mm for the top and 15mm for the bottom, end of head tube to edge of top tube /down tube. At one time I measured some lugs and got these dimensions. But with good design there's no reason why these dimensions have to be any specific amount, just what the design calls out for.

As to welding close to the head tubes ends and distortions I would hope the welder you pick is already up to speed with these details and knows how to correct for any distortion (thicker walls, heat sinks, after joining machining). If in your discussions with the "manufacturer" they seem to not be able to describe their method to mitigate I would take that as a red flag.

I have no recent experience with Asian contract builders but the second hand ones and stories from back in the 1970s have had me be quite careful in how much I would assume is being taken care of by the contract fabricator and how much HAS to be called out by the designer. Back then if the design or contract didn't include really tiny details the product you got might not be what you expected. Andy

[Andrew R Stewart]

Sure. I build with the head tube extending beyond the lugs or reinforcing bands. I have a lower locating cone on my jig that is sized to have x amount of HT (20mm in my case) more than the final frame calls for below the HT, the upper end doesn't need to be positioned like the lower end does WRT geometry and such. Many jigs use a puck, not a cone, to positively locate the HT lower end and these jigs would need some modding to allow the HT to extend past the designed end point. Andy

[hans736]

I thought about this again: does welding near the top/bottom really carry a risk of greater warping? If not then it's more a matter of convenience and skill. The head tube wall thickness is greater near the top and so there is less risk of warping. The one reason I could think of why welding near the top carries a higher risk is that this is where the bearings and so this is the only area where warping has any negative consequences.

[shand]

I thought about this again: does welding near the top/bottom really carry a risk of greater warping? If not then it's more a matter of convenience and skill. The head tube wall thickness is greater near the top and so there is less risk of warping. The one reason I could think of why welding near the top carries a higher risk is that this is where the bearings and so this is the only area where warping has any negative consequences.

The warping thing is a bit of a red herring. All tubes will warp when welded. That's why the headtube is reamed and faced post weld. If you have really thin wall tubing and a lot of distortion, then sometimes reaming the tube will result in a wall that is too thin. But that's an extreme example. Welding near the end of the tube is trickier as there's nowhere (or not as much area) for the heat to dissipate, This could result in more distortion or in extreme cases blowing holes in the edge.

It's also worth pointing out that a lot of headtube material is plain gauge, so not thicker at the ends.

Does your fitting info take into consideration that those stems also require a decent amount of space under the stem? I recall when I looked at them, you needed spacers under the stem to stop the linkages hitting the headset ot top of the headtube.

[hans736]

The warping thing is a bit of a red herring. All tubes will warp when welded. That's why the headtube is reamed and faced post weld. If you have really thin wall tubing and a lot of distortion, then sometimes reaming the tube will result in a wall that is too thin. But that's an extreme example. Welding near the end of the tube is trickier as there's nowhere (or not as much area) for the heat to dissipate, This could result in more distortion or in extreme cases blowing holes in the edge.

It's also worth pointing out that a lot of headtube material is plain gauge, so not thicker at the ends.

Does your fitting info take into consideration that those stems also require a decent amount of space under the stem? I recall when I looked at them, you needed spacers under the stem to stop the linkages hitting the headset ot top of the headtube.

Hi Steven,

thanks for providing some context regrading head tube warping.

The specific stem I plan with (https://www.vecnum.com/en/products/freeqence) does require about 13mm of spacers below the head tube for their 90mm length. But the 105mm length doesn't require any amount of spacers acc to the manufacturer, see https://www.vecnum.com/en/products/freeqence/servicefaq

[steve garro]

I like 13.
it just looks right unless you need to avoid something like a suspicion fork knob, then you adjust as necessary.

13mm gives you enough room to lay down a nice fillet - BITD the guys who made tons of fillet bikes like TET or Ritchey would take it right to the edge so you could clean it up real quick with a dynamic-file - when you faced them not uncommon to take some bronze with you but it's all good

- Garro.

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