Which style seat tube do you use for your short chainstay bikes and why?
OFFSET
To me this seems easier, cheaper and aesthetically I like it better. I can understand that in theory it might not be as strong but you’re also adding a lot more weld surface to the ST-DT junction which might balance it out.
Clearly there is a limit to how far you can offset the st on the bb but what is that limit? Has anybody tested this? With a T47 bb and offsetting the tube beyond the shell like this Allaince it seems like you could achieve about as much offset and you might want.
I’ve been going with offset for my carbon seat tube/mast frames but for sure prefer bent when possible! Imo it just looks nicer and it seems smart to have as much seat tube as possible in contact with the BB.
This one has 13mm offset and lands on the down tube. I think it’ll be fine but it doesn’t give me warm fuzzies:
I prefer straight seat tubes and will offset the tube if I can get away with it, but I won’t adjust chain stay length just to run a straight seat tube. And I won’t place a seat tube directly on the down tube because those will eventually crack.
Kevin, do you know if the Alliance frames connect the DT to BB first and cope the ST around the DT? I guess that’s the way to get more offset for the ST.
See I love how it looks! Once the chainring covers it you don’t even realize it has an offset. If I were to go the bent route, I’d do a tight radius bend and keep it as low as possible so it’s hidden by the chainring.
I’m not sure if Erik welds the DT or ST first but I reached out and will follow up when I hear back. On the full suspension he built me, he also offset the DT a bit beyond the BB as well.
Mountain bikes typically need >20mm of offset, which does not work well with offsetting the seat tube.
For gravel bikes, the bent seat tube limits the tube you can use. Some seat tubes have a thinner butted section in the middle, which can kink during bending. This is not a problem with the 34.9 seat tubes because the bottom portion is straight .9mm thick.
Structurally, I don’t think there is much difference. If anything, bump up the BB thickness. It’s mostly an aesthetic choice between bent and offset.
Have you seen this happen a lot? I’ve only ever seen one photo of such a failure, but I hear people say this a lot so I am led to believe that it’s pretty common. If you have photos of the DT cracking as a result of this construction method could you share them here? I’m curious to see what the failure looks like/where exactly it occurs.
A friend’s bike cracked doing that. I feel like I’ve seen other examples of it too. To me it doesn’t make sense transferring more loads to a .9mm tube when there’s a perfectly thick BB shell to take that on.
The way I see it, purely intuitively, is that a seat tube joined to the BB shell will ultimately be a stonger join.
A T47 BB shell from PMW will have a wall thickness of 1.5mm whereas a DT will likely be thinner. For comparison, the thickest wall Columbus (Zona) DT is 1mm and my brain-FEA visualisation tells me that it wont’ be as strong. How much the difference is I don’t know, but it’s also something I’d prefer to not find out the hard way.
I’d personally always avoid mitering a seat tube to the DT, so if going for an offset solution I’d rather have the ST “hang off” the BB at the front as long as it’s by let’s say no more than 1/3 ST diameter (which also gives a handy internal opening for maybe a dropper cable) and miter the DT to the BB and ST.
I’ve recently drawn up a frame for myself based on some leftover tubes I have sitting in the garage. Since I have no means of bending tubes myself, and I want to avoid the offset, I’ll instead be getting an elbow 3D printed from stainless steel and I’ll slip braze two sections of straight cromoly to it. That way I end up with zero offset at the BB shell.
On the first one I did, I welded the little seat tube stub to the BB then the down tube to that. But that distorted the socket significantly so this time around I’m welding the DT on first and the ST stub atop that and the BB.
This is my thinking as well, That 1/3 measurement is exactly what I’m after. I’m wondering if anyone here has gone to that limit or beyond. That would equate to the offsets below.
BSA x 28.6 ST = 14.3mm Offset (@ 1/3 Overhang)
T47 x 28.6 ST = 20.6mm Offset (@ 1/3 Overhang)
BSA x 31.8 ST = 10.6mm Offset (@ 1/3 Overhang)
T47 x 31.8 ST = 20.1mm Offset (@ 1/3 Overhang)
