Andrew R Stewart 

Break away bike? As in a folding/travel/coupled one? If so i would avoid thin wall tubing. Traveling can be hard on a bike.

As to the difference in tubing between the two riders- Of course the stronger/bigger rider's frame should be made with bigger diameter and/or thicker walls IF ALL ELSE IS THE SAME (as in intended use and riding styles). Andy

guadzilla

It would be a 56 or so.

In case it matters: I am going to be using this bike for travel and climbing, so some stiffness in the BB would be nice (with my weight, i dont spin up hills but grind up at 70-75 cadence). Would that change things?

guadzilla

Yeah, a folding design where the rear triangle hinges. I have a nice, secure case I can pack it in, so not that worried from a travel safety point of view (travel a lot with my bikes and know how to pack them securely). I am, however, unsure as to whether the folding/breakaway design would add more stress to the materials or something like that.

Do you have suggestions on tubing size that might work?

guadzilla

Another update - order for my wife's frame placed. Figuring out the finer points of the geometry, but I had a question re the tubing.

We have settled on:
- TT: 1", 7/4/7
- DT: 9/8", 7/4/7
- English BB

Suggestions for tubing sizing for chainstay, seat tube and seat stays? I would assume a slightly beefier chainstay would provide better transfer while skinnier tubing on the ST and seat stays would provide more comfort. But not sure about what sizing to get that would provide a lively ride without being too flexy.

I'd like to be able to use 27.2mm seatpost, if I can..

guy153

Sounds like it's going to be a sweet bike. I like road stays but they don't allow for as much tyre clearance as the beefier curved ones (which are more intended for MTB, gravel and touring bikes.

I don't think anything is going to be too flexy on a frame that small. Reynolds road SS are 0.6mm wall and the Columbus ones are mostly 0.8 I think. I really like the Reynolds ones and I have a bigger frame size (555mm ST centre to centre).

Cynikal 

For stays, I would focus on the tire and crank interaction not the butting profile. For example, if you want to fit a 50mm tire (or fender) with a double crank coupled with a short chainstay you will need curved stays or some modification of the stays. That should be your first decision point. If there are butting options in the shape of stays that you need, the make that decision. Most likely there will not be many options.

guadzilla

Ah, sorry, I should have mentioned - this is going to be a gravel bike, and so have clearance for up to 40mm tires. And yes, will be run on a double, as 70-80% of the usage will still be on tarmac. Good point re the clearance and curved shapes, will focus on that. Let me see what options are available and I will come back and ask you guys for more advice if need be (we are going with regular CroMoly tubing, fwiw).

Thanks again!