landing massive oversize tubes on a very narrow BSA bb shell

[Crowemagnon]

I've tried them, and they do a job, but I'd like to stop using pf30 bb shells, but keep using bb30 cranks (road , short spindle) and massive tubes (42mm and 33,5mm dt and st respectively).

With the help of a skilled welder, I'm building tig steel bikes (6 so far, all oversize, 2 with pf30 shells, 4 with BSA shells, also with flattened, MAX , or Aero downtubes) and my reason for moving away from pf30 is that I believe threaded bb shells are a better compromise than pressfit ones for my hobby level of building , the practicalities of my workshop, and for bikes that will last a long time.

Just how to make this compromise is proving tough enough for me to ask for thoughts on what I have come to as a possible solution. Here's where I am at....

A BSA threaded bb shell at 46mm wide with two 11mm wide screw-in BSA-to-bb30 adapter cups leaving me with a 68mm wide bb30 'shell'.

Obviously this has some issues....
So my questions are , does anyone land big round tubes on little BSA bb shells without crushing them to fit or if you were in a similar situation , how would you go about fitting these tubes?

I also feel 46mm is a tight ask for chainstays and tyre clearance, but that is something Im willing to bet on being worth the compromise.

Any thoughts appreciated.

Patrick Crowe Rishworth.

[Crowemagnon]

YMaybe I can ask more clearly,

How do people feel about squashing oversize tubes to make them fit onto bb's and headtubes which are smaller in diameter?

When people do this, how do they go about it? Cold?

in the past I have simply Put the tube in the vice with some flat wooden soft jaws extending out one side ( a way up the tube) and carefully wound it up?

can anyone imagine this causing problems with the tube re fatigue?

Patrick Crowe Rishworth.

[bencooper]

I've done it both ways, by squishing or by simply building up a fillet. Either works fine. I'm mostly doing it with 50mm tubes for recumbents.

[Crowemagnon]

All the reading I can find suggests that in low carbon steel, ductility ( plastic deformation) is decreased by bending as work hardening, but elastic deformation is not. which means this cold bending ( crushing the tube ) shouldn't be a problem up untill it begins to fracture. Though I can't imagine getting anywhere near fracture!? Failing any other thoughts, I'll try it and see how it goes.

[Rody]

Patrick, tubin

I've been swaging large diameter tubes to fit on standard bottom brackets since the early 90's, using as large as 2.00" .035 tubing.

I use dies that I've machined that have a taper to gradually move the swaged area back to round as the mechanical action travels down the length of the tube, helping to distribute stress. This is done with the tube in it's normal state, no heating required.

There are plenty of pics on my blog.

cheers,

rody

[David Tollefson]

Considering what we put chainstays through when crimping for tire/chainring clearance, I can't imagine it'll have any appreciable effect.

[Andrew R Stewart]

Agreed that well done the reshaping should have no structural effect. The cosmetics are likely to be trickier to get nice, how one blends the 4 tubes together, that intersect the shell, is where I'd spend a lot of thought. Andy.