azza_333

So I did some number crunching and worked out the deflection (stiffness) of some different titanium tubes, with a standard length of 550mm, and a force of 91kg. I got the following results;

Diameter 31.7mm, Wall thickness 1.2mm = Defection 31.061mm
Diameter 34.9mm, Wall thickness 0.9mm = Defection 29.922mm

Noting that there is not a lot of difference between the two (Yes I am aware that 31.7mm x 1.2mm will weigh more)
Would there be much difference between the weld strengths and stiffness when comparing 31.7mm x 1.2mm, and 34.9 x 0.9mm, welded to a tube of the same diameter?

For some extra information, the bike will be used as a touring bike, and I am trying to decide on the seat and top tubes. the down tube will be 44.5 x 0.9mm

unterhausen

joining tubes of the same diameter gets a little funny. Both options will be strong enough, I don't think that's an issue at all. The real question is if you will be happy with the ride, which is virtually impossible to answer with an approximate stiffness calculation.

I assume you did fixed free end conditions with a tip load. That's probably a decent proxy for the actual end conditions, which are significantly more complex.

azza_333

How is it funny?

I was just hoping to get a bike stiff enough for up to 85kg rider, and 15kg of luggage. Several websites recommend a 34.9mm TT and ST for "bigger riders" but they don't list what a bigger rider is. They also don't list the wall thickness of the 34.9mm tubes.

If I can I would like to use a 31.7mm seat tube, so I can use a 27.2mm seat post for good compliance. I also don't really want an ovalize the TT, so the TT would need to be 31.7 if the ST is also 31.7mm. This all said if having that compromises the stiffness or strength of the frame, then that for me is a no go. I am fairly confident the the both tubes will have roughly the same stiffness, I am just trying to work out if the welds/joints will be significantly weaker or less stiff using the narrower 31.7mm tubes.

Yep, that's what I calculated.

unterhausen

the "wings" of the mitered tube are very long and goes to the centerline of the unmitered tube. I think most builders do something to cut the miter short, but it's still pretty long. A smaller tube intersecting a larger tube has a nicer miter.

I think"bigger rider" really depends on your style. As someone that is 85kg, but usually not carrying quite that much weight, I don't feel the need for a particularly robust frame. A very strong 85kg rider might not agree.

azza_333

Thanks for the answers, I might go with 31.7 x 1.2mm then.

azza_333

Bugger, they don't have 31.7x1.2mm tubing. I guess 34.9 its is then.

Tourist in MSN

My 2017 Lynskey Backroad has a top tube and seat tube of the same diameter. My very cheap caliper measures 35 mm but the caliper is low in precision, could be 34.9.

There is a shim built into the frame, 27.2 seat post. I think the shim is aluminum, not titanium, I am making that guess based on color of the metal.

The only change in the frame that I can see in their website and my three year old frame is that my head tube is cylindrical.

They have lots of photos on their site, you can see how the welding looks where those two tubes are joined. And they cite more tube data on their website.
https://lynskeyperformance.com/backroad-touring-frame/

I pasted a pertinent photo below from the Lynskey website.





I have only done one tour on my Lynskey, but it handled a load just fine.



That said, if I recall you tour much lighter than I do.

Most of my riding on that bike is unladen. And it rides quite nice with or without a load.

Since you are using a pretty big downtube, thought you might want to see what the bottom bracket shell joints look like. Downtube was ovalized at bottom bracket shell.

azza_333

Wicked, thanks, that puts my mind at easy.