Tandem frame tube sizing

[mrgunner]

I have been going back and forth with my frame builder and finalized on the geometry of my titanium tandem bike frame. However after discussing the frame with my local bike shop they were questioning some of the tube sizes and wondering if it would be too stiff, making it uncomfortable.

My stoker and I weigh 300 lbs (136kg) combined and no more than 325 lbs (147kg) with a fully loaded bike, so not a heavy team.

The tubes that my shop questioned were: TT 44.5x0.9, DT 50.8x0.9, CS 25.4x1.55 (ovalized & dimpled for clearance), SS 22.2x0.9.

This will be the builders first tandem but he’s excited to build one. So, looking for any help with giving some feedback to the builder. Since I’m wanting S&S couplers, the TT is limited to a few different sizes. The next size down would be 38.1.

[Luniz82]

Hey there… Maybe you can contact Biagio at @HILITE , they have had a few titanium tandems built for their customers in the past.

[bulgie]

Sounds like aluminum sizes to me. Though I made a couple hundred tandems they were all steel, so I am not an expert on Ti. We made a few Ti tandems while I worked at Ti Cycles but owner Dave Levy was the designer. ISTR they were more like 1-1/2" DT, 1-3/8" TT, but that was 30 years ago so I wouldn’t be surprised if Dave has changed his designs since then. I know they were ridden hard and raced by large/strong riders, with no complaints.

My own steel tandem might be on the flexy side, and I am a clydesdale+, fast-twitch dominant (sprinter, not a climber except for short “sprinter’s hills”), but I wouldn’t change a thing, I love it to bits. It’s a combo of Max and Max-OR. DT is 1-3/8" .8/.5, TT is 1-1/4" .7/.4. Keel tube is not available anymore, a custom-drawn 4130 in 1-7/8" x .028 (0.7 mm), you won’t find anything that thin anymore, but just as a benchmark you might want to calculate what it takes in Ti to get the same stiffness.

The Max chainstays are 1-1/8" if you re-round them at the BB, as I did, I don’t like c.stays to be oval at the BB. Obviously kept the oval at the tire/chainring clearance point, plus added indents. 1-1/8" untapered would make an awkward joint at the dropout, so your choice of 1" probably makes sense.

“How stiff is best” is a neverending question that you have to answer for yourself, but to me (again, a heavy sprinter), most frames are too stiff. YMMV. But I don’t think you need to worry about too stiff being uncomfortable, since tandems are inherently compliant up/down due to the long wheelbase. Your choice of an “open” frame design, no diagonal bracing between cap’n and stoke, also adds vertical compliance. Stick the fattest rubber you can fit in there, and you’re golden for comfort.

With the caveat that the stoker’s seat is usually perceived as less comfy than the same on a single. That’s due to other factors than tube selection, so mitigate it with fat tires and optionally, a suspension seatpost like thudbuster. Keeping stoker happy is rule #1, even before Don’t talk about fight club.

[DEVLINCC]

Given it’s an open design in Ti, I’d be erring on using bigger tubes. Also, what experience in building bikes has the bike shop had?

[bulgie]

@DEVLINCC

Given it’s an open design in Ti, I’d be erring on using bigger tubes

One test we did at R+E (Rodriguez/Erickson) while I was there about '79-'80 was ride the same frame with two different internal bracing schemes, and then with none (open frame). We actually brazed in tubes, and then hacksawed them out before the next test. Between the ‘direct internal’ and the double-diamond, no one could feel the difference. Not that there wasn’t one, just that experienced riders couldn’t detect it. With the open frame, there wasn’t any more “windup” in the frame like from sprinting or climbing, but there was a perceptible amount of bounce when your cadence matched the natural resonance frequency of the frame. I’d describe it as just perceptible, not at all unpleasant/distracting/inefficient. And that was with old-school small tubes, like 1-1/8" toptubes. I think with modern tube sizes, you’d be hard pressed to notice.

That’s why I think for modern tandems, any mid-tube is just a place for a second H2O battle for the stoker. Not worth the extra weight, but no real harm done if you want one.

I think the open frame has been proven plenty stiff enough considering it’s been in use since at least the 1940s (Singer, Herse et al.) and quite a bit more popular in recent decades.

One interesting factoid about open frames: If the stoker TT is horizontal, parallel to the keel, and the two STAs are the same, you have a parallelogram. If you imagine the joints as pinned, free to pivot up down like a hinge, then I think you can see that as the cap’n seat goes down, the front of the frame is rotating CCW around the front hub (as viewed from the right), but the rear seat is forced to rotate CCW also around the rear hub, which means the stoker saddle is forced UPWARD. The captain being so often heavier than the stoker means cappy might be jamming stokie’s saddle up in her nethers.

No one uses pinned joints in this way, it’s just a visualization aid. And tandem rear frames aren’t usually parallelograms, not exactly. But still I think this might be part of the reason the stoker saddle is usually reported to be less comfy than that person’s single.

Stokie not being able to see bumps coming is probably part of it too. I don’t know what other factors there may be. Could there be a doctoral thesis in this? it’s an important question. Future generations might thank you if you solve this one!!

Or just use some suspension and/or fat tires, problem solved.

[DEVLINCC]

I will put my hand up and say I’ve not built or even designed a tandem so definitely take my comments purely as from the gallery. Just my ‘gut feel’ on where I’d be starting my approach from. Always interested in elarning those that have expereince.

[project12]

Zero experience here with building tandems, but a little bit with the difference between steel and titanium for similar bikes. Going with the same dimensions in both steel and titanium makes the titanium definitely not stiff enough. It might be strong enough but the bike didnt ride nice. So for my Victor fully I went from 38mm DT in steel (Zona or 853) to 44.4 x 0.9 in titanium and for the toptube went from 31.7x0.9 in steel to 35x0.9 in ti. Bikes feel pretty similar where the ti is a little bit more stiff (but that might also be the carbon wheelset or the different yoke in the back.)
If you would come to me for a titanium tandem I might have considered for a 44.4 DT and 38mm DT with your weight, but maybe I would also go the safe route and use 50.8 and 44.4. Either way I think yours will be a sweet ride!