I’ve seen some discussion on here about twin top tubes, and pictures of some very pretty bikes sporting them. I love the look of twin top tubes, especially when they transition into the seatstays. However, I still don’t really understand why someone would or wouldn’t use them for practical reasons.
It seems to me like a twin top tube would have a lower stiffness/weight ratio than a single top tube due to the smaller diameter tubing. I know there must be more to the discussion, and I would love to hear why and when y’all do or don’t use twin top tubes.
(Discussion of twin down tubes or other alternative frame designs is also very welcome)
For the side-by-side tubes shown on the Stooge below I’d say most people would be doin it for the old school looks. Some might argue that there’s more vertical compliance too but any properly built bike frame will have negligible vertical movement within the front triangle since it’s a fully braced structure.
If you’re talking about an over-under double like Riv do then there’s an argument that this re-establishes triangualtion lost in frames with extra long headtubes. Although choosing a stiffer/larger diametre top tube would achieve the same thing so again I’d say it’s mostly done for looks.
Also it’s worth adding that I reckon chosing a tubing arangement for aesthetic reasons is as logical as chosing one for practical ones. Sean Burns (Oddity) is a builder that does a lot of crazy/curvy bikes and a lot people seem to think that there is a performace benefit to it. But he even admits this in an interview (listen from 5:45) that it was mostly an aesthetic/business/branding decision for him to move away from the traditional diamond frame and it turned out to be a pretty good one.
Welcome @zach ! It is a great question. I agree with you: fundamentally, the stiffness-to-weight ratio is less with a double-top tube. You also cannot use butted tubes, which makes it even heavier.
I agree with @bushtrucker 99.99999999% about the looks, but I am all for it.
Here is an amazing twin toptube bike that @Meriwether built. I helped determine the TT bends digitally, and he was able to fabricate it:
Using 3D CAD allowed us to calculate the exact bend and clocking angles. I am still amazed Whit pulled it off. @Meriwether was there ever a complete photo of this monster?
I’d love to see some kind of analysis on compliance done with each type of frame and each type of material. It’s all so theoretical and subjective since it’s so hard to do side by side comparisons with exactly the same bike with exactly the same components built with just one thing different at a time. But one would think a twin top tube would be more vertically compliant, even though with a suspension fork will anyone notice that? That was the idea behind the ovalized top and downtubes of the old Mountain Goat frames and of all those since that have tried the same thing in their own way.
The one I made myself didn’t feel flexy with a rigid fork but did ride very nicely (other than my knees brushing the top tubes since I’m knock-kneed). Another that I built for a customer (Klunkpacker) doesn’t seem to notice any added compliance, but loves the bike and the ride quality.
I think like usual it’s what tires, PSI, fork settings, and contact points you use (Ti bars? Ti layback seat post?) that make the greatest difference in what we feel. I notice that the Ti frames I’ve made and ridden are in general are “softer” than comparative steel frames and float over rough stuff smoother. That’s when I transfer the parts over from one to another but the geometry is usually a little different. It’s easy to make a too stiff steel frame but hard to do that with Ti.
Twin TT is mostly about aesthetics. I always used to drool over Retrotecs!
The Ti frame in that blogpost for Paul isn’t built yet, waiting on a Squid fork from Oddity to complete it. That was a tough one! But couldn’t have done it without the CAD work of @Daniel_Y (without ruining a ton of expensive tubes).
Thanks for all the replies and beautiful photos. @Meriwether and @Daniel_Y that klunkfat looks amazing!
It does seem like mostly an aesthetic choice, which as others have said, is totally valid. I could see some added vertical compliance, but as @Meriwether said, its probably further down the list than tires, bars, saddle, etc.
