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Discussion over in the families and kids forum got me thinking -- has anyone ever tried making a frame with multiple headtubes with a short piece / pieces of tubing welded in between as a spacer to make one frame "fit" more than one size?

For a smaller rider / smaller wheel size, you'd put the headset and fork in the headtube closer to the rider. As they get older you could move everything to the front headtube to increase the reach & stack.

A well-designed insert could let you use the spare headtube as storage for a multitool or snacks.

Is there something I'm missing that would make this a terrible idea or reasons why I can't find any instances of anyone trying it?
 

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Interesting concept.

Here's a pic from the link hillm posted. Holy what the?!?!

quad_headtube.jpg
 

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yeah its cool. I think he built it to be able to test out different geo's. Also has some sort of analogue read out that gives some information about the riding. I cant remember where i read that so i could be totally wrong hah.
 

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Yeah, I can't figure out everything that's going on in that pic. It looks like there's some kind of push-pull device that goes from the steering to the rear end but for what? Also the second chainring on the NDS, what the what? Don't forget the 12 different front axle positions, not to mention the multiple headtubes. Oh the possibilities...
 

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Cycologist
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Looks like the second chainring on the NDS drives a roll of paper that is recording like a seismograph, providing info on flex on various tubes of the frame.
 

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Looks like the second chainring on the NDS drives a roll of paper that is recording like a seismograph, providing info on flex on various tubes of the frame.
I don't think it's frame flex. I think it's recording steering inputs. But the mechanism seems pretty complicated to do so, so I think it's related to stability of riding the bike with different geometries. It almost seems like there's some sort of steering damping of sorts going on, as I don't think the stem clamps straight to that intermediate steerer tube of sorts.

What I can't figure out is the lever arm that drops vertically down from the seatstay lever. It looks like it's connected to the same apparatus that records steering inputs, but why, I dunno.
 

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What I can't figure out is the lever arm that drops vertically down from the seatstay lever. It looks like it's connected to the same apparatus that records steering inputs, but why, I dunno.
Is it dragging the ground to try to detect if the rear wheel leaves the ground?
 

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Looks like it measures bike lean and steering yaw.

The design itself seems to decouple HA from all other things that people attribute to HA: reach, ETT, cornering feel, steering directness/stability, front center, etc.

For example, someone might ignorantly say that they wouldn't ride a 90 degree HA bike, as they fear it would be much more OTB prone and twitchy. That is more a function of a too short front center (distance between the BB and front axle). The twitchiness is a function of the front wheel's "mechanical trail"--it has multiple front wheel mounting locations (fork offset) to adjust the steering feel. Having the contact patch behind/trail the fork's steering axis gives it stability.

In essence, the HA's role is reduced to determining the front axle path. If it were a telescoping fork, any force acting on it that isn't along its axis would be translated to harshness and flex.
 

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The twitchiness is a function of the front wheel's "mechanical trail"--it has multiple front wheel mounting locations (fork offset) to adjust the steering feel. Having the contact patch behind/trail the the fork's steering axle gives it stability.
On this subject, would I be right in thinking that the bike would handle the same (assuming flat pavement) and not know the difference of wether the fork is completely in front of the front wheel and offset to the rear (as pictured) as opposed to a typical steel fork which is either inline or offset to the front of the fork? (this assumes the wheelbase is the same)
 

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As a side note I like that the steering looks to be connected from the steer tube attached at the handlebars to the forks steertube by a belt under the lower headset cup.

Not too often you have to clarify which steer tube and headtube you're referring to.

Next up, steering axis, inclines, declines, wheelbase, and.. OH Dammit I just burst a vein in my brain.
 

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Cycologist
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Looking at the linkage at the top of that vertical rod, it is designed to move vertically and then translate that movement horizontally when recording on the paper. I'm starting to think this was just an art project.
 

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Back to the original question of a bike with two headtubes to accommodate growing riders...

Some company is already doing something like this, but using an oversized eccentric headtube similar to eccentric BB on tandems. This lets you stretch it out by rotating the headtube insert. I can't imagine you get much more than an inch or so of adjustment, but that can make a big difference. I think that two headtubes would just be too heavy if you built it stiff enough, and if you adjust the front that much you need to adjust other things too, so at that point you just need to get a different frame.

I actually have built kid bikes with eccentric BB specifically to adjust BB height as the kid grows. But the impact of an eccentric is pretty low, maybe 100g heavier and probably makes the frame stiffer if anything.
 

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