I delivered my first titanium commission about a year ago and got recent feedback from the owner. Overall he has been happy with the bicycle (1x gravel - large frame) but he did mention that he can get a high-speed shimmy (20+mph) when hands are not on handlebar. The only thing I have done differently for this build was internally routed cabling from levers to derailleur/calipers. Has anyone had a similar problem they have successfully solved?
Dims + tubing dims + cockpit dims?
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Thank you for the reply.
Gravel frame stack 626mm reach 398mm. HT/ST angles 72.5/73.
Tubing DT 37.3, HT 44 (47.6 OD), TT 31.8, ST 34.9 tapering to 31.8.
Wheelbase as delivered 1055 (sliding dropout). Gera Curvy fork.
Rider is 188cm in height, 71cm shoulder to palm and mass of about 80kg.
Tyres are Schwalbe G-One 45mm.
Check the balance on the front wheel.
This has only happened to me once, it turned out the front wheel had a heavy spot in it…
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My original thought was that the bike (top tube specifically) might be underbuilt but it doesn’t seem so.
I’m with Kirk: start by looking for equipment issues. That’s an easy fix:
- Reverse the tires (front to rear, rear to front).
- Try a different front wheel.
- Double check the frame/fork for issues.
- Has anyone checked the headset?
I’d also be asking the following:
- Does the bike shimmy when the sliders are all the way forward and all the way back?
- When the shimmy appears is the rider sitting up and back?
- Does the shimmy appear if they keep their body in a forward position and just remove the hands from the hoods?
- What happens if you place a fully loaded handlebar bag on the front and the rider goes hands free?
Who is riding no-handed at 20mph???
It seems like a pretty big frame. I assume the DT is butted? I would have gone with a 42mm DT and 35mm TT if these were butted.
The beam deflection equation is a L^3 relationship, where L is the length of the beam. Larger frames get much more flexy:
That being said, some people get shimmy, others don’t. I think the rider-in-the-loop has a lot to do with it. Even if the hands are not on the handlebar, you steer with your body’s micro-movements. Frame stiffness is the only thing we can control.
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I had this discussion with a framebuilder friend of mine at some point. He had the same issue. A frame style he’d built before, but this one rider got a shimmy where others didn’t. This was a steel frame though.
The issue was eventually solved by brazing in a tube brace low in the front triangle, between the DT and ST.
My theory is that the small change in frame stiffness changed its resonant frequency enough to bring it out of the speed band where the rider would often find himself, and where a shimmy would previously occur.
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