Montague FIT folding bike with “snaking” frame

Question

My 2012 Montague FIT folding bike has started to “snake” especially when riding at speed downhill. I have tightened the frame quick release and inspected and tightened the pivot nut but the snaking still persists.

The wheels are ok and there is no play on the bottom bracket yet if I put weight on the pedals I can get the frame to shift/“snake” sideways.

When cycling up hill there is no issue but there is slight snaking on the flat which gets worse when gathering speed downhill.

Any explanation and resolution would be much appreciated since in the U.K. there is a dearth of Montague outlets and repair shops willing to service/repair their folding bikes.

Answer 1

I think you'd benefit from an assistant here. Get someone technical to look closely around the bike while you lean on the pedals.

Based on "it gets worse speeding downhill" the problem is speed-related - that suggests wheel axles/bearings. Check for horizontal play on each wheel at the rim.

Another possibility is a frame crack which has weakened a specific point, but not enough to fail yet. Wash the bike, then inspect all over for cracks. If you find something questionable, put your hand across the crack and flex the bike as you have done. The sides will move subtly-different and you can often feel that.

Good luck !

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Additional: I've recently ridden a folding bike that had a sloppy pin in the main hinge. It rode fine until one brakes hard when the hinge area "bucked upward" like a rearing cat, a particularly horrible sensation that made me walk.

Check your frame pivots for play in all directions and adjust it out where possible. If the pin is worn or the holes are enlarged, you may need replacements or bushings.

Answer 2

TL;DR: This bike looks scary as heck and I would not dare to ride it.

This bike is unstable by design. The entire front part of the frame, where normal bike would have a triangle, is much more flexible than a triangle would be. Because of this flexibility, even smallest steering and braking forces bend the frame, and trying to compensate for the flexing can bend the frame in other direction, which leads to the snaking. The snaking when pedaling has the same reason: the frame isn't rigid enough to withstand pedaling. The wheels shown in the picture are very flexible and make the problem even worse.

If the bike used to be more stable than it is now, the frame is probably cracked. I would not ride it. The manufacturer has probably found a way out of any responsibility, and I think most repair shops don't want to touch it to avoid being held responsible if someone injures themselves with it.