6

What are the signs and symptoms of a bent mtb frame? Or bent forks? What does it feel like when riding it?

3
  • 1
    If you post pictures of your bike, especially any part that might be bent, we might be able to confirm and offer solutions
    – David D
    Sep 18 at 15:54
  • 1
    I thought I had somehow badly buckled my rear wheel. The bike was wobbling significantly and pedaling felt spongy. Turns out my frame was broken. The point is if you think it's bent you should check for that by examining the frame not going by how it feels to ride.
    – Eric Nolan
    Sep 19 at 9:05
  • Same experience as @EricNolan with a broken frame at night - so I didn't notice the frame was broken in front of me and I suspected that the problem was in the front wheel, the fork or the stem. However that wobbling started when one frame tube was already broken, so not actually an early warning sign. At most it warned that a few seconds later a second tube would broke and I would end with two separated half bicycles (and fortunately no worse consequences).
    – Pere
    Sep 19 at 21:30

4 Answers 4

6

Don’t count on any warning signs.

A few possible signs of impending frame/fork failure or a bent frame which are all pretty non-specific:

  • Creaking
  • Soft feeling when pedaling or going over bumps (especially if the rear is damaged)
  • Strange steering behavior if a bent/cracked fork or frame changes the steering angle slightly
  • Veering to one side
  • Shifting not working correctly or it changes gear when putting a lot of force on the pedals because the frame is bending/twisting and changing effective cable length

It’s a good idea to look over your frame and fork occasionally and check for any cracks or bends.

3
  • Can confirm that a frame close to failure in the rear triangle can change gears on you unexpectedly Sep 18 at 20:23
  • Yes when I'm pedaling hard there's a wierd grinding crunching like sensation as though my pedals are attached to a mill in sandy gravel. Then my gears in the rear start to slip. My back tire seems to want to slip when riding on dirt, gravel or anything other than smooth concrete even though its a mtb. Plus I've gone through 3 rear tires. They're straight and true when put on bike but before long they wobble until its so severe I have to replace it once again
    – Alison
    Sep 19 at 7:49
  • @Alison: This could be many things, not necessarily a damaged frame. Maybe it’s just a combination of worn pedal or crankset bearings and the wheel being out of true. If you have rear suspension it could also be worn bushings.
    – Michael
    Sep 19 at 8:08
5

We can infer from Jobst Brandt's article that the bicycle will not track straight and will always veer off to one side if ridden no-hands if frame or fork are out of alignmnt.

See: https://sheldonbrown.com/brandt/align-fork.html

See: https://yarchive.net/bike/frame_bending.html

His repair suggestions are for steel bikes and shouldn't be attempted on alu, carbon or titanium.

5

FRAME I broke a bike (several bikes) at the seatpost junction.

enter image description here

This started as an odd-noise - an occasional tappy sound, and a general lack of speed where everything felt sluggish, like there was brake drag. Then I felt a clonk and the bike settled - I thought the seatpost was slipping down. This resulted in a long walk to work.


FORKS I had an 80s road bike which turned out to have bent forks. It wasn't visible unless you laid the fork crown flat and the measured the height of the dropouts from a flat reference surface. The rim brakes were about centered when assembled.

The symptom was if the bike ever hit a bump like a stone, stick, crack or crossing a ramp then the bike felt like it had "stepped to the right" If it was a MTB you'd wonder if the front had slipped off a rock, requiring some effort to hold your balance. Not ideal if pedalling at full road speed, and I got that thing up into the 40 km/h range.

I did repeated checks looking for cracks and other issues like this, but the only way it was diagnosed was that one dry day I rode through a shallow puddle then onto dry road. I glanced back (perhaps I was about to turn across traffic?) and noticed that the bike was leaving two distinct wheel tracks of water on the dry road.

Upshot was that under a sudden load, the fork acted like a spring, loading up one side and throwing the front of the bike to the right.

You can just make out that the for tines are bent back (relative to the headtube) in this photo, and that the wheel is turned slightly less than the fork tines/legs.

enter image description here

Also notice the rust on the top tube and down tube, around 2 inches/50mm aft of the head tube. This is characteristic of a frontal impact that has tweaked both tubes and allowed moisture to the steel underneath.


Here's another bike with a frontal impact. This one was straight, but pushed back.

This one rode correctly, but it actively wanted to turn to either side. So it required the rider to keep both arms in tension, holding the wheel in the middle. None of this casual self-centering or hands-free riding. If you let go, it would throw you at any speed.

enter image description here


SEATPOSTS This happened at least three times, always due to a hard landing and far too much seatpost stickout. It feels like the saddle clamp has failed and the saddle has rotated so the nose is upward.


MEASURING Bicycle frames are symmetrical. So you can use a string pulled taught from one rear dropout, up and around the head tube, and back down to the other rear dropout. As long as the string isn't fouling cables or chain, then the horizontal gap to the frame should be the same on both sides of the bike, within a millimetre is fine.

Suspension can also throw off this measurement, if the bushings, bearings, pivots are worn/sloppy. Any joint should have rotational movement, but not sideways movement. Is it possible you're feeling slop in suspension?

5
  • 2
    I was expecting an answer from you on this one ;)
    – MaplePanda
    Sep 18 at 13:46
  • 2
    I actually disagree with the claim that bicycle frames are symmetrical. Sometimes, they are not, like in Cannondale Ai.
    – juhist
    Sep 18 at 17:02
  • @juhist true - the right hand chainstay is frequently dented for clearance from the chainrings. Sometimes the stay takes a different path, higher or lower. I believe there's also a rear wheel standard for MTB where the right hand dropout is further outboard from centerline than the left dropout, but its pretty rare and unusual and the owner would know that. The vast bulk of frames display symmetry across the centerline.
    – Criggie
    Sep 19 at 9:24
  • 1
    @MaplePanda I seem uniquely qualified to talk about broken bikes don't I :) MTB experience is a bit light though.
    – Criggie
    Sep 19 at 9:25
  • 1
    @Criggie That would be Cannondale’s AI offset. The owner is not guaranteed to know about it—it’s only 6mm so not blatantly obvious from inspection, and shops are unlikely to mention it during the sale.
    – MaplePanda
    Sep 19 at 12:22
1

The front tire is very close to the down tube, or may even touch it periodically.

This happened to me long tone ago after the hard forward hit. The workshop said that frame is not safe to ride and not repairable.

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service, privacy policy and cookie policy

Not the answer you're looking for? Browse other questions tagged or ask your own question.