What could cause this knocking sound when pedaling? (see video)

Question

The sound stops when I don't pedal. It is most bizarre, like someone tapping lightly on the frame with a hammer, as you can see/hear here:

I took it to the bike shop and they tightened the bottom bracket, but that didn't fix it.

• The noise only happens in 4th or 5th gear for some reason.
• Only happens when pedaling.
• The knock is associated with the revolution of the pedal.
• Happens even if I stand and pedal (i.e. not seat-related).
• I can't reproduce it on the bike rack. Apparently, there needs to be some torque/weight.
• I checked the chainring bolts, they are not loose.
• It happens regardless of which ring the chain is on on the front derailleur.

I am at my wits end and would appreciate a tip.

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The knock sound is there even if I pedal without touching the handlebars.

"Does it change speed when you change gear" - The speed is associated with the pedal revolution. Regardless of the gear, there are two knocks per revolution.

"What if you pedal backward" - Then there is no knock sound

"small arc" - Impossible to test that, as there needs to be a certain cadence (a few pedal cycles) established before the knock emerges. Stopping pedaling breaks the rhythm.

"does noise go away while out of the saddle" - Nope, the noise is still there even if I stand while pedaling.

UPDATE: So I just got back from the shop. He tried a different freewheel because mine had some play. That didn't fix it. He then concluded that it was the rear wheel's hub. He said on these cheap bikes they compress the hub too much and this can result in the ball bearings within making sounds. Unfortunately, he did not have a matching wheel on hand which we could use to verify. Side note: After I left the shop, I went to where I originally bought the bike and they had two other of the exact same model still for sale. I took them both for a ride and whaddya know- they both have the same exact issue. So, stay away from the Hyper Spinfit.

Answer 1

Great video - we can see that the knock is twice per revolution of the crank, and that its when the cranks are about horizontal.

• Does it change speed if you change gear?
• What about if you pedal backward ?
• If you pedal forward/backward over a small arc of travel rather than doing full revolutions? I suspect that each alternate click is something moving back and forth, and "toggling" across one of them won't click.
• Lastly, does the noise go away if you pedal while out of the saddle ?

At this point, I'd check pedal bearings, look for play in the pedal-axle/crankarm and the crank/BB-axle joint.

Also look for witness marks - if something's hitting, it might be marking paint and leaving a visible scratch. I was suspicious of your kickstand but that doesn't seem to be it.

Do report back with your findings.

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I'm going to attribute the cause to the Bottom Bracket and bearings first, the cranks, or maybe the pedals. You've eliminated the rest of the bike with your tests.

I'd be checking for play all the way through from pedal to pedal, through the crank and BB. Try and replicate the weight down on both pedals, and look for something that sets and resets.

Hypothesis I'm wondering if there's some kind of bearing damage and it is taking a couple of turns to wind-up and bind and then release. Some exploratory opening of the BB might be the next plan.

Last resort would be to replace the BB with a new cartridge unit. They're not too expensive - even a cheap shimano one will last years.

Answer 2

Excluding contact between moving parts, sounds from bicycle are mysterious. They may arise from resonance phenomena (if you have some music expertise, you know, if not it is the same issue that brought the infamous Tacoma Bridge to collapse due to the wind having a certain speed and direction).

From this phenomena, there is a rather counter-intuitive consequence: the sound may originate in a different part of the bike.

I will then investigate the unlikely occurence: the seat post.

Does it still happen if you remove the seat-post/saddle? My reasoning is that you hear the click because your body is loading and unloading the frame at the moment when the pedals are more or less horizontal and the seatpost flexes a little bit, but just enough to click on the frame somewhere.

Again, this is the unlikely cause, but ... your wits end is there to be stretched ;)

ps: with physics I am ok, but I have a grammar doubt "your wits end is" or "your wits end are"? Thanks!

Answer 3

Sherlock Holmes said that "when you have eliminated all which is impossible, then whatever remains, however improbable, must be the truth" — and isn't that appropriate given the mystery you present to us? I'll show that after "eliminating all which is impossible" what remains is an issue in the bottom bracket ball bearing/axle or the pedals caused by the slight deformation that occurs when you load the pedals and the frame.

The reasoning is as follows:

• Not related to the chain, like a faulty link: That would not be synchronized with the pedal revolution.
• Not related to the back sprockets or back wheel: Would not be synchronized either.
• Not related to the seat (which is exposed to changing loads by the pedaling).
• Therefore, must be related to the pedals or cranking, either something touching or the mechanism.
  • Nothing external is touching: You verified that, and we can see nothing. It must originate from the mechanism.
  • It cannot be a mechanical fault on the chain ring: It happens with both of them.
  • Remains the bearing/axle mechanism, or the pedals.
    • Does not happen on the bike rack: Needs load.
    • Load must change the bearing/casing/pedals in a way that produces noise. The only change induced by loading the frame and pedals is mechanical deformation.

This is, of course, a bit tongue in cheek: Some unknown reason other than the inner mechanism may have escaped my imagination, and the "only in 4th and 5th gear" detail fits only marginally (the stress from pedaling must be directed just right to make the inner parts touch). But still, I think the reasoning is sound.

Edit: Re-reading your description the issue may be handle bar related: You exclude the seat by standing in the pedals, but did you exclude the handle bar by pedaling without holding on? (Actually, you may have when you took the video...) <-- The OP said that it's not the handlebar.

Edit 2: Andrew made the case for an issue in the back of the bike which is triggered by the oscillating loads when pedaling. That would also be synchronized with the pedaling, not the back wheel. Good idea. Actually, there may be a frame fracture anywhere that has the ends moving against each other under load. Even a broken frame may hold for a while if the parts are under compressed load. Worthwhile to look for.

As criggie said: Please tell us the solution to the riddle when you found it! :-)

Answer 4

Also check for loose spokes, cracked rims, or other wheel issues. A few years back I had an extremely similar knocking I couldn't find for months.

The fact that this noise only happens when you're on the bike pedaling means there's some correlation with your weight loading the bicycle and the pressure on the pedals. Those pressures aren't symmetrical, which means they can apply sideways forces to your wheels that aren't there when you're not pedaling - and they're not there when the bike is on a rack, like you've noted.

Why only in your 4th or 5th gear? Maybe you're hitting a resonance related to pedal stroke pressure/stress and wheel rotation speed. And that resonance isn't there in any other gear.

In my case, I had a cracked spoke hole in my front rim that was making noise in sync with my pedal strokes - somewhat like you're hearing, although my noises were once per pedal rotation instead of twice per rotation. My noise finally stopped when the crack propagated enough to allow the spoke to pull fully through the rim hole, go entirely slack, and the wheel to go out-of-true. I could see from the dirty edges on some of the rim shards that the crack had been there for some time.

Answer 5

TL;DR: re-torque the crank arms to the bottom bearing shaft. Good odds it is the tolerance between the crank arms and the bottom bearing shaft.
Until the 80s we used to have a horrible design of a single offset "cotter pin" and nut that would secure the crank arms to the bottom bearing shaft. Example here: https://www.sheldonbrown.com/cotters.html
Everything would work great when the whole set was brand new. As you took it into the shop to have it cleaned, steel balls/bearings replaced and re-lubed, they would remove said cotter pin and nut to pop off both crank arms from the bearing shaft. Long story short, cotter pin wears a little each time it is removed, bearing shafts were not properly hardened, and the notorious click would return twice per crank rotation, and only under heavy torque/load. Home mechanics used to hack it with tiny pieces of aluminum foil inserted with the cotter pin to avoid replacing the bearing shaft, crank arms or cotter pin. In your case, short of replacing crank arms and bearing/shaft assembly, try re-torquing the crank arms to the bottom bearing shaft.

Answer 6

It looks like a bottom bracket problem to be. They cost around $20 so I would replace that and see if it fixes the issue.

I had a similar issue and refused to believe that a bottom bracket could go out so quickly so I wasted a bunch of time and stress trying to troubleshoot the issue but when I finally replaced the bottom bracket, it solved my problem and the bike felt amazing to ride.

Answer 7

My 20 cents:

You checked the chainring screws, but do you still have grease on it?

When you have your chain at a specific angle (explaining it's happening while on the 4-5th speed), the crank is "constrained" in a very specific direction, making the screws moving slightly. This can cause this kind of sound.

Unmount all screws, put some grease on them and remount.