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I bought a new bike about a month ago. And it made a strange sound from the first ride , I thought the sound was because it was new and will disappear with time but it didn't. The sound is like a sudden collision of the chain with the gears and it lasts for about a second and happens when I am pedaling under pressure ( for example pedaling up hill ) so I filmed the rear part of the chain in slow mo.

Some important points:

  • the derailleurs are fully adjusted and the shifts work fine
  • I tried using WD 40 on the gears but didn't work
  • the rear derailleur hanger is not bent
  • the slip happens every about 5 minutes on straight roads and about every 30 seconds on uphills
  • At the point of the sound, the pedaling feels very easy for that second ( almost no resistance at all )

Thanks

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    In practical terms, the answer to any question of the form "I bought a brand new thing and it doesn't work properly" is "Take it back to the shop and get them to fix or replace it."
    – David Richerby
    Commented Jul 27, 2018 at 15:35
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    I did take it to the shop and they couldnt fix it
    – Omar Magdy
    Commented Jul 27, 2018 at 15:43
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    Then they need to replace it.
    – David Richerby
    Commented Jul 27, 2018 at 16:35
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    +1 for posting the video
    – Argenti Apparatus
    Commented Jul 27, 2018 at 16:45
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    Also, don't use WD-40 as a lubricant.
    – Sparhawk
    Commented Jul 28, 2018 at 0:42

2 Answers 2

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You can see in the video how the teeth on the cassette rotate quickly at the moment it slips(at :05 seconds), while the spokes of the wheel actually stay slow, then jolt forwards to catch up. This means that the freewheel mechanism in the rear hub is failing to engage.

It's actually quite dangerous because as it gets worse it could cause you to lose control, fall off etc. in the worst case.

It shouldn't be like that on any bike, especially a brand new one. So take it back to the shop and have the freewheel, or freehub replaced (whichever type it is).

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    To be clear, you shouldn't be paying any money for this repair! The bike is too new
    – Swifty
    Commented Jul 27, 2018 at 15:37
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    It's really weird. I have had worn pawls/teeth in freehubs, but I never got the effect mentioned in the 5th bullet point. If it's one pawl/tooth that's worn, the next should catch. When it occured to me, the metallic sound was super loud, but the pedals would just kick to the next ratchet tooth. OP's issue seems different. Could it be that the freehub is just so packed with grease that it impedes ratcheting and pawls can skip several teeth at once?
    – Gabriel
    Commented Jul 27, 2018 at 17:43
  • @gabrielc plausibly, especially given that it’s new, rather than worn through use. I’ve had bullet point 5 happen on a bike though. Not fun!
    – Swifty
    Commented Jul 27, 2018 at 17:52
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Confirming @Swifty's diagnosis. It's the freewheel mechanism. Take it back to where you purchased it and demand full refund or at least a complete new rear wheel.

Don't let them try to repair the freewheel or the hub. Accept replacement only. The store has already demonstrated incompetence and a disregard for customer satisfaction by being unable or unwilling to fix this for you.

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    This should be a comment under Swifty's answer ...
    – Lamar Latrell
    Commented Jul 28, 2018 at 1:12

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