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Got a bike 3 weeks ago. It now slips gears WITHOUT shifting. only slipping when I pedal hard or uphill.. It shifts good. I weigh 250lbs.

edit: with a new derailleur

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    If it's a new bike, especially if it's that new, you should be able to take it back to the shop for minor tune-up items. (For example, it's somewhat expected that indexing may need tweaking.) – DavidW Aug 26 at 20:44
  • slipping gears as in shifting speeds or slipping on the same speed? – Tooniis Aug 27 at 10:17
  • With new bikes or newly serviced bikes chain/gears/derailleur can sort of ~bed in~ after the first few days or riding and this can result in the gears needing tweaking to shift smoothly again. As DavidW says the shop should be happy to take it and do the tune-up. If it doesn't tune up check chain/cassette cogs wear, if the cogs are sharp and pointy it is a bad sign. – gaoithe Aug 30 at 14:41
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Even if a derailleur appears to shift well, it could be out of adjustment. My shifter cable recently snapped and I replaced it. I put on a new one, and after adjusting it to the point of the shifting being "good", there was still some skipping when pedaling up a steep hill; some finer adjusting took care of it (while also improving the shifting from "good" to "excellent").

You mention that the derailleur is new, which suggests that the bicycle isn't. A used bicycle can have a worn out drive-train.

If the chain slips on the cogs, a new derailleur won't fix it. If the chain is slipping on the rear cassette, both the chain and cog need to be replaced. If it skips on the front, then the chain and front rings will have to be replaced, and likely the rear cassette also (because the new chain will quite possibly skip on the new one).

A second-hand bicycle doesn't have to appear to be well-used to need drive train component replacements. A bike that is used daily for year-round daily commuting can need a new cassette and chain well before it's two years old, especially with lower-end components, even if kept clean and well-maintained.

  • Hmm... If it's used year-round, I'd think the chain should be changed at least yearly... By not running a chain to its absolute end of life you get more life out the rather more expensive cassette. – DavidW Aug 27 at 2:26
  • @DavidW That economics doesn't work out for me. $45 cassette, $30 chain. Not gonna change the chain 3x more frequently to add a bit of life to the cassette. If you have some $500 cassette, the reasoning will be different. – Kaz Aug 27 at 15:49
  • Obviously individual economics will vary. But for me it's 2 chains + 2 cassettes in 2 years vs. 3|4 chains + 1 cassette, and the spread in cost between cassette and chain is greater. – DavidW Aug 27 at 16:37
  • @DavidW I'd love to use try expensive cassettes, but they don't make them in 8 speeds! Looks like these ones you're using don't exactly last longer, either; I also use around 2 chains and 2 cassettes in 2 years (5200-ish km/year). I have these sprocket sizes: 12-13-14-15-17-19-21-23. I almost never use the 23, and not without downshifting in the front first (where I have an unused third ring!). I may switch to a 12-21 that includes the 16T sprocket. Can't imagine what I'd do with 11 speeds; e.g. a 4.5% ratio difference from 21T to 22T doesn't seem useful. – Kaz Aug 30 at 0:14
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I'm guessing that this is a used bike as you specifically point out it has a new derailleur.

The most likely cause is worn chain and sprockets. When you pedal hard you are literally pulling the chain off of the sprocket teeth. A new derailleur won't do anything to fix this.

Another possibility of ghost shifting - where the chain moves between sprockets on its own, but that feels different than the chain slipping.

Check the wear on the chain and cassette sprockets. A $10 chain wear gauge is a good investment. Sprocket wear can be assessed visually.

  • This slipping is definitely caused by worn by chainrings and/or sprockets. A new chain will not fix this problem as the teeth are worn with a ramp that allows the chain to pop from one tooth space to a tooth space farther down the chain. It's also dangerous! If it pops while standing on the pedals, you will go over the bars... from personal experience. :-) – Scott Lundberg Aug 28 at 19:34
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what about your gear or sprocket . is there any clogs or it might been use for so long and have misalignment or become gear too sharp as when you pedaling it hard easily make the chain loose .. a picture of it will help to understand

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