Gears shift and chain jumps while coasting

[Souza]

Hi, I bought a barely used mountain bike from a friend. It rode great with no problems for a while, then I started noticing the rear derailleur loosening - like I was changing gears and making a clicking noise - whenever I was coasting and not pedaling. When I did begin pedaling again, the chain was sometimes still slack but would catch after a second. It got progressively worse and now whenever I stop pedaling or brake, the chain gets slack and jumps off onto the axle - (almost killed myself at a busy intersection). I did some research and watched a couple of videos on adjusting tension, etc. and think that is probably the problem, but wanted to get some "expert" advice from ya'll before I started tinkering on my own. Any ideas? Thanks!

[cyclerUK]

I'm not sure if I understand you right but-
Does the chain go slack on the top run when you stop pedaling?
If you manually push the bike forward does the chainset also turn because the chain is going round?
If so then it sounds like your freewheel is jamming up. Turn the pedals backwards and watch the dérailleur. It should stay hanging down but if it screws upwards then take out the back wheel and try turning the freewheel by hand. It should spin backwards with ease. Make sure nothing has got between the freewheel and the hub spokes (string or rag).
If not like the above then ignore my theory. ಠ_ಠ

[Souza]

Thanks Cycler. Yes, the chain goes slack on top. Yes, I think the chainset turns too - but I'll have to check for sure when I get home. The derailleur pulls upward whenever I stop pedaling and I'm sure it does when I pedal backward too. I took the back wheel off and cleaned everything and I'm pretty sure the freewheel could spin, but I don't remember it being with ease - by any stretch. Thanks for the tips!

[yg713]

Hello Souza,

The problem is not your derailleur but your freewheel/freehub body.

There is too much gunk inside and it just makes it stick. When you stop pedaling to just coast, your rear wheel is still rotating obviously and the stickiness inside your freewheel makes it want to go forward anyway. This causes your untensioned chain (the bottom part) to be pulled upward over the freewheel and as it does, it ends up tensioning your derailleur at its maximum.

You can try and remedy the situation by using some sort of degreaser that you would shoot through the back of a freewheel or of a freehub body. Which means you need to disassemble your hub in order to do so. And then you need to relube once the degreaser has completely been drained from the system.

If you are using a freewheel, a replacement can be had for as low as $10 nowadays so maybe go this way instead of degreasing. It will save you time and you will be sure that it will not happen again.

I hope this helps.

Have a great day!

[Souza]

Thanks! That makes perfect sense and your description is exactly what seems to be happening. I'm all about spending $10 rather than playing grease monkey. Thanks again!
Souza