Bike changing gears random

[Megster]

I was biking home with my college friend and her chain was jumping at random changing gears on the back wheel from the smallest cog (is that how you call it?) to the second smallest. She has 7 speed on back and one chainring in front. She has been ignoring this chain jump for few weeks, but now it has become too annoying. Can we somehow fix it easily by ourselves? One guy said that the chain looks okay and tight if that info helps. #studentlife #moneyissues #cannotpaybikeservice

[Joe_W]

If the chain and sprockets are OK, you just need to set up the rear dérailleur correctly. See other posts for that. Most likely the cable is just too loose, so you can tighten it with the barrel adjuster.

• shift up all the way, loosen barrel adjuster a turn or five
• check if chain runs correctly, use limit screw to dial in
• shift down a gear, check if chain follows
• tighten barrel adjuster a bit of the chain stays (or rattles) and shift up, then down again
• repeat on next bigger sprocket, etc. Until it works
  

If you cannot get it to work, check cables and housing. Then, check chain for wear (12 full links should be 12 inch, plus 1/8 at most, AFAIK, check Sheldon Brown's site for the numbers)

[Painkiller]

It sounds like it needs inspected and tuned, If you cannot true a wheel and own a Derailluer alignment gauge then you cannot do it at home correctly. Seek out a local co op or a Local bike shop for service.

[Joe_W]

A bent dérailleur hanger might be an explanation as well, right. But you do not need to true the wheel to correct that: always register the alignment tool next to the valve[1]. I would start with the simpler stuff (if I had no alignment tool...). More often than not it is a simple turn of the barrel adjuster, or a frayed cable. At least that has been my experience so far, which is not representative and just circumstantial evidence I admit


[1] having round wheels makes biking more fun, though

[Megster]

Oh thank you both )
Now we have a lot of information to digest, but all in all I see that she can't avoid spending some $.

[Painkiller]

Some of the problem that I see when people set up a geared bike is that they kind of go in the order of 1,3,2. when I get a used or new bicycle in I always start from scratch. disconnect cables, check axle and crank/bottom bracket for play, check front rings for run out, true wheels and make sure they are bottomed in lugs correctly and centered/dished proper. Now the bike is ready to set up the derailluers. Check and align the derailluer hanger. This is where the 1,3,2, comes in. if you just start by turning barrels thinking it is set but it just is not right you start looking for ways to make it right scratching your head. then ultimately tracing back to what should have been done in the first place. Proper process and order 1,2,3,
Now you can proceed with the set up. set the high limit screw, set shifter to the highest gear in the rear, screw barrel adjuster all in then back out a couple turns, connect cable making sure the housing is seated in all stops. shift down 1 gear, if it does not hit the gear then cable was attached to loose. if it does shift then back out barrel adjuster until you hear the chain clanging for the next gear then back off barrel adjuster indent by indent til the clanging sound stops. Now run up and down the gears to see how they go and set the low limit screw. That is the condensed version. for a newbie it can be a bit of trial and error til you do a few and understand what action and out come have to do with each other. But learning bad habits from the get go is never a good thing. One must have the tools to perform certain tasks properly. there are no short cuts to fine tuning a bicycle, anything else is sub standard. Its just the hard truth! In your case if you follow the correct processes and it is not performing well you can go forward with more diagnosis not backward because you have done the checks to rule them out already. you will only need to take a closer look at your chain, cluster or rear derailluer and shifter its self. If you want to be your own wrench you have to go "All in" and start collecting tools as you need them. down the long haul it will still be cheaper that paying someone else.