Gears randomly shifting?

Gears randomly shifting?

Not sure what the issue is here but hoping for some ideas/advice...

I am riding a 8 speed hybrid (3 x 8) bike.

When I am pedalling along (typically in 3:4 or 3:5) sometimes I experience the bike shift from a comfortable high'ish gear to a very low gear (on it’s own!). After almost losing control of the bike:

• Sometimes it returns immediately back to the high gear (on it's own!); or
  
• Sometimes it remains in the low gear for several revolutions before going back to normal; or
  
• Sometimes it remains in the low gear and I need to change up/down to revert to normal.
  
  

I have tried to repeat the effect however I cannot. It seems to happen randomly.
It does not look like the front chain ring is changing but it may well be something happening on the rear cogs or derailleur?

The chain was replaced (and not the chain wheel) a long time ago. There was some chain slipping in 3:4 but this stopped happening a while ago. The problem happening now feels and acts differently.

I did a search around but not sure how to technically describe the issues.

If your chain was replaced a long time ago, you may need a new chain again, also maybe a new cassette. Check for wear. Your Gear hanger may also be a little out of alignment but it sounds more like wear.

Check chain for wear (using a chain ware guide tool), replace parts as needed. May as well replace cables at the same time, degrease the remaining drive chain, check hanger alignment or take it to a shop to check alignment as it can be hard to do so without the proper tool.

Congratulations, You have invented the automatic, quick patent it.

As tommy said is the chain slipping or actual derailleur moving?

Thanks for the replies.

The chain was changed within the last 12 months, so hoping that does need replacing. I will have a look for any noticable signs of wear.

It feels like some sort of derailleur issue because of the slip into low gear? ie I am riding in a hard'ish gear (3:5) and then suddenly it feels too easy like (3:1). Although I haven't actually looked down to see the position of chain and gears.

Not sure if it makes any difference to mention, but this happens on flat road. I don't think I am applying significant force on the pedals. Thinking back, the problem did occur more and worse when riding 2:4 or 2:5 (front on the middle cog).

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The chain was changed within the last 12 months, so hoping that does need replacing. I will have a look for any noticable signs of wear.

A chain might last 3 years is might only last 3 months depending on conditions and mileage you need to check it with a chain ware tool visually does not cut it. Checking the chain will be you first step to resolving this problem.

I reported some strange gear shifting behaviour a few months ago. I suspected an issue with the chain. I had a play around and everything seemed okay until recently.

Now when I turn the pedals the chainwheel does not turn at all. The chain just moves over the rear freewheel (ie no teeth are grabbing the chain). During the week the issue got worse and worse until now I cannot ride the bike at all. Issue happens in all rear gear settings.

I have tried adjusting the rear derailleur adjustment screws. This doesn't help.
Any suggestions?

I have attached photos but don't think they will help.

2 Attachments

pics are pretty blurry. how old is your cassette? could be that the teeth on your cog are worn and need to be replaced.

but as tiswas said, i'd adjust the L&H screws before replacing cassette.

why mess with the L&H screws? These limit the movement of the rear mech to stop overshifting at either end, or undershifting i.e. not picking up 1 or 8 in the example given...

Sounds like an issue with cable tension, possibly rust in the cable housing stopping the cable operating proper. I'd recable the whole thing and re-index from there to get the cable tension spot on.

Worn cassette + not enough tension in the mech arm. (?)

rhb and tynan are right. leave the limit screws (not sure what i was thinking), check your cable tension and if your cassette is worn.

Thanks for the responses.

It seems that adjusting the L&H screws may not be the solution. The chain is not slipping off the freewheel - ie no over or undershifting is occuring. The gears are shifting correct.

The slipping happens in any gear so don't think it is worn teeth on the cassette.
Chain tension or arm tension sounds closer to the problem. When I turn the pedals the chain just doesn't have enough tension to take hold of the rear cassette.

How do I resolve this? Would I still need to adjust or reindex cables?

Thanks for the responses.

It seems that adjusting the L&H screws may not be the solution. The chain is not slipping off the freewheel - ie no over or undershifting is occuring. The gears are shifting correct.

The slipping happens in any gear so don't think it is worn teeth on the cassette.
Chain tension or arm tension sounds closer to the problem. When I turn the pedals the chain just doesn't have enough tension to take hold of the rear cassette.

How do I resolve this? Would I still need to adjust or reindex cables?

have a look on youtube by searching 'adjusting rear derailleur cable tension' and there are a few videos that will show you how to do it. also, as was mentioned, your cable might have rusted inside its housing, so it'd be worth checking that. you can adjust the tension all you want, but if the cable is sticking, you'll always have problems.

good luck.

You have to reset them whilst re-indexing.

Thanks for the responses.

It seems that adjusting the L&H screws may not be the solution. The chain is not slipping off the freewheel - ie no over or undershifting is occuring. The gears are shifting correct.

The slipping happens in any gear so don't think it is worn teeth on the cassette.
Chain tension or arm tension sounds closer to the problem. When I turn the pedals the chain just doesn't have enough tension to take hold of the rear cassette.

How do I resolve this? Would I still need to adjust or reindex cables?

Clean and re-grease all the moving parts of the rear mech also.

Chain too long could also contribute perhaps? Would need to get hands on to really get to bottom of problem, but there's enough info now in this thread to help you get to the solution I reckon.

Hasn't he already messed with the limit screws? Might be worth putting those right before re-indexing.

Naturally, yes.

http://bicycletutor.com/adjust-front-derailer/

http://bicycletutor.com/adjust-rear-derailleur/

You have to reset them whilst re-indexing.

You set them first, then index (set cable tension). Otherwise you're indexing within the wrong range of movement.

My point was that you shouldn't be doint one with out the other.

All part of the same job innit.

My point was that you shouldn't be doint one with out the other.

All part of the same job innit.

Pretty much, although if you were to re-cable an already setup geared bike, you'd not adjust the limits as they're not influenced by the cable but remain a constant within the rear mech. It'd be good practice (and a couple of seconds job) to check the limits before fitting the new cable though.

Slight tangent, have found limit screws useful to help bodge a slightly spinnier single speed for a riders gear cable after it snapped on a club ride, otherwise don't touch once set.

Difficult to tell from the photos, but is the chain correctly wrapped thru the derailleur jockey wheels??

Pic 1 does not look right but difficult to tell.

Thanks again.

I will read up on the links provided. I will first look for any rusting inside cable housing then any rear derailleur (or cable) adjusting.

Hopefully it will be an easy fix.

I will try and take some better photos tonight.

The reason I posted the pic was I wasn't sure if the chain and deraulier arm were positioned correctly. I was thinking the loss in tension was caused by the arm being too upward?

From the picture posted, I think this is the relevant techdoc from Shimano for correct installation and setup of the rear mech on your hybrid.

Print ( or enlarge on screen) and follow step by step.

http://techdocs.shimano.com/media/techdocs/content/cycle/SI/Alivio-Acera/RearDriveSystem/SI-R920A-En_v1_m56577569830612828.pdf

Before doing all that though, grab a bare bit of rear mech cable (under the downtube is a good place) and give it a good pull outwards from the tubing. If this pulls the mech over without any trouble it's probably not seized. If it feels tough / binding - replace inner and outer(s).

P.s. Position of arm changes relative to gear selected, so difficult to answer your last point on that.