Have to change up a cog when shifting down to the small ring

[Gingerbreadman]

Hi,

I cant change from the smallest cog/big ring to the smallest cog/small ring without the chain coming off (e.g. when I run out of gears in the big ring going up a hill). I have to change UP a cog first and THEN I can shift to the smaller ring without the chain coming off. Only this specific situation (smallest cog) leads to a problem. The setup works fine otherwise. (I have been running it like this for quite a while).

What could be afoot ?

 

[Joffey]

You shouldn't be changing from the smallest cassette cog / big ring to the little ring - it crosses the chain too much. If you do this often it wears the chain out - maybe your chain has stretched due to the crossing and now causes this when you change gear. Use your cassette more! ;-)

 

[winjim]

Your chain is trying to run in a straight line and doesn't like the angle it's being forced to make to try and move to the small chainring, so it comes off.

ETA: And if that's what's happening, why are you shifting like that? There is no reason to change from the big to the small chainring whilst remaining on the smallest sprocket. And if you are on the smallest sprocket, how have you run out of uphill gears? Some clarification would be helpful

 

[sight-pin]

What they say ^. Also it's possible your derailleur could be springing over a tad too much and may need the limit screw adjusting.

 

[Karlt]

OPer is talking about running out of gears going uphill - do they mean biggest cog/smallest gear?

 

[Dogtrousers]

OPer is talking about running out of gears going uphill - do they mean biggest cog/smallest gear?

In which case a similar thing applies really. If going up a hill in the big ring, change all the way down on the cassette run out of gears (and start cross chaining) then drop down to little ring the chain can drop off on the inside. Same thing happens occasionally to ... er ... a friend of mine (I would never do such a frightful thing).

The answer is to only use the extreme left/right sprockets with the left/right rings. eg everything to the left when going up a big hill. Change rings in the middle-ish of the cassette according to the upcoming terrain.

 

[boydj]

If you have a longish hill you should be getting onto the small chainring before you are halfway up the cassette.

 

[Karlt]

If you have a longish hill you should be getting onto the small chainring before you are halfway up the cassette.

This. Plan ahead. You don't want to be having to do front changes under load if you can avoid it.

 

[bykeshed]

I'm not sure which gear you're in when the chain comes off.

If you mean you're in gear 1 at the rear, I'd say the stop screw (others call it the limit screw) isn't set properly on the front derailleur, causing the chain to move too far left and dropping off. Changing to gear 3-4 at the rear before changing to the small ring at the front would change the angle of the chain enough to stop it moving too far left and jumping off, if the stop screw isn't out by much.

If you mean you're in the highest gear at the rear (smallest cog), I'd say your chain might be too slack and the 'jump' from big ring to small ring at the front (assuming you have trigger type shifters) is causing your chain to 'leap' off - although I'd say the stop screw would probably stop the chain coming off completely to the left if set properly.

Could be a combination of both - a slack chain not helping?

If you've been riding like this for a while, I would rule out what others have said (with due respect) about the gear choice (smallest cog rear/small ring front) causing the chain to come off due to its angle. If you've been riding like this for a while, the chain should be worn enough to allow enough sideways movement to accommodate small ring/small cog riding without causing any problems other than increased chain wear.

 

[Gingerbreadman]

Thanks for all your advice. Sorry, I did mean lowest gear (i.e. the largest cog) not the smallest cog. (duh!).

I do try to change between rings in the middle of the cassette but there are situations where (for a number of reasons) I dont - e.g. I think I can stay in the big ring as I approach the crest of the hill but fail...

My setup (mostly Shimano 105) never used to do this when I first inherited the bike and it does this EVERY time now, so, whilst I appreciate I may be "asking for it" by swapping rings when in the lowest gear (largest cog) - not necessarily under load - that leads me to suspect that something may well be afoot and I can improve the situation.

@bykeshed I think your suggestion about the front mech limit sounds bang on...

 

[Karlt]

There are no medals for making the top in the big ring. Use the overlap between big and small ring ranges to go up in the small ring, unless the hill's so gentle that you absolutely know you won't need the small ring.

The Rules are humorous...