Confused about gearing

[Neil G]

Just say you were in 36/30 and felt the need for 26/30 but mistakenly pushed with your left thumb towards 46/30, instead of clicking with your left finger. The chain isn't long enough so it won't actually happen, but is your left thumb strong enough to break the chain? Mine isn't.

It's not a question of thumb strength. Once the chain is picked up by the chainring, it's your leg strength that completes the shift. The chain isn't usually the first thing to give: it's more likely to be the chainring or derailleur hanger.

A more likely scenario is sprinting in the big chainring to crest a rise and running out of gears. It's easy to forget that the chain is too short to allow the bail-out shift from 46/26 to 46/30. Once the big sprocket picks up the chain it's the same situation as before: something has to give. It might be the chainring (which is seeing quite a big lateral load under cross-chaining) it might be the sprocket (likewise) or it might be the derailleur hanger. If that puts the derailleur cage into the wheel, that can lead to a host of other problems, all in less than a turn of the cranks.

It's easy to say "just avoid big-big" - but I know I'm not always aware of which sprocket the chain is on at a given moment.

Has happened to me at around 2/3 in the morning in a 12 hour race. Rear mech let go and hangar bent (luckily steel frame so sorted easily)

 

[Anthony]

It's not a question of thumb strength. Once the chain is picked up by the chainring, it's your leg strength that completes the shift. The chain isn't usually the first thing to give: it's more likely to be the chainring or derailleur hanger.

A more likely scenario is sprinting in the big chainring to crest a rise and running out of gears. It's easy to forget that the chain is too short to allow the bail-out shift from 46/26 to 46/30. Once the big sprocket picks up the chain it's the same situation as before: something has to give. It might be the chainring (which is seeing quite a big lateral load under cross-chaining) it might be the sprocket (likewise) or it might be the derailleur hanger. If that puts the derailleur cage into the wheel, that can lead to a host of other problems, all in less than a turn of the cranks.

It's easy to say "just avoid big-big" - but I know I'm not always aware of which sprocket the chain is on at a given moment.

I must confess that I found one particular hill a bit more effort than usual the other day. Of course it couldn't possibly be my age, so I thought perhaps I need more pressure in the tyres. Went to change into the big ring at the top and 'aha, that's why it was more effort'.

But still I had been in big/3rd rather than middle/4th. Say I had still struggled, would I get anywhere near 1st before changing rings? I really don't think I would. You should always change rings early rather than late IMPO.

And the chance of my making that mistake during a race at two in the morning is zero.

 

[one-eyed_jim]

You should always change rings early rather than late IMPO.

I agree with that as a general principle, but the world is full of exceptions.

Big-ring climbs happen more often on the road than off it. The point of my example about sprinting to crest a rise is that it's a situation where you're trying to avoid changing chainrings at all. Rear shifts are almost always smoother than front shifts, and there are plenty of occasions - racing or not - where a front shift can be a liability.

If you're absolutely certain you'll never accidentally shift into big-big of course there's no problem, but I like to have the option from time to time, and on principle I think it's a bad idea to have a self-destruct mechanism hard-wired into the transmission. A slack chain in small-small carries no analogous risk.

 

[sgw]

Hello...remember me

Only kidding.

I have decided the sensible thing is to get a decent long cage mech.

Thanks for all the advice, I am at least a lot better informed to get the right bits now. I don't trust myself enough not to miss change. I wouldn't be going anyway near as fast as some of you racer types but I don't want to risk any of that.

I think it might be just as eye watering on an overweight tourer being stepped up a big hill. ouch!