Fitting a new chainring: anything to look out for?

[onlyhuman]

A new 39t chainring has arrived, to replace the 42t ring on my my Campagnolo-equipped road bike. I haven't done this before, is there anything I need to know? Specifically (come to think of it) how can I tell if the chain is the correct length?

 

[TheDoctor]

Undo the chainring bolts such that you're pulling up and away from the teeth, not down and towards them.
It hurts otherwise...

 

[Theseus]

When you put the new ring on, be patient and spend time getting it properly centered. I have only done this for a fixed with a tight chain. This may not be so important for a geared bike

The technique I use for chain length, and others exist, is to put the chain over the big ring and the big sprocket without going through the cage. Add 1 complete link to this length and that is what to go with.

 

[onlyhuman]

When you put the new ring on, be patient and spend time getting it properly centered. I have only done this for a fixed with a tight chain. This may not be so important for a geared bike

The technique I use for chain length, and others exist, is to put the chain over the big ring and the big sprocket without going through the cage. Add 1 complete link to this length and that is what to go with.

Thanks Touche. A couple of follow-up questions: first, I don't mind being patient, but how do I know whether the ring is centred? Second, if I am only changing the smaller of two rings (42t to 39t), does that mean the chain length will be ok as it is?

 

[I am Spartacus]

IMO the chain length will not be an issue for you.
However when replacing the driveside crank with new inner ring, be careful as you replace and begin to tighten up to correct torque.
I did similar swap to compact and after 1st attempt there was an imperceptible drift on the chainring which heard itself after a test ride.
Took it off and greased again and replaced and was dead on.

 

[jay clock]

I strongly advise one of these for doing up the new ones http://shop.2wheelsonly.co.uk/park-tool-chainring-nut-wrench-289-p.asp

Without it they tend to be hard to get tight enough

 

[mr Mag00]

jay never needed one of those a flat head screw driver and an allen key

 

[I am Spartacus]

2 pence piece

 

[Crackle]

Undo the chainring bolts such that you're pulling up and away from the teeth, not down and towards them.
It hurts otherwise...

Can I just emphasise how important that advice is

 

[Theseus]

Thanks Touche. A couple of follow-up questions: first, I don't mind being patient, but how do I know whether the ring is centred? Second, if I am only changing the smaller of two rings (42t to 39t), does that mean the chain length will be ok as it is?

1. I am not sure how important it is for a geared bike, but it is quite important for fixed/ss. The technique is explained in detail (or was, I can't find it yet) on Sheldon's site. I will take a longer look later to see if I can find it.

2. Your existing chain length should be OK.

 

[Theseus]

I strongly advise one of these for doing up the new ones http://shop.2wheelsonly.co.uk/park-tool-chainring-nut-wrench-289-p.asp

Without it they tend to be hard to get tight enough

Oh! Is that what that thing is for? Got one in the repair kit and never understood what it was meant to do.

 

[jay clock]

re the 2p piece and flat head screwdriver options.... on my chainset the centre of the bolt protrudes meaning it can be hard to use those options. The tool I have I got in a Spanish bike shop for a couple of quid - not often used but does the job perfectly

 

[MartinC]

There's no need to centre the ring on a derailleur transmission. It's very unlikely that you'll need to change the chain length (assuming it was right before),

 

[youngoldbloke]

- don't know what chainset you've got but recent Campag UT chainrings have little diamond shapes stamped on them that must be lined up as the chainrings are 'timed for optimum derailment'. Make sure you note the position of any thin spacer washers that might come to light and replace in the same position. (Also from the Campag info. : "Final assembly must be carefully performed to avoid an accident, personal injury or death" !)

 

[Theseus]

I don't mind being patient, but how do I know whether the ring is centred?

1. I am not sure how important it is for a geared bike, but it is quite important for fixed/ss. The technique is explained in detail (or was, I can't find it yet) on Sheldon's site. I will take a longer look later to see if I can find it.

There's no need to centre the ring on a derailleur transmission.

In which case I won't bother hunting down the Sheldon article. Thinking about it you can get eliptical rings for derailleur transmissions so centering is probably pointless.