Mismatched drivetrain?

[_chris]

Hi there,

I've got an old Peugeot that I've gradually been fixing up over the last couple of years. Could someone explain why my drivetrain doesn't work together properly (the chain keeps slipping off)?

I had:

• Shimano HG30 7 Speed Cassette
• Shimano HG70 7/8 Speed Chain
• Old (~70s), bent, double chainset

I decided to take the bike to my LBS to replace the chainring with this one, which I thought would work. I was told that I would need a 1/8 inch chain for the chainring, which was fitted, and now seems to be rubbing on the cassette when I'm in the highest and lowest gears, and occasionally falls off.

So I guess my question is, do I now have a 'single speed' chainring, or is there no reason why I can't go back to a 7/8 speed chain?

Thanks,

Chris

 

[RecordAceFromNew]

1/8th inch chains are for single speed and are not suitable for your cassette because they are too wide - 7 speed sprockets are spaced 5mm apart centre to centre but a 1/8th chain is nearly 8mm wide, leaving too little space for the sprocket's thickness while the internal width of 1/8th inch between the chain's side plates leaves too much room for it to slosh around sideways. 7 or 8 speed chains on the other hand are 3/32" between the plates internally and are under 7mm wide externally.

The shop should never have sold you a single speed chain which is why it interferes with the cassette. While it is possible that the chainset you bought is too thick for 3/32" chains, that doesn't make it right, if they did, for them as pros to flog you a single speed 1/8" chain knowing you have a 7 speed cassette at the back! Quite appalling service if that was indeed the case.

In terms of what to do, it seems a sensible way forward is to return the chain since it should never have been sold to you in the first place, see whether a 3/32" chain will fit the chainset, if not get rid of that chainset and get one that matches. If and when you pick another chainset you should get one that matches i) the existing bottom bracket (unless it also needs changing), ii) a 3/32" chain, and iii) the front mech (if you wish to continue using the front mech).

Why did you decide to get that single chainset to replace your old double?