Change double chainwheel

[taximan]

Just a quick question. My pub/work bike is currently fitted with a 52/40 chainwheel. If I were to replace it with a 48/34, would it be a straight swap or would I need to shorten the chain slightly. I live in a very hilly area hence the need for lower gearing.

 

[Paulus]

You will probably have to shorten the chain by at least one link

 

[Dogtrousers]

If putting a new chainset on I'd be getting a new chain anyway. (But would make it shorter than the old one)

 

[roubaixtuesday]

Alternative viewpoint: 4 teeth reduction only requires one link down, and that only matters in small/ small with a worst case scenario of minor chain slack in a gear you shouldn't use anyway.

I wouldn't bother, just replace when you need to.

The only thing that really matters is that the chain is long enough for big/big.

I would suggest ensuring the front derailleur has the capacity you need and will go low enough if its a braze on.

 

[Kevberlin]

The Sheldon Brown advice as follows:

”The best technique for setting chain length is to thread the chain onto the large/large combination, without running it through the rear derailer. Mesh the two ends on to the large chainwheel so that one complete link (one inch, -- one inner and one outer half-link) overlaps. In almost all cases, this will give the optimum length.”

All the best

 

[fossyant]

Are you just replacing the chain rings, or the whole chainset - I suspect it's the whole Chainset as you are running a standard rather than a compact that you want. Standard BCD is 130mm (Shimano etc - 135 Campag) and can't go lower than a 38.

If you are going square taper, you might need a new bottom bracket with a shorter/or longer axel depending upon the recommended length.

You may want to look at the newer sub-compact chainsets, but those use hollowtech BB's.

Budget in a new chain, cassette and bottom bracket (cassette will be worn to the chain, so a new chain alone may skip.

 

[taximan]

Thanks for the advice guys, perhaps I should have mentioned that the bike is a vintage Dawes and was recently fitted with a new chain and 5 speed freewheel. As I said earlier the bike is only my pub bike so I do not need to go overboard with parts. I may do a restoration on it sometime but I have no immediate plans at the moment.

 

[fossyant]

You'll struggle to get smaller gears without a new chainset. Look at SJS Cycles or Spa for a traditional looking compact chainset

 

[T4tomo]

Old school derailleurs usually had a bit more capacity than our modern day short cage stuff, so it will probably cope without shortening the chain. The answer is try it and see with the new chain-set, and only take a link out if necessary. You'll need low gears if your pub(bike) crawl involves going up Golden Lion Bank.

 

[taximan]

Old school derailleurs usually had a bit more capacity than our modern day short cage stuff, so it will probably cope without shortening the chain. The answer is try it and see with the new chain-set, and only take a link out if necessary. You'll need low gears if your pub(bike) crawl involves going up Golden Lion Bank.

Or the Stiddy at Lythe

 

[Ajax Bay]

Repeating bits from above
OP has a square taper chainset. Very likely 130mm BCD so won't get a 34t inner ring on that.
For a 48t/34t they'll need a new chainset, and that may or may not be OK with the current BB spindle length. I would look to fit an 'MTB' triple (from my 'box' but widely and cheaply available second+ hand) and use only the outer two rings (eg 42/32). If rivetted, check chainstay clearance of small (unused) ring.
With a 48t large ring, assuming the current chain is the correct length (with the 52t), shorten chain by one (1") link, but it will roll on entirely happily at current length.
Dropping from 52t to 48t will mean dropping the FD down maybe 8mm on the seat tube to keep the cage 1mm above the large ring, for best shifting (drop 2mm per tooth).
There is zero reason to fit a new chain if you're changing chainrings.
OP could 'upgrade' to a 6sp freewheel (assume friction shifting if "vintage") but the largest sprocket is normally 28t. Theirs might be 14-25t 5sp block, so that'd get an extra 12% lower gear.

 

[IanSmithCSE]

Morning,

I recently swapped a 52/42 for a 46/34, with the 11-28 cassette it is a bit iffy with small/small but fine with any of the combinations that would normally be considered as not cross chaining.

This is with a medium cage 105 rear mech from around 2014, but the big issue is that we don't know if your existing chain is a bit short, a bit long or the correct length.

However and this a big however if replacing the crankset rather than just the rings BB length may be critical.

On my Raleigh I have been using an old Shimano 6207 crankset with a 113mm BB, this places the chainrings into the correct position.

I replaced the chainset with an unused Thule 52/42 which rubbed on the chainstay and then a new Shimano Claris that sits so far out that small/small is a straight chain line and the front mech can only just about move far enough to change rings and only if I am on the small sprocket.

I would get a new BB but am planing another change.

Bye

Ian

 

[Big John]

'Compacts' are definitely the way to go. You'll notice a marked difference and a 34 small ring should get you up most stuff. As we get older we quite rightly try to make life easier for ourselves so we can get out and enjoy it. I hope you get sorted soon 👍