Changing rear sprocket size

GuyBoden

Fat bloke, pedalling slowly and enjoying the ride.
Location
Warrington
I am trying to select a rear socket for my Wife's bike, so it is easier for her to pedal.

Currently my Wife's bike has a 45T front and a 18T rear sprocket.

The chainstay length is 435mm.

The current chain is 51 links (51inch), which includes about a 3/4 inch slack.

The bike has vertical drop outs, so there is no room for moving the wheel backward/forwards.

There is no chain tensioner on the bike and I don't really want to install one.

I've been using this chain length calculator:
http://www.machinehead-software.co.uk/bike/chain_length/chainlengthcalc.html

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18T (Current sprocket)
For a 18T sprocket the recommendation is:
Recommended length = 51 inches or 51 links (Which is what is currently on the bike.)
Unrounded result = 50.2706255585656 inch
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22T
For a 22T sprocket the recommendation is:
Recommended length = 52 inches or 52 links
Unrounded result = 51.197128486344006 inch

Do you think the result for the 22T sprocket would work, with enough slack?
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24T
For a 24T sprocket the recommendation is:

Recommended length = 52 inches or 52 links
Unrounded result = 51.664740048919334 inch
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The 24T does not seem to have enough slack.
But, would a half link chain work with the 24T?

I've calculate that adding a half link to a 52 link chain would work:
Subtract the Unrounded result from the Actual chain size give the slack.

52.5 - 51.67 = 0.83 inch slack

So, am I correct in thinking that a 52 link chain plus a half link would be ok for a 24T sprocket?

Thanks
Guy
 
Last edited:

CXRAndy

Guru
Location
Lincs
With vertical drop outs and refusal to use a chain tensioner, its going to be a compromise fitment.

Another option is a internal geared hub, like Shimano Nexus or Alfine 8
 

TheDoctor

Resistance is futile!
Moderator
Location
Stevenage
With vertical drop outs and refusal to use a chain tensioner, its going to be a compromise fitment.

Another option is a internal geared hub, like Shimano Nexus or Alfine 8
Which would also make the chain tension a bit of a compromise.
It's an expensive option, but if you don't want a tensioner derricks eccentric hub could be the way to go.
Personally, my SS / fixed uses slanted ends.
 

12boy

Veteran
Location
Casper WY USA
I've found a sweet spot on a couple of bikes with verical drops although as soon as the chain wears that perfect tension is lost. Here's a cheap fix in you are going SS and not fixed. Throw in a wheel with a shimano or sram freehub, find the gear that works tension wise and replace all the others with spacers. You can consult Sheldon Brown's gear inch calculator, or another one ,if you wish, to figure out different chain ring sprocket combos in terms of pedalling effort. The sweet thing about the freehub for SS is the ability to get a good chainline. I would probably just buy a $25 shimano tensioner and skip all this work. That will allow easy future chages as well. Bromptons have verical dropouts and use a tensioner which I find works fine. This also allows for 2 chainrings so as to have a granny gear with a manual shift for those really steep hills.
 
OP
GuyBoden

GuyBoden

Fat bloke, pedalling slowly and enjoying the ride.
Location
Warrington
I got a Half link chain and a 24T rear sprocket in the post today and fitted them, I went for a short ride, it seems to work very well indeed..................
 
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