11-25 to 11-28

bikeman66

Senior Member
Location
Isle of Wight
Hi all,

I'm going to be purchasing an 11-28 cassette for my bike, which will replace the original 11-25. I'm doing this as a bit of extra insurance to help get me to the top of Mont Ventoux in a few months time, but would appreciate some advice from the learned folk on here as to whether a new chain will be required too.

The chain that is currently on the bike is fairly new, and has only covered about 200 miles, so meshing with the new cassette shouldn't be an issue, but are the extra three teeth on the large sprocket likely to cause a problem?

The rear mech is a Shimano Sora, with what I believe is a long cage (about 73mm centre to centre of jockey wheel pins. Chainset is a 50-34

I know a new chain won't break the bank, but don't want to purchase one if there really isn't any need.

Thanks in advance for any advice you guys might be able to give.
 

robjh

Legendary Member
I wouldn't expect the chain length to be a problem - unless that is your chain looks ultra-tight on the 25-tooth sprocket already and is pulling the derailleur out at an extreme angle. I made the change from 11-25 to 11-28 a couple of years ago, and though I put on a new chain at the same time I don't remember sizing it any differently from the old one.
 

neil_merseyside

Veteran
Location
Wirral
I always keep the bits of trimmed chain so I could a link back if needed - you still got your offcuts?
Can you use big-big (on a stand, not riding) and not have mech fully forward? AND small-small and have some movement left in mech?
If so you might get a way with chain as it is. 3 teeth is only a link and a half.
 
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bikeman66

bikeman66

Senior Member
Location
Isle of Wight
Just measured things up, and the chainstay is 408mm, current chain is 53 links (or 106 depending on which way you count). The big/big combination does not appear to be close to maxing out the rear mech. Obviously the big/big and small/small set-up is not one you would normally use, so as long as the mech has the capacity to deal with these extremes and taking on board the information you guys have come up with, it would appear my current chain would be OK. I guess I'll give it a go with just the cassette and then get a new chain if it all goes pear shaped.
 
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bikeman66

bikeman66

Senior Member
Location
Isle of Wight
If you measure the chainstay length you can use an online chain calculator to work out how many links you need for each cassette. The chainstay length is the distance from the centre of the bottom bracket to the rear wheel bearing.
http://www.machinehead-software.co.uk/bike/chain_length/chainlengthcalc.html
Thanks for the heads-up with this Slowmotion. Just typed in my info and dimensions and it recommends 53 links, which is what is on the bike already. Sorted!
 
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