Cassette size and chain length

[PRD76]

Classic question relating to change up from 12-25 to 11-28 cassette and whether or not i need a longer chain. A quick test run seems to show changes all ok up to both large cog combo. Derallieur quite, but not fully, extend and chain is taught but not stretched seemingly. Check out the image - any more experienced eyes spot anything awry?

 

[raleighnut]

That's fine if it's on the large front.

 

[Spiderweb]

Looks ok to me, I would run as is until you need a new chain the add a link.

 

[arch684]

Looks ok to me

 

[PRD76]

Thanks guys - yes on large cog front and back. Position of the derallieur look ok? It's extended but not what I would class as under duress. It is fairly close to the cassette in this position but that's normal isn't it? To be fair it is a combo i would rarely if ever use, but of course my principal concern is safety and damaging hard earned equipment :-)

 

[themosquitoking]

You'll get a better idea of how close it is on the small ring. You're more likely to use it on that ring anyway.

 

[raleighnut]

You'll get a better idea of how close it is on the small ring. You're more likely to use it on that ring anyway.

You're right, might need the 'b' screw winding in a bit.

 

[PRD76]

Exactly, thanks both.

 

[Ajax Bay]

You'll get a better idea of how close it is on the small ring.

Why? The capacity of an RD to cope with a large sprocket is surely the limiting factor here. The wrapping capacity will cope with a drop from 12t to 11t smallest. The set up looks fine to me, perhaps experiment with a turn on the B screw, but I don't see it's necessary, from the image.

 

[slowmotion]

I've always used this chain length calculator. It's never wrong.

http://www.machinehead-software.co.uk/bike/chain_length/chainlengthcalc.html

 

[raleighnut]

Why? The capacity of an RD to cope with a large sprocket is surely the limiting factor here. The wrapping capacity will cope with a drop from 12t to 11t smallest. The set up looks fine to me, perhaps experiment with a turn on the B screw, but I don't see it's necessary, from the image.

The issue is not on the smallest cog but rather on the largest which has increased from a 25 to a 28, this may call for an adjustment to the 'b' screw.
The reason for checking it in a smaller front gear is that with the chain at that length (which is within acceptable parameters) is the 'b' screw probably isn't on its stop with the RD at that extension but will be when a smaller front ring is selected. I'd also hazard a guess that the mech is about at its operating limit (But is still within it, just) and that extra links in the chain might cause problems in small-small with it not being able to tension the chain sufficiently.

 

[Ajax Bay]

The issue is not on the smallest cog but rather on the largest which has increased from a 25 to a 28, this may call for an adjustment to the 'b' screw. . . . The reason for checking it in a smaller front gear is that . . . the mech is about at its operating limit (But is still within it, just) and that extra links in the chain might cause problems in small-small with it not being able to tension the chain sufficiently.

Pretty sure we're singing from the same song sheet. If the OP's chain is the length the image implies on large/large then it will be fine on small/small - the new cassette is only one tooth smaller. As for capacity (assuming 50-34 chainset), 11-28 plus 50-34 capacity is well within spec at 33 and a 28t largest sprocket is an entirely 'normal' cassette to use. Even if the chainset is a triple 52-42-30 the wrapping capacity will be enough (39t). For a very hilly, long ride I am using 52-42-28 with a 12-30 and the wrap is not sufficient - but the RD soon tells me I've inadvertently (say in the dark) got into 28/13 or even 28/12 and I use that audible catalyst to prompt to change up to the middle ring pronto.
[/mini-rant] Why do the manufacturers produce these cassettes with an 11t cassette? With a 12t you still have a 110" gear length with a 50t chainring. Unless racing/time trialling why would one need a longer top gear? At 90 rpm cadence, 50/11 gives you 52kph; 50/12 gives you 48kph. And furious pedaling downhill will get one another 22% (at 110 cadence, if desired). I can only think that Strava may be a part culprit here. The only reasonable requirement for an 11t is with use on a 1 x 11 or 1 x 10 using a 44t chainring (say). But YMMV

 

[PRD76]

Pretty sure we're singing from the same song sheet. If the OP's chain is the length the image implies on large/large then it will be fine on small/small - the new cassette is only one tooth smaller. As for capacity (assuming 50-34 chainset), 11-28 plus 50-34 capacity is well within spec at 33 and a 28t largest sprocket is an entirely 'normal' cassette to use. Even if the chainset is a triple 52-42-30 the wrapping capacity will be enough (39t). For a very hilly, long ride I am using 52-42-28 with a 12-30 and the wrap is not sufficient - but the RD soon tells me I've inadvertently (say in the dark) got into 28/13 or even 28/12 and I use that audible catalyst to prompt to change up to the middle ring pronto.
[/mini-rant] Why do the manufacturers produce these cassettes with an 11t cassette? With a 12t you still have a 110" gear length with a 50t chainring. Unless racing/time trialling why would one need a longer top gear? At 90 rpm cadence, 50/11 gives you 52kph; 50/12 gives you 48kph. And furious pedaling downhill will get one another 22% (at 110 cadence, if desired). I can only think that Strava may be a part culprit here. The only reasonable requirement for an 11t is with use on a 1 x 11 or 1 x 10 using a 44t chainring (say). But YMMV

Thanks for those fantastic and detailed responses guys. Very helpful. Yes, i did indeed read it that you're both singing from the same sheet. The reason for the change was fairly simple - a change of wheels (upgrade) and my old cassette was jammed on. My LBS only had the larger cassette in stock for my set up - we discussed at length and i decided the extra range might prove useful both on climbs and on the flat with a couple of sportives coming up, so I took the plunge in the knowledge i would have to check chain length. Certainly nothing as calculated or vain as Strava times

 

[Ajax Bay]

Depending on how many miles the chain had done on the old cassette, you might find it prudent to change the chain (same length though) at the same time. (Old cassette 'jammed' on??) How?

 

[PRD76]

Depending on how many miles the chain had done on the old cassette, you might find it prudent to change the chain (same length though) at the same time. (Old cassette 'jammed' on??) How?

Agreed, thanks. The locking nut was completely jammed - 3 of us attempted to no avail. The bike builder was a little over zealous methinks.