Rear cassette compatability problem?
 

Hi everyone,

I have a 2008 Cannondale System Six 3 which, until recently, has been running on stock components. I recently changed the rear cassette from the 12-25 to a more forgiving 28 (both Ultegra parts) but it seems like cassette is the wrong size. For example, when I'm in the largest rear cog, it looks as if the top jockey wheel is too close to the cog teeth and the chain can't run through smoothly enough. Is a 28 teeth ring just too large or can I just adjust the derailleur arm (using the b-screw IIRC) to move everything down to create more clearance? I hope that this makes sense and that someone can help。

Sounds like the issue is with the RD, and it's capacity, not the cassette, if probably have a short cage (SS), when you need a medium cage (GS). To work out exactly what you need, you need to know the number of teeth on the cassette & crank, see here for more details http://sheldonbrown.com/harris/derailers-rear.html

Did you get a longer chain ? Perhaps a slight b-screw adjustment would help move the pulley away a bit ?

Thanks for the reply. I thought that all RDs were the same size-wise but maybe I'll need a new one. It would be a shame to have to change the RD as it's DuraAce, though. And thanks for the Sheldon Brown link - I haven't used that site in years.

I put on a new chain but kept it at the same length as the old one. I'll try moving the pulley down a bit with the screw. Thanks.

If you get a larger cassette, you should get a longer chain. If your chain is too short, you run the risk of damage when shifting into big-big.

The B screw should move the pulley away from the cog. I have also heard of people swapping the pulley itself for a smaller size (typically 11t to a 10t) to get more clearance.

IIRC you can get a joining link and add that to your existing chain, but you may need to add two to get the length you want, and not sure if adding 2 is recommended.

1. Why do people say "rear" cassette?
2. 25t to 28t doesn't necessarily mean you need a longer chain - don't change it unless it doesn't pass the big-big test.
3. 28t is not too much for Dura Ace, and it doesn't have anything to do with the cage length.
4. B-screw will almost certainly fix your issue.
5. The top pulley is the "jockey", and the bottom pulley is the "idler" or "tension" pulley.

Rear cassette is like front windshield.

As DiabloScott posted, cage length normally has absolutely nothing to do w/ max cog size...a longer cage only does one thing...it wraps more chain. It does NOT allow larger cogs to be used. A derailleurs capacity is not related at all to its max cog size.

Advice from not-so-experience armchair mechanics...no one here knows for sure if you need a longer chain. If your chain was sized correctly (small/small) you most certainly do NOT need a longer chain. If your chain was originally sized using the big/big method, you probably do need more links.

As posted by a couple of people a B-tension screw adjustment will most likely take care of your problem as even the older D/A derailleurs w/ a stated max cog of 27 will work just fine w/ a 28. Carefully check your chain length...​big ring and slow shifting up to the big cog in a work stand.

From practical experience, yes it does, had a case a few years a go, with a 11-34t cassette, was getting the issue as described by the OP using a GS RD, changed to a SGS, and the issue went straight away.

This is your personal experience ...the Shimano spec for the derailleur is what it is designed to work with. Let me repeat my previous post, THE ONLY THING A LONG CAGE DOES IS WRAP MORE CHAIN, generally for use w/ a triple. That is all. Unless the geometry of the derailleur body is changed, then it makes NO DIFFERENCE in the max cog size that will work w/ that particular derailleur. Shimano has always spec'd the same max cog size for their SS and GS derailleurs of the same model/time period. While you may have had good luck using one w/ a mtb cassette, this is not what the longer cage is supposed to accomplish.
Bear in mind there is "what works for me" and there is "the right way to do things so it works for everyone"...the reason it worked for you has to do w/ the length of your derailleur hanger not the length of the derailleur cage.

Yup -

Series XTR
Shifting Action Top-Normal
Mount Type Direct Attachment
Maximum Sprocket 32-36T
Minimum Sprocket 11T
Front Difference 18T
Total Capacity SGS: 41T GS: 35T

As for the OP's Dura Ace - he mentioned 2008 components, so I figured 7900 series which lists 28t largest sprocket. 7800 listed 27t, and 7700 listed 26t. (Usual weasel words here about pushing those boundaries by a tooth or two.)

Basically, there are derailers designed for big cogs versus derailers designed for small cogs and derailers designed for triples versus derailers designed for doubles/singles. The cage length, as others have mentioned, is designed to offer more wrap capacity; this is the numerical difference between the smallest/largest sprockets as well as the smallest/largest cogs. (Other strategies have been used as well; feast your eyes!:http://www.disraeligears.co.uk/Site/...system%29.html. Max cog is determined by geometric differences on the cage that go beyond my understanding and the scope of this thread, but consider that SRAM offers, for example, x9 rear derailers in short-, mid-, and long-cage versions, all of which have a max cog of 36t--but divergent chainwrap capacities.

It's also worth noting that both wrap and max cog is influenced by the length of the derailer hanger, too.

As a note, it seems that a lot of 'net bike mechanic "conventional wisdom" has been informed by Sheldon's site and the ubiquitous Shimano offerings that dominated the market at the dawn of the internet. When Sheldon wrote the bulk of his site, Shimano only made long-cage (sgs) r.derailers for mtb, and nothing longer than gs models for road groups. Which, in a way, made Sheldon's accurate crib sheets confusing to some, who got the correct info, but didn't understand the math/mechanics behind it all.

OP: read up a minute, then mess with the b-screw. Therein lies your fix. You may not necessarily need a longer chain, but you ought to make sure you're running the correct length regardless before proceeding.