JC 911

This may be a stupid question but I’ll ask it anyways….

I have DA7800 with a 12-27 cassette. Shimano’s cross reference chart on their site says that I cannot use an 11-28 cassette with DA7800.
Why is that?

pmt

Because Shimano didn't sell one at the time that was made. It works fine, go right ahead.

Garfield Cat

It works, the Ultegra 11-28 on the Dura Ace 7800. I have that on right now. Climbs nice too.

JC 911

Thanks - I'll make the change!

ericm979

Shimano says not to do it because their rear derailleur spec says 27t max. But 28t will work.
If you pair it with a 50/34 crank then you will also be exceeding the derailleurs wrap spec, but by quite a bit instead of a small amount. That may not work right in all gear combinations.

Psimet2001 

+1 to what has been mentioned so far. The "issue" is/was total chain wrap capacity.

scirocco

Yup, 11-28 shifts perfectly with DA. But, as others have said the chain wrap issues may mean you also need to change the rear derailleur to the long cage version. The short cage derailleur with an 11-28 cassette with front 53-39 has the chain just sagging slightly in the 39-11 which you will probably never use. But with a 34-50 compact it will not be taut in several of the small rear cogs which could lead to problems.

pmt

Nah, compact crank works fine. Nobody is going to do something dumb like cross-chaining small to small, so it really makes no difference if that works or not. Short-cage is fine.