What is it that allows a different derailleur to handle a larger cog?

[No]

It's strange, out of the 5700 and 5701 105 rear derailleurs they both look identical but one can take a 30t and the other a 28t cog. It's not the cage that allows this. What is it that allows this though?

Actually, make that the 5700 and 5700A. The instructions for the 5701 aren't up (but the parts view is???).

[NZPeterG]

Hi No,
Its in the design of the Derailleur, I think in the angle the two link arms of the derailleur move!

As I have said before you can run a XT, XTR, or any of the good top end 9 Speed Derailleurs with your 10 speed shifters

None before I get cut down Why Not 10 speed Mountain Bike Derailleurs?
Well Shimano changed the pull of there Mountain Bike Derailleurs to help with better shifting on there mountain bikes (because of the dirt and mud I think  ) the road 10 speed derailleurs and the mountain 9 speed derailleurs have the some pull and anyone of them will work with a 10 speed and 9 speed shifter.

I hope is of help to you 

Pete

[No]

If only it didn't add so much to the cost!

I just needed a new cassette. To add a new rear der is to much at the moment.


SO! Apparently I can use a 30t cog. I have no idea what the model number for my derailleur is. It's nowhere to be found. But, the cage says 30t and the instructions say 30t, so 30t it must be!

[triaesthete]

Hi No

Rear mech. maximum sprocket size capacity for a given rear mech. is ultimately limited by the clearance available between the largest sprocket and the upper jockey wheel (guide wheel).

(Not to be confused with gear range capacity-ie tooth difference from big to small sprocket- which is governed by cage length)

Shimano ratings are normally conservative and can "sometimes" be exceeded by a small margin.

In this instance some rear mech. hangers sit lower than others. A lower hanger (distance from hub axle centre to hanger bolt centre) de facto increases max sprocket size any given mech will handle. Altering the mount pivot adjuster screw may also vary this clearance a little.....

Put your bike in low gear and check the gap between the upper jockey and your 28t sprocket. If it is more than 10mm you "may" well get away with 30t.

It's a "fit and try" job.

If it does not work you can insread fit a smaller front ring to get the gearing drop. I like to use http://www.bikecalc.com/gear_inches to compareratio changes.

Happy fiddling
Ian