[top_ring]

I`m using a Ti 8 spd 12-28t cassette with a 9 spd chain that I wish to keep and I`m considering the purchase of a DA 7800 rear derailleur. Will it handle a 28t maximum and will it shift precisely with 8 spd? Thanks.

 

[mward]

It'll work fine with 8 speed, it's the shifter that determines the spacing, not the derailleur. No idea about the capacity, however.

 

[Fungazi]

The shifting is no issue (the der moves far enough, and the spacing is controled by the pod) but Shimano specs a 27t max sprocket for the DurAce rear derailuer, and a 29t total tooth diffrence. Your 12-28 is techically a no-go.
However, they spec a 27t max sprocket for ALL thier road products, and I run a 28t sprocket on my 105 equiped road bike (also 8 speed). It shifts very well and has lasted at least 1000 miles, so I think you'll be fine with the DA setup runing a 12-28, as long as you keep the 29t total in mind, or at least avoid certain "cross chain" combos.

 

[pimpbot]

Uh, that's true except...

It'll work fine with 8 speed, it's the shifter that determines the spacing, not the derailleur. No idea about the capacity, however.

... for Dura Ace. Dura Ace pulls at a different rate than 105, 600 or LX or XT. I pulled my hair out with my Dura Ace 8 speed STIs with a 105, a 600, a Sora, an Alivio and an XT derailleur trying to get them all to work in all the gears (I could only get five to work) until my sage-like LBS told me that I need a Dura Ace derailleur to go with my Dura Ace STIs. I found one from a bud for cheap (thanks Aosty!) slapped it on an it worked right out of the box.

This may not be true of Dura Ace 10 speed, but it was for DA8.

 

[Cloxxki]

I only learned about this very recently, but indeed up till 8spd DA used a different ratio. 9spd and 10spd are normal again.

I've seen DH bike with Ultegra derailer and 11-32 cassete, which seemed to work fine. I tried (minimal effort) 11-32 with a 9spd DA, but it wouldn't run smoothly on the 32t. Some b-tension tweaking might have solved that, but I used the stop screw to limit to the 8th position 28t, which was anyway better than the 27t I was used to up to that point.

 

[mward]

Well I'll be damned. You learn something new every day. Thanks for the info, pimpbot.

 

[SocalSuperhero]

I only learned about this very recently, but indeed up till 8spd DA used a different ratio. 9spd and 10spd are normal again.

I've seen DH bike with Ultegra derailer and 11-32 cassete, which seemed to work fine. I tried (minimal effort) 11-32 with a 9spd DA, but it wouldn't run smoothly on the 32t. Some b-tension tweaking might have solved that, but I used the stop screw to limit to the 8th position 28t, which was anyway better than the 27t I was used to up to that point.

I run a short cage 105 on my dh bike with 11-32. Works well, just have to throw a couple extra links in for all the suspension travel stretching the chainline; found that out the hard way after snapping an ultegra in half.