Bicycle Mechanics - Choosing a rear derailleur

Hi

I have to replace the rear derailleur on my MTB and would want some help for choosing it.

I actually have a Shimano Deore XT RD-M735 (model from about 1993).

I looked at Shimano website for a replacement model and was surprised to see that there is 4 models of Deore XT rear derailleur. (RD-M751-GS, RD-M751-SGS, RD-M760-GS and RD-M760-SGS)

My front sprockets are 28, 38 and 48. I have 7 rear sprockets ranging from 13 to 30.

I suppose I need a SGS model because (48-28)+(30-13)=37 and the total capacity for GS is 33T. The difference between RD-M751 and RD-M760 seems to be the link length but I don't understand what it means.

Also, I don't know if theses derailleur designed for 9 rear sprockets will work with my 7 rear sprockets and shift lever.

Any tips on derailleur choice would be welcome.

The rear derailleur isn't specific to the number of cogs in the cassette, it's the shifter that has to be in sinc with the cassette. To use any of the larger cog-cassettes, like a 30, 32 or 34 cog you should use the super long cage model (SGS). The wide pivot link model is designed to be a bit more rigid and allow better/smoother shifting under a load than the narrow link. Maybe, I don't know. My Ultegra RD is a wide pivot link design and it shifts nicely, my Hybrid has the XT SGS with a narrow pivot (can't remember the model number) and it shifts fine too.

Hi-

Given that you have a triple and a wide range cassette you want a long cage derailer. I suggest an acera rd. They are cheap ($22). When the pulleys need replacing, don't get new pulleys, replace the entire derailer.

It really doesn't matter as long as it's a long cage Shimano derailer. You can even run a 9 speed Ultegra if you want to.

http://community.webshots.com/photo/549737266/2846979690068014369opcapm#

The 760 is a low normal, or rapid rise derailler, which works the opposite way round to most deraillers. The 751 is the more conventional top normal (could be the other way round, I can't really remember). As far as I'm aware you should have no compatibility issues.
Just get the one that matches your current derailler the best, which I assume would be a 751 long cage. Personally I'd stick with XT; XTR is unnecessary, LX is noticably worse after a few months riding (but still not a bad setup).

The 760 is a low normal, or rapid rise derailler, which works the opposite way round to most deraillers. ... I thought that entire concept went out with the old Simplex and Cyclo bandspring rear derailleurs. I admittedly haven't snapped a shift cable in years, but I would worry that slippage or breakage of the control cable could throw the chain into the spokes. I'll stick with normal high on the rear derailleur, thank you. (In the early 1970s, I did use SunTour's reverse-shift normal high front derailleurs, which worked fine for half-step gearing, but less well for large drops and/or triple chainrings.)

They brought it back when they released the flappy dual controls. At the moment I'm using one rapid rise and two conventional deraillers, and honestly couldn't say which is better. All the criticism I've heard levelled at the newer rapid rise stuff seems to be along the lines of "I don't like it" rather than any real reason why it shouldn't work.