Replacing rear derailleur and shifters when upgrading from 3x7 to 3x9 drivetrain

Question

I am considering upgrading my old trekking bike from a 3x7 to a 3x9 drivetrain. The bike is currently equipped with a 26/36/46 Shimano Exage FC-M320 which I intend to keep due to its wide sprocket spread and relatively good condition (bottom bracket will be replaced). Front derailleur is a Suntour FD-XR15 which you cannot find very detailed information about, but should have a chain line of 47.5 mm.

As the upgraded bike should be used for touring, I would like to have a wider gear range, especially for the lower gears to make climbing steep passages with heavy load easier.

Since the rear wheel will be replaced, switching to a more modern drivetrain became an option and I fancy with a 11-34 9-speed cassette (e.g. Shimano CS-HG400-9) combined with an SRAM X5 rear derailleur (long cage version).

The maximum capacity of the 9-speed X5 rear derailleur is specified as 45 teeth (given in reseller 'fact sheet'). I calculated the required capacity as

(46-26) + (34-11) = 20 + 23 = 43

which seems to be within specification.

Can I keep the existing front derailleur and crankset? Will the chain line be affected by changing the cassette and rear derailleur resulting in compatibility issues which I missed? Do have to take care about the derailleur hanger?

Note: The existing Shimano shifters/triggers will also be replaced with SRAM X5 3x9 ones and all shifting cables will be swapped out.

Answer 1

Your proposed changes should work together well. Important points are that the shifter and derailleur are the same brand (and the shifter speed matches the number of cogs in the rear cassette). Using a Shimano cassette in a SRAM system (or conversely a SRAM cassette within a Shimano shift system), has NO compatibility issues as the inter-cog spacing is the same between brands. You'll want to get a new 9 speed chain for 1) 9 speed systems have narrower spacing at the cassette and require a chain that has a narrower outer diameter. Brand of chain doesn't matter. All 9 speed chains are compatible. 2) running a new chain on a new cassette allows them to wear together. Running a stretched (used) chain on a new cassette will cause premature wear on the cassette and subsequent replacement of the chain on to the abnormally worn cassette may cause slipping of the chain. New cassette, new chain for drivetrain longevity

At the front, your current set-up is fine as it seems to be working well now. Replace the bottom bracket with one having the same size spindle so that the 47.5mm chain line is preserved. Obviously the BB spindle shape must be compatible with the crank (square taper for square taper, Octalink for Octalink, etc). Overall it's helpful to think of the front and rear drivetrain as somewhat independent of each other. In other words, the speed class doesn't have to be the same (don't need a 9 speed marketed crankset when changing to 9 speed rear drive) if you keep things +/-1 speed class. You can consider the 7 speed front drive like an 8 speed since the chain wheel spacing is nearly the same. Overall, with a 9 speed rear drivetrain, one can run an 8 or 10 speed front drive and expect perfect shifting and function. A caveat here is that the chain you use is designed for the size (speed class) of the rear drivetrain. As mentioned above, you'll require a 9 speed chain.

There shouldn't be any radical departure of the chain line imparted by the 9 speed rear drivetrain, which utilizes a 47.5mm chain line on mountain bikes. One aspect to look at is the Freehub body of the new wheel. It's width. You want to make sure is an 8,9, 10 speed mountain Freehub and not a too short 7 speed freehub. Also if the wheel happens to be equipped with a road 11 speed freehub, spacers will be required before cassette installation so the lock ring can tighten things up. Again you'll want an HG (Shimano style) freehub to fit that 9 speed cassette on to. These are by far the most common design out there but new wheels can come equipped with SRAM XD drivers or Shimano microspline free hub bodies, both of which are incompatible with your cassette selection.

Answer 2

7 speed front derailleurs work awkwardly and are prone to chain drop with 9 speed chains. Internet advice about front derailleur compatibility loves to get this wrong, but the difference from the width of the cage is meaningful. It would be difficult to get it to index right even with a compatible shifter, which a SRAM shifter isn't.

If you're getting SRAM 3x9 shifters and an X5 RD, just get any SRAM or Shimano mountain triple front derailleur that will work well with the 46. FD-T4000 is an example. The crankset will be fine with a 9-speed chain and FD.

Many bikes with Suntour 7-speed parts (or from that era) are 130mm in back. Presuming it's steel, the good approach is usually to cold set the frame to 135 if you're replacing the wheel. If it's 700C you could also use a chunky road wheelset, but if it's 26" then a production replacement wheel in sizes other than 135 doesn't really exist. Cramming in a 135mm modern wheel isn't ideal for a bike that will see good mileage going forward; people do get away with it but there is risk of breaking a dropout, which will be worse the more you ride it and/or the stronger you are.

Answer 3

Perhaps no need to change the whole crankset, but fwiw I would expect to have to change the current 7-speed front chainrings to 9-speed compatible equivalents.

9-speed chains are a bit narrower than their 7-speed equivalents in order to allow them to work correctly with cassettes that have the sprockets packed more tightly together (more here). Without changing the chainrings would end up having to choose between either a 7-speed chain that's too wide for the rear cassette. Or a 9-speed chain that might be a touch too narrow for the front chainrings.

Might work - no harm in carefully trying with a 9-speed chain (don't want to risk new components) - but unfortunately having to replace chainrings seems likely.