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I am a beginner and I have a Trek Marlin 5 2020, it has quite a crap Shimano Tourney derailleur. I was wondering if I could swap it out with a SRAM GX DH 7 speed derailleur and shifter? Would it work with the existing ‘cassette’ which is a Shimano HG200 freewheel, 12-32 7 Speed from what I know, I may be wrong. Was wondering if it would work without having to change it to a SRAM 7 speed cassette meaning having to change the whole wheel to a freehub.

Please help me out, cheers :)

Chris

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Unfortunately your plan will not work.

The compatibility issue is the spacing of the cassette/freewheel sprockets. I'm doubtful that SRAM uses the same spacing as older or cheaper 7 speed drivetrains. (If I were designing for them I'd make the cassette as narrow as possible to allow the drive side hub flange to be moved out for greater wheel strength).

However, even if the sprocket spacing were the same the GX DH 7 derailleur would still shift awfully on your 7 speed freehub because it does not have the ramp and tooth profile design of the GX cassette.

It's a better idea to save your money and purchase a better bike. Trying to put higher end components on cheaper bikes almost always just does not make economic sense.

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I suspect your bike has poor shifting and you believe the derailleur is the culprit. This is not always the case. The shifting is a sum of many parts:

  • Shift lever
  • Quality of cables and housing
  • Proper length of cable housing segments
  • Derailleur
  • Cassette / chainrings
  • Chain

Not only that, but the system can be damaged. For example, if the derailleur hanger is bent you won't get any better shifting by changing the derailleur to a better one. In fact, you won't get any better shifting by changing every listed part to a better one!

I used to have a really cheap bike with Shimano Tourney derailleur too. The shifting was crap, and it stayed crap no matter how much I adjusted the derailleur cable tension barrel adjuster. The cause? A bent derailleur hanger.

To get a 100% guarantee of better shifting, you first must diagnose whether your bike has a bent derailleur hanger. Then you must replace every part in the bullet point list with a better one.

Anyway, derailleurs have a wider compatibility than what manufacturers claim. You don't always need to change a 7-speed derailleur to another 7-speed one.

From the bible: https://www.sheldonbrown.com/speeds.html

Within a given brand/style of rear derailer, all "speed numbers" are generally interchangeable. This applies to all indexable models, basically everything manufactured since the late 1980s.

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