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Well i was thinking thats is it ok if you use your stock rear derailleur if you upgrade you cogs/sproket?

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Shimano has used the same rear derailleur pull ratio on all 7 to 9 speed groups and on most older road 10 speed groups too (with the exception of very old Dura-Ace and new Tiagra 4700 / GRX). So, you can use the same rear derailleur. In theory, the chain will become slightly narrower so it might be beneficial to have rear derailleur pulleys that are optimized for 9-speed chains, but I don't believe that will be a showstopper issue. So if you have Shimano rear derailleur, it will work. You will need to update the shifters (unless you have friction shifters and consider it acceptable to continue using friction shifting), the cassette and the chain. Please note that rear derailleur affects the permissible cassette slope and permissible cassette range. So if the rear derailleur supports the new big and little sprocket sizes, and the difference between big and little sprocket teeth counts added to the difference between big and little front chainring teeth counts is not larger than the rear derailleur capacity, it will work.

Chainrings will probably work. 7 speed used practically the same chain size as 8 speed, so if going to 9 speed, although strictly speaking there is 2 generation difference, in practice there is only 1 generation difference in chain inner width. The worst case issue there is that you are using big chainring front and little sprocket rear (largest gear combination), and then downshift in the front before downshifting in the rear. In this case, if using 9-speed chain and 7/8-speed chainrings, the chain could "skate" over the little ring teeth for a small amount of time before engaging the little ring. It isn't reasonable to shift that way, so the worst-case issue will almost never materialize.

However, this applies for Shimano. There are of course other manufacturers whose products may differ.

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  • The problem with front shifting is chainring spacing and outer chain width. If there is just almost enough room for chain between the rings, the chain will go there anyway and it's not fun to remove it.
    – ojs
    Commented Dec 6, 2021 at 21:42
  • Is the cable pull ratio the same for 7-9s MTB groups, maybe excepting XTR? Seems likely, but maybe worth checking as the OP didn’t specify that this was a road bike.
    – Weiwen Ng
    Commented Dec 7, 2021 at 9:51

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