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I've been given an old bike from the road which has gears where you can't tell what gear the lever is in (there is no number) just a lever in one of three positions or seven for the back).

I am wondering if I upgrade this front gear lever system do I have to upgrade one, both or neither of the (1) rear cassette and (2) rear derailuer?

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    Note that if you have a non-indexed setup (one lacking detents in the shifters), you need to replace the sprockets to make it indexed. This is because the "ramps" on the sides of the cogs are an integral part of the indexing system. In theory, though, you'd not need to change both front and rear -- only the end that you want to be indexed. But there may very likely be a chain width difference between old and new setups.
    – Daniel R Hicks
    Commented May 26, 2014 at 3:49
  • Well, it sounds like they are indexed since he states a specific number of positions. Also, not seeing which gear number you're in doesn't really matter - you just have to have a feel for if you're near the bottom or top of your gearing so you know if you need to shift up/down, you can (and you'll likely keep it in your head anyway, which is why aside from weight weenie-ing, higher component groups don't have indicators for what gear combination you're in).
    – Batman
    Commented May 26, 2014 at 13:15
  • @Batman - Yeah, it's unclear. Certainly most better quality shifters don't display the gear numbers -- such a display is mostly the realm of inexpensive twist shifters. (Of course, in part this is because it's impractical to display a number on "brifters" and bar-end shifters.)
    – Daniel R Hicks
    Commented May 26, 2014 at 14:23
  • But, to the OP: If you have indexed shifters and want to replace them (for whatever reason), you can get away with replacing just the shifters if the old and new shifters are in compatible "families" -- that is, they "pull" the same direction (only matters for front), and they have the same amount of "pull" per click. But determining if this is true can be hard if the shifters are of different brands.
    – Daniel R Hicks
    Commented May 26, 2014 at 14:26
  • Its up to Alivio level on Shimano i think, which is acceptable (at least, my Alivio stuff does).
    – Batman
    Commented May 26, 2014 at 15:57

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You can completely change the front setup (shifter, cables, crankset and/or derailleur) completely independently of the cassette and rear derailleur/shifter with no problem. If it's a used bike it may be worth checking the chain/cassette are not worn in case the do need to be changed (but would be unrelated to the fact that the front setup was changed)

Update - As per comment from @batman, if the size of the crankset has changed the length of the chain may need to be updated also

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    Nearly independently. One could conceivably change a front crankset enough to exceed the chain wrap capacity of the rear derailleur, but its an edge case.
    – Batman
    Commented May 26, 2014 at 3:13
  • Also, when upgrading old bikes one needs to beware of chain width differences.
    – Daniel R Hicks
    Commented May 26, 2014 at 14:18

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