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Is it practical (or is there any reason not to) upgrade components on a bicycle when they are replaced? For instance, I've got a 10 speed cassette on a tour bike that needs to be replaced. The bike is mostly Tiagra components, but is there any reason (other than cost, but in this case it seems like it's only about a $35 difference) to not upgrade to 105 or Ultegra? (Staying within the same 10 speed part.) Are there certain components that have to be replaced as a set if I upgrade?

I have looked at this compatibility chart, and if I understand it, it is saying that an Ultegra ST-6700 is a drop-in replacement for a Tiagra ST-4600. But I'm not sure if that is everything I have to look at, can I just do a drop in replacement of any component in any box?

I'm also concerned about whether there could be other issues with mis-matched components, for instance will a better component in one place lose some of its benefits or be degraded by a lower grade component that it interacts with?

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  • Remember to replace the chain at the same time as the cassette!
    – Qwerky
    Commented Oct 24, 2014 at 17:18
  • The CS-6700 has a solid spline on the big gears so if you are cutting into the freehub it helps. Yes it is only $35 more but that is about twice as much and it is not going to last twice as long.
    – paparazzo
    Commented Oct 24, 2014 at 18:37
  • @Blam looks like if i want a higher tooth range for 11T (e.g. 11-28T) I'll have to go to the Ultegra range anyway, as Tiagra/105 only go to 11-25T.
    – Michael
    Commented Oct 25, 2014 at 16:13

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There's no harm in doing so, as long as you identify compatible components.

You do take a hit on cost, by which I mean that components work out a lot cheaper when bought as part of the bike, rather than standalone. So economically it would probably make sense to stuff the money in a piggy bank, and save up for a new bike with better components, since when that time comes you'll get more bang for your buck. But if your pocket is willing to take that hit, go for it.

I haven't looked at your specific case but in general, upgrading from something Shimano to something Shimano will work, as long as both mechs have the same number of gears on the cassette. 6700, for example, is a 10-speed system so as long as 4600 is also 10-speed, you should be safe.

The other caveat to bear in mind (when upgrading brakes/shifters) is that a couple of Shimano mechs have different pull ratios to others. In fact, there was a question about this just a couple of days ago which I remember links to a document explaining what mechs are affected.

Lastly, while I start by saying there's no harm, there might also be no gain either. That's going to depend on you, and on the rest of the bike. So you're right to raise this in your question, but it will result in "no gain" rather than "harm".

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In my opinion, the only time it is worth upgrading is after the part in question has worn out. If that is the case for you, and if you plan to keep the bike for a while, I would say that upgrading the cassette is a decent investment. We're not talking about a huge outlay to explore different equipment. Your experience might nudge you toward or away from "better" parts all around on your next bike.

Theoretical benefits of an Ultegra cassette over your Tiagra cassette:

  • Weight savings (a bit of Googling tells me an Ultegra 11-25 6700 cassette weighs 209g and a Tiagra 11-25 4600 cassette weighs 263g, or about 2 ounces difference). Not much that you will notice, but it all adds up.
  • Better shifting. The Ultegra cassette is much more aggressively ramped than the Tiagra cassette to encourage the chain to move more quickly between gears.

Do you have to upgrade other parts at the same time/will better parts be degraded by lesser parts they work with?

  • In this case you need to change (but not necessarily upgrade) your chain or you will never get good shifting with any new cassette. Overall you won't see the full benefits of an Ultegra bike vs a Tiagra bike without changing out everything, but as I stated at the start, I wouldn't upgrade anything that wasn't worn out. It's just too expensive to do, so you may as well use what you have as long as it still works. There should be no "degradation" in terms of "damage", but the better parts won't shine like they will if working in concert with a full group of the same quality.

It all comes down to a cost-benefit analysis. The costs are obvious, the benefits are not always as clear because there is too much advertising hype. In this particular case, I would personally spend the $35 extra to try out Ultegra if I planned to keep the bike.

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  • Yeah both the chain and cassette are worn out.
    – Michael
    Commented Oct 25, 2014 at 1:55
  • I guess the main concern I had was, if I don't change the chainring too, would that be a problem, since it would have slightly different characteristics over the rear cogs.
    – Michael
    Commented Oct 25, 2014 at 1:56
  • The chainrings are really only in charge of the front shifting, so in theory that won't be a concern. BUT, if the chain is extremely worn there might be more wear on the chainrings than you realize, which will affect front shifting when you put the new chain on. Chainrings being larger than cassette cogs wear relatively slowly, but the teeth will wear (often look like shark fins) if the chain is very bad. You won't be able to use the big Ultegra 6700 ring on the Tiagra crank, because the Ultegra ring is hollow and has a different profile. 105 5600 might work, but you'd have to research that.
    – pjd
    Commented Oct 25, 2014 at 2:12
  • Also, I usually advise that unless you obviously need new chainrings and have some time to play with, you start by just replacing the cassette and chain. You will know if you need new chainrings on your first ride. If you do, you can then have your shop order the rings. If you don't, you saved a decent amount of money.
    – pjd
    Commented Oct 25, 2014 at 2:15
  • if i'm going to replace the chainrings i definitely want to tweak the tooth count. but that seems like the subject of a new question.
    – Michael
    Commented Oct 25, 2014 at 4:05

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