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Chainring size, shape and wheel size

jimothy78jimothy78 Posts: 1,407
edited October 2017 in MTB general
Hi all - been away from the forum for some time, as I've not had a lot of time for riding recently.

Unfortunately, my bikes got nicked from the garage, so I've just replaced them with an Orange Crush Pro.

Both my previous bikes (hardtail and full susser) were 26" and I was comfortable running a 32T chainring and 11-42T 10-speed cassettes on them.

Now the new bike is 650b and comes specced with a 30T chainring and 11-42T 11-speed cassette. I know there's going to be a slight increase in gearing due to the slight increase in wheel size, but I think dropping down to 30T is going to be over-compensation. So, I'm thinking that if I'm going to get a new chainring anyway, I may as well pick up a 34T oval, rather than get a 32T round one.

Is my reasoning sound, or have I under-estimated the step-up in gearing due to the wheel size issue?

Posts

  • cooldadcooldad Posts: 32,599
    Why not ride it and see?

    And why a round 32 or oval 34?
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  • cougiecougie Posts: 22,512
    Just ride it and see as suggested.

    I'd not bother with oval rings.
  • jimothy78jimothy78 Posts: 1,407
    cooldad wrote:
    Why not ride it and see?
    Waiting for other parts, and if I'm going to change the chainring anyway, then I'd rather not use the original at all, so I can sell it "as new".
    And why a round 32 or oval 34?
    I was under the impression that if you used an oval, it was standard practice to go up a size, the reasoning being that at the hardest point in the pedal rotation, the oval's effective radius is the same as a standard ring 2T smaller, so it follows that if you can turn a 32 at that position you can turn a 34 oval, because they're the same radius at the position that it matters most?
    Not the clearest explanation sorry, but sure you can follow the gist.
  • The RookieThe Rookie Posts: 27,757
    That was the perceived wisdom until the research that followed suggested that any benefit wasn't sufficient to actually justify that perceived wisdom. Those using Ovals now stick to the same size.

    Any benefit of oval rings ranges form close to nothing to about 1.5% depending on a number of factors plus experimental error depending on which research you read, and that's for road bikes, for MTB's it is likely to be less but the nature of MTB usage makes doing the research itself quite hard, fair to say the biggest benefit is for those that prefer the feel, makes them feel comfortable etc rather than a measurable performance benefit.
  • jimothy78jimothy78 Posts: 1,407
    The Rookie wrote:
    That was the perceived wisdom until the research that followed suggested that any benefit wasn't sufficient to actually justify that perceived wisdom. Those using Ovals now stick to the same size.

    Any benefit of oval rings ranges form close to nothing to about 1.5% depending on a number of factors plus experimental error depending on which research you read, and that's for road bikes...

    Ok, thanks for clarifying. However, would it be fair to assume that there might be more potential benefit for an MTBer (especially when using flat pedals) as our (or at least my) pedal stroke tends to be a lot less even and efficient than the average roadie, especially at the ragged end of our range of gears? Not arguing, just wondering...
  • Uber_PodUber_Pod Posts: 110
    On a road bike, it's pretty much all about pedalling. You don't do much else.
    On a MTB, you spend a lot more time not pedalling. So any change to pedalling makes a smaller overall difference.

    As far as oval vs round goes, the biggest selling point of a new bike is the manufacturer saying it will make you faster. If oval rings made a measurable difference, I'm pretty sure all new bikes would have them.

    Going back to your original question, you are the only person who can tell what gearing works best for you.
    As others have said, try what you have. If that doesn't work for you, spend the 20 odd quid for a new ring and go with that.
  • The RookieThe Rookie Posts: 27,757
    A 26" wheel to a 650B wheel (with the same size tyre) is an increase in gearing of about 3.8%.
    A 32T to a 30T is a reduction in gearing of 6.6%.

    So the net change is two parts of naff all from your previous bike (about 1/4 of the difference between two gears on the cassette). If the new bike has wider (so bigger radius effect) tyre it will be even less.
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