New build - Gearing decisions

lewishamilton
lewishamilton Posts: 138
edited November 2010 in Road buying advice
I know this comes up every so often but wanted your opinion.
I'm building myself a carbon bike (not specifically for racing although I have done one or two triathlons. I'll use it as my summer bike and also anytime the roads are clean and dry).

I currently have a compact 50/34 12-25 cassette, which is ok, but I would prefer to have a gear or two more to prevent me spinning out at high speeds.

I'm getting a Sram groupset and have the option of 50/34 or 53/39 and cassettes 11-23, 11-25, 12-25, 12-27.

I was thinking about getting a 53/39 with 12-27 cassette OR 53/39 & 11-23?

Am I being really naive, or is it much of a much (I've looked at the sheldon calculator and it may as well be in arabic).
Also, can I run a short cage with the 53/39?

Comments

  • Forgot to mention.

    This is my first build. I've been offered a new Sram Rival groupset for £350. Good deal? or do you think there will be bargains to be had after christmas?
  • sungod
    sungod Posts: 16,551
    having 53/11 should stop you spinning out

    what's best depends more on the low end though...

    if you've got no problems on hills with 34/25, then 39/26 will be about 10% harder, so probably will be ok, (34/23 is almost 9% harder than 34/25, so you can get a feel on your current bike for what 39/26 would be like)

    you ask about 53/39 with 11-23, that'll give you a lowest gear around 25% harder than your current lowest (about the same as 34/20)

    using your current bike, see what's the lowest you need, put the gears into sb's calculator

    then you can experiment to see what you'd need on the rear to get the same gearing with a 39 small ring

    short cage should be fine, it's maximum capacity that determines derailleur type (capacity is the sum of front and rear differences between biggest and smallest, 59/39 and 11-26 would be 14+15 = 29)
    my bike - faster than god's and twice as shiny
  • unixnerd
    unixnerd Posts: 2,864
    Go with 53 and 11-25. I have 52x12 and above 40mph I'm spinning out (on my rare visits to that speed range). At the top end of the scale I know I could go faster with an 11, but I only tend to need it going down big hills.
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  • amaferanga
    amaferanga Posts: 6,789
    I'd get a 50/34 and 11-23 and change the 34 to a 36.

    If you get compact cranks you can fit 53/39 rings to the cranks later, but as I'm sure you know you can't fit a 34 or 36 to standard cranks.

    Spinning out with a 50/12 won't have much effect on your speed anyway - I raced with a 50/12 and I can't remember spinning out.

    If you can turn a high cadence for a few seconds then 50/12 should be fine. If you can't turn a high cadence then you should learn to....
    More problems but still living....
  • Monty Dog
    Monty Dog Posts: 20,614
    This is a frequent misconception, get a cadence meter and simply learn to pedal faster / more smoothly - it'll make you a better cyclist. FWIW spinning-out is when you start bouncing at 150rpm plus - whereas most people struggle to hold 100rpm. Even racing I rarely use anything bigger than 50x12 and that sees me to over 70kph
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