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I am running on a Shimano Tiagra 10 speed (CS-HG500-10) (12T-28T) and (50-34T front). I'd like to change the smallest cog (12T) to something smaller, i.e., 11 or even 10, to improve my sprints. Is that a possibility?

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    Changing from a 12t small cog to an 11t small cog is only going to help your sprints if you're spinning out on the 50-12 combination. A 50-12 combination will reach 39 mph/63 kph if you're pedaling at 120 RPM. If you can't sprint at 120 RPM for 30 seconds or so, you'll help your sprinting a lot more if you work on your ability to pedal with power at a high cadence. – Andrew Henle Jun 17 at 15:32
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    I'd like to add the comment that even if you can do it, it might not be the best plan. Spinning out is not very likely, but even if you need higher gears, the overall drivetrain efficiency is better if you switch to larger chainring instead of smaller cog. – ojs Jun 17 at 16:53
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    I think it is easier to buy one of the many cassettes already mady with 11T. It seems to be the default or the more common combination these days anyway. – Vladimir F Jun 17 at 17:18
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    You can swap for an 11-28 cassette. There's a SHIMANO 105 CS-5700 10-speed of that range. – Carel Jun 17 at 18:08
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    @Amir bikecalc.com/cadence_at_speed calculates a table of cadences required for a certain velocity in each gear combo, and there's another table there for the calcularting speed given cadences. Have an explore ! – Criggie Jun 17 at 21:50
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Yes you can.

You could need a new cassette screw too, the current one you have will be too big for the 11T cog.

Realistically, the 1T different will not make as much a difference as improving your techique, muscle/power, tyre and air pressure choice and even nutrition.

Make sure the new 11T cog is for Shimano 10s cassette, it might not fit otherwise. Worst case scenario, you can get a complete new cassette, Like a 11T-25T or 11T-28T etc. Plenty of brands available with them.

I don't think you can get a 10T cog on it.

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  • You most definitely cannot get a 10t cog onto that cassette. That would require the new micro spline freehub, it would require Shimano to design a 10s Tiagra cassette with a 10t cog, and it would require pretty big legs to turn that 10t cog over. That would be bigger gearing than a 53 chainring to 11t cog combo. – Weiwen Ng Jun 18 at 22:02
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I believe that an 11-tooth cog is the smallest you can get on a Shimano cassette, but yes, you can replace the 12 with an 11.

A couple caveats:

  • The second sprocket will be sculpted to pick up the chain from a 12-tooth sprocket, so downshifts may be rougher.
  • Obviously the 13 > 11 jump is a lot harder than the 12 > 11 jump.
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