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If I spun my cranks backwards on all my previous bikes I would get a good few spins out of them, maybe as many as three or four on one of the bikes. But with my current road bike I can barely get a full rotation out of it.

Does this matter? Is it an indication of the friction within the drivetrain, or just the way that some setups work, either by design or as a side-effect of how they're designed?

I'm guessing that it's the rear hub that slows it down, but there's no restriction in speed when riding fast or freewheeling that I can detect, and no excessive noise. It was an almost brand new Cannondale Optimo and the BB has been serviced.

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    What are the other bikes? How heavy is the crankset/pedal combination on this bike compared to the others? A lighter crankset will have a lot less angular momentum, and with the pedals any weight difference can really change the moment of inertia of the crankset/pedal combination because the pedals are the farthest from the rotation axis. Commented Feb 11, 2021 at 15:18
  • They were sub-£300 8-speed hybrids and this is a 10-speed Tiagra road bike, so yes this one may weigh less.
    – Wilskt
    Commented Feb 11, 2021 at 15:37

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I don’t think it’s a good indicator. Plenty of viscous lube and lightweight (-> low inertia) components can easily mean that a high end bike spins worse than a 20 year old bike with dry and rusty bearings.

The most important thing is friction under load, which is hard to measure. But you should make sure that bearings spin with little force (to make sure they don’t have excessive preload), that they don’t have any play and that they turn smoothly through the full 360°.

To check if the hub’s free wheel mechanism has excessive friction, spin the rear wheel. If you have quick release axles, make sure they are not excessively tight. To check if it’s the bottom bracket bearings (more likely) take off the chain and spin the cranks. If it’s neither of them it has to be the rear derailleur.

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All things being equal the crank should spin backwards just as far for each bike.
Backward crank spin is not a standard test for drive train friction but the difference you have noticed is worth looking into and understanding.

Things that could be different causing different amounts of spin:

  • Force, did you apply the same amount of force to each crank?
  • Weight, did all of the drive trains weigh the same amount? This includes pedals, crank, chain, freewheel.
  • Derailleur adjustment, nothing rubbing
  • Friction,
    -- Chain friction chain stiffness and cross chaining or chain line come into play
    -- Freewheel friction
    -- Bottom bracket friction

Just because it's a new bike does not mean any bearing group is adjusted correctly.
Some sealed cassette bearings don't spin as freely as other bearing types.

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    I bet the weight and moment of inertia of the crankset and pedals on this bike is a lot less than on the other bikes. Commented Feb 11, 2021 at 15:19

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