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  1. #1
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    Trainer and Crank Length

    My buddy ask me this and I don't want to give him my answer, in case I am wrong.

    He has an older 26" bike on a trainer. Cranks are 170 mm.

    The bike he plans on racing is a 29er, with 175 mm cranks.

    What say you?

  2. #2
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    It'll be fine. The difference in pedaling technique is not that big.

  3. #3
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    I use 170 mm on my road SS bike, 172.5 mm on my geared road bike and 175 mm on my SS and geared mountain bikes. My trainer (it is a CycleOps 300 Pro) has 170 mm also.

    I hardly notice it. The Q factor is a bit more noticeable, but even that goes away in minutes for me.
    My rides:
    Lynskey Ti Pro29 SL singlespeed
    GF Superfly 29er HT
    S-Works Roubaix SL3 Dura Ace
    Pake French 75 track

  4. #4
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    Same here, my indoor bike has 170 mm, too. I can't tell the difference to my road bike (172.5) or my MTB (175).

    The only thing bothering me is the wider q-factor. This is the reason for not getting a fat bike though our winters would justify it. But q-factor isn't an issue for a bike-trainer-setup.

  5. #5
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    The main thing in factoring crank length is RPM. I would say that Q-factor is also a pretty important issue with trainers since generally you will spin more and change positions significantly less than when riding outside.

    But back to RPM: I find that for every 5mm of crank length, I can comfortably turn the cranks at 5-10 RPM slower. I have tried all kinds of different lengths and read lots about it, but at the end of the day, that's a prime factor. There are plenty of debates out there with the pros and cons. I still see top-level pros of my dimensions that are riding 177.5s and 180s so that's what I'm sticking with.

  6. #6
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    Quote Originally Posted by chomxxo View Post
    The main thing in factoring crank length is RPM. I would say that Q-factor is also a pretty important issue with trainers since generally you will spin more and change positions significantly less than when riding outside.

    But back to RPM: I find that for every 5mm of crank length, I can comfortably turn the cranks at 5-10 RPM slower. I have tried all kinds of different lengths and read lots about it, but at the end of the day, that's a prime factor. There are plenty of debates out there with the pros and cons. I still see top-level pros of my dimensions that are riding 177.5s and 180s so that's what I'm sticking with.
    Sorry and not trying to be a dbag but that's rubbish. The ONLY factor in crank length is fit.

  7. #7
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    Quote Originally Posted by Bdabike View Post
    My buddy ask me this and I don't want to give him my answer, in case I am wrong.

    He has an older 26" bike on a trainer. Cranks are 170 mm.

    The bike he plans on racing is a 29er, with 175 mm cranks.

    What say you?
    Ideally, one would like to mirror crank length on the trainer and the bike that will be used in competition. However, in this case there is also the addition of a wheel size/gearing difference. Either way, in the end we can all only produce so much power no matter what the length or wheel size or gearing happens to be...

    I'd say he shouldn't worry about it too much.
    The 14 warmest years have all occurred in the 16 years since 1997.

  8. #8
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    Quote Originally Posted by Walt Disney's Frozen Head View Post
    Sorry and not trying to be a dbag but that's rubbish. The ONLY factor in crank length is fit.
    Incorrect. Fit is relative to leg length, agreed (this is a rule of thumb though, there is no evidence or correlation other than experience and feel). With regard to RPM, it is plainly faster to turn a shorter arm and slower (with more leverage) to turn a longer arm, that's just a physical fact and not debatable.

  9. #9
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    Trainer and Crank Length

    Just battled this one also. Trainer (old road bike) had 170, roadie has 172.5 and mtb has 175. Ended up getting a 175 for trainer. My reasoning is wanting to train on a similar bike to what I race. I also setup trainer with a flat bar and geometry nearly identical to mtb for the same reason.

    Also I agree that crank length can affect cadence.

    All said, glad I made the changes.

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