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Crank Length

Skid SoloSkid Solo Posts: 29
edited January 2009 in Road beginners
Hi, I know there are many topics on subject but I am still confused after looking at them all, here is my scenario :

• I am 5’8” with an inside leg of 29” (trouser measurement, I have not actually measured real inside leg)
• I normally ride mountain bikes, and use a 175mm crank.
• I purchased a road bike 2 years ago for some winter training, a Trek 1200, which came with a 170mm crank length
• I have damaged the frame on the Trek and have purchased a replacement, and I am about to build up the new frame
• I want to replace the crankset on the new road bike frame and looked at buying a new set. However when I looked into cranks and there were various lengths.
• I did some further exploring and found that my ideal length would be under 170mm
• Being predominantly a mountain bike rider I don’t seem to spin high rpm, I struggle to get to a cadence of 90rpm and on my road bike I normally have an average Cadence of 60-65.
• So short cranks don’t seem to offer me an advantage as I seem to use power more than rpm in my riding style.

So can anyone offer me any advice as to what size crank I should purchase, I originally wanted to go for the 170 but don’t want to be that much difference to my mountain bikes, so am thinking 172.5

Posts

  • alfabluealfablue Posts: 8,497
    I would go for the 170. I don't see the need for similarity to the mtb, gears and riding are all different anyway. I ride 170 road, 175 mtb (this is pretty standard for most people, the thinking is you need more torque on the mtb, and more rpm on the road). That said, the differences do not make a great difference in practice.

    How about learning to spin faster on the road, you can do it, its just changing a habit. You will probably find a higher cadence more efficient.
  • rb1956rb1956 Posts: 134
    1) Please measure your leg length properly. It is crucial to many bike fit issues. I do wish the bike world would stop talking about inseam, inside-leg etc. because it's so dangerously misleading. Trouser length is so vague as to be meaningless, but it is usually substantially shorter than leg length. If you're building a new bike, please consider getting a proper bike fit done.

    2) There are lots of different formulae around relating leg length to crank length. It is one of the more controversial issues in cycling. However a 2.5mm or even 5mm difference is one that I doubt you'll even be able to detect.

    3) Unless you're a racer, a cadence of 90rpm isn't especially low. Over typical rides I average 80rpm and peak at 105-110rpm. However, learning to spin faster usually means developing a smoother, more even pedal-stroke, and that is always a good idea.
  • carefulcareful Posts: 720
    However a 2.5mm or even 5mm difference is one that I doubt you'll even be able to detect.
    - RB1956

    I agree with RB1956. I have slightly different crank lengths on different bikes. In theory the saddle needs to be slightly lower for longer cranks but a couple of mm shouldn't make any difference.
  • Ride the crank length you are comfortable with. There is very little to no difference in our ability to generate power over the range of crank lengths typically available with bikes.

    There's an item on crank length here if interested:

    http://www.cyclefitcentre.com/further%20reading.htm
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