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Why am I not faster with shorter cranks? (Oh no, not crank length again...)

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  • #31
    The mass of the leg above the knee also moves through a smaller distance when the leg is closer to straight. But there is a down side of shorter cranks that less of the range of useful muscle movement can be applied.

    Efficient transfer of energy in a mechanical system is mostly about impedance matching. Ideally the impedance of the load needs to match the impedance of the drive. Most of the time a unicycle has far too little impedance with a lot of the energy going into moving the leg rather than the uni.

    The moving leg mass can be reduced by getting some of the motion from ankle. Less force can be applied by the calf muscle but the force isn't the limiting factor at high cadence.

    At normal cadence I mostly use ankle movement like a suspension as a rapid adjustment to keep in contact with the pedal by extending the ankle as the wheel drops into a depression. A good example of this is when crossing a kerb ramp between the footpath (sidewalk) and the road. If you hit the bottom of your stroke at the bottom of the dip you need to quickly find some extra leg length.

    You don't want the mass of your body taking the whole excursion through the dip and takes too long for your whole leg to speed up. The goal is to more or less make your body go straight across, coming off the saddle while dropping the uni down the dip.

    Similarly the shock of a small bump can be absorbed by bending the ankle while the power continues to be steadily applied by the quads. The energy being provided by the quads can be elastically stored in the Achilles tendon and recovered as you come off the bump. Be conscious of keeping the ankle somewhere in the mid range of its movement until it becomes automatic.

    Try to think of the uni movement as quite independent of the rider. The massive rider should move smoothly while the uni does the excursions, just like a suspended wheel on a car. Push yourself upwards before hitting a dip to compensate for the subsequent loss of support as you let the uni drop into it and come back out.

    Ankle movement can also be used to significantly extend the pedalling arc which is vital on hill climbing. Bending the ankle allows the rider to apply forward force to the pedal earlier near the top of the stroke and similarly, backward force at the bottom by extending the ankle. It becomes less of a series of short sharp shoves to a more continuous rotation.
    Triton 36" + 29" | KH 29" | KH 26" | KH 27.5" Muni | Nimbus eSport Race 24" | Torker LX 24" | Qu-Ax Luxus 20" | Qu-Ax Profi 20" | KH / Impact 19" hybrid


    • #32
      Scoox, are you thinking about switching to shorter cranks? A couple more experienced riders mentioned they moved to shorter cranks. But that was over an extended period of time. 127/20 seems like a good combination for learning basic skills.

      Regarding efficiency: I pulled out my G20 (you read that right) last night and practiced shifting and riding around the neighborhood in high gear. For the most part, riding in high gear felt more efficient, because I was covering a greater distance for every pedal stroke. But, sometimes I started getting out too far in front of the guni, and I would have to suddenly exert myself to make a correction. I transitioned from using slow twitch (low load, repetitive) muscles to fast twitch (heavy load, sudden) muscles. Making too many sudden, big corrections in high gear wears me out. I think the same applies to shorter cranks. They have the potential of being more efficient than long cranks, so long as we're not playing tug-of-war with them.

      The OP mentioned that he wasn't going much faster on shorter cranks. While it's easier to spin fast on shorter cranks, it's more of a challenge to maintain a steady cadence. I noticed that yesterday on my G20 in high gear. I was more likely to lose momentum from corrections.


      • #33
        Originally posted by Scoox View Post
        Old but very interesting thread.
        Yeah, it's the best one I have yet seen on this perpetual topic, though it is mostly focused on efficiency for reaching and maintaining the highest possible speeds. There are a lot of other things people do on unicycles.

        Originally posted by Scoox View Post
        1. When pedalling, our quadriceps do most of the work by pushing down on the pedals.
        True, but when pedaling a unicycle, there are a lot of other leg muscles that get used besides the quadriceps, sometimes in opposition to them. Pedaling a unicycle on level ground is often described as spinning the pedals, rather than pushing them, and when riding over uneven surfaces, your biceps femoris sometimes has to work pretty hard to keep you balanced.

        The flight of stairs I mentioned previously on a different thread consists of steps that are about 1.5 meters long, so they are not exactly steps. They are each about 20 cm high, and not fully horizontal. It is more like a series of downhill ramps than a stairway. When I ride off the end of each one, I have to accelerate upon landing to absorb the impact, then decelerate to stay in control and get on top of the wheel for the next drop. I have to pull backward so hard on the top pedal, especially on my 29, that I am always scared it will shoot out from under my foot, but so far my pedal pins have never failed me. My enjoyment of this activity is part of why I switched back to 125mm cranks on my 29 from 110s, however.
        Last edited by song; 2018-06-28, 06:44 PM.


        • #34
          But with shorter cranks do you keep your feet still? I noticed when making speed, I "pump" with my feet to make the rotation, mostly for a short sprint. But at that case I try to keep my legs still - of course they go up and down a little bit since my feet aren't hobbit feet


          • #35
            Yeah, my calves seem to do more of the pedaling work when I use shorter cranks. Short cranks also put a bit more stress on the backs of my knees, I think, although the real reason that became a problem when it did may have been too much wheel walking and/or returning to the pedals, - I have never been 100% sure- but in any case the problem went away.