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New Crank System Concept Design

xxccxxcc Posts: 13
edited March 2010 in MTB general
Hi everyone, I'm new to the forum. If mods think I'm posting in the wrong place, please move the thread.

I'm a design student who'd come up with a new concept for a crank system.

The deal is to use a hollow crank design with a rolling weight inside (black ball in the image). When the cyclist pushes the crank down from its upright position the weight (ball)gradually rolls down in the same direction as the pedal - this should mean the cyclist needs to put in slightly less energy than when using standard cranks (the ball should be pretty heavy but not add to much extra weight to the bike itself...).

I know the system won't make a huge difference in speed - if any - but the project brief is aimed at ordinary people not racers. We're only trying to make it _slightly_ less strenious going uphill, just so the cyclist won't need to put in quite as much energy.

Note that this concept is in a very early stage of development. I haven't specified materials etc. yet but do you guys think this is viable as a concept at all, given the project brief mentioned above? Is it worth looking into at all or should it be abandoned for something else? Comments much appreciated, positive or negative feedback likewise!


  • Stick with me on this, I'm a bit of an ape when it comes to stuff like this I'm afraid.

    I assume you'd have both cranks being hollow? If so, how is this likely to affect the stiffness of the crank arms as a whole?

    How heavy does the weight need to be for it to make a difference?

    What meterial would you use that's small enough and heavy enough to sit inside a crank arm?

    Will all this weight bouncing around in the cranks make the ride unstable?

    and when one crank arm is at the top of the stroke, and unweighted, the other is at the bottom of the stroke and has X amount of weight that you need to pull upwards?

    Please understand, this isn't me trying to be negative or anything. I know it's still early days, but at this stage, I'd seriously doubt ADDING weight to cranks is going to makle a positive difference (although, as stated above, I am an ape so I really dont know).

    Sorry if that's a bit of a useless response, I'm sure some more knowledgable types will input in a bit.

    Oh, and it should probabbly have gone in the "general" section ;)
  • supersonicsupersonic Posts: 82,708 Lives Here
    I don't see how it would work - why would it take less energy? it would take more, surely, to accelerate the crank?
  • ilovedirtilovedirt Posts: 5,798
    I'm no physics student, but i would imagine the force added by the ball rolling down the crank would be countered by the other ball having to come up, making the whole thing pretty pointless?
    Production Privee Shan

    B'Twin Triban 5
  • chris_swchris_sw Posts: 100
    If you're relying on gravity to move the balls around it would only make it harder to pedal. Also you might find centrifugal force makes the weight stick at the pedal end of the crank at high cadence, just giving you a heavy crank.

    To make it easier the weight would need to be at the pedal end on the down stroke, and at the bottom bracket end on the up stroke, relying on gravity would give you the opposite result.
  • lugsey2k5lugsey2k5 Posts: 960
    hello. again not wanting to destroy your idea but it wont work.
    as stated centrifugal forces would just pin the weight to the bottom of the arm at a medium candecance. also as a wise man once said "you don't get out for nowt". the force required to raise the weighted end would equal the force exerted by the opposite weight.
    as the right arm is moving past 12 o'clock to come down the weight is still in the centre. the left arm weight is still at the end so acting to make the crank heavier to pull up. as the right arm passes the 3 o'clock position it would begin to move the weight towards the end due to "gravity" which yes would aid the crank a tiny amount. this help is countered by the extra force required to raise the left crank from the 6 to the 3 o'clock position.
    if that all makes sense that is im too lazy for diagrams.
    sorry. you could still follow it threw for a project anyway but write in your conclusion that it didn't work. my science teacher said the easiest conclusions to write are the ones were something went wrong.
    good luck and hope to be proved wrong
  • bike-a-swanbike-a-swan Posts: 1,235
    what he said...

    conservation of angular momentum, they'll cancel out.

    although it looks to me that the extra rotational inertia at the bottom of the stroke could help to carry the crank through the 'dead spot'. however, this could just as well be achieved by making the pedals (or shoes) heavier, because the weight won't return to centre until well into the upper section of the stroke. in fact, the fact it returns to centre for the top would probably mean the inertia effects would cancel out too. Perhaps if you were to fit it on a spring, so the weight only moved outwards once past 45 deg below the horizontal and moved back in before the main section of the stroke when the cranks are levelish. That might help carry through the dead spot. However, Q rings already have this effect and they have no moving parts. Moving parts = weight and things to go wrong, so the benefits have to be fairly hefty to justify.

    Sorry, my brain just sort of headed off on its own there
    Rock Lobster 853, Trek 1200 and a very old, tired and loved Apollo Javelin.
  • belugabobbelugabob Posts: 158
    Two words - pertpetual motion - I suggest that you Google them.

    Nice try, but then, I suppose that's what being a student is all about - learning.

    If you didn't learn anything today, you weren't paying attention!
  • belugabobbelugabob Posts: 158
    And another thing, even if it did work, the noise of the ball rattling around inside your cranks would drive you nuts - in about 30 seconds flat!

    If you didn't learn anything today, you weren't paying attention!
  • stuart_c-2stuart_c-2 Posts: 805
    Unless you could control the position of the weight, maybe using some kind of pulley system, it would probably be counter productive.

    If you're after a new concept, maybe look at variable length crank arms? Longer in the down and up stroke for leverage, but shorter across the bottom and top of the stroke? It may have some benefits for cadence, not sure. It could also help reduce pedal strike if the crank shortens at the bottom of the stroke? Advantages may be negligible, but it should still work for a design project?
    "I ride to eat"
  • Daz555Daz555 Posts: 3,976
    "Design student" needs to pop along to the mechanical engineering dept and also have a chat with the physics guys. - look at "over balanced wheels".

    History has shown that the "perpetual motion" trap is an easy one to fall into.
    You only need two tools: WD40 and Duck Tape.
    If it doesn't move and should, use the WD40.
    If it shouldn't move and does, use the tape.
  • bike-a-swanbike-a-swan Posts: 1,235
    also, more moving parts = more energy lost in friction
    Rock Lobster 853, Trek 1200 and a very old, tired and loved Apollo Javelin.
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