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At what angle can you put the front wheel of a bicycle and still be able to steer it?

  1. May 20, 2009 #1
    just like the title says at what angle can you put the front wheel of a bicycle in relation to the ground and still be able to steer it with moderate percision now when i say angle i mean like a low rider you know the bicycles that have the really long front fork and look cool so basically my question is what angle is the lowest you can go?

    thankyou,
    Kamen
     
  2. jcsd
  3. May 20, 2009 #2
    Re: at what angle can you put the front wheel of a bicycle and still be able to steer

    I'm not sure which angle you're referring to - do you mean the head tube angle, i.e. the angle between axis of the the steering tube and the horizontal? Or, are you asking how sharply you can steer the wheel and still ride, i.e. without "jack-knifing".

    The last part of your post makes me thing that it's the head tube angle. Something you should be aware of is that the stability of the bike depends not only on the head tube angle, but also on the "fork rake", which is the distance by which the hub of the front wheel is offset from the rotational axis of the steering tube - it's basically the bend that you see near the bottom of most bicycle front forks.

    The combination of the head tube angle and the fork rake determine what's called the "trail", which is the distance on the ground between the contact point of the front wheel and the imaginary point where the steering axis intersects the ground. If you had no fork rake at all, but the head tube angle is less than 90 degrees, then the trial is negative, i.e. the wheel contacts the ground at a point behind where the steering axis intersects the ground. This is very important, since it's what makes it easy to keep a bike upright (it's a myth that the gyroscopic effects of the wheels make a significant contribution to balancing a bike). As you lean the bike in one direction, say to the left, the negative trail is what causes the front wheel to steer to the left in response (you can see this just be leaning a stationary bike to one side without touching the handlebars). The effect is that if you start to fall to one side, so that your center of mass is no longer over the support points (i.e. you're about to fall over), the bike automatically steers itself in the right direction to put the contact points back underneath you.

    The fork rake is there to control exactly how much trail you have - usually it's several cm. It sounds like you're thinking of modifying a frame to give it an extreme head tube angle - i.e. far from vertical. This will give you an enormous amount trail unless you also have a fork with a long rake.

    This all gets kind of absurd if you have an extremely long front fork and a very shallow head tube angle, because the steering axis is getting closer to horizontal. This means that instead of turning the front wheel to the left or right around a near-vertical axis, you're actually leaning it to one side or the other around a near-horizontal axis. Combine that with the long fork rake you'd want in order not to have a ridiculous amount of trail, and you end up with the situation that when you steer, you're actually trying to move the front wheel across the ground in a big arc.

    There's a good reason highly maneuverable bikes aren't designed this way. Choppers look great going in a straight line, but when you steer them, they start to look a little sillier. There's no magic angle where you suddenly can't ride it any more, though - it just gets worse and worse the more extreme you make the angles.

    More than you wanted to know, I'm sure, but I was bored ...
     
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