CentreOfGravityYou’ve read frequently in cycling magazine articles and advertising:

“…this tyre/Chainstay configuration/handlebar stem/product, blah, blah, blah, will make your bike more stable…”.

It is assumed by these ‘experts’ that your bike has inherent stability.

Try this for yourself: get off your bike, stand beside it and let go of it. What happens? It falls over. You have just proved a scientific fact.

So much has car technology wormed its way into our brains that ‘a-low-centre-of-gravity-increases-stability’ dictum is now applied to all wheeled vehicles. It’s certainly true about cars, or tricycles for that matter, but not bicycles.

Just let’s make this quite clear: your bicycle is not stable, never has been, never will be. If it’s not stable in the first place, it cannot be made more stable.

In terms of bicycle design, we should be thinking more along the lines of ‘controlling instability’, or even better, ‘maintaining balance’. Using phrases like this help us all understand the basic issue; riding effectively is much more challenging if you don’t even know your bike is completely unstable because you’ve been assured by the experts that it is, at least, more stable than another bicycle.

It isn’t.

Posted by ajantom

The extreme upright position is akin to riding a horse.

The high centre of gravity means that little movements of your body can shift your weight around easily and make it easy to steer over really rough and steep ground.

Steering a bicycle isn’t always to do with where you point your front wheel; it’s more to do with where and by how much you can shift your centre-of-gravity; the ease with which this can be done is determined by its height from the ground.

These factors lead me to conclude that a bicycle with a high centre-of-gravity is not, as the experts would have it, more unstable.

All bicycles are unstable.

If designers and manufacturers cannot, or will not recognise this fact, they’ll never be able to produce a bicycle intended for off-road riding that is relatively easy to control.

A high centre-of-gravity allows you more flexibility to balance and steer, in essence the same thing, even if your front wheel is not in contact with the ground.

This fact is demonstrated by those skilled riders who can pull a wheelie (or ‘sunfish’, as this manoeuvre was called in the 60s) and ride on their rear wheel for considerable distances. A more extreme demonstration can be seen in videos of off-road unicyclists, who don’t bother with a front wheel at all!

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About gmacleland

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4 Responses to Stability…

  1. Pingback: Tiller effect… | Cleland > L A N D S E E R

  2. Pingback: Leverage… | Cleland > L A N D S E E R

  3. 2wheeltrails says:

    Not sure if you are saying the following in your post:

    The angular acceleration, a, of an object is inversely proportional to the moment of inertia, I, given the same torque. Roughly speaking, the moment of inertia is proportional to radius^2 between CM and center of rotation (the ground). Torque is proportional to radius * weight, r*w.

    Newton’s law for rotation is (simplified): I*a = r*w. Then, solving for a: a=r*w/m*r^2. Cancelling the r’s, we see that a is proportional to 1/r.

    Thus, a higher center of gravity will make the bike more stable.

    99% of the bike companies and cycling reviewers do not understand this, and it makes me cringe when I read that a lower centre of gravity makes a non-stationary object, like a bike, more stable.

    It’s just wrong, as you have noticed, too. Thank you! Cheers!

    • gmacleland says:

      Well, I try to keep my explanations as straightforward as possible. Moreover, I am an ‘intuitive engineer’. I always designed my bicycles with a high centre-of-gravity simply because it made handling so much easier.

      The difficult bit was working out exactly why, because no-one, but no-one was going to agree with me. First, I had to decide whether I was imagining the effect, or not. Once I realised the effect is real, with the help of a few friends, figured out the physics.

      Once I’d figured out the physics, no-one, but no-one understood what I was talking about. So I tried to simplify the whole thing in terms most people would readily comprehend.

      Unfortunately, after all that effort, hardly anyone agrees or believes me. Their loss. Consider yourself one of the cognoscenti!

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