Biking 101: Accelerating

One of the amazing performance aspects of a sports motorbike is its ability to accelerate.  Standard 1/4 mile times and 0-100kph / 62mph times are staggering and leave all but the most exotic supercars lying in their wake.  Getting these sorts of figures is a test of courage as much as clutch / throttle control, but the potential is there if you possess the right qualities. 

Unlike turning corners, accelerating doesn’t require any seemingly counter-intuitive input from the rider.  Having said that, there are some interesting points to make about acceleration*.   Under hard acceleration, the rear suspension of a motorbike becomes less compliantNewtonian physics states that an object at rest is inclined to stay at rest until a force acts upon it.  This is quite observable in everyday life – you can feel a weight transference when a vehicle begins to move.  This is because initially, this weight is at rest and until the energy is transferred to it, it will continue to remain at rest.  On any vehicle with sufficiently compliant suspension, this will cause the vehicle to “squat” at the rear when accelerating.   However, after an initial compression of the rear suspension, the motorbike appears to “stiffen up”.  Even though there is more suspension travel to be had, it becomes harder for it to use.  Here’s my explanation of this:

A chain driven motorcycle has a small amount of slack in the chain.  This slack is necessary, as the distance between the two sprockets changes as the swingarm moves up and down.  – This is because the front (drive) sprocket is not located at the pivot point for the swing-arm.  At rest, gravity ensures that this slack is present on both sides of the chain.

Image showing the slack in a chain

When accelerating, the chain is pulled through by the drive sprocket.  Due to the tendency of the rear wheel to remain at rest, this pulls the top part of the chain taut. 

Tensioning of chain 

The harder you accelerate, the greater the difference in inertia of the two sprockets.  As a result, the distance between the top of the sprockets is minimised.  This is achieved with the aid of the weight transference and the suspension squats.  Once this shortest distance has been achieved, further suspension travel requires the distance between the tops of the sprockets to be extended again.  It’s not that this can’t occur, it is just an additional force that needs to be overcome.  Any let-up in this force will see the suspension return to the state where the tops of sprockets are minimally spaced.  As such, under hard acceleration, the rear suspension becomes distinctly non-compliant.

The second point to make about hard acceleration is the tendency for the bike to “wheelie”(or “wheel-stand” if you prefer to sound like a boffin).  In its simplest explanation, this is just a characteristic of a large weight transference to the rear of the bike.  Normally, the speed of the sprockets at their outer radius is the same.  If you can increase the speed of the front sprocket such that it exceeds the rear, then the front sprocket will “climb the chain”.  This can be demonstrated with two pens and a rubber band:

  1. Place the rubber band around the two pens to represent the chain and sprockets of the bike.  Keep the rubber band under enough tension, to ensure it grips the pens.
  2. Hold one pen in your right hand on the surface of a desk.
  3. Twist the pen in your left hand anti-clockwise (or counter-clockwise if you live in the US!)
  4. If you’re holding the right hand pen still, the left hand pen will “climb” in a clockwise direction around the right-hand pen.

This characteristic also holds true in shaft drive motorcycles, but the right-angle gearing makes it more difficult to demonstrate with mere office stationery.

Modern sports-bikes and drag bikes run longer swingarms than older bikes.  This helps prevent the bike from wheel-standing, for the same reason that a fat kid needs to sit closer to the middle of a see-saw to balance a light kid on the other end.  That is, the amount of torque required to lift the front of the motorbike becomes greater, the longer the swing-arm.  If you don’t have offspring of wildly differing weights (or a see-saw) you can try my second desktop experiment.  For this one, you will need a ruler and a smallish weight.
1. Place the ruler on the desk, such that one end extends 5cm (2 inches) past the edge of the desk.
2. Place your weight on the opposite end of the ruler.
3. Now push down gently, on the end of the ruler that sits over the edge of the desk.
4. Move the weight closer to the edge of the desk, and repeat step 3.

You will note that as the weight gets closer to the pivot point, it becomes easier to lift. (By now, I expect most of you are going “well duh!”).  It’s this same idea that makes the longer swingarm a less wheelie-prone bike.  Like every element of design, there is a compromise that must be reached – as swingarm length increases, suspension performance is reduced as is the turning ability of the bike.  But that’s a story for another day.
* Like my previous entry on cornering, what I state here is based on my observations and my understanding of physics.  Please feel free to leave a comment if you think my statements are not correct.

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