Tag Archives: Riding theory

Biking 101: Decelerating

So far we’ve covered going faster and we’ve covered going around corners.  The last essential ingredient is of course, stopping (or just slowing down).  Motorcycles use two separate hydraulic braking systems which operate independently of each other and (for most cases) affect one wheel each.  The front brake is controlled by a hand-lever on the right hand side whilst the rear wheel is controlled by a pedal activated by the rider’s right foot. 

Ironically, my current motorcycle (a Honda VFR 800) uses a system that sends a differing proportion of the braking force to both wheels when using either the brake lever or pedal.  It does this in an attempt to make braking a safer venture than it may normally be in the hands of an unskilled operator.  The weight transference (towards the front of the motorcycle) that occurs when braking, adversely affects the amount of braking force that can carried out effectively by each wheel.  Some figures suggest in dry conditions as much as 90% of the braking force can be delivered via the front wheel.  The “linked brakes” of the VFR are Honda’s solution to removing this judgement from the rider.

When you look at the contact patch that the front wheel has on the ground, you begin to realise that there is a lot of momentum being shed through a very small area.  Motorcycle training will teach riders that they need to “set up” their braking: transferring weight progressively to the front and thereby compressing the suspension and tyre gently.  As this weight transfer occurs, the tyre is flattened out on the ground, increasing the size of the contact patch.  This in turn allows more force to be applied in a controlled manner. 

There is a theory in physics known as the “Conservation of Energy”.  It states that “energy can neither be created nor destroyed. – It can only be converted from one form to another”.  A motorcycle, or indeed any mass when moving is said to have “kinetic energy”.  The faster it goes, the more kinetic energy it has.  Therefore, stopping a motorcycle reduces the amount of kinetic energy the bike has.  But, because of the “conservation of energy”, we know that this energy hasn’t been lost.  What has happened to it?  Chiefly, it has been converted into heat energy.  – That’s what brakes do, they turn kinetic energy into heat energy.  This heat energy is then dissipated through both the air and the braking components, thus doing its own little bit to help keep the planet warm…

Now the astute amongst you may be thinking along the lines of “it takes a lot of power to accelerate a motorcycle quickly, how can I generate the strength required to stop it as quickly, simply by squeezing the brake lever?”  If this thought has crossed your mind: Well done!  It shows you’ve been paying attention…  The answer lies in the fact that you are utilising a hydraulic brake system.  In cars and some top-end motorcycles featuring ABS systems, the brakes include a mechanical/electrical system to increase the force you can apply to the brakes yourself.  I’m not going to go into how these systems work, rather, I’ll stick to a plain-vanilla style brake set up found on most “conventional” motorcycles.

Hydraulics work on the principal that you can’t compress liquid.  In our case this liquid is brake fluid.  At the lever (or pedal) end, moving the level pushes a piston, which in turn pushes the liquid through the brake line(s).  At the other end of the brake line is the “brake caliper” which contains one or more pistons of its own.  With nowhere else for the liquid to go, these pistons are now displaced too, which pushes the brake pad onto the brake disk.  The disk is attached to the wheel, and so is rotating, whereas the pads and caliper are fixed.  When the disk and pads come into contact, there is friction which converts the kinetic energy into heat energy and “voilà!” you are slowing down!  (Hopefully slowing fast enough to avoid a sudden impact with the scenery…)

This still doesn’t explain how you manage to provide enough force for the brake pads to grip the disk with the necessary bite to stop.  Well, the really cool thing about hydraulics is known as “Hydraulic Multiplication”.  If you change the size of the piston at one end, you can increase the force this piston pushes with.  If this sounds too good to be true, it isn’t…  Although you are gaining more force, the distance you are moving the piston at the other end is reduced.  Fortunately for us, we don’t have to move the brake pads very far to make them grip the disk.  For a more in depth look at how hydraulics work, you may want to look at the brilliant “How stuff works” page.

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.

Biking 101: Turning corners

Believe it or not, riding a motorbike and knowing how one turns are two different things.  Professional rider training organisations will introduce you to the concept of “counter-steering” and some may even attempt to explain how this phenomenon works, but, you don’t have to understand it to ride a bike.  Here’s the briefest summary I can give you on what counter-steering is:

If you want to turn left, you turn the front wheel to the right. 
If you want to turn right, you turn the front wheel to the left.

After you’ve read that, I think you’ll understand why the technique is called “counter-steering”.  What’s more is, it actually works!  Here’s my attempt at something between a layman’s explanation and the physics nerd’s explanation.  The explanation given is based off my understanding and what I’ve observed first hand.  I promise I won’t go close to using mathematics in my explanation!

The gyroscopic effect of the turning wheels is what holds a motorcycle up once it is moving at any sort of speed.  (Say around 20kph / 12mph).  The two wheels on the bike have different roles to play.  If we discount the effect of suspension travel, the rear wheel remains with its axis fixed relative to the rest of the motorcycle, whilst the front wheel allows its axis to pivot left and right (when viewed from the rider’s perspective). 

The rear wheel is responsible for keeping the motorcycle moving in the same direction of travel.  The front wheel is responsible for changing this direction of travel.

Lets look at the rear wheel effect first:
If you spin a gyroscope where the top of the wheel is not centred above the bottom, it will maintain this angle, providing the gyroscope does not lose momentum.  Given the freedom of being able to move, it will circle in the direction matching the side the top leans to.  Therefore, once a motorcycle is leaning, it will move in an arc in the direction of the lean. 

Figure 1: Trajectory of leaning wheel 

Once the rear wheel is spinning with a fair degree of velocity, the weight of the rider and motorcycle become insignificant compared to the gyroscopic effect of the rear wheel.  Although you can use your body-weight to lean the motorcycle into a corner, it’s a slow and arduous process unless you can influence the direction the front wheel is pointing.

Here’s where the front wheel comes in:
Forcefully altering a gyroscope’s orientation will cause it to behave in strange ways.  This is best demonstrated with a loose pushbike wheel.  Spin the wheel up whilst holding the ends of the axle. 

A badly drawn arrow indicating a spinning wheel

Push the left end of the axle “forward” and pull the right end toward you.

Oh look, now there are dodgy green arrows as well! 

You will feel the wheel “react” to this movement and the wheel will lean to the left. 

Dodgy red arrow removed to make blue arrow easier to spot

The easiest way to return the wheel to the vertical plane, is to reverse the action you just did.  That is: pull the left hand toward you and push away with the right.

Putting it all together:
With our increased understanding of what is going on, we’re ready to “hit the road”.  (That should be taken as a “figure of speech”, rather than a “literal interpretation”)

  1. Travelling forward on the bike we push the left handlebar away from us.  As explained above, this will cause the front wheel to lean to the left.  The rest of the motorcycle will follow, resulting in both wheels now leaning to the left.
  2. We stop pushing the left handlebar, allowing it to resume a “neutral” position.  It requires some force on our part to remain at this current lean angle, as the gyroscopic effect of the front wheel will now make it “want to” turn in more.
  3. Because the wheels are leaning, the bike travels in an arc.
  4. Once the joy of turning left has worn thin, we need to stand the bike back up.  So, we reverse the process and push the right handlebar forward.

And that’s the simplified version of turning corners on a bike!  I will leave “turning right” as “an exercise for the reader”. 

Some points in closing:

  • I’ve heard it claimed that the Wright brothers (as in the bicycle makers who forgot that push-bikes weren’t meant to fly) noted that you counter-steer bikes.  Later observations (such as “look, my brother is flying”) seem to occupy most text that you see written on the duo.
  • Whilst counter-steering works for push-bikes, the relative weight of the rider compared with the bike means it is much harder to observe the effect.  Body weight / balance play a bigger role.
  • Rider training will teach you to push  the bars, not pull  on the opposite bar.*  I believe this is taught to stop you gripping the bars too tightly.  A loose relaxed grip with your hands is a safer way to ride.
  • Throttle control also plays a large part to how well you can ride around a corner, but that is a story for another day. 

* Personally, I find it easier to feel the gyroscopic effect of the front wheel by pulling on the bars, probably because my arms are tense when doing so.  From changing between the two techniques, I find pushing the bars easier to control.