How
does a bike steer?
After
a ROG's Run on a very pleasant day and leaving fliers at Ryka's where
we set off from and lunch at Eastbourne it is time to reflect on what
makes the bike go around a corner and what keeps it stable in a
straight line after a very slippery ride along country lanes on the
way home. The basic component to all this is the spinning wheel. If
you have ever held a bicycle wheel by the spindle and spun it around
the wheel likes to be upright and it is hard to move it to make it
run at an angle but once there it will remain so. It is the
gyroscopic effect. The wheel is a gyroscope. On the road this is your
forward momentum and if you have seen lots of bike racing many times
a rider has been dismounted with a high side and the bike stabilizes
itself and continues to run on in a straight line. Usually it is not
quite upright so it then has a sideways momentum combined with the
shape of the tyre that sends it on a decreasing spiral trajectory
until it hits the ground.
The sideways momentum is created by the
shape of the tyre, it being curved, so that the further from the
centre line the overall diameter of the wheel reduces. When the
diameter gets smaller you have to do more revolutions to cover the
same distance so the effect is that speed is reduced. Just by leaning
into the turn will set the bike on a curved course because you have
moved over to a slightly smaller diameter wheel that has slowed the
speed. A slower speed means less gyroscopic effect and helps to tip
into the corner. Giving power to increase the speed helps bring the
bike upright after the turn by increasing the gyroscopic effect. This
is only part of the story. What helps to make the bike turn is
around the front end. The best example is seen on the extended forks
of a chop. The wheel is raked forward so far you can see and feel
exactly what is happening. In a straight line all is fine when the
handlebars are turned either to the left or right you will see the
wheel being rotated by the forks around the headstock.
The effect is
to move the wheel form the centre line in the opposite direction to
way the handlebars are being turned. Pulling the left side will
displace the wheel to the right and vice versa. This is also changing
the plane in which the wheel is rotating so that turning to the left
moves the wheel to the right and tips the headstock to the left. In
slow manoeuvring if the tip is too large you end up having to put
your foot down or be a heap on the ground.
You counter balance this
with your weight to keep upright because you have little gyroscopic
effect from the wheels. Next to look at is how far forward is the
front wheel. On a chop its miles away and the steering is a bit slow,
bring the wheel closer to the frame and this make the rake less and
the steering will become much quicker. Many of the earlier Japanese
bikes had quite steep fork angles to help make the bikes turn as the
weight distribution was not quite right. They have made impressive
improvements over the years but have a look at the older British
bikes that seem to have the front wheel a long way from the frame
with shallower angled forks and why did they handle so well?
The
answer is Trail. This is the distance from the projected angle of the
headstock to the centre line of the wheel where it touches the
ground. The shorter the distance the faster the bike turns. This
affects the straight line stability and there is a compromise to get
good handling at speed and low speed manoeuvrability. One affects the
other.
Where does counter steering come into this? Look at the chop
again, if you have pulled on the left side of the bar the headstock
tips to the left, to make it go more to the left you need to change
the plain in which the wheel is spinning so do do that you need to
push the left bar forward. You can feel this if you lean the bike
over to the left and push on the left bar. This puts the wheel more
to the edge of the tyre so making the wheel even smaller and turning
tighter still
To bring the bike upright you pull back on the left
bar. Amazingly opposite!! None of this takes into account the effects
of where the engine is placed in the frame or of the position of the
rider and where the centre of gravity is. That's for another time.
