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This article was first published in 2006.
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Despite every car ever built having all of ‘em,
amongst the most complex of things to understand in a car are the angles built
into the front suspension and steering. Even their names are bewildering:
steering axis inclination, camber, castor, scrub radius, toe, Ackerman... So let’s
take a beginners’ look at what all of these mean.
In Part 2 we’ll look at the implications of these
angles for how the car feels and corners on the road.
Suspension and Steering Angles
Camber is the angle that the wheel leans away from
vertical when the car is viewed front-on.
If the top of the wheels lean inwards towards the
centreline of the car, the wheels are said to have negative camber. If the top
of the wheels lean outwards, the camber is said to be positive. In the past,
cars were built to have a little positive front camber but these days, the
camber is either set at zero or a little negative.
Camber is measured as an angle expressed in
degrees and is not usually adjustable without adding a suspension kit.
Toe refers to how parallel the wheels are when
viewed from above.
If the leading edges of the tyres are closer
together than the trailing edges, the car is said to have toe-in. If the leading
edges of the tyres are further apart than the trailing edges, the car has
toe-out. With toe-in present, each wheel is steering a little towards the
centreline of the car.
Toe can be expressed either as an angle in degrees
(better) or as the difference in the distance between the front and rear edges
of the tyres (not so good but more common). The reason that the latter
measurement style isn’t so good is that even if the distance measurement is the
same between different cars, the actual angle of the toe will be affected by the
diameter of the wheels.
Toe used to be commonly set to having a little
toe-in but the factory spec of many cars today is zero toe. Toe is adjustable in
all cars.
Castor refers to the angle of the steering axis
away from vertical when the car is viewed from the side.
To understand castor it is necessary to realise
that all car steering systems have two pivot points on each side to allow the
wheels to be steered. For example, in a wishbone suspension system, there are
upper and lower ball-joints. The line passing through these swivel points is the
steering axis. In a McPherson strut system, the steering axis passes through the
top of the strut (where there’s a bearing or bush) and then through the lower
ball-joint. The steering axis is therefore very much along the line of the
strut.
Cars use positive castor. That is, when viewed
from the side, the steering axis is further forward at the bottom than the top.
The affect of this is that when the steering axis is extended right down to the
road, it touches the road ahead of the contact patch of the tyre. In other
words, the contact patch of the tyre is behind the steering axis. When it’s
expressed like this, it’s easy to see that the term has the same meaning as when
applied to castors on a supermarket trolley – in both cases, the contact patch
of the wheel is behind the steering axis. In a car, that axis is imaginary
immediately ahead of the wheel, whereas with a supermarket trolley the swivel
can actually be seen ahead of the wheel.
To achieve positive castor, the lower suspension
ball-joint must be physically ahead of the upper swivel point. Castor is an
angle so is measured in degrees. Castor is not usually adjustable without adding
a suspension kit.
The distance between the point that the extended
steering axis line touches the road and the centre of the contact patch is
called the castor trail. If the axle is located on the line that extends from
the top ball-joint through the bottom ball-joint (ie the steering axis), the
castor trail will be determined solely by the castor angle. However, there’s
nothing to stop the axle being placed ahead of this axis (which will decrease
castor trail) or behind this axis (which will increase castor trail). Castor
trail is a distance measurement.
In this diagram the line A – B is the steering
axis. Note how the centre of the wheel (ie the axle, shown in purple) has been
placed behind the steering axis, so increasing castor trail.
Castor and castor trail are most easily seen on
bicycle front forks. Straight forks that extend forwards at an angle and have
the axle mounted in line with the forks have no additional castor trail – all
the castor trail is provided by the forks’ castor angle. However, if the forks
are curved forwards (or the axle is offset forwards in one step at the base of
the forks), the castor trail is reduced.
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Steering Axis Inclination
Steering axis inclination – previously called
kingpin inclination as it is in this diagram – refers to the angle of the
steering axis when viewed from the front of the vehicle. (So, to be clear:
castor is the steering axis angle when viewed from the side; steering axis
inclination is the steering axis angle when viewed from the front.) As with
castor, what’s important is where the imaginary line through the steering pivot
points reaches the road.
If this line touches the road halfway across the
width of the tyre contact patch, the steering is said to have zero scrub radius.
(Sometimes this is called ‘centrepoint steering’.) If the steering axis
inclination line touches the road on the inside of the tyre’s centreline (ie
closer towards the centreline of the car), the steering is said to have positive
scrub radius. If the steering axis touches the road on the outside of the tyre’s
centreline, the steering has negative scrub radius.
Steering axis inclination can only be achieved by
placing the lower ball-joint further outboard than the upper ball-joint. (This
diagram of old Jaguar suspension shows just that.) Therefore, to achieve
steering axis inclination and castor, the upper ball joint must be
further rearwards and inwards than the lower ball-joint.
Steering axis inclination is an angle, so is
measured in degrees. It is usually not adjustable. However, scrub radius can be
altered by changing the wheel offset - and so where the centre of the tyre’s
contact patch is relative to the steering axis.
When a car turns a corner, the inner wheel needs
to turn at a tighter angle than the outer wheel. Well, it doesn’t have to, but
if it doesn’t then bad tyre scrub will occur. The difference in the two wheel
angles achieved during cornering is called the Ackermann angle.
Ackermann is usually achieved by angling the
steering arms. The extension of these angles meet somewhere along the centreline
of the car. The point at which they meet was traditionally at the rear axle
line, however in practice, this point can be quite a lot further forwards. Note
that Ackerman steering geometry can also be achieved without angling the arms in
this way.
Picturing the Angles
I for one have taken years to even remember what
all the terms mean. But one easy way of getting it all clear in you mind is to
make a simple model.
Get a paper clip and a round piece of rubber.
Straighten the paper clip out until it is in an ‘L’ shape. Push the rubber onto
the short arm of the ‘L’ – that’s your wheel/tyre on the stub axle. Hold the
assembly vertical and turn the ‘wheel’. Then rotate the paper clip’s vertical –
that’s the steering axis. So you can now rotate the wheel on its axle and steer
it. Note that at this stage, you have zero camber, castor and steering axis
inclination.
Now tilt the paper clip’s vertical backwards –
you’ve added castor. (Rotate the steering axis and note how the wheel’s camber
now changes as the wheel is steered. Note also how the wheel no longer remains
level when steered.)
Bend the vertical of the paper clip backwards so
that the angle between the steering axis and the ‘stub axle’ is no longer 90
degrees. Lean the steering axis towards the centreline of the car. You now have
steering axis inclination.
Bend the paper clip further to give the wheel
negative camber.
Watch closely and see how an increasing steering
axis inclination – especially when mixed with lots of castor - gives some very
weird wheel angles when the wheel is steered.
People will think you mad steering a paper clip
and a round piece of rubber around in the air but it is a brilliant way of
seeing how the angles interact – as well as getting the definitions of the terms
much clearer in your head.
Next week: the practical implications
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