Steering Geometry
Author Bob Carter
With no suspension, the F24
car steering geometry is simpler than your “full sized” car but many of the
same principles apply. The designer can choose what angles and factors to apply
and I will try to explain what each factor does and what was chosen for Raptor.
Camber Angle
Roads slope down at the edges
so that water drains off them. This is called the camber of the road. Very
early automobiles were often designed with positive camber, where the tops of
the front wheels were further apart than the bottoms. This meant that the tyre
tread was square on to the road surface. This practise died out as cornering
speeds increased, the cornering forces are trying to roll the tyre off the rim!
Much more recently it has
become fashionable with sport and rally cars to have negative camber on the
front wheels (tops of the tyres IN compared to the bottoms). The theory here is
that as the car rolls in a corner, the outside front tyre will rotate to a near
vertical position (which becomes quite important with a wide tyre!). A Greenpower car, with narrow bicycle tyres, and which has no
body roll (no suspension) sees no benefit from negative camber unless its tyres
are way too soft! I know the Greenpower technical
pages say negative camber is a good thing – this page says it isn’t! The main problem is a general problem
with a non vertical tyre, which the load on the contact patch causes the rubber
in contact with the road to “squirm” and that must be a waste of energy!
Raptor has zero camber angles.
King pin inclination and scrub radius
The pivot around which the
front wheel moves when steering is historically called the king pin. Often in Greenpower cars this steering axis is angled sideways (this
angle is called “king pin inclination”) so that the axis of rotation actually
passes through the contact patch of the tyre. Such an arrangement is called “centre
point steering”. Among full- sized cars, the Mazda MX5 is notable for using centre
point steering.
Pro:
- No “kickback” through the steering
if a wheel hits a bump
Con:- Can feel a bit dead
Tendency for steering to oscillate in the
straight ahead position
The sideways distance between
where the steering axis meets the road and the contact patch of the tyre is
called the scrub radius. Among full size cars it is usual to have a small
inboard scrub radius (in a ford cortina was up to
40mm) although one or two (e.g. early audi 100) had
an outboard scrub radius & claimed it was good for braking stability.
The king pin inclination
enables one to engineer a small scrub radius, and imbues a useful self centring
tendency when stationary, but moves the wheel away from vertical in corners.
Raptor has about 25mm scrub
radius and about 15o KPI
Castor angle and trail
Dynamic self centring is
achieved by having the point where the steering axis hits the road ahead of the
tyre contact point. This can be achieved by angling the steering axis backwards
when viewed side on (caster angle) or by having the wheel axis behind the
steering axis (trail).
Ironically, the casters on a
hostess trolley have zero caster angle, and rely on
trail to work!
Full size cars caster angle
is usually in the range 5o to 7 o with no trail. Raptor
has 10 o caster angle and 10mm trail.
Ackermann angle
At last – the important one!
This principle was worked out by a German guy (for stagecoaches!) I’ve
forgotten his name – it was NOT Ackermann. No, Mr Ackermann was his
OK whenever you go round a
bend, the outside front wheel always describes a larger radius turn than the
inside one. So it needs to steer slightly less. Ackermann is a neat way of
doing this, by making the steering arms line up with the centre of the back
axle.
Strangely, some full sized
cars do not have proper Ackermann steering. They are the ones you hear
squealing in car parks! Their theory is that the outside tyre is loaded more
than the inside, so it should have a larger steering angle.
For Greenpower,
correct Ackermann steering is VITAL
Steering Controls
At CAUC we have tried a
number of control systems over the years; Blue Bug had a steering wheel; Carbon
QT and Maxx Factor had handlebars. Brian and Raptor have both used “tank
levers”. These are certainly no better than the other systems, but they do
allow the driver in and out without having to have a removable steering column
(which inevitably confers undesirable backlash to the system). And in the case
of Raptor, they allow a very low profile car.
Furthermore, we put brakes,
throttle and gears on the tank levers too. No pedals. Thus short and tall
drivers can use the cars without adjusting anything but the safety harness.
Brakes
All our cars use bicycle disc
brakes on the front wheels. On Blue Bug these were hydraulic but have been
changed to cable operation because they proved rather high maintenance. Carbon
QT, Maxx Factor and Brian all had cable callipers and a “balance lever” to
share the braking force between the 2 wheels. This has worked OK but has been
prone to cable binding and requirement of excessive force to achieve reasonable
retardation. Raptor has placed the two front wheels’ callipers in series – thus
requiring half the force but twice the travel on the brake lever. This seems
better but has not yet been raced….
Front or back wheel brakes
This has been discussed on
the Greenpower forum. I would argue against the use
of rear wheel brakes because a lock-up would cause a spin - period.
Some cars just use a brake on
one rear wheel. Such a car won’t spin because the un-braked wheel will keep it straight,
but it won’t brake very well!
I appreciate that some cars
have a very rearward weight distribution and this would limit the effectiveness
of front brakes. Fortunately at CAUC we are not in that position!
Greenpower regulations
The F24 regs
state that the car must be able to stop FROM MAXIMUM SPEED in
25m. This would be quite a challenge for a fast car. Something like Seaford’s “
Raptor is designed to challenge
“phoenix” so it’s a bit of an issue for us too….
Stopping from 25mph in
25metres > retardation = 0.25G – worryingly ineffective brakes.
Stopping from 50mph in
25metres > retardation = 1.0G – Ferrari class brakes…
Note 1G is the acceleration
due to gravity – about 9.81m/s2. It’s how fast you accelerate
downwards when you fall off a cliff…..
Height of Centre of Gravity
It is usual in the design of
a racing car, to try to get the centre of gravity as low as possible. There are
2 main reasons for this: -
1)
A car with a high centre of gravity will see
significant weight shifts during braking and acceleration. For us this is a
braking issue. The braking force, say 1000N, is applied AT ROAD LEVEL
and affects the centre of mass which is some 50cm up. The net result is a
torque applied to the whole car pushing the front down and lifting the rear, a
torque of 1000N x 0.5m = 500Nm. That’s a lot of torque. A 140kg car may
nominally have about 700N pressing down on each axle. If we apply the above
torque to the car, and the car has a wheelbase of 1.5m, the force on the front
axle increases by 500/1.5N to 1033N. The force on the rear axle reduces by the
same amount to 367N. So the weight shift is good news if you have front wheel
brakes, bad if you have rear wheel brakes. The car above is limited to
significantly less than 0.5G braking with rear brakes (at which point it would
do a handbrake turn) but with front wheel brakes it could stop safely at 0.73G.
2)
A car with a high centre of gravity is more likely to
tip over when cornering. If we assume that the tyres are able to generate 1G
sideways acceleration when cornering, then for the car to slide (relatively
safe) rather than roll (dangerous & costly) the track must be more than
double the height of the centre of mass. It’s easy to do this test, load the
car up in racing trim & see at what angle it balances on the two wheels of
one side. If it’s over 45o that’s good!
A car following this
guideline should only roll over if it hits something!