Rolling Frame
A very important part of any vehicle is, quite obviously, the frame
or chassis. For a small electric vehicle - and in particular one
for which there is a strict maximum price limit imposed - the design
and construction of the chassis is even more important.
The Stealth II eV is a good example
of one type of cheap but effective and robust chassis. It is made
from two standard BMX bike frames welded together with cross-members.
Inserted in-between is a seat for the driver and the steering mechanism
which is one handlebar linked to both front wheels by linkages.
This type of construction also allows plenty of room for the motor
and batteries and other equipment. As the wheels have a fairly large
diameter, it also gives the Stealth II eV very good ground clearance
and as the driver is quite high up and in an upright seated position,
has very good visibility. However the disadvantages of this is that
the centre of gravity is quite high and care must be taken when
cornering to prevent overturning the vehicle.
Another type of construction is to use a purpose built racing-cart
type of frame. These have small wheels - typically 15 to 20 cm diameter
- and a very low seating position and with some types the driver
is in a reclining position. This gives them a very low centre of
gravity which makes it almost impossible to tip these types of vehicles
over and can also help with the aerodynamics of the vehicle, making
them more efficient.
Racing-cart type frames can either be made from steel or aluminium
tubing or can be bought ready made and second-hand carts are occasionally
available. Obviously if this is the case then the price limit must
be taken into account.
Wheels and Tyres
The Stealth II eV uses standard bike
wheels and tyres and these have a number of advantages - as well
as disadvantages - over small racing-cart and other small wheels
and tyres.
The main advantages are that they are cheap and readily available.
Furthermore they are available in a wide range of sizes, treads
patterns, are light in weight and can take fairly high pressures.
These last points are very important. Firstly the range of sizes
means that smaller or larger wheels and tyres can be chosen to change
the transmission ratio of the drive-train - fitting smaller wheels
and tyres has the effect of gearing the motor down while larger
diameter wheels and tyres have the opposite effect. This may be
important as it can help optimise the speed and distance traveled
by the vehicle. The gyroscopic forces generated by larger diameter
wheels and tyres also help the vehicle's stability.
Secondly, the tread pattern is very important - wet weather tyres
have tread which allows the water to be 'pumped' of the road and
into the pattern. This allows better contact between the tyre and
the road. However in dry weather, slick tyres give better traction
and, under with fairly smooth and hard surfaces' less rolling resistance.
Thirdly, lower weight helps efficiency as there is less 'dead'
weight that must be dragged around. Also lower weight can mean better
vehicle handling as there is less mass that must be forced to go
around a corner.
Lastly higher pressure tyres also greatly help efficiency by also
reducing the rolling resistance.
On the down side, large diameter wheels and tyres may increase
the wind resistance and if the gyroscopic forces are too great,
the vehicle maneuverability may also be effected.
Small diameter wheels such as racing-cart wheels and wheelbarrow
wheels also have advantages and disadvantages. Proper cart wheels
and tyres are quite expensive and only come in limited sizes. On
the other hand, their small size may lead to less wind resistance
and help improve efficiency.
Brakes
Quite obviously, brakes are also very important in any vehicle
and in the Murdoch University eV Challenge, any vehicle with inadequate
brakes will be disqualified from entry.
There are a number of types of brakes, but the main three that
could be used are normal bicycle 'scrub' brakes, cart and bicycle
disk brakes and regenerative braking systems.
Scrub brakes come in a number of different forms but they
all basically work in the same way - two blocks scrub against the
side of the wheel rim and the friction caused slows the vehicle.
In effect, they use the rim as a large-diameter disk or drum, just
like the disk and drum brakes of cars and other vehicles. There
are a number of variations and the common cable-operated 'Vee-brake'
is just one of the more popular types. Another type - although more
expensive - uses a hydraulic system to actuate the pads.
Disk brakes have recently become popular on some mountain
bikes as they have a greater stopping power than do scrub brakes.
This is mainly due to disk being at the hub of the wheel which allows
them to remain cleaner and dryer than the rim. Also they can have
a larger area in contact with the pads than do scrub brakes and
if they are made from special materials such as machined disks of
cast iron, have very friction properties.
Disk brakes are also available for racing-carts and in this case
are usually mounted on the axle rather than on the wheel hub. As
the axle usually is solid and has no differential, only one disk
is needed to stop the two rear wheels simultaneously.
Regenerative braking is a special type of braking that is
only used on electric and hybrid vehicles. What it does is recapture
some of the energy that is lost when the vehicle slows down. It
does this by using the motor or a special generator as an electric
'brake' and the kinetic energy (energy in the motion of the vehicle)
is turned into electricity which can be used to charge the batteries.
With normal braking systems, this energy is simply lost as heat
in the brake disks and pads. By using regenerative braking, the
range of an electric vehicle can be extended by as much as 30%.
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