Remember, while weight may not be as important as some other areas such as build
accuracy, aerodynamics and effective power utilization, it does have a significant effect
on performance. On a car capable of a 20 second run in 100% sun, an extra 200 gm will
slow the car by about 0.8 seconds over a one lap race, while 200 gm extra at 20 % sun
will slow the car by about 2.5 seconds over the one lap race and may even prevent it
from climbing the hill.
At the end of a race a significant proportion of the energy provided by the motor will be
stored by the car in the form of Kinetic Energy. Lower weight means less energy is stored
in this form and more will have been available to overcome other losses during the race.
Weight also directly influences wheel rolling resistance as well as the side forces acting
on the guide system when cornering. Lower weight reduces these forces.
7 Guiding
Guiding on the outside of the U channel is now MANDATORY as there are joining
pieces placed inside the “U” to improve horizontal alignment. Remember there can still
be some mismatch at the joints so any guiding system used must be able to cope with
this. (The mismatch can be both vertical and horizontal.)
To keep friction low, use ball bearings either alone or supporting a roller wheel.
Remember a car capable of a 20 second run will be experiencing a side force up to about
80% of the car’s weight while traversing the last corner in high sun. Consequently guide
rollers deserve as much consideration as wheels.
Mounting the guide rollers is important as they are subjected to quite high side forces
plus impact loads as they pass over the guide channel joints. The rollers, their support
shafts, and the section of car they are mounted to, all must have adequate strength to
survive the pounding they will experience during the course of racing. As a rule of thumb
the side forces on a fast car can be equal the car weight and that is not counting the
impact loads as the guides traverse the joints in the guide.
A word of caution: ensure whatever guides you use are high enough above the track not
to catch on the track mismatch at the joints. Rollers have been ripped off cars when this
has happened. As a rule of thumb 3 mm clearance between the guide rollers and the track
has been found to work well.
What appears to be a good idea for guide rollers was seen at the 2008 National event in
Hobart. It is shown in the photograph below.
It is simply to cant the rollers so that they run on the guide rail down close to the track
surface. This allows the car to bounce upwards nearly the full height of the guide rail
before becoming disengaged.
This angle on the guide roller reduces the probability of catching on any step that may be
present at the track joints. The angle tends to assist the guide roller to ride up and roll
over the step instead of catching on it and bringing the car to a dead stop, with the
probability of significant damage.