33
the car smoother in the bumpy sections.
MOTOR GEARING
You can get the most from your motor with the
correct gearing.
STOCK. A recommended starting gear ratio for four
cell racing with stock motors would range from about 3.5:1 to
4.2:1. For six cell racing with stock motors the gear ratio
would range from 4.0:1 to 4.75:1. This is based on using a full
diameter (or new) tire. Your gear ratios will also be influenced
by the type of stock motor plus factors such as your track's
length, traction, surface composition; tight or open track; and
your car's rear tire diameter. A basic 24° stock motor would
start normally at the low end of the gearing range (3.5:1 for
four cell or 4.0:1 for six cell). While a 36° to 40° short stack
stock would normally have a starting gear ratio of 4.2:1 (for
four cell) or 4.75:1 (for six cell).
If you are not familiar with determining gear ratios
and tire diameter adjustments, then the formulas that follow
this section will assist you.
MODIFIED. For modified racing there are different
motors used for outdoor (asphalt) and indoor (carpet) as well
as six cell and four cell. With the current technology in
batteries for six cell racing our team is using 14 or 15 turn
motors on large asphalt tracks or 15 or 16 turn motors on
carpet tracks. For four cell carpet racing we use 12 to 14 turn
motors depending upon track size and layout. For six cell
racing the gear ratio would range from 4.5:1 to 5.3:1. For four
cell racing the gear ratio would range from 4.2:1 to 5.0:1. The
lower the wind of the motor the higher the starting gear ratio
should be.
Formulas
Determing the gear ratio. If you already have a pinion gear and
spur gear you can calculate the gear ratio as follows:
Examples:
Spur gear = 78, pinion = 17
Spur Pinion Results Gear
Gear Gear Ratio
(78
÷ ÷
÷ ÷
÷ 17) = 4.59 = 4.59:1
Determining pinion size based on a given gear ratio.
How to
determine starting pinion size based on a recommended gear
ratio and chosen spur gear size. Examples: Spur gear = 78,
pinion gear ratio = 4.30:1
Spur Gear Results Gear
Gear Ratio Size
( 78
÷÷
÷÷
÷ 4.3 ) = 18.139 = 18
(always round to nearest whole
number).
If your results are not close to a whole number (example
18.652) you can get closer to the actual gear ratio by trying
different size spur gears.
.
Tire diameter adjustment.
If you change tire diameter you can
affect your gearing. You can calculate any gearing adjustments
by using the following formula.
Old New
Tire Tire Factor
Dia. Dia.
( 2.1"
÷÷
÷÷
÷ 1.9" )
=
1.105
Old Pinion Factor Results New Pinion
Gear Gear
18 X 1.105 = 19.89 = 20
(round to nearest whole number)
BATTERY CHARGING
It is important to understand the characteristics of the
battery pack in your car. How you charge and use your packs
will greatly affect both its performance and its life span. With
proper care your packs will perform well for many hundreds of
cycles.
The ROAR legal battery for use with your car is
composed of four or six “sub-C” size cells with a rated capacity
of between 1.2-1.8 amperes for one hour, or 2.4-3.6 amperes
for 1/2 hour, etc. This charge capacity is the same regardless
of the number of cells in the pack because the cells are
connected in series and the same current passes through each
one.
CHARGER. A good quality automatic charger will last
longer than an economy unit, so please do not cut yourself short
here by trying to save a couple of dollars. Any good name brand
charger will do the job correctly. Associated recommends a
peak detection charger as opposed to the timer charger.
Timer chargers increase the chance of making a mistake when
charging the battery. This also increases the chance of damag-
ing the battery pack. Peak detection chargers have an internal
circuit that monitors the voltage and charge rate of the battery
pack. When the pack is fully charged, the voltage will begin to
decrease and a peak charger will detect this and either turn the
charger off, or down to a trickle charge. Some chargers have
even more sophisticated features that make charging less
time-consuming. The better chargers like this can easily handle
the abuse of heavy back-to-back type charging that is common
when racing or playing for a long time. The choice of a DC only
or an AC/DC charger should be based on personal needs
(where you will be using your car, etc..) and usage.
OVERCHARGE. There is no way to make a Ni-Cd cell
accept more charge than it is designed to hold. This means that
as the cell approaches a fully charged condition, the portion of
charging current not being stored becomes heat and pressure.
If charging continues after the cell is fully charged, all of the
current is converted to heat and pressure—about 40 watts
worth, or the equivalent of the heat produced by an average
soldering iron. High temperature and pressure is damaging to
the cells, so overcharging must be avoided.
Ni-Cd cells have a built-in process for recombining the
accumulated gas (actually oxygen) produced by overcharge,
but the process produces heat and takes a lot of time. If you
overcharge your battery and it seems to take a long time to cool
down, it’s because this pressure reducing reaction is taking
place. Once the gas is recombined the temperature drops.
HOW TO TELL WHEN YOUR
CELLS ARE CHARGED
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