TECH TALK -- GEAR
A karter asks, "What is a gear ratio, and
why do I care?"
Your gear ratio will ultimately determine how quick or
how fast your kart will go. The right gear set up is dependent on variables such
as: track, weight, weather, tires etc. In general you should have a 15 tooth
clutch with a variety of rear gears ranging from 58-63 teeth for 4-cycle karts
(2-cycle karts will have different needs). You will determine the optimum
gear for your kart by practicing prior to the race.
Gear ratios are figured using rear (axle) gear divided by clutch gear (aka drive
gear). For example if you were running a 15t clutch gear and a 60t rear gear it
would be 60 / 15 = 4. Simply put, this means 1 revolution of the axle to 4
revolutions of the crank.
A gear ratio chart can be extremely useful to save you from doing the math.
We’ve all head this before: "My
buddy George uses a 10T drive gear and he beats me by half a lap.
So I want to change my 9T drive gear to a 10T so I can be as fast as
Now, all we need is some
scrap paper and a gear chart to explain that George’s gear of 10-70 is the
same as his buddy’s 9-63. Sometimes this is easily seen; other times it's difficult.
Divided out, 70 / 10 = 7.0 or (7.0 to 1) and 63 / 9 = 7.0 or (7.0 to 1) are the
same, clear and simple. Old math or new math, slide rule or calculator, it's still 7.0 to 1.
To continue, 7.0 to 1 is referring to 1 axle revolution to 7 revolutions of the
crank. This represents a 7.0:1 gear ratio. Rule #1: Gear ratios, no matter how
you arrive at them, are the same. The front sprocket with a 9 tooth or 10 tooth
will still rotate 7 times with the above rear gears. There are slight exceptions to this
Rule #1, such as tire circumference -- but with everybody running the same tires to make things equal, this should not be
a significant factor.
is a gear ratio chart. You are
welcome to print it out for future reference. A nice idea is to
take it to a local copy shop and have it laminated and put it in the lid of your
toolbox, or on the wall of your trailer next to your gear selection.
CLICK HERE for GEAR RATIO CHART (.pdf format --
Adobe Reader required)