Transmission
What Is A Transmission?
A Transmission is the mechanism by which power is transmitted from an engine/motor to the wheels of
a motor driven vehicle.
What Does a Transmission do?
Transmissions convert the RPM and Torque of the motor along the drivetrain. The objective is to
maximize the efficiency of the motor which means to achieve the lowest amp draw and voltage
consumption with the greatest speed and torque.
How does a Transmission achieve the maximum efficiency of the motors power output?
To maximize efficiency, Transmissions feature gear reductions, which help us create more torque.
Higher final drive ratios will create more torque than lower final drive ratios. Higher final drive ratios are
best suited for lower turn motors that have much higher RPM’s and much lower torque. Lower final
drive ratios are better suited for high turn motors which have far greater torque, but much less RPM.
Final Drive Ratio Cheat Sheet:
- Higher FDR = More Torque, Less RPM (Low turn motors, bigger, heavier tires and chassis)
- Lower FDR = More RPM, Less Torque (High turn motors, smaller, lighter tires and chassis)
What are the various types of Transmissions?
Traditional 3-Gear Transmission:
This style of transmission is optimal in medium to high bite, Spec and Modified Racing on Foam Tires.
2-Gear Transmission:
This style of transmission is used in Foam-tire high-bite racing where lower rotating mass and friction is
desired, and the driver does not want to use a direct drive transmission. Often, this setup is used with
the motor running in reverse direction.
4-Gear Transmission:
This style of transmission is optimal in low to medium bite, rubber tire classes with Modified Motors.
The main advantage to this style transmission in lower bite conditions is that it uses the natural weight
jacking effects of the motor to create more forward drive while on throttle and more off power steering
under deceleration.
Direct-Drive Transmission:
This style of transmission is optimal in high bite, lower power classes. It has the lowest rotating mass
and least amount of friction of all the transmission types. In high bite SPEC motor racing, if a Direct Drive
Transmission is legal in your racing division, you better be running one if you want to compete.
Ball Differential vs. Gear Differential
Perhaps one of the most frequently asked questions that I receive, is “When do I want a Gear Diff as
opposed to a ball diff?”
Well, in 90% of situations, a ball diff will give you the most consistent and comfortable feel. Problem
with a ball diff is that they wear out, and require maintenance and rebuilding on occasion. What about
the other 10%? Well, Typically, Gear Differentials will unload off power during corner entry and increase
off throttle steering and rotation. Likewise, Gear Differentials will “Lock Up” on throttle and thus
increase forward drive tremendously.
What about Gear Differential Fluids?
Typically, the MurfDogg Team always uses Losi 3,000wt Diff Fluid in our Gear Diffs for high bite racing,
and Lucas Oil “Red ‘N’ Tacky” grease in our gear diffs for Loose Dirt Racing. This eliminates variables, and
helps us to tune our chassis with two basic diffs for each type of car. Yes, there are an infinite number of
various fluids and varying viscosities of these fluids available for you to play with. No, we don’t play with
them, and do not recommend you do so either. We NEVER do, and frankly do not see any reason to,
knowing that we have a firm grasp on what each style diff does already, and how changing between the
two styles of diffs will affect our chassis’. For those of you, who must know what changing the diff fluids
will do, keep on reading.
Tuning with Diff Fluids:
Differential Fluids are just like shock fluids in that the larger the number the thicker the fluid. Using
different viscosity fluids in the diffs is a way we can control how much power is applied to each wheel by
controlling the action of the gear differential.
- Thinner fluid in the gear differential provides more off power steering and increases forward drive
coming out of the turn. This also results in less on power steering. - Thicker fluid in the gear differential provides less off power steering and decreases forward drive
coming out of the turn. This also results in more on power steering.
Chassis Tuning 101
Matt Murphy’s Dirt Oval Chassis Tuning Guide