In a previous article we discussed the gear ratio and its importance in safeguarding the life of the motor and electronics. In this article we want to explain how the gear ratio is calculated.
Several mechanical elements are a stage of RPM reduction in an RC car. We will analyze the transmission of a scaler/crawler model, but the method apply to any type of model.
Definitions and important concepts
In a pair of meshing wheels, the wheel that transmits motion is called driving wheel while the other is called driven wheel.
The transmission ratio of two meshing wheels is the ratio of the driven wheel teeth to the teeth of the driver wheel.
The final ratio of a transmission made of several stages is given by first stage times the others
keep in mind that only meshing wheels have effect on the gear ratio, while wheels mounted on the same axle do NOT influence the final gear ratio.
Stages of a complete drivetrain
Let's analyze all of the transmission stages of a Scaler drivetrain.
1^ stage: motor pinion – spur gear
The first pair of meshing gears are motor pinion and spur gear:
ST = spur gear teeth (driven)
PT = pinion teeth (driver).
The gear ratio of this stage is:
GR1=ST/PT
2^ stage – main gearbox
The spur gear is mounted in axis with wheel 1 in the gearbox. Spur gear and gear1 are do not mesh.
Gear 1 engages gear 2 and gear 2 engages gear 3; there are two consecutive stages with respective gear ratios T2/T1, T3/T2.
The overall gear ratio of the gearbox is:
GR2=T2/T1xT3/T2=T3/T1
Note that gear 2 does not affect the gear ratio. Its only function is to make the direction of rotation of gear 1 the same way of gear 3.
The drive shaft transfers motion from the gearbox to the axle. This connection does not disturb the gear ratio, because gear 3 and the bevel pinion in the axle are axially connected.
3^ stage – axles
Inside the axle, two bevel gears introduce another gear ratio:
GR3 = diff gear theet (driven)/bevel pinion teeth (driver)
Final gear ratio
We found 3 transmission parts that contribute to the overall gear ratio. The final gear ratio is the first gear ratio times the others:
GR=GR1 x GR2 x GR3
If the RC Scaler is equipped with portal axles, the portal gear ratio must also be calculated. In fact, some portal axles further increase the gear ratio, while others are neutral.
Internal gear ratio
We often hear about the internal gear ratio. It is the ratio of the RC car that cannot be modified by the the user.
When the internal gear ratio iGR of the RC car is known, the calculation of the final gear ratio gets easier, because the teeth of the spur gear and pinion are all you need. In this case, the formula for calculating the total ratio changes to:
GR = iGR x ST/PT
Conclusions
Calculating the gear ratio is a tricky matter because you need to count the number of teeth of all the gears involved in the transmission. Fortunately, the model manual often gives the internal ratio iGR and you need to check just the motor pinion and spur gear teeth.
Finally, sometimes manufacturers include some even more simplified charts in their manuals, where the final ratio is directly stated according to the motor pinion used.