Gearing is one of the first things an RC car enthusiast should learn about. Changing the gear ratio is an easy way to alter the performance of your car so you can use the available power more efficiently for the track or area you are driving on.
While changing wheel diameter can affect the final gear ratio of your car, the most common way to alter it is by changing the gearbox configuration – the pinion gear and the spur gear.
The pinion gear is the very small gear that fits onto the motor, and is usually made from some kind of metal. Common metals include aluminium, steel and titanium.
The spur gear is the larger gear that the pinion turns. These are often made from plastic and sometimes composites or even Kevlar.
To alter the gear ratio you can change either the pinion, the spur gear, or both for maximum effect or fine adjustments. Changing the pinion will have a greater effect so if you want to make more subtle changes, change the spur gear.
The photo below shows a gearbox with dust cover removed showing these gears.
The photo shows a spur gear with the markings 83T, indicating it has 83 teeth. Pinion gears also usually have the marking engraved into it, although some manufacturers omit this.
You can also see 48dp written on the Spur gear – this indicates the pitch, or size of each tooth, on the gear. 48dp is a common size for 1/10th competition cars. The higher the pitch the smaller the teeth.
How does changing the pinion and spur gear affect the performance?
By increasing or decreasing the amount of teeth on one of the gears, this will affect how many times the pinion gear has to rotate to turn the spur gear one rotation.
Low Gear (or high gear ratio)
- Smallest pinion, largest spur gear.
- Fast, punchy acceleration, lower top speed.
- Usually less strain / heat on the ESC and motor
- Longer run times from a single charge
- Fast pull away, but easier to control at top speed
Your car will accelerate off the mark very quickly, but the top speed it reaches won’t be as high.
You’ll benefit from longer run times, less heat from the ESC and motor due to less strain (unless you go too low, then the motor may get hot due to high RPMs).
High Gear (or low gear ratio)
- Largest pinion, Smallest spur gear.
- Slower acceleration, higher top speed.
- Easier to pull away due to slower acceleration which can mean more grip (less spinning out) on some tracks.
The motor has to work harder while the car is accelerating, and this additional work means a hotter motor and ESC, higher currents, and can reduce your run times per battery charge.
Getting the balance just right
A small winding track with many curves or tight corners will be suited to a low gear, while a high gear will be better for a large open track with long straights.
On the other hand, a high gear will mean slower acceleration so may be easier to drive in some situations.
Experiment to get gearing just right for the kind of area or track you want to race on.
Select the correct gearing for your motor
A higher turn motor (13.5t, 17.5t etc) will have more torque but less top speed or RPM. Therefore, a higher turn motor will need larger pinion / smaller spur gear combination to make use of all this low end torque.
Low turn motors such as 5.5t or 8.5t will generally need to use a small pinion large spur combination.
Check your manual to see what the recommended pinion size is for your motor.