Changing the gear ratio is an easy way to alter the performance of your RC car. You can gear for a higher top speed at the expense of slower acceleration, or vice versa.
You can alter your car’s gear ratio by using a different pinion or 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.
Changing the pinion will have a more noticeable effect, so if you want to make more subtle changes, change the spur gear. Change both for maximum effect or fine adjustments.
The photo above shows some markings on the spur gear; 83T, indicating it has 83 teeth. Pinion gears usually have the number etched on them too, but some manufacturers omit this.
You can also see 48P written on the Spur gear – P (or DP) 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 affects how many times the pinion gear needs to rotate to turn the spur gear one full rotation (hence why it is called the gear ratio).
Low Gear ( High Gear Ratio): Quicker Acceleration, Lower Top Speed
- Smallest pinion, largest spur gear.
- High Torque, Low RPM
- Quick, punchy acceleration, lower top speed.
- Less strain and heat on the motor and ESC
- Longer run times from a single charge
Your car will accelerate off the mark very quickly, but the top speed it reaches won’t be as high.
The car will be easier to control at top speed, but the high torque and instant power may make the car feel aggressive; especially around the infield of the track when you are on and off the power a lot.
Higher end ESC’s allow you to program some throttle softening or punch reduction in. This can make the car much easier to drive resulting in smoother driving and quicker lap times.
With a low gear, you’ll benefit from longer run times and cooler running temperatures on the ESC and motor. This is because less strain is being put on the motor. However, be careful not to go too undergeared as this can also cause overheating.
High Gear (Low Gear Ratio): Higher Top Speed, Slower Acceleration
- Largest pinion, Smallest spur gear.
- Low Torque, High RPM
- Slower acceleration, higher top speed.
- More strain and heat on the motor and ESC
- Shorter run times from a single charge
The car won’t accelerate as quickly but it will reach a higher top speed.
With less torque, the motor has to work harder while the car is accelerating. This extra strain on the motor will cause hotter running temperatures and higher currents flowing. This will reduce your run times per battery charge when compared to a low gear.
Gearing too high should be avoided, or measures taken to ensure the motor and ESC do not get too hot. Installing an additional fan directed at the motor and cutting some ventilation holes in the body shell should help.
Thin motor wires and cheap battery connectors can get very hot and melt if too much electrical current flows through them.
Getting the balance just right
In theory, 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.
However, the softer acceleration from a high gear may be easier to drive in some situations; a low-geared car on a high grip track may be difficult to control for beginners.
On low grip tracks, or wet/icy conditions, a high gear will give you more traction out of the corners, so this setup is worth trying even on small, technical layouts.
Smooth driving will help you get those consistent lap times, so 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
Despite being able to select any gear ratio you like, you should still take into consideration the motor being used. Your RC Car manual should give recommendations on gear ratios.
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 a larger pinion / smaller spur gear combination to make use of all this low end torque.
Lower turn motors such as 5.5t or 8.5t will generally need to use a small pinion, large spur combination.
If you find your car seems to lack power, you may be over or under geared. Check your RC car’s instruction manual to see what the recommended pinion and spur range is for your motor.
Gearing has a similar effect to Timing. Be sure to check out my Simple Guide to Motor Timing, Boost and Turbo