RC Final Drive Ratio (FDR) Explained
What Is Final Drive Ratio (FDR)?
Final Drive Ratio (FDR) is the total gear reduction between your motor and the wheels. It represents how many times the motor spins for one full rotation of the tires.
FDR is calculated by combining:
• The internal transmission ratio (built into the car)
• The spur gear tooth count
• The pinion gear tooth count
Basic Formula:
Final Drive Ratio = (Spur Gear ÷ Pinion Gear) × Internal Ratio
A higher Final Drive Ratio means more torque and less top speed.
A lower Final Drive Ratio means more top speed and less torque.
How Does Final Drive Ratio Affects Your Car?
Final Drive Ratio directly controls how your car delivers power to the track.
Acceleration
• Higher Final Drive Ratio = stronger acceleration
• Lower Final Drive Ratio = slower acceleration
Top Speed
• Higher Final Drive Ratio = lower top speed
• Lower Final Drive Ratio = higher top speed
Motor Temperature
• Higher Final Drive Ratio = cooler motor temps
• Lower Final Drive Ratio = hotter motor temps (higher load)
Runtime Efficiency
• Higher Final Drive Ratio = more efficient, longer runtime
• Lower Final Drive Ratio = less efficient, shorter runtime
Drivability
• Higher Final Drive Ratio = smoother, easier to drive
• Lower Final Drive Ratio = more aggressive, harder to control
How To Change Final Drive Ratio?
Changing Final Drive Ratio is done by adjusting your gearing.
1. Change the Pinion Gear
• Smaller pinion gear – increases Final Drive Ratio
• Larger pinion gear – decreases Final Drive Ratio
This is the most common and easiest adjustment.
2. Change the Spur Gear
• Larger spur gear – increases Final Drive Ratio
• Smaller spur gear – decreases Final Drive Ratio
Used for fine-tuning or when pinion options are limited.
3. Adjust Internal Ratio (Advanced)
Some cars allow internal gear changes, but this is rare and typically not done for track tuning.
When To Change Final Drive Ratio
Final Drive Ratio should be adjusted based on track conditions, layout and motor performance.
Increase Final Drive Ratio (More Torque)
Use a higher Final Drive Ratio when:
• Track is tight and technical
• You need better acceleration out of corners
• Motor is running hot
Decrease Final Drive Ratio (More Speed)
Use a lower Final Drive Ratio when:
• Track has long straightaways
• You need higher top speed
• Layout is fast and flowing
What to Check After Changing Final Drive Ratio
Final Drive Ratio doesn’t work in isolation. Changing it does affect other areas of the car, mostly electronics.
Motor Temperature
Always check temps after a run:
• Safe range: typically 140°F – 170°F (60°C – 75°C)
• Too hot = gearing too aggressive (FDR too low)
ESC Settings
• Punch and timing may need adjustment
• Lower FDR can require softer throttle control
Battery Performance
• Lower FDR increases current draw
• Watch for voltage sag or shorter runtime
Tire Selection
• More aggressive gearing may require more traction
• Adjust compound or tread if needed
FAQ
Q: What is a good starting Final Drive Ratio?
A: Use your manufacturer’s recommended gearing as a baseline. Most have a good starting point posted in the manual. Adjust in small steps from there.
Q: How much should I change Final Drive Ratio at once?
A: Make small changes, 1-2 teeth on the pinion gear at a time. This keeps changes predictable and easy to track.
Q: What happens if my Final Drive Ratio is too low?
A: There can be multiple signs – your motor starts to overheat and reduced runtime, both of which can cause possible ESC or motor damage.
Q: What happens if my Final Drive Ratio is too high?
A: You’ll notice signs like a lower top speed. Another key sign is your car feels slow on straights.
Q: Is Final Drive Ratio more important than motor choice?
A: They work together. Even the best motor performs poorly with incorrect Final Drive Ratio.
Q: Should I adjust Final Drive Ratio for different batteries?
A: Yes. Higher voltage batteries (like LiPo vs NiMH) often require a higher Final Drive Ratio to keep temps safe.
Back to the RC Suspension Tuning Guide.
