In radio controlled vehicles, the Kv rating of a motor significantly impacts its performance and speed capabilities. Optimizing motor Kv by methodically increasing it along with proper gearing adjustments is key to safely unlocking maximum speed potential without risking mechanical failure. This guide examines best practices for swapping motors and tuning Kv to hit peak speeds.
Table of Contents
Key Takeaways on Optimizing Motor Kv
- Higher Kv yields more RPM and power but requires load reduction to avoid damage.
- When increasing Kv, reduce gearing like spur or pinion size to proportionally lower speed.
- Monitor motor and battery temperatures to ensure you remain safely below maximums.
- Incrementally increase Kv while verifying you don’t exceed amperage ratings.
- Balance torque vs. top end speed needs based on vehicle type and electronics.
- Improper Kv selection risks burnt components and reduced reliability.
Carefully balancing Kv, gearing, and load results in responsibly achieving maximum speed targets.
Relationship Between Kv, Current, and Power
Before optimizing Kv, it’s important to understand how it relates to current draw and power:
- Higher Kv = More RPM – Increasing Kv produces higher motor RPM.
- More RPM = More Power – RPM directly contributes to total power output.
- More Power = More Current – Current draw must increase to generate additional power.
- More Current = More Heat – Higher amperage results in increased temperatures.
This interdependency requires carefully scaling gearing and load to avoid exceeding electrical limits when raising Kv.
Why Increase Motor Kv?
There are a few key reasons you may want to swap in a higher Kv motor:
- Increase top speed – More Kv equals more RPM to raise speed if geared properly.
- Unlock more power – Additional Kv provides access to more power from the electronics.
- Utilize battery output – Higher Kv taps into unused battery discharge potential.
- Add performance headroom – Allows gearing down for more acceleration and torque.
When sized appropriately, higher Kv motors unleash more total performance from existing electronics and batteries.
Swapping Kv – Things to Keep Constant
When swapping motors for more Kv, these factors should remain unchanged:
- Motor can size – Ensure new motor uses the same can as old.
- Battery voltage – Keep using the same battery voltage (cell count).
- Gear pitch and ratio – Maintain the same type of spur and pinion gears.
Holding these variables steady isolates Kv itself as the only significant change to the power system.
Calculating Kv Change Impact on RPM
Use this formula to estimate impact of a Kv increase on maximum motor RPM:
Max RPM = Kv * Battery Voltage
For example:
- Old motor Kv = 2500
- New motor Kv = 3000
- Battery voltage = 14.8V
Old Max RPM = 2500 * 14.8 = 37,000 RPM
New Max RPM = 3000 * 14.8 = 44,400 RPM
44,400/37,000 = 20% Max RPM Increase
Even small Kv changes drastically impact potential maximum RPM.
Option 1: Reduce Pinion Gear Size
Decreasing the pinion gear size lowers transmitted speed:
- Start by dropping 2-3 teeth size from existing pinion gear.
- Calculates speed using new Kv and pinion gear to verify acceptable velocity increase.
- Monitor motor and battery temperatures to watch for excess heat.
- Increase pinion size incrementally until temperatures remain in safe range.
Watch RPM limits – excessive gear reduction risks motor damage from over-revving.
Option 2: Increase Spur Gear Size
A larger spur gear also reduces transmitted speed:
- Boost spur gear size in proportion to the Kv increase.
- Larger spur gear drops overall ratio and lowers RPM to the wheels.
- Provides more torque compared to just smaller pinion.
- Allows using existing pinion gears.
Too large of a spur gear reduces efficiency and lowers runtime by increasing loads.
Option 3: Adjust Pinion and Spur Combination
Combing pinion and spur changes provides more tuning flexibility:
- Calculate new ratio needed to reduce RPM proportionally to Kv increase.
- Divide by old ratio to determine required gear teeth adjustment.
- Alter pinion and spur sizes to achieve target new ratio.
- Verifies lower RPM while optimizing torque.
Complex but allows optimizing for balanced speed and acceleration.
Monitoring Mechanical Power Limits
Regardless of gearing, exceeding mechanical power thresholds risks catastrophic failure:
- Calculate Total Watts – Multiply battery voltage by max motor current rating.
- Compare to Manufacturer Limits – Stay safely below motor and battery maximum wattage.
- Listen for Bad Sounds – Strange noises can indicate problems.
- Check Temperatures – Excess motor and battery heat decreases safety margin.
- Gear More Conservatively if Unsure – It’s easy to go up in pinion size later once confident limits are not exceeded.
Leaving power headroom ensures longevity when pushing to higher Kv setups.
Indicators of Inappropriate Kv Selection
Signs of inadequate load reduction with increased Kv include:
- Motor overheating – Temperature exceeds max rating.
- Battery overheating – Heat beyond safe lithium polymer levels.
- Cogging or stuttering – Occurs when accelerating from low RPM.
- Excessive current draw – Motor amps spike abnormally.
- Mechanical noise – Odd grinding or whining sounds.
- Reduced runtime – Higher loads drain batteries faster.
Adjust gearing to eliminate these failure symptoms when raising Kv.
Maximizing Speed Safely
Follow this process to properly optimize Kv for maximum speed:
- Calculate new maximum potential RPM based on desired higher Kv.
- Determine required gear reduction to proportionately lower transmitted RPM.
- Monitor motor and battery temperatures to identify safe gearing balance.
- Incrementally raise pinion size while ensuring temperatures remain in spec.
- Perform final speed tests only once system stability is verified.
Careful testing eliminates risks when pushing components to their performance limits.
Conclusion
Optimizing motor Kv enables tapping into more power and speed without exceeding electrical or mechanical limits. Meticulously increasing Kv along with proportional gearing reduction allows drawbacks like excessive heat and load to be managed safely. Patience is required to find the optimal balance through methodical real-world testing. But the reward is responsibly achieving maximum speed with reduced risk of failure. So take the time to properly analyze, plan, adjust, and verify each incremental change when tuning Kv. The results will be blazing fast speed from your RC vehicle that leaves everyone wondering what performance mods you’ve mastered.
Frequently Asked Questions
How is torque impacted when raising Kv?
Higher Kv lowers torque at equal RPM. But by gearing down appropriately, more total torque can be maintained despite less per RPM.
Can I achieve the same RPM with different Kv motors?
Yes, by modifying gearing you can achieve equal RPM from different Kv values. But higher Kv allows accessing more power at those RPMs.
What causes cogging when accelerating?
Cogging is caused by motor poles interacting with magnets at low RPM. Higher Kv amplifies it but multiple gear meshes can contribute too.
How do I calculate impact on current draw from a Kv increase?
Use power (watts) increase relative to Kv increase as a guideline. More power requires proportionally higher current to maintain voltage.
Can I run higher Kv if I don’t care about speed?
Yes, higher Kv allows using lower gearing for more torque and acceleration while limiting top speed. But ensure temperatures remain in check.
What are signs my motor Kv is too high?
Excessive heat, low runtime, frequent mechanical failure, ESC/battery damage, burning smell, and odd sounds indicate Kv is likely exceeding system limits.
Should I gear lower with brushless versus brushed motors?
Yes, brushless motors have higher efficiency so require less gear reduction. A good starting point is brushed motor gearing * 1.5 = brushless gearing.
How do I find the actual maximum safe RPM for a motor?
Divide Kv by voltage. But stay 15-20% under that calculated max RPM to allow a safety margin. Exceeding true max RPM damages motors.
