We’re tackling a common misconception in RC power systems: Kv and brushless motor torque are often misunderstood. Many assume that a lower Kv motor automatically produces more torque, but the reality is more nuanced. Using the Limitless V2 as our test platform, we’ll break down how Kv, current, and gearing interact to determine torque at the wheels.
Comparing 1257 Kv and 2200 Kv Motors
The lower Kv motor draws 42 amps, while the higher Kv motor can handle 73 amps. Despite the difference in Kv, both motors produce nearly identical motor shaft torque: around 319 mNm. This is determine by taking the Kv value and computing Kt. You do this by taking 1/ Kv. Convert Kv so that the units are not in RPM but rad/s, then multiple by the max current of the motor. Even though the low Kv motor makes more torque per amp, it does not have the potential to consume a lot of amps resulting in the same torque output as a higher Kv motor. However, when factoring in gear ratios—2.72 for the 1257 Kv motor and 4.77 for the 2200 Kv motor—the higher Kv motor delivers about 74% more torque at the wheels. This shows that the Kv rating alone does not determine torque at the wheels.
Gearing Matters
The key factor is gearing. To reach 150 km/h, the low KV motor needs a final drive ratio of 2.72, while the high KV motor requires 4.77. This adjustment allows both motors to produce almost identical torque at the motor output: roughly 319 mNm. When multiplied by their respective gear ratios, the high KV motor delivers about 74% more torque at the wheels, despite having a lower torque constant.
Optimizing KV for Voltage
Kv selection should match the voltage you intend to run. Choosing a low Kv motor does not automatically improve torque. A high Kv motor at the correct voltage can achieve higher wheel torque when paired with proper gearing. The main takeaway is that Kv influences motor RPM and Kt, but overall performance depends on how you integrate it into your setup.
Final Thought
When considering Kv and brushless motor torque, focus on the system as a whole. Low Kv doesn’t automatically provide more torque. Select a motor that matches your voltage and optimize gearing to get the best torque at the wheels for your speed goals.