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KV & Max Speed Calculator Guide - ESC Log Usage

Tool Overview

This tool uses ESC logs/telemetry and vehicle setup information to estimate motor KV, theoretical maximum speed, and RPM efficiency. Use the tabs to switch between KV Easy Measurement, Max Speed Estimation, and Run Data Evaluation.

Main Features

  • KV Easy Measurement: estimate KV from no-load max RPM and min voltage
  • Max Speed Estimation: estimate max speed and FDR from gearing, tire diameter, and run max RPM
  • Run Data Evaluation: use RPM efficiency and temperatures as load indicators

Required Equipment

To use this tool effectively, you need an ESC that can record driving logs (RPM, voltage, temperatures, etc.).

  • An ESC with logging/telemetry. Example: Hobbywing XeRun XR10 series
  • A way to access the ESC’s stored data. For example, the manufacturer’s program box or data link/adapter. Whether this is required depends on the ESC

How to Use

KV Easy Measurement - estimate KV from no-load RPM

Spin the motor with no load, then pick "No-load max RPM" and "Min voltage" from the ESC log to calculate KV.

  1. For safety, place the car on a stand and keep clear of rotating parts. Remove the pinion gear to make it no-load
  2. Make sure ESC logging/telemetry is enabled and that RPM and battery voltage are available. Set ESC timing to zero (blinky)
  3. Connect a fully charged battery and power on
  4. Hold full throttle continuously for about 5 seconds, then release and stop. Some ESCs require a few seconds of full throttle to record the log
  5. Some ESCs require an extra action to finalize/save the log (for example, powering off once). Follow your ESC’s procedure
  6. Check the log and note the max RPM and the minimum battery voltage during the full-throttle segment. Use them as no-load max RPM and min voltage
  7. Enter both values and click Calculate to see KV

Max Speed Estimation - estimate max speed and FDR

Estimate theoretical max speed from gearing, tire diameter, and run max RPM from your ESC log.

  1. Vehicle model selection is optional. If selected, tire diameter, internal ratio, and spur options can be auto-filled
  2. Enter spur teeth, pinion teeth, internal ratio, and tire diameter
  3. Enter "Run max RPM" from your ESC log
  4. Click Calculate to see estimated max speed and FDR
  5. Compare results before/after gearing or tire changes to move toward your target speed/load

Run Data Evaluation - check RPM efficiency and temperature

Calculate RPM efficiency from run max RPM and no-load max RPM, and use temperatures as an overload guide.

  1. From your run log, note run max RPM, motor temperature, and ESC temperature
  2. Enter the values and click Calculate
  3. Check RPM efficiency and its status: Optimal, Overload, or Underload
  4. Use the temperature status too as a guide for gearing, cooling, or current/punch settings

Formulas for Max Speed, FDR, and KV

This tool calculates results using the following formulas based on your inputs.

KV Easy Measurement - estimate KV from no-load RPM

Estimate KV (rpm/V) from no-load max RPM and min voltage.

KV = No-load max RPM / Min voltage

For min voltage, use the lowest voltage recorded in the same segment where the no-load max RPM was recorded. Note: the KV calculated by this method is an estimate and may have error.

Final Drive Ratio (FDR)

FDR = (Spur Teeth / Pinion Teeth) × Internal Ratio FDR indicates how many times the motor rotates for one tire rotation. It is a key metric for matching recommended gear ratios for classes like Touring Car or Off-road.

Max Speed Calculation

Speed(km/h) = (Motor RPM / FDR × Tire Diameter × π × 60) / 1,000,000

Tire diameter in mm, RPM in rpm, result in km/h.

What is RPM Efficiency? (Visualizing Load)

A load indicator based on how much RPM drops from no-load to running conditions. It is not a direct measure of driveability. • Low Efficiency (< 70%): Over-geared (High gear). The motor is under heavy load, which may cause overheating. • High Efficiency (> 85%): Under-geared (Low gear). The motor has torque to spare. You might have room to increase speed.

RPM Efficiency(%) = Run max RPM / No-load max RPM × 100

Temperature Status

  • Motor temp: < 80°C = Optimal, ≥ 80°C = Overload
  • ESC temp: < 80°C = Optimal, ≥ 80°C = Overload

FAQ

Q: Which values should I enter from the ESC log?

For KV Easy Measurement, enter the max RPM during a no-load full-throttle segment and the minimum battery voltage during that same segment. For Max Speed Estimation / Run Data Evaluation, enter the run max RPM and temperatures from your driving log.

Q: How long should I hold full throttle for the no-load test?

About 3–5 seconds is enough. Avoid long no-load runs for safety.

Q: My KV looks way too high/low. What should I check?

Double-check units (rpm/V) and your inputs. Watch out for logs that report eRPM (electrical RPM) instead of mechanical RPM, or voltage fields that are per-cell rather than pack voltage.

Q: Do I need to enter no-load max RPM every time?

No. When you enter it in KV Easy Measurement, it is saved in your browser and can auto-fill the Run Data Evaluation field (you can still override it).

Q: Why does RPM efficiency exceed 100%?

Most of the time it's a mismatch in inputs (mixing logs from different runs, too-small no-load RPM, different RPM scaling, etc.). Make sure the values come from the same motor and comparable conditions.

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