Calculation methodology

Car gearing and speed

External reduction is spur teeth divided by pinion teeth. Final drive ratio is external reduction multiplied by internal reduction. If you enter total FDR directly, it is used once. Motor RPM is either KV multiplied by selected pack voltage or a measured RPM input. Wheel RPM is motor RPM divided by FDR. Rollout is tire circumference divided by FDR. Idealized speed is motor RPM multiplied by rollout and divided by 60.

These equations describe kinematics for the stated inputs. They omit load-related RPM change, voltage sag, drag, drivetrain losses, slip, and tire deformation unless you explicitly enter a tire growth assumption. A GPS result is a measured input. Realization is GPS speed divided by idealized speed, not drivetrain efficiency.

Reverse gearing and tires

Target FDR is motor RPM multiplied by tire circumference, divided by 60 and target speed. Fractional target pinion is spur teeth multiplied by internal reduction, divided by target FDR. Whole candidates are recalculated through the forward speed equation. Tooth arithmetic does not establish gear fitment or enough motor power.

Changing tire diameter by a factor of q changes rollout and idealized speed by q at fixed gearing and RPM. The equivalent pinion on the original tires is original pinion × q. The pinion that preserves the original rollout with new tires is original pinion ÷ q.

Battery and electrical tools

Reference values per cell are 3.7 V nominal, 4.2 V full and 3.8 V storage for standard LiPo, and 3.8 V, 4.35 V and 3.85 V for high-voltage LiPo (LiHV). Chemistry is chosen with the pack and changes only these three figures; anything saved before the choice existed is read as standard LiPo. ESC voltage limits are compared against the full-charge figure, since that is the worst case the ESC sees. Rated energy is pack voltage multiplied by capacity in Ah. Input power is voltage multiplied by current. Runtime uses capacity, a user-visible usable fraction, and representative average draw. C-rating current is a label-derived claim.

Pack series count multiplies voltage; parallel count multiplies capacity. Voltage per pack is derived from cell count and chemistry rather than entered, so a combination cannot describe a cell count and a voltage that disagree. Pack arithmetic assumes matched packs. No calculation here approves wiring, discharge, thermal behavior, or component compatibility.

Crawler reduction and overdrive

Shared reduction is the product of the stages before the axle split. Front and rear total ratios each multiply that shared reduction by their own axle reduction. Each stage is included exactly once. Front axle RPM overdrive is 100 × (rear axle reduction ÷ front axle reduction − 1). Travel difference also includes the front/rear tire diameter ratio. Separate axle speeds do not establish whole-vehicle speed when the drivetrain is coupled.

Corner weights and nitro runtime

Total weight is FL + FR + RL + RR. Front and left percentages use their respective pairs. Cross weight is explicitly FL + RR divided by the total. No target balance is prescribed. Nitro runtime is tank volume × usable fraction ÷ fuel burn rate. Pit interval subtracts the entered reserve in minutes. A negative interval is invalid.

Spring and suspension estimates

For a close-coiled cylindrical spring, rate k = G × d⁴ ÷ (8 × D³ × N), with supplied shear modulus G, wire diameter d, mean coil diameter D, and active coils N. GPa input is converted to N/mm². Wheel rate is spring rate × motion ratio², where motion ratio is spring displacement divided by wheel displacement. Undamped frequency is √(wheel rate in N/m ÷ sprung corner mass in kg) ÷ (2π). This simple linear model does not predict damping, traction, or an optimal setup.

Servo and scale conversions

1 kgf·cm = 0.0980665 N·m; 1 ozf·in = 0.00706155183333 N·m. Rated servo travel time scales linearly with angle, without modeling acceleration or load. Scale dimensions multiply by the current scale denominator and divide by the target denominator. Visual scale speed multiplies model speed by scale; the separately selected Froude convention uses the square root of scale.