RC car gearing and setup sheets

I kept doing the same sums at the track: what pinion gets me the rollout I ran last time, will it fit the motor plate, and what was the oil in the rear shocks two meetings ago. This page does those, and keeps the setup sheets in the browser.

How to use it

Gearing — pinion, spur, the car’s internal ratio (the transmission or diff ratio from the manual; 1 for a direct-drive pan car) and the tyre diameter give the final drive ratio (FDR) and the rollout, the distance the car moves for one turn of the motor. Give the motor as a Kv, as a sensored brushless or brushed 540 turn count, or as a no-load rpm, plus the battery, for the theoretical top speed. The load factor (0.8 by default) is the rpm the motor keeps under load.
Gear table — a grid of pinions against the spurs you list, showing FDR, rollout or speed. Set a target (a speed, an FDR or a rollout) and the cells within the tolerance are highlighted, the closest one outlined. If you have entered the motor-mount slot on the Mesh tab, pairs that don’t fit are struck through.
Mesh — pick the pitch (48P, 64P, 32P, Mod 0.5/0.6/0.8/1) for the pitch diameters and the centre distance. Measure the motor plate’s slot (motor shaft to spur axle, motor in and motor out) and it says whether the pair fits, and which pinions fit with that spur. For tooth shapes and contact ratio, the gear designer does the full job.
Oils & springs — shock oil between cSt and the scales below, diff oil in “k” and cSt, spring rates in N/mm, lb/in, kgf/mm and gf/mm, and an approximate wheel rate from the motion ratio and shock angle. To work out a spring from its wire and coils, use the spring calculator.
Setup sheets — make a car, fill in the sheet (camber, toe, ride height, droop, caster, shock oil, pistons, springs, anti-roll bars, diffs, gearing, motor, tyres, track and surface, notes) and save it. Copy gearing in takes the pinion, spur and FDR from the Gearing tab. Pick two saved sheets to compare them; differences are highlighted. Export JSON saves every car and sheet to a file, Import merges one back in, and Print prints one sheet on its own.

For the battery, battery pack does series and parallel packs.

How sure the numbers are

FDR, rollout, pitch diameters and centre distance are exact arithmetic.
Top speed is theoretical. Tyres grow (rubber) or shrink (foam) at speed, drag rises with the square of speed, and the pack sags; timing and boost raise the rpm. Use it to compare gearings, not to predict a radar gun.
Kv from a turn count is a rule of thumb, marked as approximate on the page: for sensored brushless 540s, Kv × turns ≈ 39 000 (so a 13.5T is about 2 900 Kv, a 17.5T about 2 250), and makers differ by 10–15 %. For brushed 540s it’s about 57 000 ÷ turns (a 27T stock motor about 15 000 rpm on 7.2 V), and brushed motors vary a great deal more.
Shock oil. The Associated weights are Team Associated’s own published conversion (10 wt = 100 cSt, 20 wt = 200, 25 wt = 275, 30 wt = 350, 35 wt = 425, 40 wt = 500, 45 wt = 575, 50 wt = 650, 60 wt = 800, 70 wt = 900, 80 wt = 1000 cSt); weights between those are interpolated and marked approximate. Most Japanese brands (Kyosho, Yokomo, Mugen) number their oil close to cSt (#300 ≈ 300 cSt) — also marked approximate. The “wt × 10” rule some people use only matches at the thin end. “wt” is not one scale across brands: if the bottle gives cSt, trust that.
Diff oil “k” is thousands of cSt (7k = 7 000 cSt) on Associated, Losi/TLR, Xray and Kyosho.
Wheel rate = spring rate × motion ratio² × cos(shock angle) is the usual first approximation; the real one changes through the travel.

Motor temperature

The Gearing tab has a short note. In brief: check the can a few minutes in and at the end of the run; most makers put the limit near 80 °C, and finishing under about 70 °C leaves margin. Too hot means less pinion or less timing.

Privacy

Nothing leaves the browser. Gearing and setup sheets are kept in this browser’s local storage; Export JSON is how you move them to another device or keep a backup. A share link carries the gearing numbers only, never the setup sheets.