E-Moto Gearing Calculator
Change the rear sprocket on a Light Bee class bike and two things move at once: the speed you can reach before the controller stops feeding the motor, and how hard the bike pulls at the wheel. This calculator gives you both, for two setups at a time, so you can see the trade before you buy the sprocket.
It is built for bikes with no gearbox. If you have used a motorcycle gearing calculator and found the fields did not match your bike, that is why — the explanation is on the page about why generic gearing calculators give wrong answers for electric dirt bikes.
The calculator
This calculator runs entirely in your browser and needs JavaScript switched on. Everything else on this page — the assumptions, the sources and the explanations — reads normally without it.
What the numbers mean
Total reduction is the number of motor turns it takes to turn the rear wheel once. It is also, directly, the torque multiplier: a total reduction of 8.0 means the wheel sees roughly eight times the torque the motor is making, minus losses. Nothing mystical about it. A higher number is a bike that pulls harder and runs out of speed sooner.
Top speed here means the speed the bike reaches when the motor is spinning at the rpm ceiling, on flat ground, with no wind and no slope. That is a ceiling, not a promise. It is the fastest the drivetrain can geometrically go at that rpm. Whether your bike gets there depends on power, rider weight, tyre drag and how much battery voltage is left, none of which this page models.
Chain speed is how fast the chain runs over the countershaft sprocket, in metres per second. Higher chain speed means faster wear and more noise. It rises with countershaft teeth and with motor rpm — and it does not rise with rear sprocket teeth, because the rear sprocket is downstream of the countershaft. This catches people out.
Put together: a bigger rear sprocket lowers top speed, raises wheel torque, and lowers chain speed at the same motor rpm. A bigger countershaft sprocket does the opposite on all three counts, and raises chain speed as well.
Calibrate to your bike
A theoretical ratio is built from published numbers. An effective ratio is built from what your bike actually did. If you have one GPS-verified top speed run, you can solve for the second, and it will beat the first every time. Open the panel and enter the run.
Enter a top speed you have actually recorded on GPS, along with the sprockets and tyre you recorded it on. The motor RPM limit and rolling factor are read from Setup A.
- Enter a GPS-verified run to solve for your effective ratio.
The gap between the effective ratio and the theoretical one is not an error in your measurement. It absorbs drivetrain loss through the belt or primary chain, the real rolling radius of a loaded tyre rather than the assumed one, speedometer error, and any field weakening the controller is doing near the ceiling. A calibrated figure beats a theoretical one, because it is the only one that has met your actual bike.
What this tool assumes
Every assumption below is a place this page could be wrong about your bike. They are listed rather than buried.
- Rolling radius is 97 % of the free tyre radius by default. That is an assumption about tyre squash under rider load, not a measured value, and it is editable in both setups.
- The motor reaches its rpm limit. On a loaded bike on a hill it will not, and the top speed figure will be optimistic by however much the motor falls short.
- No drivetrain loss is modelled. Belt loss and chain loss are real and they are not zero. The calibration panel is the way to absorb them into a number that reflects your bike.
- Preset values are vendor-published figures, attributed inline next to the field they filled. The ones marked UNVERIFIED are exactly that: we could not confirm them for that model, and we have said so rather than presenting them as fact.
- The 5400 rpm figure is the stock controller limit. If you have changed controller, it is wrong for your bike, and every speed on this page is wrong with it. See what sets the rpm ceiling and how to find yours.
What it does not do
It does not tell you whether a sprocket will physically clear your swingarm or your tyre. Tooth count and clearance are separate questions and this page only answers the first — choosing a rear sprocket size by terrain covers where the two collide.
It does not tell you what chain you need. That is the chain length calculator, which asks you to measure your own centre distance rather than assuming one.
It does not model acceleration, battery sag, thermal derate or regen. It will not tell you what your bike will do on a hill. A gearing calculation is a geometry calculation; everything on that list is a power and heat question, and those need a dynamometer and a battery log, not arithmetic.