Quick answer: the motor sprocket is a torque-versus-speed trade. A smaller motor sprocket raises the overall reduction ratio, which multiplies torque at the wheel and lowers theoretical top speed. A larger one does the reverse. The size of the change is not a feeling — it is arithmetic, and you can compute it exactly, provided you have the inputs. What you cannot do is read a top speed or a climbing ability off a sprocket tooth count alone.
This article gives you the formula, the seven inputs it needs, a worked 9T-versus-11T comparison, and the limits of what the result means.
Four steps. Every quantity is defined so you can substitute your own numbers.
Step 1 — Overall reduction ratio
R = rear sprocket teeth ÷ motor sprocket teethStep 2 — Wheel speed
wheel RPM = motor RPM ÷ RStep 3 — Wheel circumference
circumference (m) = π × wheel diameter (m)Step 4 — Theoretical vehicle speed
speed (km/h) = wheel RPM × circumference (m) × 60 ÷ 1000And for torque, at the wheel:
wheel torque = motor torque × R × drivetrain efficiencyOn that last term: drivetrain efficiency is not a single published number — it depends on your chain condition, alignment and lubrication. Treat it as an input you choose and state, not as a specification you can quote. If you want a conservative theoretical figure, apply an efficiency below 1.0 and say so. Leaving it out gives you an ideal figure, which is fine as long as you label it as ideal.
You cannot skip any of these. If one is missing, the result is not a prediction — it is a guess with units.
Inputs 1, 2, 3, 4 and 6 are enough to produce a theoretical speed. Inputs 5 and 7 are what you need before the number means anything about acceleration, climbing or thermal load.
Stated inputs (illustrative — substitute your own):
Read that table as one sentence: moving from 9T to 11T makes the same motor spin the wheel 22.2 % faster and delivers 18.2 % less torque — assuming the motor can still reach 4000 rpm under the new load, which is the assumption this calculation cannot make for you.
Notice the two percentages are reciprocals (11 ÷ 9 = 1.222, and 9 ÷ 11 = 0.818). That is the whole mechanism: a sprocket change redistributes, it does not create. If you want both more speed and more torque, the sprocket is the wrong component to change.
The MY1030 family has five reported standard sprocket options:
The rule that catches people out: these sit in two shaft groups.
The two groups are not interchangeable by default, and swapping across them normally means changing the output shaft type as well. When you order, confirm the sprocket and the output shaft type together — not just the tooth count.
Two further cautions worth carrying with you. First, do not treat 25H and #25 as automatically the same standard — check the chain designation and dimensions before you buy. Second, always check the chain standard against what is actually on your vehicle.
This is where sprocket maths gets oversold, so here is the boundary clearly:
The thermal link deserves one extra sentence, because it is the one people miss: gearing changes the load the motor sees, and load changes current, and current changes heat. A ratio that looks great in the speed formula can push the motor into sustained current it cannot shed — which is exactly the situation where a temperature sensor earns its place, provided the controller is configured to act on it.
Rather than guessing, work in this order:
Is a smaller motor sprocket better for climbing? It raises the reduction ratio, which multiplies torque at the wheel. Whether that makes the vehicle climb better also depends on whether the motor can deliver that torque without exceeding its sustained current — which is a thermal question, not a sprocket question.
9T or 11T for the #35 sprocket? Compute both with your own rear sprocket, wheel diameter and motor RPM. In the worked example above, the 11T gave 22.2 % more theoretical speed and 18.2 % less wheel torque. There is no universally correct answer, because the correct answer depends on your weight, terrain and target.
Can I mix a 25H sprocket with a #35 rear sprocket? Not as a like-for-like swap. 25H and T8F use the A-type shaft; #35 and 420 use the B-type shaft. Cross-group changes normally involve a shaft type change too — so treat this as a system decision, not a sprocket decision.
Can I use a 420 sprocket on a #35 shaft? No — 420 sits in the B group alongside #35, but it is a different chain standard. Confirm the chain, the rear sprocket and the target ratio together, and confirm the specific combination with Kunray before ordering.
Will a bigger motor sprocket make my vehicle faster? It raises the theoretical speed at a given motor RPM, and reduces torque at the wheel by the same proportion. Whether you actually see that speed depends on whether the motor can still reach that RPM under the higher load.
Doesn't more speed also mean more top-end power? Not automatically. Power is a function of torque and speed together, and what the system can sustain depends on the controller's current limit and the motor's thermal capacity. A taller ratio can make a build slower in practice if the motor cannot hold the RPM you assumed.
Do I need to change the chain when I change the sprocket? Check it. The chain standard must match the selected sprocket, and the length and tension range must suit the new geometry. Keep the existing chain only if the standard matches and the chain line and tension still work.
