What’s the Difference Between an Inrunner and an Outrunner?
If you build RC cars, boats, or drones, you’ve seen the two shapes on a parts shelf a hundred times: the stubby, can-shaped motor and the slimmer, longer one. Those two shapes are not cosmetic. They describe where the rotor lives — and that single detail sets almost everything that matters in a build: torque, RPM, cooling, and how you mount the motor.
An outrunner has the magnets on the outside and the copper windings on a fixed core in the center. The whole outer shell spins. An inrunner is the reverse — the magnets are on a rotor that spins inside a fixed ring of windings, and only the shaft turns.
Here’s the practical payoff up front, so you can decide whether to keep reading (you should):
- Outrunners deliver more torque at a given size and run cooler, which is why nearly every direct-drive drone prop is spun by one.
- Inrunners spin much faster and package into a smaller diameter, which is why geared high-speed RC cars, boats, and ducted fans favor them.
Everything below explains why, so you can pick correctly the first time instead of learning from a motor that overheats or a build that’s gutless.
Structure at a Glance
| Feature | Outrunner | Inrunner |
|---|---|---|
| Rotor location | Outside (spinning bell) | Inside (central shaft) |
| Magnets | Bonded to inner wall of bell | On the central rotor |
| Windings | On fixed center stator | On fixed outer ring |
| Rotating part | Entire outer shell | Only the shaft |
| Torque lever arm | Long (magnets at max radius) | Short (magnets at min radius) |
| Self-cooling | Yes (bell acts as a fan) | No (conduction only) |
How Each Motor Works
Outrunner — the rotor spins outside
In an outrunner, the permanent magnets are bonded to the inside of a rotating outer bell. The stator — the coil of copper windings — sits on a stationary core in the middle, usually around the mounting face. When the ESC commutates the phases, the magnetic field pushes against those outer magnets, and the entire bell rotates around the fixed stator.
Two consequences follow directly from this layout. First, because the magnets sit at the largest radius of the motor, they generate torque with a longer lever arm — more turning force for the same electrical input. Second, the spinning bell acts as a built-in fan, pulling air across the windings as it turns, and the magnets are directly exposed to airflow. Outrunners are naturally good at shedding heat.
Inrunner — the rotor spins inside
An inrunner puts the magnets on a central rotor shaft and surrounds it with a fixed ring of windings housed in a cylindrical case. Only the shaft protrudes and spins; the case stays still.
The trade-off is the mirror image of the outrunner. The magnets sit at a smaller radius, so an inrunner produces less torque per amp than a comparable outrunner — but because the rotor is lighter and more rigid at high speed, it can spin much faster, often 40,000 to 100,000+ RPM in ducted-fan and geared applications. The catch is cooling: the windings are buried against the case with no built-in fan, so inrunners rely on the mounting surface or an external heatsink to pull heat away.
Torque vs. RPM: The Trade-off That Decides Most Builds
The single most useful way to think about the choice is as a torque-versus-speed trade-off:
- Outrunner = torque motor. Long lever arm, big prop-friendly grunt, moderate RPM.
- Inrunner = speed motor. Short lever arm, high RPM ceiling, modest torque per amp.
This is why a drone with a 5-inch prop almost always runs an outrunner: the prop needs low-end torque to spin a large air disc, and a direct-drive outrunner provides it without a gearbox. A 1/10-scale touring car, by contrast, often runs a small inrunner that spins a pinion through a gear reduction — the gearbox multiplies the inrunner’s high RPM back into usable wheel torque while keeping the motor tiny and light.
The rule of thumb: if the motor turns the load directly, lean outrunner; if there’s a gearbox between them, lean inrunner.
Efficiency and Power Density
Efficiency is where the two designs trade blows depending on the application. Outrunners are generally more efficient at the low-to-mid RPM that direct-drive propellers live in, because they produce the needed torque without a gearbox’s friction losses. A gearbox can eat 5–15% of the mechanical power as heat and friction, so a direct-drive outrunner that skips the gearbox altogether often beats an equivalent geared inrunner on overall system efficiency.
Inrunners, however, win on power density — the amount of power packed into a given weight and diameter. Because an inrunner’s rotor is small, rigid, and well-balanced, it can be spun to extreme speeds without tearing itself apart. That’s why a tiny inrunner the size of your thumb can deliver thousands of watts through a gearbox, while an outrunner making the same power would need a much larger bell and stator. When every gram and millimeter counts — a 1/10-scale race car, a ducted-fan jet, a CNC spindle — the inrunner’s compact high-RPM output is the deciding advantage.
The practical takeaway: outrunner = efficient direct-drive torque; inrunner = maximum power per gram at high RPM.
A Worked Example: Same Load, Two Motors
To make the trade-off concrete, consider a hypothetical 2 kg fixed-wing trainer that needs roughly 1 kg of static thrust to cruise comfortably.
Option 1 — outrunner: a 2212-size, 980 KV outrunner spinning a 10×4.5 propeller on 3S. It turns the prop directly at around 10,000 RPM, produces the needed thrust at moderate current, stays cool with its self-ventilating bell, and weighs about 55 g. No gearbox, no heatsink, nothing to fail but the bearings. This is the standard, and for good reason.
Option 2 — inrunner: a 2835-size, 3,500 KV inrunner driving the same prop through a 3.5:1 gearbox. The inrunner spins at 35,000 RPM, and the gearbox steps that down to prop RPM. You get the same thrust, but you’ve added a gearbox that needs lubrication, adds weight, and sheds 10% of the power as heat. Unless you specifically need the inrunner’s compact form factor or high-RPM headroom, the outrunner is simply the better tool here.
The point of the example isn’t that inrunners are bad — it’s that the gearing decision drives the motor choice. When the load can be driven directly, the outrunner’s geometry does the work for free. When the load demands high RPM or a tiny package, the inrunner earns its place.
Cooling and Heat: Why Outrunners Run Cooler
Heat is where these two designs diverge hardest, and it’s where most inrunner mistakes happen.
The thermal path
An outrunner’s windings sit on the stator, but the spinning bell and exposed magnets continuously move air. Heat leaves through convection almost by default. An inrunner’s windings are trapped inside a sealed-ish can; heat must conduct through the windings, into the case, and then into whatever the case is bolted to. If that mount isn’t a good heatsink, the inrunner cooks.
Demagnetization risk
Neodymium magnets permanently lose strength when they cross their temperature grade — typically around 80°C for standard N42/N45 grades, higher for N52SH and similar high-temperature grades. An over-driven inrunner that can’t shed heat will climb toward that threshold faster than an outrunner doing the same work. Once a magnet demagnetizes, the motor’s KV rises, it draws more current to produce the same torque, gets hotter still, and enters a death spiral. If a motor suddenly runs hotter and weaker after an overheated flight, demagnetization is the likely cause.
KV, Prop, and Voltage: Matching to the Motor Type
KV — RPM per volt — means the same thing in both motor types, but it plays out differently because of the torque/speed split.
| Factor | Outrunner | Inrunner |
|---|---|---|
| Typical KV range | Low-to-mid (e.g., 800–3000 KV) | Mid-to-very-high (e.g., 3000–100,000+ KV) |
| Load style | Direct drive, large prop | Geared or ducted, small pinion/impeller |
| Cooling | Self-cooling (spinning bell) | Conduction to mount / external heatsink |
| Best fit | Drones, planes, direct-drive boats | High-speed cars, boats, ducted fans, spindles |
| Failure mode to watch | Bearing wear, bell damage | Overheat / demagnetization from poor cooling |
Note that KV is not a measure of power. A high-KV inrunner and a low-KV outrunner can produce identical torque if the inrunner runs through a gearbox that converts its RPM back into torque. The choice is about where you’d rather place the reduction — in the motor’s geometry (outrunner) or in a separate gearbox (inrunner).
Where Each Type Wins
Outrunner use cases
- Multi-rotor drones and FPV quads — direct-drive props, torque and self-cooling win.
- Fixed-wing and trainer planes — large props at moderate RPM.
- Direct-drive RC boats — big props need low-end torque.
- Heavy-lift and agriculture UAVs — sustained load benefits from outrunner cooling.
Inrunner use cases
- High-speed RC touring cars and buggies — geared, high-RPM power.
- RC boats with geared or ducted drives — compact high-RPM packages.
- EDF (electric ducted fan) jets — impellers need very high RPM.
- Industrial spindles and precision tooling — rigid, high-speed, controlled cooling.
Common Mistakes When Choosing Between Them
| Mistake | What happens | Fix |
|---|---|---|
| Putting an inrunner on a direct-drive prop | Low torque, overheats, possible demagnetization | Use an outrunner, or add a gearbox |
| Putting an outrunner in a tight, enclosed mount | Starves the bell of airflow, runs hot | Leave the bell exposed to moving air |
| Ignoring KV when comparing | Assumes “same KV = same motor” | Compare torque, size, and cooling too |
| Under-speccing the inrunner’s heatsink | Heat soaks the case, magnets degrade | Bolt to a metal mount or add a heatsink |
| Judging power by motor can size | Bigger can ≠ more torque | Check rated wattage and prop match |
FAQ
Which is more powerful, inrunner or outrunner?
Neither is inherently more powerful. An outrunner produces more torque per amp at the shaft, while an inrunner produces more RPM. “Power” is torque times speed, so the real question is which matches your load and gearing.
Can I use an outrunner in an RC car?
Yes — many crawlers and slower-scale trucks use outrunners for their low-end torque and cooling. High-speed touring cars and buggies typically use geared inrunners for their higher RPM ceiling.
Why do inrunners run hotter?
Their windings are enclosed in a fixed can with no self-cooling fan, so heat leaves mainly by conduction to the mount. Without a good heatsink, an inrunner climbs in temperature much faster than an outrunner doing the same work.
What does demagnetization do to a motor?
When magnets exceed their temperature grade, they permanently lose strength. The motor’s KV rises, it draws more current to make the same torque, runs hotter, and can spiral into failure.
Does KV matter more for inrunners or outrunners?
KV matters for both, but it plays out differently: outrunners use low-to-mid KV for direct-drive torque, while inrunners use high KV and rely on gearing to convert RPM back into torque.
How do I tell which type I have?
Look at the rotating part. If the outer shell spins and only a fixed face is stationary, it’s an outrunner. If only the center shaft spins and the case stays still, it’s an inrunner.
Choosing the Right Motor for Your Build
The decision usually comes down to one question: is there a gearbox? Direct drive points to an outrunner; geared or ducted points to an inrunner. Match the motor type to your load, keep an eye on cooling — especially with inrunners — and spec a magnet grade that tolerates your worst-case temperature.
If you’re still unsure which motor fits your drone, car, or boat, X-TEAM’s engineers can help you size the right brushless motor, KV, and cooling for your specific build. Browse our brushless motor catalog or reach out for a custom KV or OEM/ODM solution.