What Do the Gears on an Ebike Actually Do?

Walk into any bike shop service department and ask what breaks on ebikes. You will hear the same answer twice before you finish the question: chains, cassettes, and derailleurs, on bikes with barely a thousand miles on them. The cause is almost never a defect. It is a rider who thinks the numbers on the display and the numbers on the shifter mean roughly the same thing.

They do not. One of them changes how the motor behaves. The other changes the mechanical relationship between your legs and the ground, and it is the only one that can be destroyed by using it at the wrong moment. Understanding which is which takes about five minutes and will save you a few hundred dollars over the life of the bike.

Two controls, two entirely different jobs

On a typical ebike your left thumb sits near a small pad marked with a plus and a minus, or with the numbers 0 through 5. That is the assist level. Your right thumb sits on a shifter with a bigger lever and a smaller one, or a twist collar. That is the gear selector.

The assist level tells the controller how much current it may feed the motor. On most bikes it is a simple ceiling: level 1 might allow a quarter of the system's maximum power, level 5 allows all of it. On bikes with a torque sensor, the assist level acts more like a multiplier on your own effort, so level 2 might return roughly one and a half times the wattage you are producing, and level 5 might return four times. Either way, the assist level is a request made to a computer. Nothing mechanical moves.

The gear is pure mechanics. Moving the shifter drags the chain onto a sprocket with a different number of teeth, and that changes the ratio between crank rotation and wheel rotation. No electronics are involved, and no amount of motor power can substitute for choosing the wrong one.

Here is the practical consequence people miss. Turning up the assist does not make the gear easier to push. It adds motor force alongside the force you are already applying, in the gear you already chose. If you are in your hardest gear on a 10 percent grade, cranking the assist to 5 does not fix the fact that each pedal stroke needs enormous force. On a hub-motor bike it means the motor now has to do essentially all of the work at a road speed where it is inefficient. On a mid-drive it means all that motor torque is being poured through a chain that is already at maximum tension. Neither is good.

What a gear actually changes

A gear ratio is a simple fraction: teeth on the front chainring divided by teeth on the rear sprocket you are currently using. If your chainring has 44 teeth and the chain is on an 11-tooth cog, the ratio is 4.0. One full turn of the pedals spins the rear wheel four times. On a 27.5 inch wheel, which rolls about 2.15 metres per revolution, that is roughly 8.6 metres of ground covered per pedal stroke.

Shift to the 34-tooth cog at the other end of the cassette and the ratio becomes 44 divided by 34, or about 1.3. Now one pedal turn moves you only 2.8 metres. Same legs, same effort, one third of the distance.

Nothing is free here, and the thing you are trading is torque. Force at the pedals produces torque at the crank, the chain converts that into tension, and tension at the rear sprocket converts back into torque at the wheel. Run the algebra and the relationship inverts: wheel torque equals crank torque multiplied by rear teeth divided by front teeth. A big rear cog gives you more twisting force at the wheel and less distance per stroke. A small rear cog gives you distance and almost no torque.

That is the whole of gearing. You are not making power appear or disappear. You are choosing how to spend a roughly fixed amount of power: as force against a hill, or as speed on the flat.

Multiply that middle gear by a normal cadence and the numbers make sense. At 80 pedal revolutions a minute in the 44/20 combination you cover about 376 metres per minute, which is 22.5 km/h or roughly 14 mph, all from your legs. Add motor assist and you are comfortably at a legal cruising speed without straining. In the 44/11 top gear the same 80 rpm would put you at 25 mph, which is why that gear only makes sense once you are already moving fast. For how those speeds interact with the legal cutoffs, see ebike classes explained.

Cadence, and why 70 to 90 rpm is the target

Cadence is how many times per minute you turn the cranks. It is the variable most new riders never think about, and it quietly determines whether cycling feels pleasant or feels like a leg press.

Human muscle produces power as force multiplied by speed of contraction, and the metabolic cost of each option is not the same. Pushing hard at 50 rpm recruits fast-twitch fibres that fatigue quickly and loads your knee joints heavily on every stroke. Spinning at 120 rpm wastes energy on the internal work of accelerating and decelerating your own legs twice per revolution. The efficiency curve for most riders bottoms out somewhere between 70 and 90 rpm, which is why nearly every cycling coach lands on that band. If you want a reference point, 80 rpm is roughly one pedal stroke every three quarters of a second.

On an ebike there is a second reason to care, and it is mechanical rather than physiological. Electric motors also have an efficient rpm band. Mid-drive systems from Bosch, Shimano, and Yamaha are tuned around a rider cadence in the 70 to 90 range, because the motor rpm is geared directly off your cadence. Grind along at 45 rpm and the motor is running well below its efficient point, drawing more current for the same output and dumping the difference into heat. Your range drops and the motor casing gets noticeably warm.

Bikes with a cadence sensor rather than a torque sensor have a related quirk. A cadence sensor is a magnet ring that counts crank rotations, and it only asks one question: are the pedals turning? It does not measure how hard you are pushing. That means the motor delivers the same assist whether you are straining or barely brushing the pedals with your feet, which is precisely how riders end up ghost pedalling for hours without noticing. Torque-sensing bikes, which are standard on mid-drives and increasingly common above the $2,000 mark covered in our guide to the best electric bikes under $2,000, scale assist to your actual force and make the gearing feel far more natural.

What gear to be in, and when

Starting from a stop

Low gear, always. From zero, the bike needs a lot of wheel torque and very little speed, which is exactly what a large rear cog delivers. Starting in a tall gear means a single enormous shove on the pedal followed by a wobble, and on a mid-drive it means the motor slams full torque into a stationary chain.

The practical problem is that you cannot select a low gear while stopped on a derailleur bike, because the chain has to be moving for the derailleur to move it. So the habit you actually need is this: downshift as you slow down, before you stop. When you see the light turning red, work down two or three gears while you are still coasting with light pedal pressure. Arrive at the line already in your starting gear.

Climbing

Shift down early, at the bottom of the hill, while you still have momentum and light chain tension. Waiting until you are halfway up and grinding is how derailleurs die. Target the gear that lets you hold 70 rpm at a sustainable effort, and let the assist level cover the rest. If you run out of gears and are still grinding, that is a gearing range problem, not an assist problem, and the answer on your next bike is a wider cassette or an easier chainring.

Cruising

A middle to high gear, chosen so that you spin comfortably at the speed you actually hold. If your bike assists to 20 mph and you cruise at 18, you want a gear that puts you around 75 to 85 rpm at 18 mph. On a Class 3 bike doing 28 mph you will need most of the cassette's top end, which is one reason speed-focused bikes ship with taller gearing. Our guide to how fast ebikes actually go covers the speeds each class supports.

Descending or riding above the assist cutoff

Highest gear you can turn. Once the motor stops helping at 20 or 28 mph, the only thing keeping you moving is your legs and gravity, and in a low gear your cadence will be too high to contribute anything. This is where riders discover that a 7-speed cassette runs out of gears embarrassingly early.

Why shifting under power breaks ebike drivetrains

A derailleur shift is a controlled derailment. The chain has to lift sideways off the sprocket it is on and drop onto the next one, and it manages this because modern cassettes have ramps and pickup pins machined into the sides of the cogs to catch it. That whole process depends on the chain being loose enough, for a fraction of a second, to lift off the teeth it is currently wrapped around.

Under high tension the chain is pressed hard into the tooth valleys and does not want to lift. What happens instead is that it is dragged across the ramps under load, chewing metal off both parts, or it lifts partway and slams down under full force. Do that repeatedly and the outcomes are predictable: rounded cassette teeth, a stretched chain, bent chain plates, a snapped chain at a weakened link, or a derailleur pulled forward into the spokes.

This is true on any bicycle. It is far worse on an ebike, and worst of all on a mid-drive, for a reason that is worth being precise about.

On a hub-motor bike, the motor drives the wheel directly. Your chain only ever carries the torque your legs produce, exactly as on a regular bike, and the motor's contribution bypasses the drivetrain entirely. A mistimed shift is no more damaging than it would be on your old analog bike.

On a mid-drive, the motor sits at the bottom bracket and drives the chainring. Every newton metre it makes travels through the same chain and the same cassette your legs use. A Bosch Performance Line CX is rated at 85 Nm at the crank; a strong rider on a good day produces perhaps 100 to 150 Nm in a hard effort out of the saddle. So the motor is roughly doubling peak chain tension, and it does this continuously rather than in the short bursts a human manages. Shift into that and something gives. The mechanism behind mid-drive torque is covered in full in mid-drive versus hub motor.

Manufacturers know this is a problem and have thrown hardware at it. Most brand-name mid-drive systems support a shift sensor, either a sensor on the shifter cable or software that detects the cadence blip of a shift, and it momentarily cuts motor power while the chain moves. Shimano's LinkGlide cassettes use thicker, differently profiled teeth designed for ebike torque, and the company rates them for substantially longer service life than standard Hyperglide under the same load. Ebike-specific chains from KMC and Shimano use tougher pins and plates. None of these are a licence to shift under full power, but they widen the margin considerably.

One more consequence worth budgeting for: chains wear faster on a mid-drive regardless of technique. Buy a chain wear gauge, check it every few hundred miles, and replace the chain when it reads 0.5 percent elongation. A chain costs $25 to $50. A cassette and chainring cost several times that, and a worn chain destroys both.

The three gearing systems you will actually encounter

Derailleur systems

A cassette of 7 to 12 sprockets on the rear wheel with a spring-loaded arm that pushes the chain between them. This is what the overwhelming majority of ebikes use, because it is cheap, light, easy to service anywhere, and offers a very wide range of ratios. Efficiency is excellent when clean, typically in the high 90s as a percentage of input power transmitted.

The downsides on an ebike are the ones described above, plus exposure. The derailleur hangs off the frame in the most crash-prone position on the bike, the cassette collects grit, and the whole system needs the chain moving to shift. On budget ebikes the derailleur is also a common cost-cutting point; a 7-speed Shimano Tourney is adequate but will not tolerate abuse the way a Deore or LinkGlide group will.

Internally geared hubs

All the gearing lives inside a sealed shell in the rear hub, as a set of planetary gear stages. Shimano's Nexus and Alfine hubs offer 5, 7, 8, or 11 speeds. Enviolo makes a continuously variable hub with no discrete steps at all. Rohloff's 14-speed Speedhub is the expensive gold standard, with a range roughly equal to a full mountain bike drivetrain.

These suit ebikes unusually well, for three reasons. First, you can shift at a standstill, which erases the biggest ergonomic flaw of a derailleur on a bike that stops at every light. Second, the mechanism is sealed against water, salt, and grit, so a commuter that lives outside stays functional. Third, there is no derailleur to bend, and the chain runs in a straight line so it wears more evenly.

The costs are real. An internally geared hub adds roughly two to four pounds at the rear wheel, costs several hundred dollars more, and loses a few percent more efficiency than a clean derailleur in its indirect gears. Critically, every internally geared hub has a maximum input torque rating, and mid-drive motors can exceed it. Manufacturers publish ebike-approved versions for this reason. If you are pairing an internally geared hub with a mid-drive, confirm the hub is rated for it rather than assuming.

Single speed, usually with a belt

One chainring, one rear cog, no shifting. Almost always paired with a Gates carbon belt on the bikes that do this well, because with no shifting required a belt becomes practical. It is silent, does not need lubricant, does not rust, and will typically outlast three or four chains. We go deeper in our guide to belt drive ebikes.

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Why some ebikes have no gears at all, and when that is fine

A meaningful slice of the market, particularly cruisers and flat-bar city bikes, ships with a single gear. That is not always a cost cut, though sometimes it is.

The logic is that gears exist to solve a torque problem, and a hub motor already solves that problem electrically. A 750W geared rear hub produces something like 75 to 85 Nm at the wheel from a dead stop, at any road speed, in any gear, because the reduction happens inside the motor and never changes. If the motor is supplying the torque to get you moving and up modest rises, the mechanical low gear you would normally need becomes redundant. What you gain is a drivetrain with nothing to adjust, nothing to bend, and nothing to lubricate.

The flat battery scenario deserves emphasis because it is the one people discover the hard way. Single speed ebikes are usually geared fairly tall, since the motor handles the starts. Pedalling a 65 pound bike home in a tall gear with a dead pack is genuinely miserable. If your rides ever approach the edge of your range, gears are cheap insurance. Sizing the pack correctly is covered in ebike battery voltage and capacity, and the bikes with the most headroom are in longest range electric bikes.

Habits that make gears automatic

None of this requires thought once it is trained in. Four habits cover essentially every situation.

1. Downshift while slowing, not after stopping. Two or three gears as you approach every light. You will never again lurch away from a green in your top gear.
2. Shift before the hill, not on it. The moment you see a grade, get into the gear you will need while the chain is still slack.
3. Ease off for half a second on every shift. Cranks keep turning, pressure comes off. This is the whole discipline.
4. Use the gear to set your effort, the assist to set your pace. Pick the gear that gives you the cadence you want, then adjust the assist level to reach the speed you want. Doing it the other way around is what produces ghost pedalling.

Two related decisions matter more than any gearing choice. The first is where the motor sits, because it determines whether your chain carries motor torque at all: read mid-drive versus hub motor before you buy. The second is what the wattage figure on the spec sheet actually means, which is less than you would think and is unpicked in ebike motor wattage.

Frequently asked questions