E-bike, bike & e-scooter glossary
55 terms, in plain English — from watt-hours and torque sensors to DOT fluid and California’s Class 3 rules. Where a term has a number attached, it’s the same sourced number the free tools use, and the tool is linked.
7 terms
Battery & electrical
- Watt-hour (Wh)
- A battery's energy capacity: volts times amp-hours. A 48 V, 14 Ah pack holds 672 Wh. Range, charging cost and airline limits are all worked out in watt-hours.Battery decoder
- Amp-hour (Ah)
- How much charge a battery holds. Multiply by the voltage to get watt-hours — on its own, amp-hours can't compare a 36 V pack with a 48 V one.Battery decoder
- Nominal voltage
- A pack's rated average voltage, such as 36 V or 48 V. Lithium-ion cells are rated about 3.6 to 3.7 V each, so a 36 V pack has 10 cells in series and a 48 V pack 13. A full 48 V pack reads about 54.6 V.Battery decoder
- Series and parallel (13S4P)
- How a pack's cells are wired. S counts cells in series, which sets the voltage; P counts cells in parallel in each group, which sets the amp-hours. A 13S4P pack has 52 cells.
- Cell group
- Cells wired in parallel that behave as one. A pack is a string of cell groups in series, and the weakest group limits the whole pack.
- BMS (battery management system)
- The circuit board inside a pack that watches each cell group's voltage and temperature and cuts power to protect them from over-charging, over-discharging, over-current and heat.
- Charge cycle
- One full battery's worth of charging, whether in one go or several top-ups. Makers quote 500 to 1,000 full charge cycles for e-bike batteries; Diamant calls 500 to 700 standard.Repair or replace?
16 terms
Motor, sensors & display
- Hub motor
- A motor built into the front or rear wheel hub, driving the wheel directly rather than through the bike's gears.
- Geared hub motor
- A hub motor with small reduction gears inside between the motor and the hub. Those gears are a wear item.
- Direct-drive hub motor
- A hub motor with no gears: the motor itself is the hub. Usually heavier than a geared hub, with no gears to wear out.
- Mid-drive motor
- A motor at the cranks that drives the chain, so it works through the bike's gears. Bosch and Shimano STEPS drive units and Bafang's BBS kits are mid-drives.
- Controller
- The electronics that read the sensors and throttle and meter battery power to the motor. On a hub-motor bike it's usually a separate box; on a mid-drive it's built into the drive unit.
- Display (HMI)
- The handlebar screen that shows speed, assist level and battery, and reports error codes. Bafang calls it the HMI.Error code lookup
- Error code
- A number the display shows when the system detects a fault. Each maker numbers its own — 21 is a speed sensor fault on a Bafang display but abnormal current on a Rad.Error code lookup
- Pedal assist (PAS)
- Motor help that comes only while you pedal, at the level you pick. Class 1 and Class 3 e-bikes are pedal-assist.
- Cadence sensor
- A pedal-assist sensor that detects the cranks turning, usually with a magnet ring at the crank, and switches assist on. It doesn't measure how hard you push.
- Torque sensor
- A pedal-assist sensor that measures how hard you pedal, so assist rises with your effort. It zeroes itself at start-up, which is why Shimano and Bafang say to restart with your feet off the pedals.
- Throttle
- A lever or twist grip that runs the motor without pedaling. In California, a throttle that moves the bike on its own makes it a Class 2 e-bike, limited to 20 mph.California e-bike class checker
- Walk assist
- A mode that moves the bike on motor power while you walk beside it. California allows it on Class 1 and Class 3 e-bikes up to 3.7 mph.
- Speed sensor
- A sensor on the frame that reads wheel speed from a magnet on a spoke. Bafang's spec is 10 to 20 mm between magnet and sensor; if it can't read speed, most systems cut or limit assist.Troubleshooter
- Hall sensor
- A magnetic sensor. Inside a motor, hall sensors report the rotor's position to the controller; motor hall faults have their own codes on Bafang (08), Rad (24), KT (03) and Segway-Ninebot (18) systems.
- Phase wires
- The three heavy wires that carry power from the controller to the motor's windings.
- Brake cut-off switch
- A switch in the brake lever that tells the controller to stop assist the instant you brake. One stuck "on" stops assist — Bafang's code 03 and Rad's 25 point at it.Troubleshooter
7 terms
Brakes
- Hydraulic disc brake
- A disc brake worked by fluid in a sealed hose instead of a cable. Each one is built for either mineral oil or DOT fluid, never both.Brake fluid finder
- Mineral oil
- The fluid in Shimano, Tektro, TRP, Magura and SRAM's newer brakes. It doesn't absorb water, so it's bled less often — SRAM says every two years.Brake fluid finder
- DOT fluid
- Glycol-based brake fluid (DOT 4 or DOT 5.1) used by Hope, Hayes and SRAM's Code, Level, Guide, G2 and road brakes. It absorbs water and damages paint; SRAM bleeds DOT brakes once a year. DOT 5 is a different, silicone fluid that bike brakes don't use.Brake fluid finder
- Bleed
- Replacing a hydraulic brake's fluid and pushing out any air. It's the fix for a spongy lever.Maintenance planner
- Brake pads
- The friction pads the caliper squeezes onto the rotor. SRAM replaces them at 3 mm including the backing plate; Shimano and Tektro at 0.5 mm of pad left.Maintenance planner
- Rotor
- The steel disc the pads clamp. Rotors wear thin too: Shimano replaces them at 1.5 mm.
- Bed-in
- A set of firm stops after fitting new pads or rotors so they grip properly. SRAM says skipping it costs power and adds noise.
7 terms
Drivetrain
- Chain wear ("stretch")
- Wear in a chain's rivets and rollers that lengthens it — Park Tool's "illusion of stretching." Replace at 0.5% on 11-speed and up, 0.75% on 10-speed or fewer, 1.0% on a single-speed.Chain wear checker
- Cassette
- The stack of rear cogs on a derailleur bike. A chain run past its wear limit wears the cassette to match.Chain wear checker
- Derailleur hanger
- The small replaceable bracket that holds the rear derailleur to the frame. It's made to bend in a crash, and a bent one causes bad shifting.
- Gear inches
- A gear's size as the diameter of an equivalent wheel: wheel diameter times chainring teeth divided by cog teeth, in Sheldon Brown's definition.Gear & speed calculator
- Cadence
- How fast you pedal, in crank revolutions per minute (rpm).Gear & speed calculator
- Belt drive
- A toothed belt in place of a chain. It needs no lube, but it needs correct tension; Riese & Müller says inspect it for damage every 300 miles.Maintenance planner
- Internally geared hub
- Gears sealed inside the rear hub instead of a derailleur and cassette.
5 terms
Wheels & tires
- Tubeless
- A tire sealed to the rim with no inner tube, with liquid sealant inside to plug small punctures. Stan's says top up the sealant every 2 to 7 months.Maintenance planner
- Hookless rim
- A rim with straight inner walls instead of hooks to hold the tire bead. The ISO standard caps tire pressure on hookless rims at 72.5 psi (5 bar).Tire pressure calculator
- ETRTO / ISO tire size
- The tire size code that tells you what actually fits: width and bead-seat diameter in millimeters. 50-559 is a 50 mm tire for a 26-inch (559 mm) rim.Tire size decoder
- Pinch flat
- Two small parallel cuts in a tube where the rim pinched it against something hard — a sign the pressure was too low.Tire pressure calculator
- Solid tire
- An airless scooter tire. It can't go flat, but it rides harder than a pneumatic one.
2 terms
Suspension
- Lower-leg service
- A basic fork service: the lower legs come off to be cleaned and re-oiled. RockShox's interval is every 50 riding hours.Maintenance planner
- Sag
- How far the suspension settles under your weight when you're simply sitting on the bike.
11 terms
California law & standards
- Class 1 e-bike
- Under California Vehicle Code §312.5: assists only while you pedal, and stops assisting at 20 mph. Motor up to 750 W.California e-bike class checker
- Class 2 e-bike
- Can move the bike on the motor alone (a throttle), with no assistance above 20 mph. Motor up to 750 W.California e-bike class checker
- Class 3 e-bike
- Assists only while you pedal, up to 28 mph, and must have a speedometer. No riders under 16, and every rider and passenger wears a helmet.California e-bike class checker
- Moped (motorized bicycle)
- In California: pedals (or all-electric), under 4 horsepower, top speed 30 mph or less. It needs an M1 or M2 license, a moped plate, a helmet and a rider 16 or older.California e-bike class checker
- Electric scooter
- A stand-up motorized scooter. In California the rider needs a driver's license or permit, it tops out at 15 mph, carries no passengers, stays off sidewalks, and riders under 18 wear a helmet.California e-bike class checker
- eMoto (off-highway electric motorcycle)
- A two-wheeled electric motorcycle with a straddle seat and no pedals, built for off-road. Since 2026, California treats it as an off-highway vehicle: an OHV permit or plate, and off-highway riding areas only.California e-bike class checker
- SB 1271
- California law, in effect since January 1, 2026, that bars selling or leasing an e-bike unless its battery has been tested by an accredited lab to a standard referenced in UL 2849 or EN 15194; e-scooters must be tested to UL 2272. It governs sales, not bikes people already own.Battery safety
- UL 2849
- The US safety standard for an e-bike's electrical system — battery, charger, motor and controller evaluated together.Battery safety
- EN 15194
- The European e-bike standard. California accepts it as an alternative to UL 2849.Battery safety
- UL 2271
- The standard for light electric vehicle battery packs — one of four battery standards UL 2849 accepts.Battery safety
- UL 2272
- The standard for e-scooters, hoverboards and other personal e-mobility devices. Not an e-bike standard.Battery safety
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