Bike Maintenance Glossary

The bead is the reinforced edge running around both sides of a tire, made with either a steel wire or a bundle of aramid fibers. Air pressure pushes it outward into a hooked shelf in the rim, and that is the only thing holding a tire on. A bead that is not seated evenly all the way around shows up as a wobble at the tire molding line, and it should be corrected before the tire is brought to full pressure.

Where it comes upPatch or Replace a TubeInspect Tires for Wear and Cuts

Assembly paste is a light compound containing fine particles, applied to clamping surfaces such as a carbon seatpost in a frame. It increases friction so the part holds at a lower clamping force, which is the point: carbon fails from being overtightened rather than from being underclamped. Use it only where the component manufacturer specifies it, and use their torque figure with it.

Where it comes upCheck Bolt Torque

The casing is the fabric carcass of a tire; the rubber tread is a wear surface bonded on top of it. When a cut is deep enough to show woven threads, the tire has lost part of the structure holding it together and it is finished, however good the tread still looks. Casing construction is also what makes two tires of the same width feel completely different.

Where it comes upInspect Tires for Wear and CutsPatch or Replace a Tube

The cassette is the cluster of sprockets on the rear wheel, splined onto the freehub body and held by a lockring. Its teeth wear together with the chain, which is why a worn chain that is not replaced in time takes the cassette with it. Sprockets worn into a hooked shape will skip under load even with a brand new chain fitted.

Where it comes upClean and Lube the ChainDerailleur Indexing Tune

A chain does not stretch in the sense of the metal getting longer. The pins and the inside of the rollers wear against each other, each link gains a fraction of a millimeter of play, and over a whole chain that adds up to a measurable increase in length. A chain wear gauge reads that directly, and replacing the chain at the limit the manufacturer publishes is what saves the cassette.

Where it comes upClean and Lube the ChainWipe Down the Drivetrain

Cross-chaining means being on the largest chainring and the largest rear sprocket at the same time, or the smallest and the smallest. The chain runs at an angle it was not designed for, which is noisy, wears the chain and sprockets faster, and can rub the front derailleur cage. There is usually an equivalent gear ratio available in a straighter chain line.

Where it comes upDerailleur Indexing TuneClean and Lube the Chain

On most modern bikes the rear derailleur hangs from a separate aluminum tab designed to bend or break before the frame does. It bends easily when a bike falls on its right side, and a bent hanger makes shifting impossible to tune however carefully you set the cable. Straightening one properly needs an alignment gauge, and hangers are specific to the frame.

Where it comes upDerailleur Indexing Tune

The rotor is the braking surface on a disc brake bike, bolted or splined to the hub. It has a minimum thickness the manufacturer publishes, often stamped on the rotor itself, and it wears down over time like a pad does. Rotors are also the part that must never meet degreaser, chain lubricant or oily fingers, because contamination transfers straight to the pads.

Where it comes upCheck Brake PadsDeep Clean and Degrease

ETRTO is the European standards body for tires and rims, and its sizing scheme is what put an unambiguous number on the side of every modern bicycle tire. In practice you meet it as the two-number marking, such as 37-622, where the second figure is the rim bead seat diameter. It is the same system referred to as the ISO size on most packaging.

Where it comes upPatch or Replace a TubeCheck Tire Pressure

Indexed shifting works because each click at the shifter pulls or releases a fixed length of cable, and that length is matched to the spacing of the sprockets. Cables stretch and housing compresses, so the fixed length slowly stops matching, which is what a barrel adjuster corrects. Indexing has nothing to do with the limit screws, which only define the ends of the derailleur travel.

Where it comes upDerailleur Indexing Tune

The ISO size is written as tire width in millimeters, a dash, then the bead seat diameter of the rim in millimeters. The diameter has to match exactly; the width has to fall inside the range a tube or rim allows. Inch names such as 26 inch or 700C are nicknames that several different diameters have shared over the years, which is why the ISO number is the one to buy from.

Where it comes upPatch or Replace a TubeInspect Tires for Wear and Cuts

Glazing happens when a pad gets hot enough for its surface to harden into a smooth shiny layer, usually from dragging the brake on a long descent. A glazed pad squeals, feels grabby rather than progressive, and gives less stopping power than a healthy one. Light sanding can recover a rim pad; a disc pad that is glazed or contaminated is replaced.

Where it comes upCheck Brake Pads

PAS stands for pedal assist system, the arrangement on an e-bike where the motor contributes only while you are pedaling. The assist level is chosen at the display and changes how much of the work the motor takes on, which directly changes range. Assist is not a throttle: it responds to your pedaling rather than replacing it.

Where it comes upE-Bike Connector and Motor CheckE-Bike Battery Care

A Presta valve is narrower and usually longer than a Schrader, and it has a small knurled nut at the tip that has to be unscrewed before air will go in and closed afterward. The narrower rim hole takes less material out of the rim, which is why road and gravel wheels use it. A Presta valve does not fill a Schrader-sized hole without a grommet or adapter.

Where it comes upCheck Tire PressurePatch or Replace a Tube

A quick release is a skewer through a hollow axle with a cam lever at one end and an adjusting nut at the other. It works by clamping, not by threading, so it has to be tight enough that closing the lever leaves a mark on your palm. A lever that closes easily is not doing its job, and a wheel is the wrong place to find that out.

Where it comes upThe ABC Check Before Every RidePatch or Replace a Tube

Rim tape is a band of cloth, plastic or adhesive strip covering the spoke holes in the rim bed. Without it the tube presses into each hole under pressure and eventually punctures on the spoke end. Tape that has shifted, split or gone brittle is a common and easily missed cause of repeated flats on the rim side of a tube.

Where it comes upPatch or Replace a Tube

A Schrader valve is short, wide, and works exactly like the valve on a car tire, with a spring-loaded pin in the center that opens when a pump head presses on it. It needs the larger of the two rim hole sizes. You will find it on kids bikes, most hybrids and mountain bikes, and a large share of e-bikes.

Where it comes upCheck Tire PressurePatch or Replace a Tube

The sidewall is the thinnest part of a tire and it carries no tread rubber to protect it, which is why cuts and age cracking show up there first. It is also where the manufacturer molds the size marking and the permitted pressure range. That printed range is the boundary for every pressure decision on this site.

Where it comes upCheck Tire PressureInspect Tires for Wear and Cuts

A thru axle passes all the way through the hub and threads directly into the frame or fork, which locates the wheel more precisely than a quick release and suits disc brakes. It is tightened to a torque figure the manufacturer publishes rather than clamped by feel. Diameters and thread pitches vary between bikes, so a spare is not a generic part.

Where it comes upThe ABC Check Before Every RideCheck Bolt Torque

A torque sensor measures the force you put through the pedals and scales the motor output to match, which is what makes an e-bike feel like a strong version of a bicycle rather than a switch. The simpler alternative, a cadence sensor, only detects that the cranks are turning and delivers a fixed level of assist. Both are sealed assemblies and neither is a home repair.

Where it comes upE-Bike Connector and Motor Check

A torque spec is stated in newton-meters or inch-pounds and describes how hard a bolt is being stretched, not how tight it feels. Figures belong to a specific part from a specific manufacturer, and there is no general table worth trusting across brands or materials. Where no published figure exists for the part in your hands, that is a question for the manufacturer or a shop rather than an estimate.

Where it comes upCheck Bolt Torque

TPI counts the threads per inch in the casing fabric. A higher count generally means finer threads, more of them, and a casing that deforms more readily around bumps, which usually rides better and cuts more easily. A lower count means thicker threads and a tougher, heavier tire. It is a useful rough signal rather than a specification you can compare precisely across manufacturers.

UL 2849 is a published safety standard for electrical bicycle systems, covering the drive system, the battery and the charger together rather than the cells alone. Certification to it is a claim a manufacturer makes and has tested, and it is increasingly asked for by insurers, landlords and local rules. Check what the manufacturer of your specific bike states and read the standard from the organization that publishes it rather than from a listing.

Where it comes upE-Bike Battery CareE-Bike Connector and Motor Check

A watt-hour is voltage multiplied by amp-hours, and it is the only figure that lets you compare two e-bike batteries directly. A larger watt-hour number means more stored energy and, all else equal, more range. Actual range still depends on rider weight, terrain, assist level, tire pressure and temperature, which is why manufacturer range claims cover such wide bands.

Where it comes upE-Bike Battery Care