Guides / Wheel and rim words, explained
Wheel and rim words, explained
Plain meanings for the rim flange, bead, sidewall, diameter, width, offset, and curb rash, drawn from NHTSA and Michelin.
What is the bead?
NHTSA's tire standard says the bead is the part of the tire made of steel wires, wrapped or reinforced by ply cords, and shaped to fit the rim.
What is the sidewall?
NHTSA says the sidewall is the portion of a tire between the tread and the bead.
What is the tread?
NHTSA says the tread is the portion of a tire that comes into contact with the road.
What is the rim?
NHTSA says a rim is a metal support for a tire, or a tire and tube, upon which the tire beads are seated.
What is the section width?
NHTSA says section width is the distance between the outsides of the sidewalls of an inflated tire, not counting raised lettering, decoration, or protective bands.
What is the rim diameter?
Michelin says rim diameter is the diameter of the rim bead seats that support the tire. Michelin's overall diameter is different: the diameter of the inflated tire, with no load.
What is the aspect ratio?
Michelin says aspect ratio is the relationship of a tire's sidewall height to its section width.
What is the offset?
Michelin says offset is what locates the tire and wheel assembly in relation to the suspension. Zero offset means the mounting face lines up with the wheel's centerline. Positive offset puts that face toward the street side and moves the assembly in toward the vehicle. Negative offset puts the mounting face closer to the brake side and moves the assembly out of the fender well.
What is the rim flange?
Michelin says the rim flange is the surface of the rim that contacts the side of the tire bead.
What is the rim lip?
NHTSA's tire standard does not define a lip. This site's install guide calls the outer edge of the wheel the lip, which is where a stick-on strip sits.
What is curb rash?
NHTSA's tire standard does not define curb rash. This site uses it for the scuffed or scraped look on the outer lip of an alloy wheel.
Things you should know
- The federal definitions are from NHTSA's standard for new pneumatic radial tires for light vehicles, 49 CFR 571.139. Offset and rim flange are Michelin's glossary, not that standard.
- The printed size code on the tire is a separate walk-through. Find your wheel size reads the rim diameter code. What is curb rash? is the longer note on that word. How to install a rim protector is where this site uses lip.