The air tightness is very important feature when tubeless tires are installed on the wheels. Proper touch between rim and tire ensures sealing the air inside tubeless tire.
The air tightness and exact fitting between tire and rim are provided by bumps and recesses at the place where the tire sits on the rim. They fulfill important functions and we will call them rim profiles:
- In accordance with the shape of the rim for passenger vehicles there are a few variations of commonly used Drop Center Rim
- In accordance with the shape of rim flanges there are a few Rim Flange Contours or Bead Profiles
- In accordance with the shape of bumps there are a few types of Safety Contours (Humps)
The terms "wheel" and "rim" are often used as words with the same or nearly the same meaning, but technically speaking it is not really so. Many people use term "wheel" as a "rim" meaning the entire metal part to which the tire is mounted because rim and wheel are usually cast or pressed from a single piece of metal.
The offset of a wheel is what locates the tyre and wheel/rim assembly in relation to the suspension.
More specifically, it's the measured distance between the mounting pad (the mounting surface of the wheel) and the center line of the rim. Sometimes It's used interchangeably with the term "backspace".
A similar concept to an offset, a backspace is simply the space between the wheel's mounting surface (the mounting pad) and the inboard flange (lip) of a wheel.
Therefore the backspace depends on both the overall width of the wheel and the offset of the wheel.
It shows where exactly the mounting pad is in relation to wheel's overall width. Terms: the offset and the backspace are often used interchangeably. Both terms determine where the wheel sits within the wheel well, how much of the wheel will protrude inboard, towards the suspension components or jut out, towards the fender. A wheel with the positive offset has more backspace, conversely a wheel with the negative offset has less backspace.