More than a Rim: How a Wheel Is Structured
Accelerating and braking, fast curves or a pointed curb – driving places heavy mechanical demands on the wheel. Not to mention the heat stress resulting from friction and braking. A wheel must be engineered to withstand all of these strains. Modern wheels represent the state of the art and carry in them the professional expertise of many generations of engineers. Their design is fine-tuned to meet all the demands of driving while also having additional safety features.
Rim or Wheel? Don’t worry: this blog isn’t here to dissect linguistic subtleties, but to deliver some technical background information. One thing is important to clear up from the start, though. When people casually talk about aluminum rims, they are typically referring to an aluminum wheel with a rim, spokes, and a connection face with a bolt ring and bolt circle. And experts refer to the combination of wheel and tire as a complete wheel.
Wheels for passenger cars are almost always produced in one piece these days. With this in mind, it isn’t very important whether you colloquially say “rim” or use the more technically correct “wheel”. Wheels used to consist of several parts that weren’t permanently attached to each other, which meant that you could screw them apart and, for example, change the rim along with the tire.
As explained above, a wheel consists of a rim, spokes, and a connection face. The design of these parts involves several construction types and terms that are important to note when selecting the perfect wheel – here’s an overview:
The flange is the rim’s external ridge, which is curved out slightly like a horn (in German, it is called the rim “horn”). The rim flange is standardized as laid out in the DIN 7817 standard. Its design variations carry the letters J, H, P, K, and JK; J is the most common variation for passenger cars. The rim flange is the first site of damage when a wheel is driven over a curb too forcefully.
Between the rim flanges is the rim well, which is seen almost exclusively in the drop center variety today.
The bead seat connects to the outer rim flange. This is where the tire sits.
Rim diameter and rim width
Significant for wheel sizing are the rim diameter and rim width, which are both measured in inches. The rim diameter is the distance between the bead seats. The rim width is the distance between the rim flanges and corresponds to the inner diameter of the suitable tires.
The contour of the rim is commonly referred to as the hump. It ensures that the tire sits firmly on the bead seat even when driving into sharp curves.
Wheel offset refers to the distance from the wheel’s inner contact surface to the middle of the rim. It determines the vehicle’s axle track and can have positive or negative values.
All the bolt holes of a wheel are located on the bolt circle. Its dimensions, as well as the size and number of wheel bolts, vary according to vehicle type. The wheel bolts fasten the wheel to the axle hub – turning the wheel into an integral component of the overall vehicle system.