Understand Wheel and Rim Terminology
At first glance, wheel terminology can seem a little daunting if not cryptic. When considering a wheel upgrade or even knowing more about the wheels you already have fitted to your Porsche, understanding the codes and terminology is a vital piece of the puzzle. Here's a guide to helping you speak the language of the wheel and rim experts.
First, A Little History
A little history as to the origins of the wheel will help understanding some of the terminology. The story of the history of wheel began when the human race began to use the log to transport heavy objects. The original design of the wheel of round slices of a log was gradually re-inforced and used in this form for centuries on both carts and wagons.
This solid disc changed to a design having several spokes radially arranged to support the outer part of the wheel keeping it equidistant from the wheel centre.
A wooden wheel which used hard wood stakes as spokes was very popular as a wheel for many vehicles up to about 1920. Afterwards the disc wheel, in which the spokes were replaced with a disc made of steel plate, was introduced and is still being used to this day. Most recently, a light alloy has come to be used as a wheel material for many types of vehicle.
Rim Size Designation
This is the way in which a wheel's specification is captured in a string of digits. For example a 7" x 16" Wheel with an offset of 40mm might be denoted as follows 7J16 ET40. This appears at first to be hard to decypher, but actually its quite simple. Take a look at the Rim size designtion picture below as well as the nomencature diagram for definitions:
The "J" in our example denotes the specification for the Rim Flange. This is the letter that typically sits between the rim width and diameter figures stamped on the wheel, and indicates the physical shape of the wheel where the tyre bead meets it. In the cross-section shown you can see the area highlighted as "Flange and Hump". Common wisdom has it that the letter represents the shape. ie. "J" means the bead profile is the shape of the letter "J". This is not the case, although "J" is the most common profile identifier. 4x4 vehicles often have "JJ" wheels. Jaguar vehicles (especially older ones) have "K" profile wheels. Some of the very old VW Beetles had "P" and "B" profile wheels. Our Beautiful Fuchs wheels on older Porsches are "J" profiles.
It is important to understand that term 'Rim' is often used to mean 'Wheel'. This is not the case. They are different things. There's only a few things to remember (See also cross section diagram below):
- Wheel: Wheel is generally composed of rim and disc.
- Rim: This is a part where the tire is installed.
- Disc: This is a part of the rim where it is fixed to the axle hub.
- Offset: This is a distance between wheel mounting surface where it is bolted to hub and the centerline of rim.
- Flange: The flange is a part of rim which holds the both beads of the tire.
- Bead Seat: Bead seat comes in contact with the bead face and is a part of rim which holds the tire in a radial direction.
- Hump: It is bump what was put on the bead seat for the bead to prevent the tire from sliding off the rim while the vehicle is moving.
- Well: This is a part of rim with depth and width to facilitate tire mounting and removal from the rim.
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