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)
Drop Center Rim - Well Base Rim
Most passenger cars and light trucks are of the drop center design. The inner section of the drop center rim is lower than the outer edges allowing the bead of the tire to be pressed into the recess on one side while the opposite side may be levered over the rim flange. The well between the bead seats makes tire mounting and demounting easier.
Inflated pneumatic tire sits on the rim by tapering the bead seat toward the flange. Both bead seats of passenger car rims are tapered at 5°.
There are a few variations of drop center rim like Wide Drop Center Rim and Wide Drop Center Rim with a hump.
Wide Drop Center Rim has expanded rim width, flatter well and smaller height flange. Wide drop design is mostly used on passenger cars and particularly on rims for low profile tires.
Rim Contours or Bead Profiles
Wheel/Rim designation contains some letters for marking the shape of the rim flanges where the tire sits on the rim. Rim contour (bead profile) specifies the shape of the rim flange. For example, the letter J means that the rim flange is J-shaped. Every tire is made to be installed on a certain type of wheel, the right rim contour letter is required. The wrong rim countour can cause tire slipping from the rim.
There are several types of rim contour designated by letters J, JJ, K, JK, B, P and D. J is the most common shape of a rim flange on passenger cars. The letter B appears at the smallest wheel sizes primarily for older car models, with diameters 12" and 13" and rim widths up to 5.0". J rim contour has 17.5 mm high rim flange but B has only a 14 mm. JJ designation is most common on 4x4 and SUV vehicles.
Rim Safety Contorus
Wide drop center rim may have elevated portion of the rim bead seat, safety contour called hump, at least at the outer bead seat area. Hump enables better tire fitting on the rim, prevents bead of slipping down when vehicle moving at a high speed and stops air escaping when cornering at reduced tire pressure.
The usage of these rims is very important for road safety. Therefore implementation of rims with safety contours on passenger cars is mandatory in many countries.