Wheel Rims

Wheel rims is the part of wheel on which tyre is mounted and supported.

Passenger cars normally use rims which are of well based, or drop-centre design. The drop enter is used for mounting and demounting the tyre onto the rim. Well-based rims can also be used on heavy commercial vehicles for tubeless tyres. [1]

The rim must be accurately shaped and dimensioned and strong enough to support the tyre under the load of the vehicle and the forces generated by the motion of the vehicle. [1]

Wheel rims must also provide the required distance from the centreline of the wheel, to the face of the mounting flange i.e. offset. [1]

Various metals can be used for the rim. Commonly seen are alloy (magnesium and aluminum), mag (magnesium), aluminum, and chrome. Teflon coatings are sometimes also applied for an extra layer of protection. [1]

The tyre must be an exact fit on the rim, to fulfil a number of functions; [1]

1. It ensures that the narrow contact area between the beads of the tyre and the rim will seal the air in a tubeless tyre.
2. It transfers all the forces between the tyre and the wheel, without slipping or chafing.
3. It ensures the friction between the tyre and the rim prevents the tyre turning on the rim.

Rim Designs: [2]

Rims are having the functions of holding the tyre as well as sealing function in case of tubeless tyres. They are classified as follow based on their construction.

1. Drop center rims
2. Flat base rims
3. Slanted taper bead seat rims
4. Taper bead seat rim

Wheel with taper bead seat rim [2]

Drop center rims and Taper bead seat rims are made up of one piece, slanted taper bead seat rims are multi piece rims. A raised part all around the bead seat in case of Taper bead and Drop center rims, improves the fixation of the tyres.

Rim Sizes & Designations:

To ensure correct fit between a tyre and rim, all manufacturers of wheels and tyres comply with standard dimensions, as recommended by automotive manufactures. The width of the rim is the distance across the rim flanges, at the bead seat. Its diameter is the distance across the centre of the rim from bead seat to bead seat.

Rim diameter [1]

Rim designations types [3]

The shape of well-based rims is provided by a letter code, such as J, K, JJ and KK. The width of the rim and the diameter is traditionally stated in inches. A rim designated 7 JJ by 14 would refer to a rim measuring 7 inches across the rim flanges and 14 inches in diameter from bead seat to bead seat, with the profile conforming to a JJ code [1].

Metric-diameter rims cannot be fitted with inch-diameter tyres, or vice-versa.

Next Article: "Wheel Balance"

Feature Post: "Wheel Offset"

References:

[1] Trade of Motor Mechanic: Module 7: Unit 1: “Wheels & Tyres” by SOLAS, In cooperation with: Subject Matter Experts; Martin McMahon & CDX Global Curriculum Revision 2.2 16-01-07 © SOLAS 2013.

[2] http://www.autobei.com/autoanalysis/wheels-and-tyres/

[3] https://www.tyresizecalculator.com/wheels/wheel-rim-profiles