Terminology  used for  Wheels and Tyres


Car wheels are divided into two main categories, Steel Wheels and Alloy Wheels. Alloy wheels are often fitted standard during the manufacturing of modern vehicles.

Steel Wheels

All steel wheels consist of two pressed components, the Rim and the Wheel Disc, which are welded together. The Rim is the part on which the tyre is mounted. Its dimensions, shape and condition must be suitable to satisfactorily accommodate the particular tyre required for the vehicle. The Wheel Disc is the supporting member between the vehicle hub and the rim. Its dimensions, shape and location in the rim, must be suited to the design of the wheel hub and the suspension geometry of the vehicle to which it has to be mounted.

The purpose of the rim is to provide a firm base on which to fit the tyre. Four vital dimensions are involved. They are the wheel diameter (a precise fit between tyre and rim is of utmost importance), the rim width, the flange height (designed to give adequate support to the tyre beads without changing the flex area of the sidewall) and the rim-well ( to facilitate the easy mounting and demounting of the tyre).

Because the inside diameter of the tyre must fit precisely onto the rim, it would be impossible for the inside diameter of the tyre to pass over the larger diameter of the rim without causing damage to the beads. Forcing the tyre bead into the rim-well opposite to the fitting-head of the machine during the fitting or removal process, allows the tyre bead enough purchase to pass over the rim flange.

Alloy wheels

Alloy wheels are often incorrectly referred to as Magnesium or "Mag" wheels. Magnesium is used in alloys. However, they are almost found only in racing rims meant for the track. Its brittle and highly flammable qualities makes it unsuited as a road rim. Low pressure, die-casted aluminium alloy wheels are used and offer certain benefits over steel wheels. It is possible to design alloy wheels that allow for a better airflow over the brakes and that are also slightly lighter and visually more appealing than steel wheels. Because alloy is lighter than steel, wider rims can be used without sacrificing unsprung weight.

Off the shelf wheels

Steel dies are used in the casting of alloy wheels and cost thousands of Dollars to make. Often one might find that one mould is used to produce wheels for two or three different models. Several problems arise here and centre-bores that are too large will cause balancing problems. It is important to establish that the off-the-shelf rim is approved by the vehicle manufacturer. 

Tyre Size Markings

The size markings of a tyre denote the physical dimensions of the tyre and indicate the configuration or shape of the tyre casing.

185/70 R14 82T. The significance of each number and letter is as follows:

  • 185 mm in width
  • 70 = Profile - percentage of width. 
  • R = Radial.
  • 14 = Wheel/Rim diameter in inches.
  • 82 = Load Index per tyre at the speed rating.
  • T = Speed Rating
Some off-road tyres use the above type size markings while others still use the older American markings, for example 31x10.5R 15 LT. The significance of these markings are as follows:-
  • 31 = The outside diameter of the tyre in inches.
  • 10.5 = The section width of the tyre, in inches.
  • R = Radial Construction.
  • 15 = Rim diameter in inches.
  • LT = Light Truck.

Low-Profile Conversions

The trend towards a lower profile has gained momentum in recent years and it is possible to convert from a narrower, higher profile tyre to a wider, low-profile tyre. Always ensure that your rims are wide enough to accommodate the wider tyre. The tyre industry requires a rim width that is 70 per cent of the tyre section width.

Also ensure that the load and speed ratings of the replacement tyres are equal to or better than the tyres you are replacing. Check that the wider tyre does not interfere with the bodywork of the vehicle.

Low-profile conversions do not necessarily remain within the limitations of the original rim-size. It will be seen that while it is possible to maintain the outside diameter of the tyre, the outside diameter of the rim can be increased by as much as three inches.


Calculating Tyre Dimensions