Rim Width & Aspect Ratio

To accurately measure the width of a tire, the tire must be mounted on a rim. Since a tire's section width is larger if the tire is mounted on a wide rim, & smaller if it is mounted on a narrow rim, each tire is measured on a specific rim width.

For example: For tires with aspect ratios from 80 to 50, the measuring rim, also called the design rim, is specified to be 70% of the section width. For tires with an aspect ratio less than 50, the measuring rim is 85% of the section width.

Correct rim width ensures flex at the designed flex point in a tire sidewall for optimum tire performance.

If the rim is too narrow, the flex point moves toward the shoulder area, creating heat buildup in the shoulder, which reduces tire life & could result in failure.

If the rim is too wide, the flex point moves towards the rim area, causing heat buildup in the lower sidewall, which reduces tire life & could result in failure.

Within the acceptable range of rim widths, one can select wider or narrower rims than the measuring rim. Selection of a wider rim, from within the approved range, (T & RA tables) stiffens the sidewall & improves handling at the expense of handling. If carried too extreme, either too narrow or too wide of a rim, it can result in uneven tread/pavement contact pressure causing uneven wear & potentially reduced traction, or increased vulnerability to bead dis-lodgement. Always check with your Dunlop dealer for permissible rim width options.

Remember - safe clearance must be determined for a particular tire/rim contender & vehicle.

Wider rims may offer some performance advantages over narrow rims. A wider rim increases the distance between the beads, which results in a straighter sidewall, which stiffens it. This results in quicker steering response & higher cornering forces.

Negatively, the straightened sidewall transmits more road shock to the wheel & suspension, placing greater stress on chassis & suspension parts & delivering a harsher ride. The straighter sidewall exposes the rim, making the wheel more susceptible to damage.

A narrower rim pulls the beads closer together, curving the sidewalls. This increased curvature allows the sidewall to flex more readily over bumps & absorb more road shock during driving. This offers a softer ride.

Aspect ratio is the relationship of a tire's height to width when mounted & inflated on a rim of the correct size. Aspect ratios are expressed in section height as a percentage of section width in two-digit numbers (80, 70, 60) & are often referred to as a tire's series. For example, if section height/section width is 60, the tire is a 60 series.

This height to width relationship determines the shape of the tire on the rim, and, more importantly, determines the performance characteristics of the tire. If the sidewall height of a tire is reduced slightly, the sidewall stiffness is increased greatly.

Higher aspect ratios deliver:
Lower aspect ratios deliver:
- Greater deflection under load - Wider footprint
- Softer ride - Quicker response
  - Less tire deviation or slip angle
  - Lower flex rate
  - Less deflection under load
  - Harsher ride

A tire with a lower aspect ratio & stiffer sidewalls will transmit more force from bumps & irregularities in the road surface. A tire with a higher aspect ratio, with its more flexible sidewalls will provide a smoother ride because sidewall flexibility allows it to deform over the impact area, dissipating the energy.

Changing tires on a vehicle from one aspect ratio to another also influences section width, which relates directly to the load carrying capacity of the tire. The load carrying capacity of the original equipment tire must always be maintained or increased.

Anything that changes the tire's outside diameter also influences the vehicle's overall gear ratio, as well as the accuracy of the speedometer & odometer.

Another factor affected by change in aspect ratio & overall diameter is the tire footprint. Typically, a high aspect ratio tire will have a long, narrow footprint, while a low aspect ratio will have a short, wide footprint.

Finally, tire diameter/aspect ratio affects the input to the engine computers on some new cars, since vehicle speed is an input to these computers.

In other words, tire changes involving changes in aspect ratios should be made in consultation with your Dunlop dealer & reference to the vehicle's Owner's Manual.


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