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Upgrading Your Tires All tires, from the skinniest space-savers to the widest high-performance rubber, maintain contact with the road through a small area of rubber called the contact patch. The contact patch is the small rectangular area of the tire that's actually pressing down on the road, and it's usually no larger than your handprint. Upgrading to a wider tire also widens the shape of your tires' contact patches, with potential benefits in cornering ability and feel. Low profile tires have a shorter and wider contact patch that translates to more responsive handling and cornering stability. The other main reason for upgrading to a wider and larger-diameter tire, and sometimes the one given more importance, is of course the killer looks of a big-wheel, low-profile setup. When upgrading to a larger-diameter wheel and wider-tire combination, it's best to maintain the total outside diameter of the original wheel-tire combo. Keeping the outside diameter is ideal because the car's speedometer, odometer, ABS sensor calibration and ride height were calibrated or determined using the original OD. To compute for the outside diameter of the original wheel-tire combo, take the tire width and multiply by the aspect ratio to get the sidewall height. Multiply this by two (since there's a sidewall on either side of the wheel), and add this to the diameter of the wheel. Remember to convert the wheel diameter to mm. (1 inch=25.4 mm). For example, a 195/65 R 15 tire will compute as:
195 mm (0.65) = 126.75 mm A Plus One application means a diameter increase of one inch, with correspondingly lower-profile rubber. If Upgrading to a 16-inch wheel, for example, to a 205/55 R 16 :
205 mm (0.55) = 112.75 mm The difference in the outside diameter is 0.4%, which is an acceptable difference. If you compare that to an upgrade to a 235/60 R 16, where the outside diameter will be 688.4 mm, or a difference of 8%. That increase in diameter will wreak havoc with your car's suspension and other settings. Plus Two means a two-inch increase in the wheel diameter with a correspondingly lower profile or aspect ratio. Keep in mind that a lower-profile tire with a larger wheel will be more susceptible to damage from potholes and curbs. That's because an inflated tire is able to absorb shocks and impacts better than a metal wheel. Damage is even more likely if the wheel is light-alloy. Given the poor state of a lot of our city roads, be prepared to suffer wheel damage if you opt for very low-profile tires. A large-diameter wheel and tire combo will also tend to be heavier than its small-wheel counterpart. A metal wheel certainly weighs more than rubber and air. This increase in unsprung weight, meaning weight that's not supported by the car's springs, may be detrimental to the car's suspension response. Changing to a lower-profile tire without increasing the size of your wheel is not a good way to go. Your car will ride lower and the under-filled wheel will look a bit silly. One good way to increase performance and/or ride comfort is to maintain your car's original tire size and select a more performance-oriented brand or comfort-oriented variant. Finally, let's take a look at the tires' speed rating and load index. |
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