One method of improving tire grip is to increase the contact area between the tire and the road. This can be accomplished by fitting a tire with a larger circumference (i.e. a larger or oversized tire) or fitting a tire with a wider thread area. A tire with a larger circumference has a larger longitudinal contact area, which will provide better straight line stability but will affect overall gearing and will increase the ride height and the car's center of gravity, and will also increase the rolling mass the bakes will have to overcome. There is nothing that can be done to rectify the increased ride height and center of gravity; but the drivetrain gearing can be rectified by fitting a differential with a higher diff ratio. The increased rolling mass that the brakes need to overcome can be rectified by upgrading the brakes. However, even though a larger tire may be able to fit under the wheel arches when the car is static, it may knock against the body or suspension components when suspension travel occurs and during cornering. Stiffer suspension springs with less travel and stiff shock absorbers may compensate for the tire knocking against the body during suspension travel but will result in a harder ride.
Wider tires, on the other hand, have a wider contact area but will need to have a smaller aspect ratio to retain the overall circumference of the wheel. Thus going from a 185/60 R 14 91H tire to a tire that is 215 mm wide will require a tire with an aspect ratio of 50% as a 185/60 R 14 91H tire has a sidewall height of 111 mm (60% of 185 mm) while a 215/50 R 14 91H will have a sidewall height of 107.5 mm. A tire with a similar overall circumference to the standard tire will not interfere with the body or suspension parts during suspension travel but will be slightly heavier, and will increase the rolling mass that the brakes need to overcome, but not to the same extent that larger tires do. However, wider tires are also prone to knocking against the body or suspension parts during cornering. This can be overcome by fitting a wider wheel arch and fitting a spacer on the wheel hubs to allow the wheels to be mounted further away from the suspension parts. This will, however, place greater stress on the front wheel bearings and on the steering mechanism.
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