Monday, September 17, 2012
Tech Tip: Tire and Wheel Fitment
The relationship your wheels and tires have with your car is a very special relationship. Your tires are (hopefully) the only part of your car that will actually interact with the road and most aspects of your car are tied to that. Your acceleration, your braking, and your suspension and steering systems all rely on the type and quality of tire you have for maximum effect. Probably the easiest thing to identify and relate to is going to be size, so lets start there.
The size of your tire has two main aspects that relate to your car: width and height. Tire width directly affects traction under all conditions. The wider your tire the more contact with the road and the better the traction. Too much width, however, and it will be more difficult to steer. The height of your tire primarily affects suspension effects directly through sidewall height and tire stiffness. If your tire sidewalls are higher, then your steering will feel very mushy and they could actually fold over under hard turning or braking. If they are very low, you will get added stiffness and a better steering response and handling, but you will also get more road noise and will typically wear through the tires faster due to the increased demand on the tread blocks themselves.
Size also directly affects the volume of air inside your tire, this is important because the amount of air and the pressure of that air determines how much weight or force that tire can safely support. A common mistake by people trying to go with smaller than stock tires is knowing the weight of the car and relating that to the "maximum load capacity" on the sidewall of a tire. However, when turning, braking, accelerating, or driving on an uneven road surface you are shifting the amount of force between your tires. If you are driving on tires that are not designed for that amount of force, the internal components could become overloaded and break down causing a tire to delaminate (where the rubber breaks its bond with the steel belts). It is always recommended to use the original equipment load index of a tire as a minimum replacement tires including different sizes.
Performance Rating (Speed rating)
Performance rating, typically referred to as speed rating, determines the performance level of your tires and how it relates to your vehicle suspension. This is the second most important aspect of your tire, your suspension and braking are primarily designed around the performance specifications of your tire. A higher performance rated tire is designed and constructed to work with a high performance suspension and braking system. This will typically include a high performance tire compound, reinforcement of the sidewall and shoulder areas, and in some cases additional or reinforced steel belt packaging. If you installer a higher performance tire than what your vehicle was designed for, you may get better handling, better acceleration, and sometimes better braking, but the effects may not be very noticeable. However, if you install a lower performance tire than what your vehicle is designed for you will typically wear through your tires faster and have noticeably worse handling, a longer stopping distance, and more wheel spin under acceleration. I would recommend trying to stay with the same performance rating or higher if you are trying to go with a specific brand or model tire, but never going with a lower performance rating.
Now that we have a little bit better idea of how tires interact with vehicles, lets visit wheels. The way that your wheels look is directly attributed to the overall diameter of the wheel as well as the width and offset of the wheel. Finish and construction can also play a role in the performance and total look of your vehicle.
Changing the overall diameter of your wheels will not only change the look of your vehicle but can also have an impact on your ride and handling. Your wheel diameter is directly tied to your sidewall height to maintain a safe overall diameter with your vehicle, so the wider the rim the narrower the sidewall and vice-versa. The width of your wheel is also tied to your tires, you want your tires to be slightly wider than your wheel width, but not too wide. Most tire manufacturers will print out the recommended wheel width for their specific tires by model if you are unsure about your fitment. Depending on quality of construction, an added benefit to several aftermarket wheels is a reduction in weight without compromising structural integrity. Typically the lighter the wheel, the more complicated the process of manufacturing and the more expensive, but there are several cast alloy wheel offerings that are lighter than stock wheels.
Wheel offset is the distance from the center of the wheel to the pad where it contacts the hub. The amount of wheel offset determines how far from the suspension the wheel sticks out. The higher the offset number the more the wheel tucks in and the closer to the suspension and the further from the lip, the lower the wheel offset the closer the wheel comes to your fender and the further from your suspension. Most performance cars are going to have a particular offset recommended for their fender and suspension set up. Most Japanese and European cars are designed for a high offset. Another consideration when determining offset should be your brakes, a higher offset will also typically put the spokes of wheels relatively close to brake calipers, especially if you run a big brake kit. Since offset is such a specific and important element of wheel fitment, most manufacturers test their products to take a lot of the guesswork out of it and design bolt patterns and offsets with specific vehicle specifications in mind.
By combining wheel size including width and wheel offset we can determine a safe fitment of tire and wheel combinations. Its a balancing act that is important for the safety and performance of your vehicle, but has enough room in it to customize or maximize the look and handling of your vehicle. If you have any questions or would like help determining the fitment of any of our great wheels and tires package to accommodate those wheels, please call or email me anytime. If you are in town, come by and see me anytime I would love to help you out.