A vehicle’s trim height is a function of the vehicle’s suspension and the added heights of the tire and rim. Some people will refer to a rim as a wheel; however, a wheel is the combination of a tire mounted onto a rim. This distinction is important because the overall size of the wheel determines the individual size of the tire and the individual size of the rim that can be combined.
Up-Sizing Wheels has become Popular
The popularity of up-sizing wheels has risen to the point that most vehicle manufacturers offer option packages that include up-sized wheels. These vehicles have been designed to accept a wider range of wheel sizes, unlike older model vehicles that had a limited size range. As such, the vehicle is able to maintain its stability, and handling regardless of the wheel size selected. The vehicle’s design was engineered so that a variation in wheel size can be used without interfering with the vehicle’s ability to be safely driven.
The Importance of Wheel Well Clearance and Vehicle Height
Each vehicle has a specific amount of clearance within the wheel well. The clearance is necessary to prevent the tire from rubbing against the wheel well when cornering, and to prevent the tire from bouncing into the top of the wheel well when the vehicle hits a bump. Tire rubbing and bouncing will ruin the tires and shorten their life. Additionally, when a road bump is encountered that causes the wheel to strike the upper side of the wheel well, rim damage can occur.
Indeed, some people will have the vehicle’s suspension raised in order to provide room within the wheel well for up-sized wheels. While this may seem like a logical thing to do, it can have dramatic effects on the vehicle’s ability to be driven safely. Ideally, when these types of changes are made to a vehicle, additional vehicle modifications should also be performed so that the vehicle’s stability and handling are maintained to factory specifications.
The suspension and the height of the vehicle are designed so they can be driven under a variety of road conditions while maintaining stability and driving control. Once modifications have been made to the suspension and vehicle’s height, the handling dynamics of the vehicle have been changed. Depending on the degree of the changes made to the vehicle’s height and suspension, the vehicle may become prone to rollovers during cornering or during panic-stop conditions.
Up-Sizing Wheels without Changing Vehicle Driving Safety
For those not wanting to pay the additional expense needed to modify the vehicle for up-sized wheels, there is an alternative. Rim size can be increased, but there needs to be an equal decrease in tire height so that the total wheel size is maintained. The tire size information provided in the article “Purchasing a Used Tire can Save Money”, can assist you in determining the maximum rim size needed to maintain total wheel size.
Total wheel size is measured from the center of the wheel’s axel to the top of tire. This number is a combination of the tire’s sidewall height added to 1/2 the rim’s diameter.
Example of Rim Size Increase that Maintains the Same Total Wheel Size
Using a tire with a size of 285/80 R13, we know that the rim is 13 inches in diameter. We also know that the height of the sidewall is 285 millimeters multiplied by 80% (0.80), or 228 millimeters. Change millimeters into inches by dividing 228 by 25.4, and the sidewall height is 8.98 inches. When we measure from the center of the axel to the top of the tire, we get a total wheel size of 15.48 inches, or 15 1/2 inches.
If we plan to increase the rim size from 13 to 14 inches, we will need to reduce the tire sidewall height by the same 1-inch change in order to maintain the 15 1/2 inch total wheel size. To find out the size of the new tire, we will need to apply our formulas in reverse to what we just did.
The new tire sidewall height will be 8.98 inches minus 1 inch, or 7.98 inches. Change inches back into millimeters by multiplying 7.98 by 25.4, and the sidewall height in millimeters is 202.69 millimeters, or 203 millimeters after we round up to make it even. The standard ratio of tire width to height is 80%, so we will stick to that percentage. Other tire width to height percentages are available and range from 65% to 95% depending on the brand of tire.
To get the new tire width, we will divide 203 millimeters by 80% (0.80), and the new tire width is 253.75 millimeters. The closest tire size is 255, so that is what we will use. Our new tire size is 255/80 R14. We have successfully increased the rim size by 1 inch without changing any of the vehicle’s driving dynamics. Additionally, because we kept the same total wheel size constant, there is no need to worry about wheel well clearances.
Up-sizing a vehicle’s wheels is not just a matter of buying bigger rims and tires. Unless you plan on making the additional modifications to the vehicle that will allow it to be driven safely, it is best to up-size the wheels using this method. Once the wheel well clearances have been measured, and a variety of tire sizes have been investigated, it is possible to up-size several inches without changing the driving dynamics of the vehicle. Just remember, as the sidewall height decreases, the vehicle’s ride becomes stiffer because the sidewall height is responsible for providing a “cushy” ride.