When it comes to exhaust systems, we know we want headers and turbo mufflers. When it comes to suspensions, we usually want it low and stiff. When it comes to the engine, we want the biggest, baddest cam we can afford, with the biggest carburetor (or injectors) that we can manage. But the question of what wheels and tires to put on almost always stumps us.
Wheels is one area where many enthusiasts completely ruin their car’s potential performance because it has an enormous impact on ride quality, handling and acceleration. How big you can go is largely governed by your suspension, and what driving qualities you are willing to trade off in order to gain other performance and aesthetic benefits.
1. Wheel Diameter
Plus sizing your wheels is one of the best ways to give your car an instant makeover while enjoying a few functional perks as well. The idea is to keep the same overall factory wheel + tire combo size by using a larger diameter wheel with a lower profile tire. Keeping the overall diameter roughly the same is crucial to keeping the gearing the same and the speedometer accurate, as well as for making sure the wheels and tires fit within the fenders. If you go overboard it can alter the accuracy of the speedometer as well as the effectiveness of your anti-lock braking system (ABS), traction control and vehicle stability system.
Lower profile tires enhance steering response and increases cornering stability, and bigger wheels allow you to fit larger rotors. But when you begin pushing the boundaries, even while working within the allowable overall diameter you’ll notice some trade-offs. Acceleration (and fuel economy) will suffer mainly due to additional wheel weight and extremely low profile tires deliver poor ride quality and offer less protection for your (probably expensive) rims. If acceleration is important to you, stick with smaller wheels. Larger sidewalls allow better ride quality, better straight line launch but do handle less precisely.
A rule of thumb for plus sizing is that for every 1-inch increase in wheel diameter, decrease the sidewall height by 5% – 10% and increase tire width by 10 millimeters.
2. Wheel Width
The forums are littered with controversy over whether or not wider tires provide better grip and while some enthusiasts claim it does, others have experienced adverse effects with handling. It’s not so much that wider tires won’t give you more grip, it’s that straying too far from the stock width or more importantly, what your suspension can handle can yield dangerous results.
When going wider, more often than not you’re going to be putting on wheels with a different offset which affects camber, toe and caster angles. Even with a re-alignment, there can be significant changes to your scrub radius (the pivot point on which the tire is turned) – and there isn’t any direct adjustment for that.
Offset is the distance from a wheel’s hub mounting surface to its centerline. If it’s dead center between the inner and outer rims then the wheel has a zero offset. In a wheel with positive offset like in most FWD cars, the mounting surface is located closer to the front or street-side surface while in a negatively offset wheel it’s located toward the back or brake side like with “deep dish” style wheels. Too much negative offset can cause the tire to rub the outer fender and too much positive offset can cause the tire to rub the inner fender or even suspension components.
Contrary to popular belief, going wide doesn’t always mean you’re getting a larger contact patch. The size of the contact patch is a result of vehicle weight (and its distribution), sidewall stiffness, and air pressure. Instead, the main difference between a narrower tire and a wider one is the shape of the contact patch.
When you fit wider tires, you’re making the contact patch more rectangular, which is better for cornering but at the expense of some straight line acceleration. Narrower tires have a more square contact patch making them better at straight line acceleration versus hugging corners.
3. Wheel Weight
The weight of your new wheels is where enthusiasts usually blow it when it comes to performance. Not only do heavy wheels contribute to the overall weight of your vehicle but wheels spin so there’s a rotational aspect as well. Dragging a 30 lb weight is much easier than spinning it around which requires more torque. Aftermarket wheels come in a variety of designs which distribute weight over the wheel differently. Some designs have most of the weight concentrated at the center while others are heavier towards the rims. The further away the weight is from the center of the wheel, the more it will affect rotational weight.
Another aspect of wheel weight is that it’s also an unsprung weight. These are components that aren’t supported by the suspension system like wheels, tires, brakes, as well as steering and suspension parts not supported by springs. When you’re driving down a typical road, your wheel will bounce off the imperfections and the lighter the wheels the easier it is for your suspension to keep them planted to the ground giving you better ride quality. As a general rule, whenever choosing parts for the unsprung portion of your car, always try to minimize the weight.
There are a lot of factors to consider before making your final decision and how you use your car is an important factor. If you drive your modified car regularly, you may want to be slightly more conservative with your plus sizing for a better balance of handling and acceleration. But if your modified car is destined for the show circuit then you may be more willing to sacrifice some performance for that trophy-winning look.
Photo credit: rocketjim54