Article 2 of my common falacies threads.
Many people know the basics of tires, but how many people know all the facts? First let us conduct a quick review of the very basics. A tire comes with a rating on its side. An example of the rating would be 255/50ZR16 (tires also come with ratings as to their tread wear, traction, temperature ratings, and type. These last 4 are a little beyond the scope of this discussion. I may visit them in a future article.) The first number in this example (255) represents the width of your tire in mm at the tread. The second number after the slash represents the aspect ratio. The aspect ratio gives you the size of the sidewall height as a proportion of the width. So in this case the sidewall height would be 255mm*.50= 127.5mm. The next piece is commonly referred to as the speed rating. In this case the car is z rated. This rating tells us the maximum speed at which the tire can be driven by referring to a table. Some common speed ratings include:
N 87 mph
P 93 mph
Q 99 mph
R 106 mph
S 112 mph
T 118 mph
U 124 mph
H 130 mph
V 149 mph
W 168 mph
Y 186 mph
The next piece to our tire rating is the type of tire construction. In this case we have an R, which means this is a radial tire. The final value gives you the diameter of the wheel your tire can fit on (in this case 16 inches). So now that we know the basics of tires let us get to the myths.
Bigger is always better?
As with all things car related there is a large push for bigger is better in a number of ways. In wheel/tire combination this manifests itself in larger diameter wheel/tire combinations, larger diameter rims, and wider tires. Let’s look at these on their own.
Diameter of the wheel/tire combo
Diameter of your tire and rim combined is a very important number. If you modify the height of your sidewall of your tire or size of your wheel independent from other changes you are going to get a different tire and rim diameter. You will also get a different diameter if you do not change them in the proper ratio (called plus sizing, this will be covered later). So how does an added diameter of tire/rim affect a car? Well the first problem is kind of obvious. If you increase your tire/rim diameter to far your car will scrub on its fenders when turning and hitting bumps. So does that mean if you could remove the fenders that increasing that diameter would be desirable? Well, that depends on what you wish to achieve. This particular diameter affects how your hp is put to the road. A smaller diameter will result in a greater acceleration at the cost of higher rpms, a lower top speed, and more shifting. A larger diameter meanwhile will result in a greater top speed and the car running at lower rpms for a given speed at the cost of acceleration. Furthermore, since your speedometer reads from your transmission resizing your tire/wheel combo will throw your calibration off giving you incorrect speedometer readings. A larger tire diameter will give you a longer contact area (I will touch on the larger volume contact area further down). Lastly, it will allow you to move the steering axis inclination allowing for less tire scrub when appropriately configured. In other words unless you are a suspension engineer you probably should not change your tire-wheel combined diameter much.
So what can you do to improve performance? How about plus sizing? Plus sizing is where you increase either the tire or wheel while decreasing the other proportionately to keep the tire/wheel-combined diameter roughly the same. Let us look first at the act of going to a larger wheel and smaller height tire. Many people seem to love this approach and it is not without merits. But, it has some costs that many don’t consider. The main advantages proponents talk about are the lessening of the sidewall height of the tire. This creates a stiffer sidewall that is less likely to bend or give to the road surface. As such it decreases the amount the tire rolls over on itself and the tires slip angle. (For those not in the know the tires slip angle is the amount the tires tread continues to go in the original direction when you are turning the wheel before they turn.). Lastly, it can give you clearance behind the larger wheel for larger brakes.
So far it would seem that a lower sidewall height was always preferable. However, as with all things there is a tradeoff. The main one is the increased weight of the wheel from being larger. Clearly this is going to increase your wheels rotational inertia and your cars unsprung weight. A wheels rotational inertia is its resistance to a change in its current state of motion whether to speed up or slow down. By adding more mass to the object that is rotating we are requiring more energy to change this state. Unsprung weight meanwhile is the part of the car that is not supported by the suspension. As such it is the part of the car that is controlled by the suspension to follow the road. Simply put the higher unsprung weight makes it more difficult for the suspension to control the tires to follow the ground over bumps, undulations, and elevation changes.
Another major tradeoff is in the loss of the tires springiness. In a car the tire is yet another type of spring. By increasing the strength of the sidewall you decrease this springiness. This cost comes in the form of a harsher ride and a car that tends to follow ruts (commonly referred to as tramlining).
Please note that going the other way of increasing a tires sidewall and decreasing wheel size would have the exact opposite effects of increasing the wheel size. In closing this portion, changing of tire diameter has pluses and minuses. You must properly weigh both to determine which way and how to proceed. General guidelines for street cars have shown that any larger then 18 inches and the benefits are outweighed by the costs.
The biggest misconception that seems to be perpetrated is that wider tires are always better. To further understand the intricacies of such a concept let us first delve into what a wider tire accomplishes. A wider tire modifies both the size and shape of the contact patch of the tire to the ground. As your tire grows in width the length of your tire contact patch decreases. Meanwhile, your contact patch width increases at a larger rate then the decrease. As such you have a wider, shorter contact patch with a larger volume. This serves a few purposes. The first is that by having a shorter contact patch the force downward is concentrated on a smaller portion of the tire sidewall. As such the tire decreases its slip angle and tendency to roll over as described above. The second is because the contact volume is larger the heat from friction is dissipated more amongst the tire. A tire creates its friction with the road chemically. As such they must be within a proper range for the tires to work efficiently. This range is both upper and lower bound. Clearly, you don’t want too small a tire because it will go over the tires operating range. You also don’t want to go too wide because you will not get the tires warm enough. Clearly this is the first important reason wider is not always better. But are there other reasons? Well yes. A wider tire will not work as well as a narrower tire in the rain or snow all things being equal. Why? Because the pressure of the weight down is spread out the tires ability to push water and snow out from under the tire is decreased. Most street tires come with grooves of some sort to aid in this process. The groves allow for the water to be routed through them. They also allow for more direct pressure to be applied to snow and water. While more effective then using a narrower tire while dry it allows for the ability to handle water. However, grooves are not the optimal tire for the dry. For more on types of grooves and their effects see “Tire glossary”. Talon Tire. http://www.talontire.com/tireglossary.html
So we have determined that too wide a tire can lead to problems with approaching the appropriate temperatures and with handling water and snow. Are these the only pitfalls? Well no. When you increase the wheels width you change the scrub radius since the center of the tires contact patch shifts (In general a wider wheel width is going to change your backspacing and/or offset). The distance from the steering axis inclination point on the ground to the steering axis inclination determines scrub radius. Zero scrub radius is a bad thing, but so is too large of a scrub radius. A scrub radius that is too large (either positive or negative) will increase steering effort. A scrub radius that is too small meanwhile will be stable in a straight line but will tend to squirm under cornering (leading to unstable steering and increased tire wear). A reasonable scrub radius can help reduce torque steer. Whether the scrub radius will increase or decrease depends on whether you have a positive or negative scrub radius to begin. (In other words whether the centerline is closer or further away from the body then the sai line).
A brief overview of some other common misconceptions
So now you know how a bigger tire is not always better. But what about other commonly held misconceptions? One commonly held myth is that you should inflate your tires to the rating listed on the tire. This is incorrect. This rating is the maximum inflation rating for the tire. You should actually inflate your tires based on application.
Another common myth is that an undulation in a tire sidewall is a weak spot. In reality this is the strongest piece of the tire and is caused by an overlap in the materials that occurs at these spots.
So what should you know? Always weigh the costs and benefits when purchasing tires. Also be sure to run tires at the correct pressure for optimal performance and fuel consumption. Make sure to replace tires when their tread wear gets too high or when they get older. Their ability to chemically bind to the road will degrade over time and wear. Also consider getting race tires shaved and heat cycled for better performance over time. Finally make sure your tires are properly matched to your wheels and have the appropriate offset.
Take this article as a guideline and feel free to use these sources for more information:
“Tire Myths”. Good Year Engineering. The Idaho Corvette Page. 2003 http://www.idavette.net/tire_wheel/tire_myths.htm
“The Tire”. Hine, Allison. http://eaglewoman.racesimcentral.com/gpl/g...amics-tires.htm
“Tire glossary”. Talon Tire. http://www.talontire.com/tireglossary.html
“Back space”. Diamond racing wheels. http://www.diamondracingwheels.com/Terms-BS.htm
Jackman, Roger. “Scrub Radius” hrsprings.com