The First Step for Improving Vehicle Dynamics: TIRES

by John Comeskey of SPS and James Walker, Jr. of scR motorsports (2000)

Like anything else, improvements to the handling of your Saturn should begin with the weakest link in the chain. Without question, this infers that tires should be the first improvement to the Saturn chassis for improved handling and braking.

Unfortunately, many enthusiasts underestimate the value of tires and fall victim to the temptation of first spending money on more “glamorous” upgrades instead. Of course, other chassis upgrades also play an important role in creating an athletic Saturn, but the effectiveness of those other upgrades is severely limited by the stock tires. We hope that an understanding of the very important role that tires play within the chassis system will prevent you from making the same mistake.

Tires are the Most Vital Link in the “System”

If you have ever driven on pure ice, then you know what happens when friction is reduced between the tire and the road surface: the car becomes difficult – if not impossible – to control. This is because the tires depend on friction against the ground to transmit forces from the chassis.

Imagine that your car was suspended in air upon a giant air-hockey table. In this scenario, your car would literally not move. As you pressed the accelerator pedal, your tires would spin, but having no friction against the ground, you would remain at rest. If the car were set into motion from an outside force, you would not be able to turn or stop – even though you could turn the wheels and press the brakes. Friction against the road surface must exist in order for a car’s controls to function properly. And it is the tire that employs friction against the road surface in order to transmit chassis forces.

Put simply, a car depends upon the tire to transmit every force generated by the chassis. As a result, the capabilities and limitations of the tires will almost always define the limits of acceleration, braking, and cornering.

Stock Tires are the Weakest Link in the “System”

Fortunately, in the real world, even the worst tires on the worst roads have more friction than the air-hockey table scenario. But do common tires have enough friction to handle the performance potential of even a stock chassis? In most cases, they do not.

If you have ever spun your tires under acceleration, skidded under braking, or slid when cornering, then you have witnessed the limits of your tires. Even a stock Saturn is capable of overwhelming the stock tires when driving aggressively. So what happens when you add more power, stronger brakes, or upgraded suspension components to a car with stock tires? Does the car perform better? As a general rule, the answer is no. If a stock Saturn’s performance is limited by the capabilities of the original tires, then a modified Saturn is still limited by the same threshold established by the stock tires. Without a change to the tires, there is no net gain in performance – it only becomes easier to reach the performance threshold and to overwhelm the tires which leads to more spinning, more skidding, and more sliding.

To understand this in another way, imagine that the tires’ capabilities are represented by a one-gallon bucket and the forces generated by the rest of the chassis are represented by water. The only forces that the car can use are those that stay in the bucket. So as soon as the suspension generates more than one gallon of force, the excess spills over the edge and is wasted. In order to use more than one gallon of “force,” we must find a larger bucket. Only then can we benefit from the extra forces generated from the rest of the chassis.

The conclusion: better tires are like a larger bucket. Start with tires to expand the performance threshold of your car!

Choosing a Better Tire

Choosing the proper tires for your car is often like choosing the proper shoes for your feet. Most of us have a favorite pair of shoes that we use for “hanging out.” But for optimal performance, we almost always have several pairs of specialized shoes that are made for specific functions – running shoes, court shoes, golf shoes, dress shoes, hiking boots, etc. And the specialized shoes usually do not cross over well into other applications. (Can you imagine wearing your golf shoes to play basketball? Ouch!)

Tires are much like shoes in the sense that many types of tires are made to meet the diverse needs of an equally diverse set of consumers. Rubber compounds and tread designs vary among these tires in order to create products that work well under a narrow set of performance guidelines. Seldom do you find a tire that claims to be all things to all people – and a wise enthusiast is skeptical of those that do since the “magical formula” has yet to be found. In fact, there are generally a series of trade-offs that create a multidimensional array of tire characteristics.

On one end of the simplified array are high-performance tires. Tires on this end have unusually large degrees of traction on warm dry roads. But in exchange for performance, they generally bring lower lifespans, reduced fuel mileage, poor cold weather performance, higher levels of road noise, harsh ride qualities, and higher costs.

On the other end of the spectrum are conservative passenger tires that offer less grip at the road surface. But, in exchange, these tires usually offer longer lifespans, better all-weather performance, less road noise, better fuel mileage, smoother ride, and lower costs. In between the extremes are a large number of tire designs that offer many combinations of the potential advantages and disadvantages.

When auto manufacturers choose the original equipment tire for a car, they must choose just one set of tires that will fit the “average” driver in a wide range of uses. They do not have the luxury of providing a “closet” full of “shoes.” The result is a tire that can perform a range of tasks suitably, but is unable to perform any single function particularly well.

As a performance enthusiast, you probably enjoy a more spirited driving style than the “average” Saturn owner. In order to accommodate this spirited driving style, you are willing to accept many trade-offs in the pursuit of increased performance capability – especially those involving costs and comfort. (If not, then you have chosen the wrong hobby!)

The conclusion: you should choose a tire on the high-performance end of the spectrum.

Understanding Tire Size

When you look at the side of any tire, you will see a series of numbers that are used to determine the tire size, such as “195/60R15” or “205/40ZR17.” Using these numbers to determine a tire’s overall diameter is easy if you understand what the numbers mean and how they relate mathematically to the overall size of the tire.  The best part is that this will allow you to choose a correct tire size by yourself!

Tire width: The first number in the series is the tire width in millimeters. So, a “195/60R15” tire is simply 195 mm wide.

Aspect ratio: The second number is the height of the tire sidewall expressed as a percentage of the width. So, a “195/60R15” tire uses a sidewall height that is 60% of 195 mm, or 0.60 x 195 = 117 mm.

Rim diameter: The third number is the rim diameter in inches. So, a “195/60R15” tire has an inside diameter of 15 inches and will fit on a 15-inch wheel.

Calculating overall diameter (OD): Looking at the equations above makes it easy to understand how to find the overall diameter (OD) of any tire. Adding from top to bottom (or bottom to top,) you add: sidewall height + rim diameter + sidewall height. The sum of these numbers gives you the overall diameter or total height of the tire from top to bottom. The only trick is converting millimeters to inches.

To make things really simple, just use this formula:

[(tire width x aspect ratio) x 2]/25.4 + wheel diameter = overall tire diameter in inches

or in the 195/60R15 example…

[(195 x 0.60) x 2]/25.4 +15 = 24.21 inches

One final note, not all tire manufacturers rate their tire sizes identically. It is likely that two “195 mm” tires from two different manufacturers may have different widths – even though they are both labeled as “195.” Be prepared to adjust for slight deviations if you are comparing tires from different manufacturers.

Fitting Tires to a Saturn

Following a few simple guidelines will make it easy to get tires that correctly fit your Saturn. For starters, use the table below to determine the tire size – hence overall diameter – that was originally used on your vehicle.

Original Tire SizeOriginal Overall DiameterSL, SL1, SW1, SC1175/70/1423.65 in.SL2, SW2195/60/1524.21 in.SC1 optional tire195/60/1524.21 in.SL2, SW2185/65/1524.47 in.SC1 optional tire185/65/1524.47 in.SC, SC2195/60/1524.21 in.

SL, SL1, SW1, SC1175/70/1423.65 in.SL2, SW2195/60/1524.21 in.SC1 optional tire195/60/1524.21 in.SL2, SW2185/65/1524.47 in.SC1 optional tire185/65/1524.47 in.SC, SC2195/60/1524.21 in.

Year Models riginal Tire Size Original Overall Diameter
91-current SL, SL1, SW1, SC1 175/70/14 23.65 in.
91-95 SL2, SW2 195/60/15 24.21 in.
93-95 SC1 optional tire 195/60/15 24.21 in.
96-current SL2, SW2 185/65/15 24.47 in.
96-current SC1 optional tire 185/65/15 24.47 in.
91-current SC, SC2 195/60/15 24.21 in.
  1. If you are upgrading to a larger tire, it is highly unlikely that you will be able to find a tire that will perfectly match the diameter of the OEM tire. This is not necessarily cause for concern. However, the tire that you select should have an outside diameter within one inch of the original tire. (More deviation than one inch will cause significant speedometer errors and/or potential fit problems.)
  2. It is also generally best to deviate toward the small side of the original diameter rather than the large side. This offers two clear advantages:
  3. It helps avoid fit and clearance concerns.
  4. It creates a smaller “effective radius” which slightly improves acceleration, holding all else constant.
  5. A wider tire offers greater traction thanks to a larger contact patch. However, it is generally wise to limit the maximum tire width for street use to 205 mm. Most tires wider than 205 mm are likely to rub against struts and fender linings unless further steps are taken (as on our ITA race car). At the same time, using a tire width smaller than 205 mm fails to take advantage of potential performance gains.

Putting It all Together

As you read through the rest of the tech information on our site you may find that there is a common thread – whenever you are trying to improve the dynamic performance of your ride, the first place you should look is at the four palm-sized patches of rubber which connect your car to the road.  Until you enable this interface to generate higher forces, your performance gains may not be a great as you would hope or expect.