Choosing the Right Gravel Tire

Rally drivers have a lot to focus on when it comes to event preparation, and choosing the correct tire can contribute to your performance at an overall  event. So, what should a driver be considering when choosing a tire to compete with?

Consideration first starts with the event itself.  There are a lot of factors surrounding an event that dictate the necessity of certain types of tires.  Depending on the location of the rally and the time of year, rallyists can experience an array of varying weather.  Additionally, the types of terrain the tires will be traveling on and the amount of miles the tires will be driven on- both on stage roads (which are primarily some form of gravel) and in transits- will all contribute to the initial decision.  Not only does road surface and weather play a key role in the decision making process, drivers must also consider the characteristics of the tire they are choosing, which include the compound, desired longevity of the tire, size and tread pattern, load and speed ratings.

Type of Tire

While there are a plethora of tires available to drivers, rally drivers choose the majority of their tires from the gravel (rally) tire category.  These particular tires play host to specific tread patterns that allow for lateral grip when turning or sliding to provide the best possible traction.  Rally tires are offered in a variety of compounds, which range from very soft, soft, medium and hard. Softer tires offer maximum grip on gravel stages, as they build heat faster, allowing them to stick to the loose surface. This compound is preferred in colder temperatures as they allow for the ultimate grip, however they do wear prematurely in hotter temperatures. Hard compound tires are best utilized in hotter  climates and on hard packed gravel or tarmac surfaces. Generally speaking, harder tires are used when temperatures are high (usually above 71°F) and a medium or softer tire is used when it is colder.  Super soft tires, such as a winter tire, would be ideal for sub freezing temperatures. In some countries, rally drivers are allowed to compete using studded ice or winter tires. 

Size of Tire

After understanding the terrain and what type of tread pattern and compound is needed, drivers must then consider the sizing of the tire.  TheyDrivers must be familiar with a tire’s width, aspect ratio, overall height, and weight.  Load and speed rating, while not quite as important in most cases, should also be considered.  So what do all the numbers on the tire actually mean?  Rally tire sizing is often written in the  form of 195/65 R15 91S.  The first number listed (195) represents the tire’s width in mm from sidewall to sidewall. The second number represents the tire’s aspect ratio, (sidewall height, or profile) and it is a percentage of the actual width of the tire.  In the example given above, the 65 means that the sidewall is 65% of the tire’s width of 195.  Therefore, this would mean that the sidewall is 126.75mm tall. The letter ‘R’ refers to the construction being a Radial tire, and ‘15’ means that it fits a 15 inch wheel. The 91 represents the Load Index, which is the tire size's assigned numerical value used to compare relative load carrying capabilities. The higher the tire's load index number, the greater its load carrying capacity.  Typical rangers are between 70-126.  In this example, the load capacity is 91, which is 1,356 lbs per tire, or 5,424 lbs of car. The ‘S’ indicates the ‘Speed Rating” of the tire.  Speed ratings are based on laboratory tests where the tire is pressed against a large diameter metal drum to reflect its appropriate load, and run at ever increasing speeds until the tire's required speed has been met.  In this case, “S” is rated for a maximum speed of 112mph.  Additional letters or numbers may follow, depending on the brand of tire.  For example, the MRF ZDM3 tires are stamped with a small ‘vs’, ‘s’, ‘m’, or ‘h’ which refer to it’s compound. You will also locate the tire’s serial number and if the tire is specified as a ‘right’ or a ‘left’ if the tread pattern is asymmetrical. 

Width

Choosing an appropriate width applies to the amount of grip a driver desires on the road surface.  Consider that the wider a tire is, the more road grip it will see; whereas the smaller the tire width, the less road grip there will be. A good example would be to compare a tarmac stage versus a snow covered stage.  Using a wider (or bigger) tire, will allow for a larger, or wider, contact patch on the road, giving the car more stability and grip; which is desired on a tarmac stage.  However, by selecting a smaller, or narrower tire, drivers will be able to get more pressure with a smaller contact patch.  Using a narrower tire makes it easier to cut through deep snow and slush for ultimate grip and flexibility. It is important to remember that any tire’s labeled width pertains to the measurement from sidewall to sidewall and only suggests the width of the tread, which is the contact point on the road. 

Height

After determining the appropriate width, one must consider the aspect ratio, or the height/thickness of the sidewall. Typically, the rougher the terrain, the thicker the sidewall should be, which is what rally tires are designed for.  Once width and aspect ratio are decided, drivers must consider the height of the tire, as different sizes will change the overall gearing of a vehicle. This refers back to the concept that smaller tires allow for better gearing for lighter vehicles, whereas a wider, taller tire is more appropriate for a heavier car. This means that with smaller tires, acceleration is improved and the gears will feel shorter or closer together.  Taller tires give the effect of raising the final drive; which means that the car will accelerate more poorly, but the driver will experience a higher top speed in each gear. One thing to remember is that changing the height of the tires will affect the speedometer and odometer to read inaccurately.  Taller tires will result in lower than normal readings, and shorter tires will read higher. 

A Quick Recap-

Factors to consider when choosing the right gravel tire:

Terrain (gravel: loose or hard packed, tarmac, sand, snow, ice)

Weather (rain, snow, humid, cold, hot)

Surface Temperature

Desired longevity 

Characteristics of a tire to consider:

Compound (extra soft, soft, medium, hard)

Size (width, height)

Tread Pattern

Types of Tires:

• Gravel (hard, medium, soft, very soft) 

• Tarmac (super soft, soft, medium, hard)

• Dry

• Rain

• Snow/Ice (studded, non-studded)

What’s in a Number?

Example: 195/65 R15 91S

195 = width from sidewall to sidewall

65 = Aspect Ratio (height of the tire sidewall, or profile. 65% of 195 = 126.75mm)

R = Radial Tire (Construction)

15 = Wheel Size (15 inch wheel)

91 = Load Index (maximum rated load capacity of the tire)

S = Speed Rating (maximum speed for the tire at load)

Not only does choosing the appropriate tire size play a large role in a competitors success, drivers must also consider the changes wheel size makes to their vehicle.  Overall consideration for total weight of the wheel and tire combination should also be given attention, as any changes in weight can directly affect the car’s handling, braking, acceleration, feel of the suspension and potential longevity of surrounding parts. The right tire can mean gaining or losing seconds in rally, and at the end of the day, those seconds could very well determine whether  a competitor stands atop the podium, or finds themself in a ditch.