How is it that rally cars can glide across gravel roads that are littered with a variety of hazards such as potholes, bumps, rocks, and other unforeseen objects and rarely find themselves stranded with punctured tires? What makes rally tires so different from the tires you would find on a street car?

Construction:

These tires are constructed in a different manner than an average DOT road tire. While there are not a plethora of companies providing gravel tires here in the United States, the ones that do, understand the importance of constructing a tire that is able to withstand the harsh environments of the rally.  There are many features that make a rally tire special, but it is primarily their tread patterns and compounds that make them unique. Unlike a typical road tire, rally tires are constructed with a lot of attention given to a thicker sidewall to protect against hazards that may cause a puncture, aid against debeading and flats, and protect the edge of the wheel where the tire sits. Steel belts are used in the sidewall, instead of the tread blocks, unlike road tires. Often, companies will add ribs, or additional rubber, to the sidewall to assist in deflecting rocks.  

Tread Pattern:

Another very important feature of a rally tire is seen in the tread pattern.  Companies utilize different tread patterns and compounds to offer maximum grip in different conditions. Compared to regular street tires, rally tires have large gaps between the tread blocks.  These blocks consist of grooves and hard edges, which help the tire cut through the loose material on top of the gravel surface and bite into the road. As these edges begin to wear off, the performance of the tire is lost. Most gravel tires offer one or two strips of straight tread blocks located on the inside of the tire to provide maximum acceleration and braking when moving in a straight line with the road. The pattern found moving to the outside of the tire begins to place tread blocks into different patterns perpendicular to the road.  These particular patterns allow for lateral grip when turning or sliding to allow for the best possible traction. 

Compounds

Rally tires are offered in a variety of compounds which are chosen by drivers based on the anticipated road conditions, stage length, and temperatures.  Most companies offer compounds in soft, medium, and hard. While the soft compound typically offers the most amount of grip on a gravel stage because they are able to build heat faster and stick to the loose surface, their longevity is short-lived. Softer tires offer the ultimate performance and are desired in colder temperatures, but they will wear prematurely in hotter weather.  Hard compound tires are best utilized in warmer climates such as California and Florida and will last the longest in those conditions. Most competitors in the States utilize the medium compound gravel tire for a good balance between traction and longevity.

Size

Lastly, just as road tires differ in size based on the vehicle, rally tires offer multiple sizes.  Smaller tires allow for better gearing for lighter vehicles, whereas a wider, taller tire is more appropriate for a heavier car.  Width and height are also considered based on the road conditions and vehicle. While DOT or regular road tires are built around comfort and the life of the tire, rally, or gravel tires are created to withstand the challenges found on the surfaces of stage roads. Road tires have a simple pattern in order to reduce noise and do not allow a wide enough tread block to bite through the gravel. Winter tires are softer and often offer a more block-style tread pattern with some directional blocks, which provides better traction on gravel than a road tire.  However, rally tires are robust, built to resist debeading or separating from the wheel when sliding sideways, optimize grip while turning, and maximize acceleration and braking. Without these special tires, you would not see the delicate dance between a rally car and a gravel road.

Final Thoughts

As you can see, rally tires are built to withstand the trials of the stage roads, and not all rally tires are designed the same. My personal tire of choice is the MRF ZDM3, which is a medium compound rally tire that provides me with the durability and flexibility that’s needed on stages here in the States. With most of the stage roads being comprised of loose gravel, clay, and sand, the ZDM3s are able to withstand a variety of surface changes and climates throughout the race season. The Asymmetrical tread pattern allows for lateral grip and optimal traction, which is extremely important during competition, for both safety and performance. With the number of stage miles my Subaru sees each season, both my budget and car appreciate the importance of high-quality rally tires.