Tread width

Many of us are moving into rallycross from some sort of pavement racing, where wider tires are often better. The reverse is frequently true for rallycross. It comes down to the tire’s traction patch, the rectangular area that is in contact with the ground at any point. Wider tires create a patch with the long dimension of the rectangle perpendicular to the direction of travel. This is great for cornering grip, but costs some grip for acceleration. This is advantageous when you’re on pavement because grip is relatively consistent and continuously available. On dirt and snow, the grip is often low and infrequent, so you need to make the best use of it. By running a narrower tire, the contact patch becomes longer in the direction of travel, and offers a better opportunity to put power to the ground. In addition, the narrow contact patch tends to get through the mud, snow, or loose material faster, giving better grip. There are some exceptions to this rule depending on the venue’s surface. If the ground is hard packed clay, such as what you might find at an oval dirt track, wider tires can be faster. This is due to the surface offering grip very similar to that of pavement. You can see some numerical data on how the size and shape of the contact changes in relation to tire width at this link.

Contact Patch Data: http://www.enginebasics.com/Chassis%20Tuning/Tire%20Contact%20Patch.html

A narrower contact patch can help power through loose surfaces.

Sidewalls

Winter tires often have sidewalls that are fairly flexible in comparison to summer tires. It is critical that you run higher air pressures to prevent debeading. You can help prevent debeads by going with smaller wheels so that your sidewalls are taller. This seems counter-intuitive, but the taller sidewall allows for more surface area for the air pressure in the tire to push against. When choosing a winter tire, pay attention to the Load Index rating of the tire. The larger this number is, the heavier car that these can be used on. This is important as it is also an indication of the sidewall strength of the tire. Some of the off-brand tires have much weaker sidewalls and some tires have several load indexes available in the same size.

Load index is often overlooked as a tuning tool. Heavier-duty tires often feature stiffer sidewalls.

Tire Size

Many racers miss an opportunity to fine tune their vehicle’s performance when choosing the size of the tires they’re competing on. Choosing a shorter or taller tire can offer advantages by changing where you are in the engine’s power band at a given speed. Most five speed cars have a second gear that tops out at 60 mph or higher. A car with a six speed may have a second gear that tops out too low for standard courses. Remember, you are losing time when you shift, so going with a setup that allows you to rarely shift is best. On most rallycross courses, it is highly unlikely that you’ll be exceeding 55 mph, so going to a short tire can really help you out in slower corners.

Tire diameter choice can optimize the time spent in a car’s power band.

For example, the SRT4 I compete in tops out second gear at 67 mph on a 25” stock height tire. This means it is under 2000 rpm during a slow 20 mph corner (something found on many courses), and well below optimal power. By going with a shorter, 24.45” (185/65R15) tire, I’m now at 2100 rpm for that same 20 mph corner and the turbo has spooled enough to be making power. Before buying tires, be sure to spend a bit of time researching your car’s gear ratios, power curve, and the effects that different height tires will have. This calculator is a great resource for this process. Keep in mind that even though two tires may have the exact same size (i.e. 185/65R15), the overall height may be different between manufacturers. Be sure to find the published specifications for a specific tire for your calculations.

Calculator: http://www.apexgarage.com/tech/gear_ratios.shtml

Tire sidewalls not only show size, but age.

Tire Age

Many of you will be buying used winter tires for rallycross. It is important to check the age of the tires when doing this. Tire rubber does outgas and get brittle over the years. You’re potentially risking a blowout or other major failure on tires exceeding seven years in age. Additionally, if the rubber has gotten hard from age, you’re leaving time on the table due to reduced grip. All tires are stamped with their week and year of manufacture on the sidewall. See the example image above to know what to look for. The first two numbers are the week and the second two are the year. Our example tire was made the 16th week of 2012. Be sure to look at this to ensure your safety.

Winter tires are a worthy investment for both speed and fun. National-level drivers may have multiple sets.

If you take away anything from this article, winter tires are well worth considering for rallycross. There’s no one perfect tire for this motorsport, so for maximum competitiveness, you likely will need to own multiple sets. No matter what winter tire you run, you’ll still have an advantage over those on all-season tires.

Bonus opinion section!

These comments are based on personal experience and some anecdotal evidence. The General Arctic series tires seem to be one of the best all around tires, only giving up significant time in mud. The various Blizzak tires, while fabulous in snow, seem to really be horrible in the dirt. The Hercules Avalanche seems to be a very competitive choice, despite being a budget tire with a blocky design. The Firestone Winterforce 2 (note, this is the second generation design, not the one pictured above) has ridiculously weak sidewalls and I would suggest avoiding. There have been a number of national champions running Nokian Hakkapeliitta tires, but these have a significant cost premium.

Thanks to David Capesius for contributing this article. - Jim