Shore Rating:
Foam tires are rated by the hardness of the foam. Most of the time you will be working in the 30-60 Shore range. The manufacturer measures the shore rating by using a tire durometer. This tool has a small steel pin that is spring loaded and indexed to a circular gauge, very similar to a tire pressure gauge. To measure the tire hardness you roll the durometer on top of the tire; the compression force that the foam has exerted on the steel pin can be read on the dial. Repeat it over several locations on the tire and the average is the tire's shore value. There are other features that must be considered when describing a foam tire; like tire construction, composition and rubber/silicone content of a foam tire. These will typically be similar if you stick to a particular tire manufacturer. Foam tire construction can be one single piece of foam or it can be multiple rings glued together to cover the rim surface. Multi-ring tires are more expensive to manufacture but are less prone to suffer damage from the occasional rub or run-in with another car.
Most racers run a front tire with a shore rating in the 60-40 range. This is more commonly referred to as tire dot color codes. The dot code for 60-40 is blue, red and orange. For the rear 30-40 shore or pink, purple, orange is more common. Tire selection is going to depend heavily on surface roughness, temperature, track preparation and chassis setup. Keep this in mind when you are going into a late afternoon main. The track is going to be hotter than in the morning. It's not uncommon for the cars that were hooked-up in the morning to be traction rolling in the afternoon. If you are on the verge of rolling over when track temps are not up to par, then change to harder tires or make suspension adjustments to anticipate higher traction levels later in the day.
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