Tire rubber is an inconsistent item, and there is definitely a science to rubber compounds found in racing tires. It is also an art to mix, apply, and cure these compounds for specific purposes. Every race tire manufacturer strives to make each type of tire have the same qualities in terms of traction and wear, and-all things considered-manufacturers really do a pretty good job.
Traction, however, is affected by things that the manufacturer has no control over-namely, heat, time, and use.
If tire rubber were absolutely, positively cured when it came out of the mold, it could be used to make hockey pucks. From the moment rubber is put in the mold, it begins to cure. During this process, the rubber is cured to the point selected by the manufacturer. Different rubber mixes will cure to different levels of hardness. Once the tire has been removed from the mold, the curing process will slow dramatically. This additional curing can last for years. The curing also seems to take place from the outer surface inward. It is as if a shell is formed, slowing the inner curing process. It would not be unusual for some rubber mixes to double in hardness when left out in the weather for some years.
Heat cycles have their own effect on race tires, such as when the tire is raced and heated, allowed to cool, and raced again. In some cases a few heat cycles can raise the surface hardness of a nearly new tire.
All of these things that cure the rubber also make it harder, thereby reducing traction. The curing process cannot be reversed because it is a process mostly catalyzed by heat. I wonder, though, if the process could be halted by keeping a tire in the freezer. This might be an interesting test by those living in colder climes or by using a large freezer.
There are, however, tire softening compounds that soften a tire's rubber. These compounds are solvent-based in some form. And, yes, they do soften the outer surface of a tire. Most solvents evaporate rather quickly. The effects of the softeners are related to the amount of time the softener is applied. I have seen tubs with rollers driven by an electric motor where the tire is allowed to rotate in the softener. A deep softening such as this can last longer. Sometimes tires treated in this manner will be bagged until time to use them. This will retard evaporation. I have also seen tires treated with softeners on the inside as well as outside. Rubber holds the cord construction of the tire together. If tire rubber on the inside is softened, the strength of the tire is lessened. This could lead to confrontations with large concrete objects lining the perimeter of the track. From what I've seen, merely wiping on a softening compound won't do much. I think softeners may have more application in the drag racing world where success is measured in thousandths of a second in very predictable time spans. Competitors who race on paved oval tracks might see some short term benefit from using tire softening compounds. Remember, heat accelerates the evaporation of a solvent. Another factor to consider is the legality of using tire softeners at your track. Also beware of flammability concerns.
There is an alternative to using softening compounds to activate a tire, and a cheap alternative at that. As we have already seen, there is something resembling a shell formed by cured rubber on the outer surface. Sanding the tire scuffs off this outer surface. With the slick outer surface gone, the remaining surface is made of thousands of prickly edges. This new surface contributes to traction even though it doesn't seem to alter the hardness.
I did find something that surprised me. When the surface was scuffed to a depth of only 0.010 to 0.015 inches, the traction improved as noted above. When I sanded the tire down to a depth of about 11/416-inch, the durometer reading changed dramatically. It showed the rubber to soften by an average of 7 points. This was on an old McCreary IMCA tire. The results on DOT tires were similar. No, I didn't test a wide variety of tires. I think these results are indicative of the results on most tires. It seems the tire forms a shell on its outer surface that seals off the inside rubber. Perhaps this seal slows the cure rate of rubber on the inside. The depth of this shell will be determined by the age and use of the tire. Therefore, it is important to use a durometer, such as the one available through Longacre, to find the softer rubber. In effect, this is a mining operation.
What all of this tells me is that on tires that have age-hardened or have been race-hardened, there can be considerable benefit to sanding tires. The downside of this procedure is obvious: Rubber is removed. Many race tires and DOT tires have a lot of tread depth. Reducing this can have its own benefit by reducing tread squirm. Still, if tires are sanded too often, the tread depth may be reduced too far.
Along with my Longacre durometer came a conversation with Jeff Butcher, president of Longacre. Jeff explained that while the durometer is a simple instrument that even I could use, there were some things I should know. Since rubber in a tire is not completely homogenous, durometer readings should be taken at many places around the tire. Throwing out the highest and lowest readings and then taking an average of the other readings will give you an accurate indication of the hardness of your tire. The durometer should be your tool of choice when selecting or sanding tires.
Stored tires should be checked periodically. If your tire dealer will allow, take your durometer to the tire store. Select the softest tires, then take them home and store them in a cool, dark place. Keep a set of old tires and wheels on your race car for the purposes of rolling around the shop or on the trailer. Save the good ones to put on at the track.
ConclusionTire rubber is inconsistent in hardness. Sanding the surface to the right depth increases traction. A durometer is a necessary tool for selecting or sanding tires. Store your select tires in a cool, dark place.
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