Issue link: http://lawandordermag.epubxp.com/i/65600
COVER Michigan State Police Tire Tests the MSP due to excessive wear – all three Cooper tires and the Goodyear on the Tahoe. Testing to failure is one thing – the tire either wore to the cords or started to chunk tread blocks. But how about perfor- mance changes on the way to being totally worn out? The real significance of these MSP tests is how the relative performance between tires may change as each wears. To see examples of the performance change as it wears, the Cooper tire is clearly not the best example. Neither is Pirelli – one is too expensive to be relevant, and the other has been es- sentially made obsolete by GM. The Nitto and Nokian are just not familiar enough to be good examples. The comparison that is the easiest to relate is the Firestone versus Goodyear on the Charger and CVPI – the two most popular police sedans and the two cars running both Firestone and Goodyear for which the MSP has the full test results. Wear Affects Wet More Than Dry In the dry braking stage, the Firestone had a slight advantage over the Goodyear when new. The Firestone kept that slight advantage when tested again in the worn condition. In each case, the stopping distances were about 10 feet longer for the worn tire, compared to the new tire. In the wet braking stage, the Firestone had an advantage over the Goodyear when new. The Goodyear turned the ta- bles for an advantage when tested worn in the wet. The same reversal happened during the wet brake-and-turn stage. The Firestone started off with a solid advantage when new. When worn, the Goodyear took over the same solid advantage. The final test of half-worn tires was the steady state circle, which shows the maximum cornering force the tire is able to give under ideal test conditions. In this maximum g-force test, the Firestone held a clear and solid advantage over the Goodyear when both were new. The Firestone held that same clear and solid advantage when both were half-worn. The key to understanding the new versus worn compari- S The steady-state, static circle showed the absolute maximum amount of cornering force from the tire under ideal conditions. sons is remaining tread depth, and its effect of performance. Remember, the tires had the same test track miles on them, but in some cases very different tread wear. For example, the entire testing process wore an average of 60 percent of the Fire- stone away, but only 32 percent of the Goodyear. That means that performance not related to tread depth (dry braking, dry cornering) would not change much. It also means performance directly related to tread depth (wet braking, wet brake and turn) could change a great deal. The Firestone gave up a lot of wet braking performance as it wore because it was S The Firestone Firehawk GT Pursuit gave first place performance, and was the last man standing, on the Chevy Tahoe. W By testing tires in both a new condition and a worn condition, the MSP proved that tread depth has an effect on performance, especially wet traction. 28 LAW and ORDER I May 2012