April 29, 2014
Kal Tire track demonstration of differences in braking distance and handling, Alan Sidorov at the wheel. Click image to enlarge
By Simon Hill; photos by Simon Hill and courtesy Kal Tire
Quick, do you know the speed rating of your tires? Unless you’re a bit of a performance enthusiast, the answer is likely “no”. With winter coming to an end and drivers switching to summer tires, Kal Tire would like to change that, because a tire’s speed rating is really the key to its performance and, ultimately, safety.
To demonstrate the link between speed ratings and real-world performance, the Canadian-owned tire retailer recently invited a number of automotive journalists, yours truly included, to take part in a demonstration aimed at illustrating the importance of tire speed ratings, what they really mean, and why we should care.
Certainly it’s easy to become complacent about so-called speed ratings – after all, according to the tire charts, even the lowest-rated tires can safely be driven at Canadian highway speed limits all day long with no problem. So one of the first things we were told when arriving at the demonstration is to think of the speed rating as more of a “performance rating.”
Speed ratings were originally developed in the 1960s to help prevent highway blowouts. Most passenger vehicle tires today are rated with a letter between “S” and “Y”, although to keep things interesting, H-rated tires slot in between U- and V-rated tires because “H” traditionally stood for “high performance.”
Technically speaking, a tire’s speed rating indicates the tire’s ability to stay cool when run at highway speeds and, by extension, during aggressive cornering. From a consumer perspective, this matters because of the way in which heat is generated in tires, and the construction techniques higher-rated tires use to combat heat buildup.
Heat is generated as a tire flexes, deforms, and squirms. That’s why underinflated tires risk blowouts: with inadequate air pressure to support the tire’s shape, the tire flexes more with each rotation, creating excessive heat and compromising the tire’s structure.
To help combat heat-generating flex and squirming even when properly inflated, higher speed-rated tires are built with features including additional bead belts, more reinforcement at the tire’s shoulder, and more stable tread patterns. This not only helps reduce flexing and deforming as the tire works, thus reducing heat buildup, but also means that the tire’s contact patch stays in better contact with the road, with less tread squirm. The result is that, generally speaking, higher-rated tires offer much-improved cornering feel and noticeably better traction than lower-rated tires.
Firestone FR710 – rated “S” & Nokian eNTYRE – rated “V”. Click image to enlarge
So how much better is the traction? In back-to-back testing with expert driver Alan Sidorov at the wheel, using identical cars and reputable name-brand tires of the same size, the car with V-rated tires (Nokian Entyre) stopped from 100 km/h a full car length before than the car with S-rated tires (Firestone FR710), making the stop in 35 metres compared to 39 metres.
Even in my own less-than-expert hands, stopping from a more sedate 50 km/h, the car running on V-rated tires came to a halt almost a metre before the car running S-rated tires. Not a big deal on the test track, but if you were on the road and that final metre was occupied by a stopped pickup truck…
In cornering tests, the “V” rated tires responded more crisply and predictably, and offered appreciably better grip. In a 35 km/h decreasing-radius turn, I ran clear off the course when running on S-rated tires, obliterating several traffic pylons in the process. On the V-rated tires I came in a little hot at 37 km/h, but still managed to negotiate almost the entire corner, just clipping the last two pylons.
Perhaps equally importantly, the car felt more planted and predictable with the higher-rated tires, and easier to get back in line after abrupt manoeuvres such as the emergency lane change.