Sunday, February 24, 2008

Speed Rated Tires

After having the bushings of the XJ40's upper control arms replaced, the ride quality was notably worse. Before the repair, the car floated across the road quite well (not withstanding the occassional clunk that sacred me into making the repair). After the repair, there was a notable shake in the steering wheel. A pair of new tires would eventually solve the problem, but which tires would work with the Jag?

I stopped by several tire stores and received differing advice. Most were more interested in getting the correct tire size as opposed to the speed rating. But Earl Gibbs was emphatic that the car would need V rated tires as came originally from the dealership. However, his price was a bit out of my league -- something like $600 a pair! So, I kept looking for a while.

I eventually purchased a pair of Goodyear Confort Tread tires at $109/ea. But I only consented after reading a pamphlet in the shop that explained the rating system. The Comfort Treads were "S" rated while the original Pirelli tires were "V" rated. Earl had said the car would wander all over the road "at speed" with a lesser tire. But is that really the case? I think both ideas are correct.

Earl was right to tell me to buy "V" rated tires. But this advice is only correct if I am planning on driving at high speeds. For instance, if I was racing the car or would be driving the Autobahn with this car I would need to use "V" rated tires. However, the pamphlet was also right. The "S" rated Goodyears were rated at 137 mph as opposed to the V series at 149 mph. In my puny mind, I figured I would reach neither speed unless chased by a tornado. So, the Goodyears were installed.

But in case you'd like an expert's advice, read the following section of Kirby Palm's XJ-S Help Book:
If you live in some parts of Europe, you are required by law to fit tires with suitable speed ratings for your car. In the case of the Jaguar XJ-S, that means a V rating or better.

Interestingly, the V rating has changed over the years. In the 80’s when many of our Jaguars were built, a V rating meant the tire was rated for 220 Km/h, or 137 mph. Today a V rating means the tire is rated for 240 Km/h or 149 mph. Just as well, since this rating better fits the capabilities of the Jaguar anyway. ...

For Jaguar XJ-S intents and purposes, a tire with a speed rating of V, W, Y listed with the load index and/or a Z in the size is acceptable. Any lower speed ratings (H, U, T, S, whatever) will be illegal in some areas and should be used with caution anywhere else.

Speed ratings are intended to indicate suitability for Autobahn-style driving -- continuous high speed. Generally, the limiting factor is heat buildup in the tire. High-speed-rated tires either run cooler or are made of higher-temperature material, or both. Much of the heat generated is dissipated to the air inside the tire and away through the rim, so inner tubes invalidate the ratings (the flexing of the inner tubes themselves adds to the heat buildup). There are also concerns about how patching holes affects speed ratings; it is recommended that the owner check the literature that comes with the tires before opting for a low-cost “plug” repair to his tires.

Since the flexing of the tire is what generates the heat, tire manufacturers usually achieve a high speed rating by making the sidewalls stiff. Therefore, the high-speed-rated tires drive differently than normal tires, even at low speed. For this reason, many recommend the V-rated tires even to those who don’t drive fast. This is especially true for the XJ6 and XJ-S, because the soft suspensions get downright mushy with the softer, lower rated tires. There are reports that S-rated tires used on these heavy cars will flex so much that they will wear out quickly.

The heat buildup due to tire flexing is also the reason you must have fully inflated tires when driving fast. Blowouts aren’t caused by the tire pressure being too high; they’re caused by the structure of the tire failing due to excessive heat. If you’re going to go fast, it would be better to err on the side of too much tire pressure than too little.

As the tread wears, the speed rating of the tire will actually improve. A thinner carcass generates less heat when flexing and dissipates heat better. There have been promotions claiming street tires were successfully used for racing; in these cases, typically the outer half of the tread was shaved off before the tire was even put on the car.
Time will tell if I'll regret saving $200 per tire.

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