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Introduction Heavy or top-heavy vehicles all have something in common - the need for strong sidewalls. I was told many years ago (mid 80's) by a tire guru at Sears, that the Studebaker required strong sidewalls - it was a heavy car, with narrow 14" rims. Sears made a special 14" Roadmaster tire that had reinforced sidewalls for this reason. It was the tire specified for my '65 bus - and many other cars of the era. Those tires worked very well for my 1965 VW Bus. But at the time, I had no idea why. Another car I have owned, the Volvo 240, shared this problem - the need for a strong sidewall in a 14" size. The Volvo is a very heavy car - like a tank. Low, but still rough on tires due to alot of weight from the reinforced roof. I purchased budget Goodyear tires, but the car was susceptible to drift in heavy sidewinds. I also had one sidewall go - which wasn't catastrophic, but resulted in a flat tire in the morning when I needed to get to work. What's the difference between a "passenger" tire and "truck" tire? A stiff sidewall prevents flexing under load - which prevents sudden lane changes due to wind gusts (trucks passing you by) or sudden failure under load (just ask an Explorer owner). The Volvo isn't prone to rollovers (heavy and low) but in an SUV or truck (including VW Bus or Vanagon), a rollover is more likely. Passenger tires rarely include additional sidewall reinforcement - for obvious reasons. Cars are designed for commuting, not transporting heavy loads. Trucks, though, can carry heavy loads. Unfortunately, all too commonly the tires used in these applications are inadequate when the vehicle is at its load-carrying limit. If you are going to put any weight at all on your tires, you should consider the ability of your tires to handle that weight. I put this page together as I was searching for tires for my Vanagon. But the information applies to all types of tires and for all vehicles. This information is not exhaustive - but should be the core information you need to know to make an informed decision. I don't claim to own the information here, so I have linked to the resources that I found helpful. Tire specifications (obviously) are very important. But interestingly, when you walk into a tire store the salesperson will likely not even talk about specifications. Instead, they talk about "grooves" and "profiles" and use adjectives like "aggressive" or "quiet". Nothing to do at all with specifications.
Let's take a look at each specification. How fast are you really going to go in your vehicle? You should never exceed the speed rating of a tire - nor should you be anywhere near the speed rating for a tire. The speed rating of the tire is determined in a laboratory environment usually at a moderate ambient temperature. This does not factor in load due to cornering, nor heat due to hot asphalt or long periods of driving, nor inflation pressure that is not at the recommended level. Nor does a high rating rule out a defective tire. When you drive near or over the speed rating of a tire you are risking your life because there are always factors that cannot be taken into account. Car & Driver Editor Don Schroeder's life ended tragically as a result of high speed tire failure (second story on page). As it turns out though, the higher the tire speed rating, the less likely you will have catastrophic sidewall or tread failure. It makes sense that higher speed-rated tires hold up better than lower speed-rated tires. This is not to say that you can overload a tire safely, or that you can safely have a blowout at 200 miles-per-hour - this just means that if a failure does occur at normal speeds, higher speed-rated tires generally handle the failure better. The speed rating indicates the maximum speed (miles per hour) the tire is rated under normal conditions. The ratings are as follows:
Speed ratings vary greatly depending on the type of tire. For example, Q-ratings are typical among off-road tires. R-ratings are typical for average passenger tires. T-ratings are typical for performance tires. Provide for a 15-20% buffer and you should be OK. In other words, If you have a Q-rated tire, your sustained speed should not exceed 80-85 mph (unloaded) and less if you are carrying a significant load. (My Vanagon barely does 70, so a Q-rated tire is alright for my application.) If you have a Porsche, and your maximum sustained speed is 135 mph, then you should not consider anything less than a W-rated tire. (Of course, this is merely an example and not advice to travel that fast.) Again, the speed rating is determined at on an unloaded (unstressed) tire at a moderate ambient temperature. With that in mind, the next spec to take a look at is temperature. Temperature and traction are probably the least marketed specifications for tires. However, in hotter climates, you should be especially concerned with temperature ratings. The temperature rating indicates how well the tire handles heat (including how well the tire dissipates heat). There are three ratings:
Traveling long distances in hot weather, at a high-rate of speed, is very difficult on tires. Consider how many times you have seen a family on vacation in summer - with dad hauling down the highway with the wagon loaded down with the gear. This is a common situation that many times results in catastrophic tire failure. Consider, also, how much more often you see semi-trailer tire tread on the highway during the summer. Luckily those big rigs have more than four tires. In wetter climates, the traction specifications may be more important than temperature. The traction rating indicates how well a tire grips the road surface. This is an overall (balanced) rating for both dry and wet conditions. Tires are rated:
What is interesting is that many times, a tire's traction rating will be sold as how well the tire handles in wet weather. This isn't always the case - as the traction rating is a combined score. You could have an A-rated tire that performs excellent in dry weather, and acceptable in wet. For most car owners, speed, temperature and traction are the most important specifications to consider. Now we get to the important specifications for heavy (or top-heavy) vehicles - load index and load range. There are two load ratings:
The load index indicates the maximum weight that a tire can accommodate vertically at a given PSI (pounds per square inch). Typically, the load index of the tires for passenger cars and light trucks range from 80 to 100.
Sources: Keep in mind that load ratings are determined using an applied vertical force against a tire under laboratory conditions (usually 65 ambient degrees). The heat rating for the tire can affect the maximum load the tire can handle at a given temperature. You will see often the load index as a combination of Load and Speed, such as 97T. This indicates that the tire is rated for 1609 lbs. at a maximum of 118 mph. However consider that at that rate of speed, if you were to need to perform an emergency maneuver, you could easily exceed the load index of the tire. Remember, the load index is how much weight you can put on top of your tires. Load Range Load Range is another way of indicating the ability of a tire to handle stress and weight. This time, we are indicating how much load can be placed on the tire horizontally. Often, these tires are associated with towing because the back end of the car tends to wallow while towing a trailer. In other words, you are exerting stress on the tire from side-to-side. While there is no industry-wide definition of ply rating, truck tires are frequently marked with ply rating and equivalent Load Range. These markings are used to identify the load and inflation limits of that particular tire, when used in a specific type of service:
Source: Bridgestone/Firestone North America, Sizes and Classifications Remember, load range indicates horizontal and sidewall strength. What load rating and/or load range do I need? Consider the following questions:
Driving a heavy vehicle, fully loaded, during the summer, at high rates of speed puts much more stress on a tire than light-duty low-speed driving. Light trucks such as the Volkswagen Vanagon have a fairly narrow track and a high center of gravity. The higher center of gravity tends to put more stress on tires due to body roll and load transfer. This kind of stress is exerted in a horizontal direction. Volkswagen suggests a Load Range of C for its commercial vehicles (Bus, Vanagon, Eurovan). They do so because a 6-ply tire can withstand more side load and will not flex as much under these conditions. [Most manufacturers do not offer 6-ply tires, nor do they document their load range.] You should select the tire with the highest load rating and speed rating (and highest PSI) that you can afford that is in the size needed for your vehicle. Lastly, while a Vanagon Westfalia has a higher center of gravity, and possibly more weight at the top of the vehicle, other models are lower and do not carry weight above the mid-section of the vehicle. The lower the weight is, the less force will be transferred from side to side when cornering or in heavy cross-winds. I recommend using Tire Rack to research your tires. Reading the user reviews is a very good way to find out how others liked the tire. Some of the results are surprising. |
