The Rubber that Meets the Road

Saying that tires come in all shapes and sizes, isn’t exactly right. They do come in all sorts of sizes but the shapes are pretty much the same, round with a hole in the middle. They are all black and they all use air pressure to keep them working properly, although with all of the strange things that automotive designers have done over the years to change the style of the cars and trucks we drive, it’s kind of funny that they haven’t had cars with blue tires or red tires. The only thing they have ever tried is varying thicknesses of a white stripe around the sidewall.

Tires are an extremely important part of the car and yet they are always neglected or forgotten. Without decent tires a car turns into an instrument of death and chaos. That may sound silly but it’s pretty much true. The tires keep the car in control and sticking to the road at all times. When it’s going straight, and when it’s turning, when it’s on smooth clear pavement, and when it’s on rough, bumpy or slippery surfaces. The tires also contribute substantially to the ride quality. When considering that the tires are bouncy air cushions that the entire weight of the vehicle is resting on, it’s easy to see that a tire problem is going to affect the way the ride feels and the way the vehicle handles.

A very interesting tread pattern. Each row of tread is different
and grips differently from the rest.

Tires are meant to provide a way for the vehicle to stick to the road. The rubber compound that makes up the tire is what does the sticking. The depth of the tread doesn’t have much to do with causing the tire to grip. The tread on the tire is only there to move water and snow out from under the tire so that the rubber can meet the road. Some tread patterns are not meant for use in snow and ice and some are designed specifically for that purpose. If you drive your car in all types of whether then all-weather tires are what you want. They will give you adequate performance in all conditions, but they will not give you the best performance in any conditions.

Tire Sizes

The most confusing thing about tires is perhaps the way that tire sizes work. If you have ever looked at the sidewall of the tire you might have seen something such as P215/65R15 89H. Most of this seems like nonsense but numbers of course, mean things. Starting with the letter at the beginning, this is usually a P or an LT. P stands for “passenger” meaning the tire is intended for passenger cars. The LT stands for “light truck” and is intended for; you guessed it, light trucks. This is any truck or truck like SUV even if it’s a full size truck it is still only a light truck.

The number 215 is a direct measurement of the width of the tire in millimeters. This is pretty easy to understand but the next number 65, is the aspect ratio. This ratio is the measurement of the height of the sidewall of the tire expressed as a percentage of the width. So in this case the side wall height is 65% of the width of the tire.

The next letter R refers to the type of construction. In this case R stands for “Radial” meaning that it is a radial tire. A radial tire is a tire that has the steel cords inside laid out in a radial pattern. All common tires today are radial tires so it is really pretty rare to see something other than an R in this spot. Old tires might have a B for bias belted, or a D for diagonal bias.

The number after this letter refers to the diameter of the rim in inches. Why they size part of the tire in millimeters and part of the tire in inches makes no sense but that’s the way it is. Many old tire sizes were all in inches and some off-road tires can still be purchased this way. Something like 31X10.5 R15 would be an example of a tire size in inches. The total diameter of the tire is 31 inches, and the width is 10.5 inches, made to fit on a 15 inch rim.

The next number, in this case 89, refers to the load rating of the tire, or how much weight the tire can safely carry on a regular basis. The number is kind of an arbitrary value, the higher the number the more load the tire can carry.  These standards are set by the Department of Transportation and will normally range from 70 to 110.

The last letter which is an H on this tire, refers to the speed rating of the tire. A tire with a speed rating of H can safely be driven at speeds up to 130 mph without centrifugal force ripping the tire apart. The highest speed rating is Y which means the tire is safe up to 186 mph. Many people think that Z is the highest rating but the letter ratings skip around a bit. On very high speed tires a Z is added before the R so the tire will be called Z rated but this is not the case. The Z in this position indicates it is a tire that is intended for use over 149 mph but the actual speed rating is still at the end of the tire size number. A good example of this is 285/35ZR19 99Y. This is a Y rated tire not really a Z rated tire. Exceeding a tire’s speed rating is very dangerous and can lead to a high-speed blowout which may lead to a loss of control of the vehicle. Some light truck tires might have a P or Q rating which is somewhere between 90 and 100 mph. The vehicle these tires are mounted on may easily go faster than this. Driving a vehicle with one of these tires faster 90 to 100 mph would not be a good idea.

One more marking worth a mention, all-weather tires will either say all-weather or all-season on the side, or they will be marked with M+S which stands for mud and snow. Unless you live in a place like Phoenix, Arizona or your vehicle is only used under certain weather conditions, all-weather tires are probably the best option.

Tire Maintenance

Of all of the important systems on a car that require maintenance, tires are probably the easiest and the cheapest to maintain. All that really needs to be done is regular pressure checks and an occasional tire rotation. Despite the fact that this stuff is cheap and easy, most people do not have these things routinely performed on the cars that they drive. That’s their choice but remember the instrument of death and chaos? That is what results from a lack of maintenance.

Every vehicle on the road should have a tire pressure gauge in the glove box so that tire pressure can be checked a couple of times a month. Most gas stations have an air hose that can be used to inflate the tires if needed, and probably 25% of those air hoses are actually in proper working order so you should be able to find a place where you can check your own tire pressure, and top them off if needed. Checking tire pressure with a run of the mill tire pressure gauge requires no skill, and neither does filling the tire with air. Most automotive technicians will gladly charge you to check your tire air pressure for you, and maybe they will even do it for free, but guaranteed they will mock your laziness or ineptitude behind your back because you cannot check them yourself.

Wear indicator between the tread blocks touching the
pavement. This means that the tires are worn out.

In order to find out what your tire pressure should be you can look in the owner’s manual, but if this is lost, or just too confusing, the other place that you can look is on the tire information label. This label is usually posted somewhere on the inside edge of the driver’s door or driver’s door frame. If the label is not there it could also be on the inside of the glove box on under the lid of the center console. Do not look on the side of the tire. This will only tell you the maximum pressure that the tire can stand before it becomes dangerous once it gets hot.

Tire rotation is something that you could easily do yourself with a few simple tools but since the car must be lifted you may just want to have this done by a professional. Tires should usually be rotated about twice a year. This usually works out to be about every 6,000 to 7,000 miles. Some special off-road tires or snow tires with very soft tread may need to be rotated more often.

The reason tire rotation is important is that it allows the tires to wear more evenly. The front tires on a vehicle with front wheel drive will wear out the front tires much faster than the rear tires because the front tire both steer and propel the vehicle. Vehicles with rear wheel drive usually wear the rear tires a bit faster than the front. If the wheel alignment of the vehicle is not perfect the tires will wear even more unevenly. Sometimes the tires on a particular vehicle just won’t wear well even if the alignment is good. Proper rotation will always make the tires last longer.

Tire Problems

Uneven wear is the most common tire problem. When the tread doesn’t wear the same across the entire tread surface on the same tire, it is called uneven wear. Things that usually cause this are wheel alignment problems or tire inflation problems. Wheel alignment problems can be caused by a lack of proper adjustment, or worn out or damaged parts. This must be diagnosed by someone who knows what they are doing. If the tires are worn more heavily on one edge of the tire compared to the opposite edge then there is an alignment problem. If the wear in the tire appears wavy or if individual tread blocks seem to be wearing on an angle then this is probably an alignment problem.

This tire has good tread over most of the surface, but the
right side is so work out the steel cords are poking through.
This is a major alignment problem.

Tire inflation problems that cause uneven wear can be easily diagnosed by anyone. If the center of the tread around the circumference of the tire is worn more than the outside edges of the tread then the tires have likely been run while overinflated. This is common because many people don’t use a pressure gauge when they put air in their tires. They put air in and just look at the bulge in the sidewall where it sits on the ground to come up with a guess as to when to stop pumping the tire full of air.

If you see the opposite condition, wear on the outside edges but not down the center, then the tire has been underinflated. This is common because it means that the tire pressure is never checked and the car is driven without enough air in the tires. If either one of these inflation conditions is caught before the difference in wear is extreme then the pressure can be corrected and monitored and the tires will usually even themselves out.



The tire on the left is underinflated. The tire in the middle is overinflated.
The tire on the right is normal.

 Other problems or forms of tire wear are an indication of a defect in the tire, or the inner carcass of the tire coming apart. If the tread of the tire seems to be worn around just a portion of the circumference, but the rest of the tread along that same line around the tire looks better, then the tire is experiencing tread separation. This means that the tread layer of the tire is detaching itself from the carcass. Another condition that may be noticed with tread separation is a vibration that begins around 30 to 35 mph but then disappears around 50 mph. Sometimes this is mistaken for an imbalance but it is not. An imbalance in the tire and wheel assembly will cause a vibration that will usually begin at a higher speed but will only get worse as speed increases, and will never just vanish. Tread separation is very dangerous and the tire must be replaced to correct this condition.

Major tread seperation. Notice the big deep crack on the right side.
This is a very damgerous tire, not to mention that the ride quailty of this
vehicle was terrible.

A true tire blowout of the worst kind.

A bulge in the sidewall is also a dangerous yet common condition. This means that the steel cords in the sidewall have broken allowing the air pressure in the tire to cause the bulge in the rubber. This is a weak spot that will be prone to complete failure at some point, but will most likely fail at high speed when the tires are their hottest and the pressure is the highest. Many times the broken cords are the result of the vehicle taking a curb shot. If the car is driven into the curb or slides into the curb in the winter, the sidewall gets pinched and it can break the steel cords. A tire with a bulge in the sidewall must also be replaced and cannot be repaired.

A Doughnut You Can’t Eat

If the tires are very neglected, the spare tire is even more so. The main thing to remember with the spare tire is to check the pressure once in awhile. Many times people find a flat tire on their vehicle and go to put the spare on only to find that it too is flat or at least very low. The other thing is that spare tires are usually meant for temporary use and should not be used for extended periods.

If you are lucky, the spare tire on your vehicle is a regular full size tire and not one of the temporary doughnut spares. These compact spares are now the standard on most cars because they are cheap, they are light, and they are easy to stow. Anything that manufacturers can do to make a car cheaper, lighter, or easier to package, will be done, even if it comes as a compromise. The problem with these little spares is that they are not meant for speeds much above 55 mph. They are only meant to get you to a tire shop in the next town. The problem is that this is not very convenient because stopping at a tire shop in the next town isn’t always convenient.

Some four-wheel-drive vehicles and SUV’s will have a doughnut that is full-size. These don’t have quite the same restrictions as the small ones but they are not the same thing as having a regular full-size tire for your spare. If you do have a regular tire and if the rim matches the rest of the rims on the vehicle, you can rotate your spare through the regular rotation. This will allow you to actually use that tire and it will make all of the tires last a bit longer.

You Get What You Pay For

When buying new tires it goes without saying that you should buy tires that are the same size as the ones that the vehicle came with. Changing the size of the tire will change the way the vehicle rides, handles, and it will also cause your speedometer and odometer to read inaccurately. Sometimes some custom rims can add a nice look to your ride and this is okay to do as long as the overall diameter of the tire remains the same. So if you vehicle came with 16 inch rims and you want to put 18 inch rims on it,  change the tire to something with a smaller aspect ratio. The new aspect ratio must be 1 inch shorter which will add two inches to the inside opening of the tire.

The other thing to remember when buying tires is that you get what you pay for. Don’t buy cheap tires, it’s not worth it. This doesn’t mean that the most expensive tires are the best tires; it just means that you should always try to spend a little extra. The problem with the cheap tires is not just that they tread wears out quickly, but they usually suffer from other problems before the tread wears out. Things like vibration problems, tread separation, conicity issues (a condition that can cause the vehicle to exhibit problem with wheel alignment when really there are none), and other things render the tires useless while there is still tread left.

Stay Connected

To the road that is. There are a few systems on your vehicle where you never want to see failure while driving, and the tires are one of them.  If your engines blows up while driving down the road, no big deal. If your transmission falls out on the freeway, it doesn’t matter. If one of your tires gives up at 80 mph, or if you have no tread left and you hit a big puddle at high speed, then watch out because you may not be able to keep it under control. Take care of your tires; remember that your life is riding on it.