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Master Driver

Joined: Mar 27, 2010
Posts: 209

PostPosted: Sat Dec 10, 2011 3:51 pm Reply with quote Back to top

The tires, what do you really know about them? Let's find out.

There is something sad to the fact that tires have the most pronounced effect on grip and handling. Of course, you cannot change a given car all around by use of tires, because of the limitations of the chassis and suspension, but even slight changes in tire quality or tire pressure will easily overweigh any clever tweaks of the suspension or chassis or aerodynamics.

Tire structure
The structure of a tire looks simple enough, does it not? It's round, black and, well, round. But there is more to it. A tire can either be bias-ply (or diagonal) or radial-ply. The bias-ply tire is the earlier and simpler design, made simply by putting the different layers (plies) of nylon, fabric and rubber one across another diagonally to form the shape of a tire, inflated with air.

The advantage is mainly in simplicity (it's also more progressive and rides well), but the problem is that the tire is too simple, just a big black balloon that highly depends on proper pressure and is very sensitive to speed, side forces and heat.

The modern Radial tire is in fact built of two different parts, the sidewalls and the tread. The tread is wide, flat and thick and the sidewalls are thin and flexible. The tread grips the road and the sidewalls act as an air spring that dampens bumps, held by the air pressure in between them. Modern Radial tires are also steel-belted, with the tread held flat by two layers of a rubberized steel net.

The steel-belted radial rides well, reacts quickly to driving inputs, is ressistant to punctures in the tread (while being very voulnrable in the thin sidewalls), and can sustain high speeds and heavy loads, as well as more aggressive alignments. It is much less dependant on proper pressure, but it is still required to keep the sidewalls stiff enough.

The tread produces grip at the point of contact with the road. A tarmac road surface is abrasive and the rubber is basically made of little microfibers that get crammed under the weight of the car and into the little undulations of the surface. When you brake, these fibers deform backwards and slow you down. When you accelerate (and increase the speed in which the tires rotate) the fybers deform forward to increase speed. When you turn, they deform laterally into a slip angle to push you around the corner.

The ability of the rubber to grip the road depends on it's texture. A softer rubber compound is more grippy, but wears (as well as degrades) faster, breaks traction more suddenly, provides less steering feel and a less immediate reaction to steering inputs. A harder compound is less grippy, but more durable and reacts more quickly to inputs, as well as provides good steering contact. The compounds also carry small effects on the weight and dampening of the rubber (soft rubber is lighter).

Temperature changes effect the flexibility of the rubber compounds. A softer rubber heats up faster and turns more and more soft, up to a point where it begins to melt and form a film of molten rubber between the tire surface and the road, reducing grip. A harder rubber will require a higher working temperature, but also won't experience a very pronounced drop past it. The temperature range of the tire is made more flexible by use of resins, mainly silica, that help it operate from lower temperatures and allow to determind a specific optimal temperature.

The Tread
The tread of road tires has voids and sipes that help the tire perform in none-perfect conditions. On the dry, a "slick" tire with a flat rubber surface will mean more rubber on the ground and more grip. However, if there is water on the road, or even a good film of dust, the grip of such a tire will be miserable. So, tires have a tread of about 8 to 9 millimeters depth that is made to disperse water and heat.

Most road tires have a "rib" or two at the center. These ribs are not ideal for grip, but since the run across the circumference of the tire they do not ressist the rolling motion and therefore they prevent too much road noise, wear or heat build-up or gas consumption.

"lugs" or cubes are used for wet grip or for off-roading purposes, by biting through the water film or clenging into dirt when driving off-road. The lugs don't give all that good dry traction, as well as wear down quickly and generate more ressistance that adds up to heat build-up, noise and gas consumption.

Sipes are used for the intention of improving water dispersing capabilites by opening up when pressurized and siping in water which are than splashed out as the tire continues to roll around. They are also effective for biting into snow in winter tires.

In off-road tires, bigger, stiffer lugs are used to bite into the surface. Their edges near the corners of the tread are angled to allow the dirt to pour out quickly, and there are often ridges inside the tread voids or even small lugs on the sides of the tires (to bite into wheel paths). On the road these tires will be noisy, ungripp, quickly wear down and build up heat as well as being uncomfortable to ride and being highly uncontroled at or past the limits of grip.

The sidewalls
The sidewalls of the radial tire act as an integral and very substantial part of the suspension. The air pressure inside the tire practically hold them up straight. As you roll over even small undulations of the road surface, the sidewalls bend up and down continuously, dampening the ride. But the sidewalls also govern the tread. When we turn the wheel, we turn the front wheels (rims) and this turns the sidewall which turn the tread which turns the car. A shorter or stiffer sidewall will transfer this force more quickly so the tire will react more instantly, sharply and accurately.

Also, when you turn, the side force pushes the sidewall of the tire towards the outside of the turn, making the tire cram aside, making the radius of the turn wider and distorting the tread and making some of it fold away from the road surface. Stiffer or shorter sidewalls mean less such deformation and therefore more grip and predictability. Even when the tire has began sliding, it could be controled more easily and grip could be regained.

Other than using a tire with a shorter sidewall or with a stiffer one, you can make this adjustment with the tire pressure. Higher pressure holds the sidewalls stiff and keeps them from cramming. This maintains grip, responsiveness and reduces wear as the sidewalls flex up and down over bumps or deform sideways in turns. With under-inflation, the tire will flex all too much, which in long-term highway driving in hot weather, mainly when the tire itself is aged, will create heat that will cause the different layers (rubber, steel, fabric) to segregate and this is the traditional blow-out. This is also why the tire blows out at the point of contact between the tread and sidewall, where the rubber is most thick and the heat buildup is most severe.

In high-speed turns, the effect of such a condition is even worse. While being much more rare, it is possible for an unbalanced tire to cram sideways so far, that it becomes detatched from the rim and loses it's air pressure. Than, the rim drops down at the road and the tire practically folds off of the rim, and as the rim bites into the road, it rolls the car over. This will only occur under high cornering loads, usually combined with braking, in extreme under-inflation and on highly abrasive surfaces.

Higher pressure will yield all of the advantages mentioned above: More immediate reactions and more grip (for well-paved roads, dry or wet). The problem is that when the tire's reactions are more quick, it will react quickly to any input, even one that will take it past it's limits, making it transition into a sliding situation more sharply. More pressure also reduces steering feel and tire noise, which are important signals for when a tire is about to begin sliding.

As said, even past the limit of grip, the tire will be more controlled, as it maintains it's shape more accurately and reacts more quickly to your corrective inputs. Lower pressure means more wear, less grip but also a smoother ride (for small decreases of pressure) and more steering feel and a more progressive transition to a sliding situation.

The pressure also effects the tread, making it a bit concave when in lower pressures, making some of the center of the tread fold away from the road, or making it more convex when inflated to a higher pressure, making the shoulders of the tread somewhat retract from the road. In a steel-belted radial, this effect is reduced and therefore there will be little to no loss of grip or excessive wear from over-inflation, unless you really over-do it.

An under-inflated tire with a concave shape is good for off-roading, as this shape traps some of the dirt and allows the tire to flex around obstacles or "float" over dry sand. On well-paved roads, over-inflation and a convex tread shape is effective, especially when the road is wet, where this helps the tire penetrate the water which would otherwise build up under the under-inflated tire. On bumpy roads, low-profile or over-inflated tires will wear faster and grip less.

Tire wear
Tires wear down not only on the tread. The rubber itself degrades over time. Any rubber surface has a tendency to age and become dry and cracky over time. Tires are no different. When driving, they heat up, causing the rubber to expand. They than cool down and the rubber compresses back. The same occurs as the tire undergoes temperature changes through the day and the year. Eventually it becomes exhausted and dries out. The dry rubber is stiffer, and thus grips less.

The effect also penetrates into the tire, causing the inside layers to expand under heat. The problem is that the different types of fabric, steel and rubber all expand differently under heat, and this leads to a certain seperation in the layers and this makes the tire prone of blowing up.

To make it worst, ultraviolet light, sunlight, oxygen, ozone, moisture and salty air all cause the resins and chemicals in the rubber to evaporate. Imagine the rubber polymers to be a series of parallel, straight-lined links of molecules. The parallel lines (macro-molecules) are linked in some spots. If more links are created by heat (like when welding metal) than they will pull the "chains" in all different directions, eventually ripping them apart, thus the crazing. This is why resins are added to prevent these extra bonds from happening, but these also evaporate.

Tires will degrade critically after about 35,000 miles of three years of age. Smoother driving can help reduce it, parking in the shade helps too, and even using a reliable tire shine product on the outside sidewalls (not the tread) can help keep the sidewalls as flexible as they should be.

Tire damage
Tires experience various types of damage or wear. Here are a few:

1. Deformations: Have reduced with the improvement of rubberized steel belts, as well as reduced use of nylon in tires. However, moist that penetrates into the tire through deep scratches or scuffs, and deforms the steel belt.

2. Buldges: Can occur due to various reasons. A serious blow from a curb or rock can cause internal seperation that will allow the air to escape into the outer layers, causing a bludge. It can also be a manfacturing problem that causes a split wire across the tire, or even a fitting problem (where the buldges will appear near the tire bed).

Beware not to mix this with the intendations that appear on some tires from the manufacturing process, where the tire is composed and the tread and/or sidewalls meet and are sewn over each other.

3. Circumferential wear: A circumferential wear that occurs half-way to three-quarters of a way around the sidewall, surrounding it as a "band" all around or almost as such. It indicates long-term driving in a situation of moderate under-inflation. This is suspect for internal wear, especially where it occurs halfway across the sidewall. If any sign of the wear is seen inside the tire (when removed) it should be scrapped.

Tire wear will appear on either of the sidewalls, or in the center of the tread. It can be light, mild or irregular ("cupping"). Under-inflation will cause wear in the corners of the tread. The wear is normally a so-called "hot tear" where the lower pressure allows the rubber to heat up more quickly, to a point where it begins to melt and reform, normally on the very edges of the tire, in the form of a grey-ish, plastic-like rubber, and the rubber exposed beneath it will crack slightly up and in an angle towards the direction in which the tire rotates.

Over-inflation, if excessive, will wear down the center of the tread in a so-called "cold tear" where the tire begins to slip when still cold and recieves a "heat burn" that causes deep cracks. This only happens in very high pressures and under high loads.

Misaligned tire camber will cause the tire to wear down on the OUTSIDE shoulder. The wear will also be more profound, and actually deform the tire tread, making it recieve an angled shape. A suspension problem of bad shocks will cause irregular wear like cupping, which will also cause the tread parts to wear down in a "heel and toe" pattern where the leading edge of each tread element will wear down into a round shape and be followed by a high trailing edge.

Bad bearings or old bushings, as well as bad shocks with misalignment, is likely to cause similar cupping on the corners of the tread, with serious tear. A skilled eye can detect very specific suspension or brake issues, as well as identify a specific driving style.
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