Why do tires wear? Answers inside

necrotimus

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I was going to post this in response to another post but decided the info was good enough read for a new thread. We all know a more aggressive rider will wear tires out faster but we may not know exactly why. The portions of articles below do a good job of explaining it.

If you decide not to read the whole thing here is one good piece of info for you found near the end of the post:
How to find tire age
If you look closely you will find a four-digit number stamped into the sidewall of your tire. It will read something like 4408 or 2101. What do these numbers mean? Well, a 4408 means the tire was manufactured during the 44th week of 2008. 2101 means the tire was manufactured the 21st week of 2001. Years prior to 2000 have a one-digit number to reflect the year, and after 2000, a two-digit number is used.


This is all taken from two articles on tires I found

Tire wear-
"The first question to be addressed is "What is tire wear?" Tire wear is the actual removal of rubber from your tire. Scuffing is what wears out your tires. Just like when you were a kid and skidded your rear bicycle tire to make marks on the driveway, the abrasion of scuffing is the only contributor to tire wear. If you never scrubbed anything off your tires, they would never wear out. It takes some pretty good stiff forces to come into play for pavement to scuff stuff off of your tires."

"When you turn, your bike really wants to go straight (Newton's First Law of Motion). You apply a force to make it turn in your wise desire to stay on the curving road. The bike's desire to keep going straight is the natural and inertial centrifugal force in this interaction and the force resisting the inertia making the bike turn to follow the road is the centripetal force. As one can imagine, there is quite a bit of force coming into play when your bike makes a turn. The only thing preventing a crash is about a 2 inch contact patch between your tires and the pavement where the forces involved in a simple casual turn exceed 1 G in space age parlance."

Left (or Right) side tire wear-
"Why is this wear more evident on the left front in most cases? Actually, excessive side tire wear is only evident on the left front in countries where one rides on the right side of the road. Riding right means that the left side of your tire will have more (and likely faster) miles on it than the right side. Left hand turns have a larger radius than right hand turns in right side driving countries, hence you ride farther (and likely faster) turning left than turning right with subsequent increased side band wear on the tire's left side. The left side of your tire has more miles on it (in some extreme cases, twice as many) than the right side of your tire. And the side of your tire only gets mileage when you are leaned in a turn, otherwise, this area of your tire does not contact the pavement at all as shown in the photo. European left side drivers find that the right side of their front tires will wear out first. Quite the opposite effect for precisely the same reasons reversed."

Center Wear-
"But if it's only the extra miles that cause the wear, wouldn't the center if my tire wear out first since I have more miles upright than leaned? Yes and some upright wear is evident upon inspection of a worn front tire as seen in the photo above left. Though this wear is not as excessive up front as the sidewall wear because of one factor... Upright miles on a properly inflated front tire are rolling miles with little scuffing taking place. If, on the other hand, you look at your rear tire, you will indeed see that the center wears out first and this wear is often exaggerated because acceleration, engine braking* and real braking scuff stuff off the upright rear tire. Each time you downshift to engine brake, upshift and release the clutch, roll on the throttle or roll off the throttle, you will scuff the rear tire at the contact patch. Along with that, the rear is your drive tire and at speed, the rear contact patch is the only thing that keeps you going (don't believe it? Just let off the throttle and see how quickly your bike slows to a stop!). Since most acceleration/deceleration and braking occurs when the bike is more or less straight up this wear is most evident in the center of the rear tire. Drive shaft bikes are the worst offenders since they are notably "herky jerky" and transfer the shock of accel/decel directly to the rear contact patch unbuffered. Belt and chain drives will "buffer" these shocks and lessen this kind of wear. This same scuffing action is minimal on the front tire because the front tire is undriven and merely rolls while the rear tire is doing all the inertial work. When brakes are applied, traction at the front tire improves minimizing scuffing while traction at the rear tire deteriorates maximizing scuffing."

Cupping-
"Cupping, which is more accurately described as scalloping (see pictures, but we will use the more common term "cupping" here), is a natural wear pattern on motorcycle tires and it will always follow the tread pattern. It is not a sign that you have bad suspension parts. It merely shows that your tire is indeed gripping the road when you make turns (thank you for that Mr. Tire!). This cupping develops within the side wear bands of a leaned motorcycle. The extreme forces that come in to play when the bike is leaned in a turn are what produce the effect and when the wear becomes sufficient, one will experience vibration and noise when one banks into a turn. Upon examination of the pictures at left of our sample rear Avon, our dusted front VTX Dunlop D256, and the picture of our chalked Dunlop D206 one can see how the cupping follows the tread pattern. The leading edge of the tread does not flex much as it grips the road and the rubber is scuffed off the tire in that area causing a depression. As the tire rotates, the pressure moves to the trailing edge of the tread pattern where the tread flexes more causing less scuffing so less material is ground off the tire. The more complex the tread pattern, the more complex the cupping pattern will be. The softer the compound of the tire, the sooner this cupping will develop. Radial tires are more prone to cupping than are bias ply because the compound of radials is softer. As one can see, the simple tread pattern of the Avon pictured produces a simpler scallop pattern while the more complex VTX D256 Dunlop is somewhat involved, though still easily seen in our photo. Cupping on the Valkyrie Dunlop D206 is very hard to photograph because of the complex tread pattern. Low tire pressure will exacerbate this wear pattern and you will lose many serviceable miles by running low. Improper balance has nothing to do with cupping on a motorcycle tire. Improper balance will merely cause your bike to vibrate within certain specific speed ranges."

On hardening from another article-
"When a tire is heated, the rubber becomes more pliable, allowing the tire to ooze into all the nooks and crannies of the surface you are riding on. However, as the tire heats and cools, it goes through a process called outgassing . Literally, parts of the tire are turned into gasses and escape, resulting in the changing of the chemical makeup of the tire. The results of outgassing cause the tire to become brittle or less pliable, hindering the tire's ability to conform to the road surface. Back in the day of flat-track racing, it was a common practice to take tires and throw them up onto a flat, southern California roof for a few weeks to bake in the sun, periodically flipping them from side to side like a pancake. The tires would heat and cool and consequently outgass until they were rock hard. Perfect for flat-track racing.

So, now back to our race compound tire. The race tire is designed for the racetrack, hence the name and usually the red sticker or other marking on the sidewall of the tire. They are designed to be used two or three times before they are discarded. Even "DOT Race" tires are typically only good for two or three days of use. That equates to two or three heat cycles and two or three periods of outgassing before the chemical balance of the tire has changed so much that it is no longer the same tire. Also, race tires are designed to be able to carry more heat before the tire melts. This is to handle the extreme speeds, cornering velocities and higher abrasion characteristics of a race track. This means it also takes more time and more aggressive riding to get the tire to its optimum operating temperature. Race tires are awesome for the track rider. They allow the racer to get the most out of the tire, but at a cost (they don't last as long).

The street rider who uses race compound tires is simply wasting their money and taking unnecessary risks. Even the fastest street rider is probably not (and should not) be able to get the tire up to its optimum temperature to reap the benefits of the added traction. Street riders are forced to stop for stoplights, police officers and traffic in general. They are also a full-on idiot if they ride at 100-percent ability, as fast as they can, in a street environment. However, the race tire is still outgassing at the same rate it would if it were being used on the racetrack even though the tire will never reach operating temperature, and never achieve optimum levels of cohesion.

Street riders will, however, get a greater level of traction and more life out of a street tire that is designed to withstand hundreds of heat cycles and operate at a lower temperature. And in fact, most modern "sport-touring" tires will provide a better level of grip for 90% of all street riders than sportier counterparts. Only an elite few, and they are probably not reading this anyway, can use the full potential of a Michelin Pilot Power or Dunlop Qualifier. In fact, its quite amazing what skilled riders can do on modern sport-touring tires like the Michelin Pilot Road or Continetal Road Attacks - skilled riders can still drag knees around corners with these supposedly "hard" tires.

Some street riders feel that it is an advantage to buy "take-offs" from racers. Take-offs are the race compound tires (sometimes race slicks) a racer has used up. Rather than discard the tire, they sell them to street riders. Take-offs do a great job of impressing your friends by allowing them to assume you were able to thrash the outside edges of your tire with your amazing riding ability. But the reality is you may be riding on a tire that has plenty or tread left but is actually worn out or has very little grip left in it.

The moral of this story is not to be so afraid of riding on tires designed to last a bit longer. Honestly evaluate the type of riding you do and make your decision based on that information. If you mostly drone up and down the interstate on your way to work with an occasional weekend jaunt up the canyon, you probably don't need as much tire as the rider does lots of high-speed track days, and has never worn out the center of a tire. Most of us will never be able to over-ride a modern "sport-touring" tire during very spirited street riding unless we are using incorrect riding form and style. I have seen all of these touring-based tires on racetracks and on colder track days, the touring tires came up to temperature quicker and provided MORE grip than the race tires. So much so, that after a track session, the race tires were, literally, cold to the touch, while the touring tires were nice and warm. Plus, you get the huge benefit of not having to change tires more frequently than you change your oil."

Tire age-
"So how old of a tire should you use? The answer for this is easy. It depends! If you need to get the most out of your tires, then the fresher the better. If you like a tire to last longer, then the age of the tire is less critical. But you should never buy or use a tire that is more than six to seven years old. But how do you know how old your tire is? Carbon dating? Serial number cross-reference with manufacturers? Actually, your tire has the date of manufacture, or the "born on" date. How nice! If you look closely you will find a four-digit number stamped into the sidewall of your tire. It will read something like 4408 or 2101. What do these numbers mean? Well, a 4408 means the tire was manufactured during the 44th week of 2008. 2101 means the tire was manufactured the 21st week of 2001. Years prior to 2000 have a one-digit number to reflect the year, and after 2000, a two-digit number is used. This is a simple and easy way to figure out the age of a tire.

But why is a fresher tire better? Well, first, rubber is organic and it decomposes. Second, after your tire is manufactured, it may sit on the top shelf of a scalding hot warehouse for four years, outgassing all the while. Then it goes to the local shop, where it sits in a south-facing window (outgassing every time the sun sets for another year before you finally buy it). So, you end up with a five-year-old tire that may already be worn out before you've ridden home. Then you'll complain every time the tire slides around underneath you. Not because it is a bad tire, but because it has gotten so old and outgassed so much it has become brittle. If your primary concern is a tire that lasts lots of miles, an older tire will do fine because it has gotten hard with age. But again, rubber is an organic material, and a tire that is seven years old is not as capable as the exact tire that is newer.

When is a tire worn out or in need of replacement? You guessed it . . . it depends! A front tire may be shot while there is still plenty of rubber on the tire. This is why you should always replace your tires in sets. If the rear is down to the wear bars, yet the front tire still has plenty of life, both tires have gone through the same number of heat cycles. The front tire may "look" great, but it is just as worn out as the rear tire. Keep in mind that your front tire is more critical to not falling down than your rear.

You may have a race tire that looks awesome but has gone through so many short heat cycles that it will feel and grip like wood."
 

deeptekkie

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Well I learned some stuff! Thanks! My front appears to be "cupping" some. I knew my suspension was okay, (or at least I thought it was), so I was beginning to worry that something else was up. I guess the tire is just doing what it's supposed to! (Thanks again)
 
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