How long do tires last & when to replace them
Tire life is quite variable. On the low end of the spectrum, some street tires can wear out in 1000 miles. On the high end, some people have reported between 12,000 and 15,000 miles. Race tires only last a few races, which can equate to only a few hundred miles.
The main factors involved in tire life are:
Front tires and rear tires also usually last different distances; it is not uncommon for a front tire to last twice as long as a rear.
Your riding style has a large effect on tire life. The more aggressively you ride, the shorter your tire life will be. This includes accelerating, braking, and cornering. Riding in conditions where you have to accelerate and stop a lot will also reduce tire life. Riding on the freeway reduces tire life in another way: they get "squared off," with a flat strip down the center. This makes turning feel weird, although it's rarely dangerous in and of itself.
In general, motorcycle tires can be categorized by the variety of rubber they use. Race tires or very sticky street tires use very soft rubber. This allows much greater friction between the tire and the road, but also wears away quickly. Sport-touring tires tend to last a bit longer, at the expense of grip. Commuting tires go further still, with still less grip. Cruiser tires tend to be very long life, but have relatively little grip. The stock tires on the Ninja 250 are generally considered to have poor grip (although not dangerously so) and good life. None of the new tires on the market have dangerously low grip, although the Cheng Shins come close in the rain.
Riding on very smooth pavement vs rough pavement also has an impact on tire life -- the sharper the road, the lower the tire life. Likewise riding in rain vs. dry: wet streets will tend to scrub less tread off the tire.
These rules are not always hard and fast. Some tires have excellent grip and still get very good life. Some tires have a dual compound, with a harder compound in the center (where you spend a lot of time but don't need a lot of grip), and a softer compound on the sides.
In general, it's reasonable to expect between 3000 and 10000 miles out of a decent set of tires. This is a very approximate guideline. It's worth checking your tires every few rides to make sure they're not getting too low. Every tire sold in the US these days has "wear bars" molded in: a raised section inside the tread which will show as a bar of no-tread across the tire when it's time to replace the tire. If your tread wears down even with the wear bars, it's time to mount up replacements.
Wear bars (if you squint your eyes and imagine a real tire) look like this:
Finding them is easy. The indicators are always listed on the side of the tire, usually in a few different spots, with the letters TWI (Tread Wear Indicator), or a small triangle. This tire has the letters TWI:
If you look across the tread, in the grooves, you'll find little blocks of rubber that stand up. These are the indicators. It's how you know that it's time to replace your tires, as running them till they're slick is a Bad Idea.
When these become flush with the rest of the tread, it's time to put new tires on the bike (or car).
For a detailed look at tire wear patterns, see rattlebars.com.
Finally, tires also have an "expiration date," after a fashion. Tires which are more than a few years old (particularly tires exposed to lots of sunlight or ozone) tend to harden and crack, making them much less grippy, and less safe. Tires have a date code molded into the sidewall, which is four numbers long. The first two numbers are the week-number, and the second two are the year-number. So the code 0206 would mean the tire was made in the second week of 2006. If you're looking at a used bike, remember to factor in the cost of tires if they're more than 3 years old.
Rear tires tend to lose their tread in the center, and are fairly even. Some front tires will "cup" or "scallop" well before they're out of tread -- this is when one block of tread becomes sloped compared to a perfect circle, and it can result in the bike feeling very "nervous," particularly in turns. A very badly cupped tread might look like this from the side:
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