|
Tech Talk
By
Ron Schmidt
Tired This Winter
So, what do all those letters on the side of your BMW’s tires mean, and why are they important?
You will see that there are lots of information areas on the sidewalls. Some is of little use and some is very important. I’m going to cover most of the data fields that are important.
First, there is the size, type, load and speed rating markings. Unless you have an early airhead, this tire information will be in a mix of metric width, aspect ratio, type of ply construction for the carcass, the diameter in inches, the load capacity and speed rating. It amazes me that we still have metric and inch measurements on the same item, but I guess it just goes to show that some things are slow to change!
For this discussion, we will use a tire that is marked 150/70R17 69H. Breaking it into parts, it reads thusly:
150 is the width of the tire, measured in millimeters. Legally, the tire width can be as much as 3 mm less or 7 mm more than the size molded onto the tire. It is easy to see, then, that one manufacturer could mark their tire a 140 and have it be the same actual size of another brand tire marked as a 150.
70 is the aspect ratio of width to tire height, in percentage of the width. In this case, the above tire will be 70% as tall (from the ground to the rim) as it is wide. So, our example tire’s height would be .70 x 150mm=105mm. The aspect ratio will generally be a larger number (meaning taller) for dual sport bikes and touring bikes than it will typically be for high performance sport bikes. This is because a high sidewall tire will soak up bumps and protect the rim better on a dual sport application, or provide a smoother ride for the touring bikes. The lower aspect ratios (smaller numbers) mean the sidewalls are shorter and stiffer, which gives a more direct feel to the tire and less flex means the tire will not heat up as much in high performance conditions.
R is the type of carcass design. In this case, it is a radial. A “B” here would mean a belted tire. Some manufacturers get a little creative here and you might see multiple letters denoting their home-brewed mixture of radial and belted and even some artistic license to the type of belt used. Almost all current BMW’s use radial tires, or in the case of the heavy LT models we see belted radials to help attend to the extra weight of these bikes.
17 is the diameter of the rim measured in inches at the bead surface where the tire mounts to it. Inches, I have been told, it’s some archaic measurement based on the distance from the King of England’s bedroom to the nearest bar or something like that. If you know a carpenter, ask him. You may get a more accurate explanation.
69 means, amongst other things, the load that the tire is made to carry at its maximum inflation and for extended periods of time at its maximum speed rating. In this case, the tire could carry 717 lbs if inflated to 42 pounds per square inch and ridden at 130 MPH (see speed rating, next paragraph). If I loaded my GS to 717 pounds it would only go 130 mph if it were dropped out of an airplane, but that is another subject altogether! Motorcycle tires are generally rated between 50 and 70. Pat is always trying to keep the length of these articles down, so I’m not including a chart. You can find it at www.metzlermoto.com if you wish.
H is the speed rating. In this case it is 130 mph. Other ratings you might see on a BMW’s tire are V (up to 149 mph) or ZR (over 149 mph). On the very high-end tires, you might see W (over 168 mph). If you have a K1200S, look and see! On R1150GS Adventures, there is a 100 mph sticker on the dash reminding you that the Conti TKC 80’s are only an S rated tire with a 112 mph limit. And, I know how you Adventure riders pay attention to THAT!
Other useful information on the sidewall includes:
Maximum Inflation Pressure: This is the pressure at which the tire will carry the maximum load it was designed for. Adding more pressure will NOT allow it to carry more weight! Also, this is not likely to be the “correct” pressure for your BMW. Refer to your BMW’s owner’s manual for that information and abide by it!
Country of Manufacture: This means the country where the tire was made. Duh. Additionally, there is a code there that will tell you in exactly what manufacturing plant in that country it was made. The list is long!
D.O.T code: This will be a 4-digit code (unless the tire is very old; if your tire has a 3 digit code, run away as it may blow out just sitting there!) The first two digits denote the week number, the last two digits tell the year, beginning with the year 2000. So, if your tire is marked 1205, it was manufactured in the 12th week of 2005. This is important, because ALL tires continue to “cure” or get harder with time. Old tires therefore do not offer the same grip as new ones, even if both tires have zero miles. This is something to consider when you see those amazingly low tire prices from mail-order places. It might mean the tires are old and hard.
Brand and model of tire: Lets you know what kind of tire you are buying. Although, sometimes the model is hard make sense of. My personal favorite GS tire is called a “Distanzia” which sounds like a tire for a touring bike. Another example would be a “Gripster” which is another dual sport tire whose name sounds like it should be on a race track somewhere! Talk to your tire salesperson to get the straight poop (hopefully).
TWI (Tread wear indicator): In several places on the sidewall there will be a mark showing where the tread wear indicators are located. The TWI might be marked with the letters “TWI” or just be an arrow or a triangle. If you follow around from the TWI symbols across the tread, you will see some little bumps that are in the tread grooves. When the tire wears down to where you can see these bumps, the tire is worn to minimum legal limits. Note that the good handling will have disappeared long before these marks show.
Two Words of Wisdom
1) If you get nothing else out of this column, get this: Get a high quality tire pressure gauge and use it often. Look in your BMW’s owner’s manual and inflate your tires to the pressure specified there. There is nothing more dangerous than riding your BMW with under inflated tires. At slight under inflation pressures the tires will wear more rapidly and the motorcycle will handle badly. Grossly under inflated tires can overheat and blow out, which will almost certainly result in a crash.
2) NEVER, NEVER, NEVER (did I mention never?) put Armor All or similar products on the tires. Not even a little bit on the sidewalls. Even just the smallest amount on the sidewalls will eventually creep around on the tread. You may wake up in the hospital wondering why your beautiful BMW slid out and dumped you on the ground when the pavement looked so good. It was because you supplied your own oil slick!
Every year or so I ask you all to give me some ideas about what you might like to read about in the Tech Talk column. Kind of like when the PBS stations ask for donations, I’m looking for help in what to write about! If there is some subject that you would like to read about in future Tech Talk articles, please let me know. E me at ron@motorcyclerandr.com with your requests.
|
