Tire Size Guide

TIRE SIZE GUIDE

Wading Through the Numerals

Peter Egan

Remember when tires had two numbers written on them? One was the rim diameter and the other was the width of the tire, and both were written clearly in inches. A 3.50-19 tire fit on a 19 in. rim and had a tread that was approximately 3.5 in. wide. That was easy. Even a kid buying a bicycle tire at the hardware store could figure it out. But now we have the added complication of aspect ratios, metric measurements, speed ratings and the amazing Alpha system, in which letters are made to stand for numbers, rather than leaving numbers to stand for themselves. If some future archaeologist ever sifts through the remains of Western Civilization, let’s hope he doesn’t run across a tire chart for motorcycles. Banks of computers and conventions of scholars will be stumped. No one will ever figure it out.

Luckily for us, we still have tire companies and motorcycle manufacturers around to explain all their numbers and codes. Some of it makes perfect sense and some of it doesn’t, but at least it’s possible to break down the symbols and find out what they stand for. It also helps to set up a comparison chart, which is what we’ve done here, to group tires of roughly the same size. That way, if you go into the bike shop and find an ML90-18 on the shelf and can’t remember exactly what that means, the chart will remind you it's about the same size as a 3.50-1 8 or a 110/80-1 8. But before going to the comparison chart, a little explanation on the various sizing systems.

In the beginning nearly all motorcycle tires were of generally round cross section; i.e. they were about as tall as they were wide (and if they weren’t no one cared, because you could look at the tire and see what shape it was). There are now called Standard Series tires and have always been measured in inches. A 4.25-18 has a cross section of 4.25 in. (there is often about an eighth inch of slop in these measurements) and fits a 18 in. rim. This simple size designation is still used on many tires.

Later, tire manufacturers began to introduce new, low profile tires. They wanted some way to separate these from the normal round profile tires and came up with sizes listed by aspect ratio. An aspect ratio is nothing more than the numerical relationship between a tire’s width and its height; sidewall to sidewall vs. bead to tread. In other words, a tire with an aspect ratio of 90 (90 Series) is 90 percent as tall as it is wide. Or, put another way, it is 10 percent wider than it is tall. A tire with an aspect ratio of 80 (80 Series) is 80 percent as tall as it is wide.

Unfortunately, the first low profile tires to hit the market, the 82 Series, did not carry an aspect ratio or series number included in their size designations. Instead, they were simply given width designations that hadn’t been used before. You had to look at the number and know it implied a low profile design. Standard size tires went up in quarter inch increments, 2.75, 3.00, 3.25 and so on, whereas the 82 Series tires were given numbers like 3.60, 4.10, 4.60 and 5.10. To confuse matters, tire manufacturers exaggerated the numbers on the 82 Series tires, going on the theory that no one wants to buy a skinny tire. Nice fat tires sell, even if they aren’t, but only sound fat. So a 4.10-18, 82 percent Series, is actually a smaller tire than a Standard Series 3.50-18. It's about the same width, but with a lower profile.

Makes good sense, no? No. You have to know the numbers, measure the tire or consult a chart before buying, or you'll come home with a 4.60-18 that will be dwarfed by the 4.25-18 you took off your bike.

After Standard and 82 percent Series tires, the next size category is Metric. Metric tires are listed by tread width in

millimeters, followed by aspect ratio, followed by rim diameter—in inches. So a 130/90-17 is roughly 130mm wide with an aspect ratio of 90 percent and it fits a 17 in. rim. Metric 80 percent Series tires are also common. A 110/80-18 has a tread width of 110mm, is 80 percent as tall as it is wide and fits an 18 in. rim. Pretty straightforward. >

Tire Size Comparison Chart

Tires Courtesy of Award Motors

The last method of tire measurement is known as the Alpha Numeric system. This uses letters to stand for the type of tire and width, followed by aspect ratio and rim size. For instance, an MH80-18 is broket down as follows. The M stands for motorcycle, the H is a letter code for tread width, in this case 3.00, the 80 means it has an 80 percent aspect ratio and the 18 is rim size. The letter size equivalents for motorcycle tires are as follows:

So an ML80-19 would be a motorcycle tire (M) with a tread width of 3.50 in. (L), an aspect ratio of 80 percent (80) and it would fit a 19 in. rim (19). Again pretty straightforward, as long as you have a chart or a terrific memory to tell you what tread widths the letters stand for.

A couple of added bits of information are sometimes tacked on to the tire size. These are load index and speed rating." The load index tells how much weight the tire can safely carry and is usually included at the end of the tire size numbers. The higher the number, the more weight the tire can carry. The speed rating is represented by one of three letters. S, H or V, and indicates maximum safe speed for that particular tire. The values are:

S =112 mph H = 1 30 mph V = in excess of 1 30 mph.

The speed rating is sometimes added to the tire size, as in a 4.25 V-l 8 or a 100/90 V-19, or it can be included at the end of the designation, as in 4.25-18 V.

With all that in mind, we’ve put together a tire chart to make size selection a little easier next time you go to the motorcycle shop and have to sort through the tire rack looking for a replacement for that vaporized rear. Bear in mind as you read across the chart that these are only rough equivalents, very rough in some« cases, and not perfectly interchangeable sizes. Tires of the same numerical size vary widely among various manufacturers, and few of the tread widths listed will be exactly that wide under careful measurement with outside calipers. If there is any tire clearance problem with fenders on swing arms, better measure both the old and the new tires carefully and determine how much room remains before mounting that fat new tire on the rim. 13