Tire Sizes explained

Tire Sizes explained

When you are looking for a fresh set of rubber for your bike you might be overwhelmed with all the different sizes that are available. When picking a new tire there are several things to consider when selecting the correct tire. Besides the size/dimensions there are a few factors that will affect the performance and safety of your bike. With all those options to choose from it might look like a neverending story. With the help of this overview you will be able to determine the best tire for your bike. To find the the best tire for your bike there are five main aspects that you have to take in consideration; width, aspect ratio, speed rating, tire construction and rim diameter. We will take a loook at each one of them in this article:

WIDTH

Regarding the width of the tire we have to look at the first number or the second letter. These figures represent the nomninal width of the tire. The width is measured in a straight line from the furthest point on one sidewall, across the tread, to the furthest point on the opposite sidewall. The different size numbering systems specify widths in different measurements. You can view a width cross reference table below .

ASPECT RATIO

You use the aspect ratio to indicate a tire’s cross-sectional profile. A smaller number means a lower profile, and the height-to-width ratio is shown as a percent. For example, if the aspect ratio is 90, that means that your tire’s cross-sectional height is 90% of its width. If the tire has the metric, alpha or low-profile inch numbering system, the aspect ratio will appear immediately after the width.

SPEED RATING

A speed rating on a tire tells you the maximum speed at which a tire can be used based on maximum load and inflation pressure. The maximum load and inflation pressure can alos be found on the sidewall of the tire. A speed rating is shown in a letter code, and that is immediately after the width and aspect ratio. It’s part of the three-digit load/speed index. This is on the tire, immediately after the complete size designation. A Z-rated tire has no maximum speed — this rating is for more than 149mph. Check out this table to see what letters correspond with what speed ratings:

Speed Ratings
Code LetterMax. MPHMax. KPH
J62100
K68110
L75120
M81130
N87140
P93150
Q99160
R106170
S112180
T118190
U124200
H130210
V220137220
V230143230
V or V240149240
V250155250
V260161260
W or V270168270
V280174280
V290180290
Y or V300186300
Zabove 149above 240
*Since Z-rated tires have no recognized speed ceiling, the rating simply means higher than 149mph.

TIRE CONSTRUCTION

After the speed rating is the tire construction, which has two options: Belted (B) or Radial (R). “Belted” means fiberglass, Kevlar or aramid fiber belts, which has added strength and load capacity. But, if a tire does not have (R), then it is bias-ply, which means that it has multiple, overlapping rubber plies. These overlapping plies form a thick layer that is sensitive to overheating and is less flexible.

RIM DIAMETER

Rim diameter is the diameter of the rim/wheel on which the tire will be mounted, in inches.

Understanding Tire Sizes

OTHER FACTORS FOR IDENTIFYING MOTORCYCLE TIRE SIZES

WW: White wall tire

TT: Tube-type tire (requires inner air tube)

TL: Tubeless tire

M/C: Motorcycle tire (in other words, for motorcycles only)

NUMBERING SYSTEMS USED IN MOTORCYCLE TIRE SIZING

Let’s check out some examples of how to read motorcycle tire dimensions:

180-55ZR-17

180/55ZR-17

  • 180 = width (millimeters)
  • 55 = aspect ratio (cross-section height is 55% of the width)
  • Z = speed rating (149mph or more)
  • R = radial construction
  • 17 = diameter (inches)

Metric

Metric tires are most commonly used and can work for sport bikes, cruisers, touring bikes and others. If there is no speed rating, the “M” will precede the metric size. After the speed rating, there will be a “B” or “R” to indicate if it is belted or radial tire construction

mt90S-16

MT90S-16

  • M = motorcycle tire
  • T = width
  • 90 = aspect ratio (cross-section height is 90% of width)
  • S = speed rating (112mph)
  • 16 = wheel diameter (inches)
  • [no construction designation after speed rating means that it is bias-ply]

Alpha

The Alpha size designation is most often used for touring motorcycle tires. Even if a tire doesn’t have a“B” designation after the speed rating, it could be belted.

3.25H-19

3.25H-19

  • 3.25 = width
  • H = speed rating
  • 19 = rim diameter (inches)
  • [aspect ratio not included, but is 100% of the width]

Standard Inch

Often, older motorcycle tires have a Standard Inch System. These tires do not have an aspect ratio designation marking. The cross-section tire height is 100% of the width. An inch-sized tire is bias-ply construction and is not belted. However, these are being phased out and are not used on newer motorcycles.

4.25-85H-18

4.25/85H-18

  • 4.25 = width
  • 85 = Aspect Ration (Aspect Ration is 85% of the width)
  • H = Speed Rating
  • 18 = Rim Diameter (inches)

Low Profile Inch

Series Size System when the aspect ratio is unspecified, so it will always be 82% unless it is specifically noted to have an 85% aspect ratio. However, manufacturers are no longer using tires with this size system, so you

TIRE WIDTH CROSS REFERENCE TABLE

Because motorcycle tires are manufactured worldwide, there are variations in size designations. Therefore, the chart below does not display exact interchangeability, but it can give you an idea of how to compare widths and replacement sizes. This table should not be used for tires with aspect ratios lower than 80%.

Permissible Rim WidthsMetricAlphaStandard InchLow Profile Inch
1.60, 1.8570MG2.75
1.60, 1.8580MH3.003.60
1.85, 2.1590MJ3.253.60
1.85, 2.1590ML3.504.10
2.15, 2.50100MM3.754.10
2.15, 2.50, 2.75110MN4.004.60
2.15, 2.50, 2.75110MP4.254.25/85
2.15, 2.50, 2.75120MR4.504.25/85
2.15, 2.50, 2.75120MS4.755.10
2.50, 2.75, 3.00130MT5.005.10
2.75, 3.00, 3.50140MU5.50
3.50, 4.00150MV6.00
4.00, 4.501606.25

Special thanks to https://www.denniskirk.com/

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Rick Cazemier

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