Question 1

Why are tires sometimes narrower than their listed size?

Accepted Answer

In order to ensure that tires have sufficient frame clearance, tire manufacturers generally prefer to keep production closer to the lower end of the permitted tolerance (+/- 3 mm). Carcass casing materials have become more and more sophisticated over time. That reduces the tire widening after the fitting. Furthermore, inflation pressure also plays a major role. With maximum inflation pressure the tire becomes wider than with low pressure. And a recently fitted tire still widens over time. This can make a difference of 1 to 2 mm. The tire width is measured at the widest point, i.e. outside the lug.

It's Important to note that a tires mounted width is directly affected by the internal rim width of the wheel it is mounted on. A larger internal rim width will yield a flatter cross-sectional tire profile. Conversely, a smaller internal rim width results in a tire that is taller, but less wide. See in the below tire schematics how a 40mm wide tire can be as narrow as 39mm on a 19mm internal width rim, but as wide as 45mm on a 25mm internal width rim: 40mm Tire Schematic on Various Rim Widths When upgrading a tire, one should consider the internal rim width of their wheel as this will dictate if the new tire will be smaller, or larger then the width listed on the sidewall.

Question 2

Where can i find the tire size on my Marathon Plus Wheelchair tire?

Accepted Answer

The tire size for our Marathon Plus Wheelchair tire is embossed on the tread cap and can be located at the transition point between tread cap and sidewall.

Marathon Plus Wheelchair size label

Question 3

What is the difference between 28" and 29" tire sizes?

Accepted Answer

There is no difference. Both tire sizes have the same inner diameter of 622 mm and can therefore be fitted on the same rims. In Europe, 28 inches is a traditional size for touring bicycles. In many countries it is even the most frequent tire size used. In countries outside Europe the rim diameter of 622 mm is only rarely used. Bicycle travelers who want to buy spare tires for a 28-inch touring bicycle anywhere in the world, can tell you a thing or two about it. A few years ago 29 inch tires were introduced as a new wheel size for mountain bikes in the US. The marking was created, as the MTB tires are more voluminous and the outer diameter measures approximately 29 inches. Both indications are, however, very imprecise. A less wide 28 inch tire, e.g. with a tire width of 23 mm, which is usual for a road bike, has in fact only an outer diameter of something more like 26 inches. In the case of a tire width of 40 mm, it is more or less correct that the outer diameter measures 28 inches. In the case of very wide tires with 60 mm or more, the actual outer diameter measures almost 30 inches.

Question 4

When is a tire worn out?

Accepted Answer

When the puncture protection belt or the carcass threads can be seen through the tread the tire has reached its wear limit and must be replaced. As puncture resistance also depends on the thickness of the tread layer, it may be useful to replace the tire sooner. The sidewalls of tires often fail before the tread is worn out. In most cases, this premature wear is due to prolonged use of the tire with insufficient pressure. Checking and re-pumping the inflation pressure at least once a month with a pressure gauge is most important.

For tires with a deep tread pattern that generate grip in loose terrain, like those used on a mountain bike, a worn out tire can lead to loss of grip and safety concerns. In such instances the user may wish to replace their tire even before the puncture protection belt or carcass threads begin to appear.

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