Bicycle tires come in hundreds of size combinations and may be sized using fractions, decimals, French or international sizing. This causes many cyclists to become confused as to which tires will function best on their rims. A tire that is too wide or narrow for your bike will not only reduce the efficiency of your ride, but it can lead to an increased number of flats.
Virtually any tire can be put on any rim with the same bead seat, according to cycling expert Sheldon Brown. The bead seat is the diameter of the inside of the rim where the tire bead creates a seal. It is typically 6 to 8 mm shorter than the rim diameter. Once you calculate the bead seat by measuring the diameter or circumference of the wheel, you can measure the width between the inner most edges of the rim and have a good starting point for what size tires you need.
Bicycle engineer Kraig Willett states that aerodynamics is one of the greatest factors affecting wheel performance. T,ires that are too narrow for your rims may increase the drag on your wheels while you are riding because of the rim lip. Similarly, if the tire is too wide you create more drag because of the extra surface area. Willett recommends purchasing a rim with a deep bead seat and selecting a tire that is exactly as wide as the overall rim to get the most aerodynamic efficiency out of the tire.
Pressure And Rolling Resistance
In simple terms, rolling resistance is the friction generated by the tire as it rolls against the riding surface. When a tire is under inflated, the width will increase significantly and thus create a spike in rolling resistance. This added width can also be a problem if your rims are narrower than tire, as you will increase your chances of getting a pinch flat on the inner tube. Excessively inflating your tire to compensate for it being too wide on the rim is equally detrimental, as you will then be more likely to have a rough ride and receive flat tires from road debris.
As the width of your bicycle rims increase, the values for the largest and smallest acceptable tires for those rims increases disproportionately. For example, if you have a narrow, 13 mm rim, the smallest tire you should use is around 18 mm and the largest is at 25 mm. When you increase the size of the rim to 25 mm, the acceptable tire sizes greatly increase to somewhere between 44 mm and 57 mm.