When you ride on gravel, you want the right tires. Whether cruising scenic trails with your road bike or giving your mountain bike a break from mud and rock gardens, gravel trails present different riding conditions than asphalt and concrete or dirt and rocks. Chances are, however, you are not inclined to change your tires every time you change your riding venue. If you have a road bike or hybrid and plan on riding in varied conditions that include gravel, choose tires that give you stability and traction in gravel, but also offer a smooth ride on the road. If you plan to take your mountain bike down gravel trails, choose tires that work in mud, dirt, rocks and gravel.
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Road touring and racing bike tire sizes are designated with two numbers, such as 700 x 23. The first number refers to the diameter of the tire in millimeters. The second is the width of the tire in millimeters. Riders who focus on speed usually prefer thinner tires, ranging from 18 to 23 mm in width. The thin tires are lighter, handle greater air pressure, have less contact with the road and offer less rolling resistance. However, for gravel travel, thinner tires provide less grip, sink more easily in soft spots and are more vulnerable to flats from road hazards. Road bikers who take long trips tend to go with wider tires, in the 25 to 28 mm range, to enhance stability and comfort, while still limiting resistance. If you plan to hit gravel trails, lean toward 28 mm. If you do a lot of riding on gravel trails, go with even wider tires in the 30 to 35 mm range.
Mountain bike tires are measured in inches. A 26 x 2.0 tire is 26 inches in diameter and 2 inches in width. A 29-inch tire ranges in width from 2 to 2.4 inches wide. Cross-country and all-mountain bikers use tires that range from 1.8 to 2.4 inches in width. Downhill and free-ride bikers, who hazard precipitous drops, rock gardens, ravines, steep slopes and variable conditions, use tires that are 2.5 to 3 inches in width. Generally, the wider tires on mountain bikes are effective on gravel.
Mountain bikes are usually equipped with knobby tires, which are well suited for gravel. Road bikes often have smoother tires to reduce resistance and enhance wear. Mountain bike tires designed for muddy conditions have more space between the knobs to prevent mud from tightly packing the treads. For larger gravel, the wider knobs with larger spaces provide traction. However, even smaller tread designs with less space between treads or knobs should provide sufficient grip on large or small gravel, especially with the wide tires and aggressive treads on mountain bike tires. For your road bike, evaluate the relative proportion of time you spend on the road compared to gravel. Treads provide grip on gravel, but unnecessary friction on the road. Combination treads marketed for road and light off-road riding may present a satisfactory compromise, but at least one expert, Sheldon Brown, says combination treads do better off road than on road. The knobs on the sides of combination tread tires provide resistance and significantly slow your road ride.
Mix and Match
For those who plan to ride on and off road, Brown recommends mixing and matching your front and rear tires to create a compromise combination that optimizes road and off-road riding, minimizing sacrifice of road speed. Brown suggests you use a wider front tire with aggressive tread to enhance control and traction on loose surfaces and prevent sinking suddenly into a soft patch. Use a somewhat narrower tire with less aggressive tread on the rear of your bike. This reduces your rolling resistance on the wheel that bears most of your weight when you are on the road, and should not compromise your control on gravel. If you do most of your riding on gravel, go with wider, knobbier tires on front and back.