I've lost count of how many times I asked my riding buddies what tires they prefer for trail riding in the Big Bear area, where conditions range from asphalt to hard-packed dirt roads to gnarly, rocky trails and sand washes. Answers have ranged from, "get the cheapest knobbies you can find and change them often," to specific brands and models that some riders swear by, and that others swear at. So, once again, rather than relying on people's personal opinions, I've had to do some research to find anything resembling to a consensus.
First of all, I had to make sense of a knobby tire's size markings. There are basically two sizing systems currently in use; inches and metric. Taking a common tire size for a rear wheel with a diameter of 18 inches as an example, a 4.00-18 inch tire is roughly the same as a 100/100-18 metric. A 4.00 tire is 4.00 inches wide from sidewall to sidewall, with a 4.00 inch sidewall height.
In the 100/100 metric size designation, the numerator represents the width of the tire's casing in millimeters, at its widest point. The denominator is what is known as the aspect ratio of a tire. An aspect ratio of 100 means that the height of the tire is 100% of the tire's width. So, in this example, the height of the tire is also 100 mm. Since there are 25.4 millimeters per inch, that means a 4.00-18 tire is 101.6 mm. wide and 101.6 mm. high, close enough to be interchangeable with the 100/100-18 tire. A standard profile tire is one with an aspect ratio of 100.
Things get more complicated when we consider low profile tires, with aspect ratios less than 100. The width of a low profile knobby tire is measured from the outside edges of the knobs at the widest point of its tread. The width of a 120/80-18 tire is 120 mm., and its sidewall height is 80% of that width, or 96 mm. As you can see, this tire's height is about the same as that of the 100/100-18. However, it more is difficult to compare widths because a 120/80-18 tire's width is measured at the knobs, while a 100/100-18 tire's width is measured at the sidewall. The manufacturer's fitment guide should be consulted to find out if a 120/80-18 tire is interchangeable with a 100/100-18. According to both Dunlop and Bridgestone, these two size designations are roughly equivalent.
You may see some tires with inch designations such as X.10 or X.60, which have an 82% aspect ratio, while the X.25, X.50 and X.75 tires have an 85% aspect ratio. These low profile tires are usually designated as X.25/85-18, for example. Only the X.00 tires have a standard 100% aspect ratio, which is not indicated on their sidewall. When choosing knobby tires, try to avoid those with aspect ratios of 70 or less, as they are difficult to interchange and their lower profile makes them more susceptible to pinch flats.
It is possible to fit wider tires than stock in some cases, but that should only be done with the approval of the motorcycle or tire manufacturer. When considering wider tires, you must factor in clearance for width and diameter, the effect on stability and handling, along with whether your rim is wide enough. If wider tires are approved for a motorcycle, it is usually permissible to increase by only one size designation. When fitting a larger tire, always allow for some tire growth from the new to used situation. All tires increase in size after they have been inflated and run for a few hundred miles.
When it comes to choosing a specific tire that offers a reasonable compromise between long wear and good traction in our local conditions, one can go with a D.O.T. approved knobby tire that is designed to hold up well on pavement and hard-pack. In this category, the venerable Dunlop D606 seems to provide better traction than the Pirelli MT21, but it doesn't handle sand well. According to Dirt Bike magazine, the new Pirelli Scorpions are the best D.O.T.-approved knobbies available. Adventure-touring guru Jimmy Lewis recommends the Continental TKC80 as the best compromise for on/off road riding with the heavier rally bikes.
An off-road tire that's on its way to becoming a favorite among our local dual-sport riders, where D.O.T. approval has not yet become an issue, is the Bridgestone M401/M402 combo. This is an intermediate-terrain motocross tire that wears extremely well. It hooks up in sand, and is stable on pavement. The IRC M5B rear is very good in sand, but it lacks stability on hard-pack. The IRC VE32/VE37 combo for intermediate terrain, VE39/VE40 for hard terrain and VE35/VE33 for a wide range of terrain conditions are also good choices. The Pirelli MT16 and Michelin M12 rears offer good off-road performance and wear. Since front tires don't wear as quickly, a less expensive front tire such as the Kenda K772 or Maxxis C6001 may prove adequate.
There are, of course, other things to take into consideration when choosing an off-road tire. Most people run four-ply tires, but some prefer the greater strength and resistance to flats offered by a six-ply. Keep in mind that it is a lot harder to change a six-ply tire. Sidewall stiffness is another factor. A tire with a more flexible sidewall will let you hang it out further on turns. The down-side is that if you get a puncture, it goes really flat and you won't be able to ride very far before you destroy the tire or damage your rim. A tire with a stiff sidewall can be kept on the rim with zip-ties or hose clamps, and be ridden flat for a short distance if necessary. If you'd like to experiment with tires, be aware that most motocross and off-road competition tires, while offering excellent traction, will wear rather quickly. Depending on a variety of other factors, your own mileage may vary.
Links to Motorcycle Tire Web Sites:
Dunlop Bridgestone Pirelli IRC Metzler Michelin Maxxis