How do dirt bikes and street bikes differ to handle the different riding and terrain types? Of course, there are the obvious things, like suspension height and ground clearance, but also subtle things, like rider position and how you steer. Of course, then there are the bikes that muddy the waters by offering different degrees of both on- and off-road capabilities, like adventure touring bikes, which are some of the most popular recent models.
On the road, you are bound to run across any number of road surfaces, from smooth to rough, but nearly all of them will be paved. Road bike suspensions are tuned to soak up the minor imperfections of the pavement, and can absorb an impact, like a pothole, but suspension travel can be as little as a few inches. Off-roading is much bigger challenge for the suspension, and dirt bikes must absorb a lot of surface irregularities, not to mention jumps and landings, so suspension travel is often twelve inches or more.
Like the bikes they are fitted to, the tires on a bike give you a good idea what they’re meant for. Knobby off-road tires are fitted to dirt bikes to provide traction on loose surfaces, like dirt and gravel. They have large empty areas and big blocks to help claw in mud, sand and dirt. You can feel the squirrelly nature of knobbies on the street the first time you corner and feel them squirm.
Street tires tend to have a smooth, rounded shape, with not much empty space at all in them. In order to deliver more grip, they put a maximum amount of rubber in contact with the road. The tread pattern is there to channel water away from the tire, but it is next to useless in sand, snow, or mud. Dual sport tires tend to list percentages, like 50/50, or 80/20, indicating how biased they are to on-road or off-road use.
Something else the suspension and tires have to deal with is the force of braking, which will tend to make the front suspension compress. Street bikes tend to move at higher speeds and weigh more, so they need more powerful brakes. Luckily, on road bikes have a lot more traction from their tires so they can transmit more braking force to the pavement. One of the first things you will learn if you venture onto a dirt road with a street bike is how quickly the brakes will lock up the tires on a low traction surface.
Dirt bikes, even ADV tourers, tend to have smaller brake rotors, and the lighter dual sport bikes and off-road only models, may have just a single disc up front. Though the front brakes supply the majority of your braking ability on any bike, when riding off road you will find you you use the rear brake a lot more. In low traction riding, the front wheel may be doing all it can to help the bike change direction and any application of the brakes will cause it to wash out or dig in.
One of the biggest differences between dirt bikes and sport bikes (cruisers are a whole other animal entirely) is the seating position. On a dirt bike you sit upright, with your feet directly below you, because when the going gets extremely rough you will likely be standing up. Adventure bikes, and to a certain degree naked bikes and standards, are the same way and that explains on of the reasons they have become so popular; the seating position is easy on your back and wrists. Sport bikes lean you forward into the wind, and move your feet upward and rearward to facilitate larger lean angles in the corners. This can make them very uncomfortable for long period if you don’t have the wind pushing against your chest – while sitting in traffic for instance.
Cruisers often times put your feet in front of you and up, as if you were sitting back in a comfortable chair. While some people like this position, it can be a literal pain in the rear if your seat is not right or your rear shocks don’t soak up the bumps. Touring bikes tend to be somewhere between a cruiser and a standard, with the rider sitting upright and the feet naturally slightly in front of you with your knees bent naturally.
On pavement, you seldom have to apply very much steering to get the bike to go where you want it. In fact, with sport bikes, you typically can’t turn the bars very far before your hands hit the tank. You only really need large steering inputs at parking lot speeds and while negotiating traffic, and most corners are taken by just leaning the bike into them.
On a dirt bike you also will be leaning, but you’ll likely be sliding and negotiating ruts, and fighting the bars much of the time. For this reason dirt bikes have wide bars for more leverage, and the ability to turn the wheel much further from side to side. There will often be occasions off-road where you will be “crossed up” with the bike sliding in one direction and the front wheel pointed in the other.
Road bikes are geared to cruise effortlessly at freeway speeds (except for the smaller displacements, which may struggle to maintain 75 mph), and can easily hit triple digits with a generous throttle application. Dirt bikes, even the street legal ones, are geared for maximum torque application in lower gears to pull you out of sand and mud, and leap to the top of steep hills. Often the final gearing is the only real change (besides adding lights) that separates a street legal enduro from a pure off-road machine, because they assume you want highway cruising capabilities. Gearing (and weight) is the reason a 250cc single can manage some off-road terrain better than a 1000cc twin with three times the power.
Speaking of weight, there is a reason dirt bikes have traditionally been smaller displacement, single cylinder bikes – weight is your enemy off-road. The current crop of adventure bikes may look like dirt bikes, but in reality, when the going gets tough off-road a 300lb single will often beat a 500lb multi cylinder ADV bike with luggage. A big cruiser or full dress touring bike can often get into trouble just trying to negotiate a muddy, unpaved parking area. Sport bikes are lighter, but they have other disadvantages in the dirt, as mentioned above.
Different Horses for Different Courses
Motorcycles come in all shapes and sizes, each suited for the intended ride. You wouldn’t use a pair of scissors to mow your lawn, and you wouldn’t use a chainsaw for fine wood working, yet scissors, mowers, saws and chisels are all cutting tools. Once upon a time, a motorcycle was a motorcycle, and they were all very similarly suited for all uses. Over time, as bikes advanced they also specialized to do their specific jobs better, and the ones that were left in the middle became known as “standards”. There are few standard motorcycles left these days, but there are sportier cruisers, lighter touring bikes, and street legal dirt bikes that serve well for commuting, canyon carving, and exploring barely there back roads.