Dirt bikes look different than road bikes and there is a fundamental difference how they handle on various surfaces. Obvious differences can clearly be seen such as the ground clearance, and suspension but there are also more subtle variations also.
And to add confusion to all this there are bikes that fit in the middle and are in reality road bikes that you can take off road. True motocross bikes are illegal on the roads and that is the dividing factor between the two. Here are some of the big differences between a street bike and a true dirt bike.
A street bike is predominately for the roads, and can handle well on any paved surface. The suspensions of these bikes are designed to soak up any small imperfections in the road surface for the smoothest ride possible. Most of these types of suspension systems will travel only one or two inches to ease out pot holes and the like. Dirt bike riding is another ball game whatsoever, and off-roading is a big challenge for the suspension of the bike. The suspension system of a dirt bike not only has to iron out the imperfections of the terrain but be able to handle jumps and landings, in these cases the travel of the suspension can be as much as one foot.
It is relatively easy to see the difference in the types of tires both bikes use. Road bikes have to have smooth tires as it is important that as much rubber as possible is in contact with the flat surface. The tread on a street bike’s tires has a clever pattern that is designed to channel water away, but is next to useless on mud, sand, or snow. The knobbly tires that are common to dirt bikes are there to provide as much traction on surfaces that are uneven such as gravel, and dirt. There are large open areas between the rubber blocks that are designed to claw as much grip in mud, sand, and loose dirt. If you ride a dirt bike on tarmac then you can almost hear the tires squeal the first time you try to corner.
There are some tire manufacturers that make dual purpose tires that have an indication on them just how good they are on both flat and uneven surfaces. This is normally listed as a percentage such as 80/20 tires, which are eighty percent good on roads and 20 percent on dirt. These tires are not ideal, if you are riding on the road then use flat tires with good tread design, but off-road then opt for proper dirt-style tires.
It is common for sudden outbursts of rain whilst you are out riding on the roads, and if you do not have the correct tires fitted on your bike you could face serious problems. That is why the F1 cars change their tires immediately if there is a storm, being on 20/80 dirt tires in the rain is a similar experience.