This is one I don't yet understand and part of the overall question.
First, what is off road? Off pavement? Two track? Or literally, just out in a field somewhere. If it's the last thing, where you do go to ride in a field somewhere?
And second, what does light enough to handle mean? Does that mean, can I pick it up by myself if it falls over? Or does weight affect it when in motion as well?
Jarrett,
Anything off-pavement generally tends to be called off-road riding. However, strictly speaking, the term is inaccurate. A road, whether it be dirt or pavement, is still a road and is not "off road" riding.
Dirt riding encompasses everything from a well maintained dirt road to the gnarliest terrain found on "The Last Man Standing" enduro competition. The term "dirt riding" is about as descriptive as the term "streeet riding". Street riding includes everything from freeways, highways, county roads, city streets, and so on. In other words, neither term is particularly descriptive.
With that being said, when riding dirt, lighter is better. It takes physical effort to ride a bike in the dirt and the more extreme the terrain (steep hills, sand, mud, rocks, etc) the more physical effort it takes to ride the bike across that terrain.
As a point of comparison most single purpose dirt bikes (motocross bikes, enduro bikes) weigh in the low 200 lb range. Light enough for a rider to pick up by himself - which is an important consideration when riding in the dirt because everyone falls when riding in the dirt, even the pros. Light enough to enable the average sized rider to handle the bike. Light enough for a rider to recover when the bike gets out of shape. If a 200 lb bike gets sideways while riding you are much more likely to be able to recover and not crash than if a 525 lb bike gets sideways while riding. The heavier the bike the more impotent the rider when the bike gets out of shape.
Dual sport bikes tend to weigh in the 300 lb range (300 - 375 lbs or so). They are bigger and heavier than single purpose dirt bikes for a few reasons, including having street legal bits (battery, headlight, taillight, blinkers, etc), comfort, and to make them more suitable for street riding than single purpose dirt bikes. These bikes are trying to balance weight and capability - they weigh more making them less capable in the dirt but more capable on pavement. They are generally at the max weight one would choose for serious dirt riding. You can probably pick one up by yourself after dropping it in the dirt.
Adventure bikes weigh 400 or more lbs, with most in the 500 lb range. They are much too heavy for any serious dirt work but are fine on easy dirt roads. They are much better on pavement than dirt bikes or dual sport bikes. They give up dirt capability to gain additional street capability and comfort. You might be able to pick one up by yourself once or twice but beyond that you will be exhausted. Despite the manufacturers and media claims of being "go anywhere bikes", they are not a particular good choice for riding anything more challenging than easy dirt roads.
When designing a bike, the manufacturers have to make trade offs. For example, a wider seat is more comfortable but it weighs more and is less suitable for dirt riding where a rider needs to stand and constantly shift his weight back and forth as dictated by the terrain. Bigger engines make more horsepower and torque but they weigh more. Small engines can be made more powerful but the trade off is reliability and maintenance - a motocross bike makes a lot of power compared to the total weight of the engine but it's rebuild schedule is measured in hours. A BMW GS 1200 engine is big, heavy, and powerful with a rebuild schedule measured in 10s of thousands of miles. Want to carry luggage? Then you need a strong enough subframe to handle the weight. Strong subframes add weight to the bike. Want to be able to carry a passenger? You need a seat, footpegs, and a strong enough subframe to hold the weight- all of which add weight. Want cruise control, heated seats, heated grips, traction control, wind protection? All add weight to the bike, making it less capable in the dirt. Want to be able to ride lots of pavement miles? You need a cush hub, which adds additional weight. And so on.
The bottom line is this:
- sub 300 lb for serious dirt riding. More than this is too much.
- 300 to 375 lbs for dual sport riding. Heavier is too much for hard dirt riding and lighter doesn't provide enough pavement capability.
- 400 lb plus for adventure riding. Too heavy for anything but easy dirt. Great pavement bikes.