Originally Posted by tedder
There's answering it on face value and reading into the question.
I think you mean "effects".
A larger front wheel helps the bike roll over bumps better and helps offer slower steering that is more conducive to dirt.
Why not increase the size of the rear wheel? Well, it has less impact on steering. Second, it's hard to put a huge wheel under a motorcycle. Also, generally a wide rear tire is desired, where the front is narrower. Note dirt bikes have 18" and 19" rear wheels, where street bikes have 17". So the size does get increased.
Some older motorcycles use/used matching sizes. For instance, look at the Ural, which uses three identically-sized tires. However, modern motorcycles have abandoned the "same size tire" model. I can think of some examples that are close (TU250), but they are retro-style underperforming bikes.
Certainly same-sized-wheel bikes can be used offroad (Versys, street versions of the F650/F800, Tiger 800, a zillion sport bikes that are ridden by poor riders offroad as well as very capable riders). They have limited tire options available and obviously imperfect geometry for offroad. This is especially true as a bike becomes more sport/street oriented- a twitchy bike is no fun offroad.
The mountain bike market has been going to larger wheel sizes too. In their case, the balanced sizes is easier.
Tedder -- you are likely right about answers and reading into a question . . . .still and all, no one actually answered the question at that point, including you ;-}
And thanks for the grammar correction.
If increasing the wheel diameter throws off the geometer of the entire vehicle, as it would, clearly, why not design for same sized wheels, as you state (correctly) has been done. Wheel sizes do not live in a vacuum, in a bike's design.
Again, I'm sure there IS a reason, but I have not seen what the reason is.