Query approved.

one reason can be - that bigger wheels provide more ground clearance, so bigger front wheel, the rear is still smaller because it takes more torque to rotate a bigger dia wheel, so better to stick to smaller rear since it is the driven wheel.

I think the probable reasons could be,..
The front being a larger wheel it can rollover some worst terrains easily & quickly having larger obstacles like stones, bumps, etc..! Also good to steer/swerve as it happens more frequently on a dirt terrain.
Rear smaller is to reduce the weight & hence good power delivery. Majorly it can allow the suspension to travel more.

There are 2 reasons precisely for the bigger front wheel, as far as I perceive:

1. Turning in mud: Dirt bikes have to have quick dirt track dynamics. Which means the front should be planted firmly and the rear should be skid-able on rider's throttle.
On normal vehicles with 17 inches on both wheels, the rider has to lean towards front to ensure that the bike turns in the muddy areas without dis-balancing.

If the rider on a normal streetbike rides hard in mud without leaning forward while steering, he will lost the front or something.. (have experienced it, don't know the exact word, the steering just turns violently and goes under sands, rider gets disbalanced and the bike stops). A bigger front wheel removes this need to lean forward all the time to stay planted.

2. Obstacle Handling: The front wheel handles all the undulations/ obstacles first. And the rule of the book is that the "Wheel Size: obstacle size" ratio should be maximum.

While obstacle size is either random or won't change according to our wish, the bigger wheel helps. That's why scooties (even with telescopic forks) lose direction at the slightest of bumps, and you can't ride them leaving the steering for even 5 seconds or so.


My college's "Gate to the Building" route is a sandy track... And I somehow manage to ride my Pulsar 180 (!) fitted with R15's tyres (!!). I manage around 25-30 kmph around the turns..