I'm a enduro trail-rider and bought recently a Kawasaki KLX250, I can now buy a 18” front wheel. Are there any advantage to running an 18” wheel over a 21” front wheel?

Yes, there advantages of running an 18-inch front wheel, but there are pros and cons to each size.

The Kawasaki KLX250 stock tire was developed for Supercross. It hooks up better on hard-packed dirt, goes across the whoops with less bounce, and provides a more accurate feel under side loads. On the downside, it delivers a much harsher ride and is prone to pinch flatting.

The 18-inch tire absorbs landings and bumps better, puts a wider footprint on the ground, and is less likely to pinch flat. On the downside, it bounces in the whoops and wallows under hard side loads.

The weight difference between the two is negligible when you factor in the rims and spokes, but there is a technology gap, as most of the new tires are designed for larger 19 to 21-inch wheels and then the technology is handed-down to the smaller 18-inch.