Is it ok to run 21 fronts and 18 rears

 

I don't see why not. The drive wheels dictate rpm's. You'll top put pretty fast with 18" rears.

 

It will change the geometry of the steering! You will be doing something similar to slacking out the head angle of a dirt bike. May or may not effect handling. No sure way to know without trying it.

 

Like stated, it shouldn't change anything but maybe the handling. The 18s will give you good acceleration.

 

You will of course lose some top end speed and ground clearance.

 

steering and all that fun stuff should stay the same like everyone said...ill be running 18s on my mo this season with 20s in the front...mostly fer mx and screwing around on my track but yea that will be fine for a yfz, they mx with them tire setups!!