Yesterday, my brother-in-law and myself were driving our 2000 Polaris Sportsman's and on a stretch where we could go over 35mph, his would start to shimmy in the front end. It wasn't bad, but a definite annoyance. His ATV only has about 180 miles on it and has never been abused. We switched quads to see if I noticed it, and it was definitely present. Any ideas on the cause and/or solution? Thanks!

 

try checking the air pressure in all four tires with a low pressure tire guage. make sure all your lug nuts are tight... and that there isn't any mud stuck in one side of the rim. Also check for water inside the tire. Do you run tire slime in them?..
Usually one of those cures it..
If not, check for a bent wheel, or obvious signs of a defective tire such as a bulge in it.
If none of this works... and since you have access to two machines.. I would suggest swapping the front wheels and tires to the other machine and take it for a ride.. If it STILL does it.. you know your problem lies in the tire/wheels.. If not, you know that you will need to look deeper into the machine itself...