For the most part, you need to travel to the Sierras or East of Fresno to find trails. Hollister is OK, but if you've been more than three times you have probably run it all.
It depends on how wet it is. If it's a large storm, then the trails are slick as snot. Normal/easy stuff becomes tougher as it's hard to get traction. For example, one V-notch which is easy/med (at the most) when dry is pretty scary when it's muddy and slick.
We went a year or two ago when it was poring, made things a little more tricky!!! You learn a lot about how well your tires clear mud!!! I enjoy Hollister Hills, but mostly for the camping/family feel.
They let both 4x4s and motor cross/trail bikes on the trails in Hollister. The trails are marked according to difficulty (just like ski runs) diamond, double diamond, etc. It has been my understanding that a single 4x4 can not enter and go trail riding without a buddy in another 4x4. Guess this is so you can pull each other out of a ditch should the need arise. Camping is allowed there also. If I am not mistaken there are hundreds of miles of trails in Hollister. Used to go when I had access to a trail bike. It is a blast.
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