What are Freeze/Thaw Rules?There's a season in the Chicago area, typically between mid-November and the end of March, when temperatures will drop below freezing at night and then warm up above freezing during the day. What this means for the trails is when actually frozen they are hard and good to ride. But when the temps go up and the moisture in the ground thaws, the dirt becomes very soft and muddy. This is because the water in the soil expands when frozen and it looses the tightly packed soil. The result is the trails can become very soft and muddy even if there hasn't been much rain or snow fall.
So, to protect the trails from damage, only ride when the ground is frozen hard, and get off once the ground begins to warm up. South facing trails will usually soften up first, and if the sun it out and the temps get into the 30's, you'll probably need to be done by 10AM.
Remember, if you're tires are leaving an imprint, it's too soft to ride.