Popular mt. bike hang out in burbs of chi-town. Nothing to radical but well maintained and lots of deer. There are small hills to climb and you can get some excellent speeds on the downhills.
There are two loops, one located east of Queinten Rd. and the other is west, the latter is the trail of choice.
There is also a nice paved loop which goes past the model airplane feild.
From downtown Chicago go east on I-90 for 20 miles, exit 53 North (Rolling Meadows) 5 miles to Dundee Rd. exit and go left (west) 3 miles entrance on the left
http://www.singletracks.com/item.php?c=1&i=729