Follow highway 16 Mount Mushmore road out of Rapid city to Hill City.take highway 385 north out of hill city to Deerfield lake.
this is my favorite hot weather trail.Deerfield lake is located about 20 miles from Rapid City and the interstate.It is the highest lake in the hills so when the temp is scorching down on the plains you can come up here in the 80 degree air and ride the trail around the lake for a great and challenging loop.The trail runs thru a forest service campground. it is best ridden counterclock-wise from lake elevation.You are then rewarded with a long and fast downhill to a secluded beach unseable just yards away from the regular beach area.Catch a pail full of crawdads out of the clear lake and have a feast at your campsite.The trail is all singletrack and for more advanced riders.It offers some tight switchbacks to open sections leading thru high country meadows.
http://www.singletracks.com/item.php?c=1&i=450