From I-90 exit 14 south. Turn right on the Colorado Blvd. Proceed one (1) mile toward the town of Spearfish and past K-mart. At the sometimes three way stop at Hwy 14A, turn left and immediately right and park behind the strip mall facing the golf course driving range.
Proceed right, north, out of the parking lot onto Hwy 14A, Spearfish Canyon. It's a 12 mile shot on a slight grade of 1 to 2 % along a serpentine road to Savoy the home of the Spearfish Canyon Lodge and Latchstring Inn. Above Savoy the road narrows, with tighter curves and changing grades. Some tough rollers take you past Elmore and onto Cheyenne Crossing. A good stop for water, snacks or good meal.
From here you can turn around and enjoy the 18 mile grade back to Spearfish.
Conversely you can turn right and ride the long gradual grade on Hwy 85 for 14 miles to O'Neill Pass which is marked with its elevation.