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Thanks to John Summerson for providing much of the content below from his great book The Complete Guide to Climbing by Bike
Colorado is filled with great riding and with its multiple mountains contains plenty of high altitude and solid climbing as well. This month’s ascent certainly qualifies as it is one of the most difficult in the state. Located near Boulder it is a steep monster and can test you.
Total elevation - 2,170 ft Length - 4.5 miles
Average Grade - 9.1% (17%) Rating - 2.40 (cat 1)
Magnolia Road is one of the steepest climbs in Colorado. This road is a tale of variability as 3 extreme sections are divided by quite mellow climbing. Be ready early on as the route is severe immediately through tight turns. The first mile is the steepest and through several big switchbacks. There are good views in spots as well if you can raise your head to see them. You encounter several flats with small descents in the middle of the climb which knock down its average grade. There are two more steep sections over the 2nd half of Magnolia with grade in the inner corners of switchbacks close to 25%. Near the top the slope eases again and the listed climb ends just before the road turns to dirt in a pleasant mountain setting. Due to grade and tight turns its descent is challenging as well.
This road is part of the famous Mike Horgan Hill Climb Race. Learn who Mike Horgan was. The actual race course starts at the Boulder County Justice Center on 6th and Canyon and finishes at the Eldora ski pond. The course usually alternates between the Magnolia version and the Sugarloaf version... The Sugarloaf version finishes at the Nederland School just north of town (1- miles) on the right side.
Directions - From Broadway and Canyon Road in Boulder head west on Canyon for ~5 miles to Magnolia Road (somewhat hidden on left) to begin the climb right after the bridge over the creek.
Life on the Mike Horgan Hill climb race, this year using the Magnolia route