most 50x36 cranks are based on a 110 bcd (bolt circle diameter). commonly known as compact cranks. the 36 is mostly used on cross bikes, but also works nicely for road bikes. the full size cranks have a 130 bcd (shimano) and 135 (campy), on the 130, you can put a 38 chainring on and 39 is the smallest on the campy. so the compact allows easier gearing for climbing. but paired with a 13 cog, you will be spinning out at about 35 miles an hour...on decents, you will be dropped...I like an 11 cog with a 50.