Undrafted free agent RB Zach Zenner latched on with the Lions.  Zenner hails from South Dakota St. but gained a cult following in the pre-draft process thanks to an impressive combination of production and measurables.  He posted 3 straight 2,000-yard rushing seasons with 58 TDs over that span.  He also caught 77 balls during that 3-year stretch.  Then Zenner checked into the Combine at 6'0, 223 pounds and ran a 4.60-second 40-yard dash with a 41-inch vertical and 121-inch broad.  That gave him the 2nd highest SPARQ score in this year's RB class -- behind new teammate Ameer Abdullah.  Detroit's backfield is currently a bit crowded with the 2 rookies, Joique Bell and Theo Riddick.  But Bell turns 29 in August and Riddick only looks like a pass-catcher.  Zenner is capable of emerging as a "big back" complement to Abdullah down the road.