not sure with bike-racing apps. but for other racing games, try cro-mag rally, it's still on wheels but with cavemen-age vehicles, and it's currently free.
or try derby quest, it's a fun horse-racing game also for free.
For some friends and me personally it's simply the best (consumer) video games of all times and all hardware and software platform: Asphalt 6: Adrenaline (3D action arcade). Especially the local multiplayer mode. Best cars, tracks, controls, gameplay, graphics, sounds, upgrades, voices… A trippy ground flight with awesome jumps.
Have a short description, some videos and the app store links here in the touchArcade forum (below the list of games).