Magnetic track! (for race games)

Ok, if anyone knows the xtreme-g games series from nintendo, then u know what i mean!

for all of u who dont, the track has loopings in them and u cen even break at the top, u dont fall off, however, gravity pulls u back down the slope.

i modeled a track, but i cant get the bikes to do the looping!, does anyone has anyidea how i can make the bikes stick to the track, (but so that they can still bounce around, and are affected by the gross- global gravity)?

i tried some things but i cant get the bikes to do the looping!

help me please!