You can bet a hand brake this bike path never would have been allowed in a hardwood hammock. In the Seventies developers dug a large swath of land off the MacArthur Causeway. When their plans for an Epcot-like trade center were abandoned, the State of Florida stepped in and bought the 1043-acre site. Opened to the public in August 1986, the Oleta River State Recreation Area became Florida's largest urban park. Although the land itself was preserved -- a good thing -- much of its native vegetation had been destroyed. A bad thing -- unless you're an Australian pine or a mountain biker. The fast-growing invader tree overtook the cleared berms and gullies left by developers. And mountain bikers scored big with more than eleven miles of narrow trails through which to bump, roll, curve, whip, and make hairpin turns. Here riders can race through dense woods thickly carpeted with pine needles, past stands of pampas grass with their feathery plumes, overlooking mangrove preserves and lagoons. Some purists say the bike trails at Markham Park in Sunrise surpass these, but tack this on to Oleta's allure: a 1000-foot-long sandy beach on Biscayne Bay, several covered picnic pavilions, fourteen primitive cabins, kayaks and canoes for rent, and a couple of miles of paved trails for Rollerbladers and less rugged two-wheelers.