Mountain bikers in general are willing to pitch in and have working bees to improve trails. There are, however, a few hoons.
Here is a trail in Sequoia NP that would be a great trail for bikers. After 3400 feet of gain, they deserve the downhill. Even though it is in a NP, Colony Mill Road, if maintained by bikers, would be an asset for hikers, too.
Open this map full screen.