But someone appears to be sabotaging these trails, to keep mountain bikers from using them.
It could get someone seriously hurt.
The mountain biking community has become increasingly concerned about this safety issue: piles of rocks and large boulders, placed right in the center of trails. One valley rider actually caught a man placing the rocks on a trail and worries that these rock piles are an intentional attempt to harm cyclists who ride at the preserve.
Andrew James has been biking at Phoenix Mountain Preserve since the 80's. But just recently he noticed a hiker on the trail up to something fishy.
"I saw him putting rocks up across the trail and I'm sitting there like this is not right," said Andrew James.
Andrew confronted the man, telling him the rocks could cause a cycling accident. The man's response?
"Well you guys should be more careful," recalled Andrew.
The hiker told Andrew he was a volunteer with the parks department, simply trying to clean up the trail. But as more and more rock piles emerge in washes and crossings, cyclists wonder who else is responsible, and why they're doing it.
"Just like every other day you'll see another line of rocks up, you clear it, and then you'll come back two days later and there will be another line of rocks up," said James.
Andrew was making his way down trial 1-A onto trial 100 when he saw the man starting to put piles of rocks right in the middle of the pathway, and he's not the only cyclist who has seen these rocks in places they're not supposed to be.
"The bottoms of wash crossings is where we saw some big ones around blind corners," said Eric Shunk.
Eric Shunk has been riding out here for 20 years and says the rocks only started piling up last fall. One of Eric's friends crashed after hitting a pile, and the blockades are proving rocky for little riders too.
"Definitely creates a problem with him having to dodge them, and of course if he hits one of them he's gonna go down," said Erik Gonzales.
The cyclists that spoke to FOX 10 say they have no ill will towards those dropping the rocks, they just wish they would stop before someone has a serious accident.
"Somebody out there has bad intents for us as far as riding goes; I don't know if this guy, in particular, is the first guy I've actually seen do it," said James.
FOX 10 reached out to the City of Phoenix and the Ranger's Office about the rock piles, but there has been no response. This evening someone has started posting images of the man Andrew confronted, with statements of ill will.
Andrew says he does not agree with the flyers and hopes his fellow cyclists will leave this issue to the authorities.