Though the Lafayette side of Briones is better for training and pain, this Alhambra side is the best all-around mtn. biking Briones has to offer. Half fire roads and half single track, it's full of everything for every level: fast fire roads, snaking, smooth singletrack through forest, and gnarly, sliding, "brake-smoking" technical downhill singletrack.
To get there, take reliez valley to the alhambra staging area (there's a big wood sign) just north of carter acres lane.
The best routes?
For beginner or intermediate newcomers:
stick to the fire roads and you'll have fun. There's a trail map here http://www.ebparks.org/parks/maps you can do lots of different loops, but for first timers, an up and back to the ponds (sticking to the alhambra creek trail) is a good ride. Stay right on your way up when the fire road splits... the map doesn't show it)
For single track lovers:
#1 coming back down the alhambra creek trail, off to your right after the fire road splits (stay right going up, left coming down... the trail map doesn't show it) there's a super fun, smooth, roller coastery singletrack. You have to get off your bike to start it but that's the only time.
#2 once you get to the ponds (after going through the gate at the top of alhambra creek trail) go around to the north side of the pond and you'll see a single track in the weeds leading to the woods to the east. take it my friend. niceness follows. it dumps you on spengler and you have to either bike up and then take briones crest back to the pond and then down alhambra creek OR... go down left, for some darn good technical singletrack that starts off to your left a few hundred yards further down spengler. some log drops, sliding, smooth jumps and then you'll exit onto fireroads that lead out to reliez valley in lafayette and you have to pedal back to the staging area on the road. worth it in my opinion.
For the advanced: dude... want to test yourself? Go north past the pond at the top of alhambra creek to the top of the hill and through the next gate to briones crest (that'll be the second gate of the climb not counting the one at the staging area). hang a left and take the briones crest trail for just a little ways, keeping an eye out for a singletrack off to the left that climbs a little hill to a short fence with a bench. hop it. stay unclipped and drop! you'll start over some rocky niceness and then hit some smooth packed stuff that totally tests your technical abilites and balance. it'll put you onto the singletrack mentioned above about halfway through that ride. either go right to finish the way mentioned to spengler, OR cross it if you want to keep testing your skills. take a left on the fire road when you hit it and it'll lead you back to alhambra valley just in time for that roller coaster singletrack on the right. I ride a hard tail and did it all, but the guys who showed me were all on duals. it takes skills but if you're