Rode with Paul, Mark (Flint), Brandt, Chris and Doug (from the Sonoran Desert Mountain Bicyclists) about 30 miles NW of Tucson in the Waterman Mountains. These are not high - around 4000' - but have many fast smooth desert trails PLUS a killer climb up and down Waterman Peak. Not all the guys made it to the top but the climb was a new experience for me and was rewarded with great views and a gnarly descent (the surface was too loose to let it rip - more of a survival run, with steep drop-offs alongside the trail in places).
The ride down ...
The famous jumping Cholla cactus ... these things come out of nowhere and are really hard to remove without a comb as they stick to your hand when you pull them out. Yes, it was painful!
This SDMB ride was about 20 miles north of Tucson, near last weeks ride at Golder Ranch. Not much single-track, almost entirely jeep roads with the only technical stuff on the climb and descent over the saddle between peaks. Rode with Dave (ride leader), Dov, Theo, Chris, and Abe. Long grind getting up there but a nice fast descent down on the way back and a flat out blast back to the trailhead
Couldn't find much evidence of mountain biking around Yuma but I did find some good Jeep trails in the Foothills close to our RV Park. Very rough rock here - looked to be volcanic - and got incredible grip.