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So far it’s a solo ride.

Baptism by Fire: Bikepacking on the Mosaic GT-X

Derek Yarra |

Just a few months ago Mosaic came on hot with a new gravel bike model dubbed the GT-X,  along with an epic video of their crew riding them deep in the backcountry on the Alaskan Denali Highway. Coming from racier variants of Mosaic gravel bikes, both Brennan and I were rather curious about these X series bikes and were eager to try them out on the trail. As luck would have it, Aaron and Mark from Mosaic were planning to be out here in California for the Sea Otter trade show, so we thought, why not order up some bikes of our own and invite our Colorado buddies on a bikepacking trip, Marin County-style? A flurry of phone calls and emails later and our frames were in the build queue, and Brennan got to work assembling the most punishing yet stunning two day dirt loop he could think of.

mosaic gtx intro 

What is the GT-X?

Not so long ago, bike brands and builders were racing to get one dedicated gravel bike into their portfolio. But nowadays, it’s commonplace to see specific models for even the most niche of gravel applications, and custom builders like Mosaic have been at the forefront of this development. With their already established line up of the GT-1 “all-road,” GT-1 “45,” and even the cross specific XT-1, the new GT-X expands on that portfolio, further closing the gap between modern gravel and traditional mountain bikes.

X essentially stands for extreme, leveling up from the “45” with extra tire clearance (it’ll fit up to  a 29” x 2.2”), extra accessory mounts (fenders, bottle bosses, cargo racks... really, whatever you want), and extra stability. Looking to ride more extreme trails than you thought possible on a gravel bike? Ready to load up extreme amounts of gear for a few days off the grid? Then the Mosaic GT-X is calling your name.

Like all Mosaic titanium models, the GT-X comes in both 1 and 2 series options. Level 1 including fully customized geometry, butted tubes, and all of their top level finish details. Level 2 frames come at a more accessible price point by using straight gauge tubes, stock geometry, and simpler finishing options. They both however offer the same rugged utility, massive tire clearance and handmade craftsmanship.

mosaic GTX building process

Our Bikes

For the sake of speed with our trip rapidly approaching, our friends at Mosaic were able to build our bikes with some 2 series tubesets they already had in progress. However both Brennan and I upgraded to their new two-tone paint finishes and customized frame geometry. We received the frames just days before we were planning to set off and built them up as quickly as we could.

brennan's GT-X

Brennan opted to build his in a flat-bar configuration with a Sram AXS Eagle drive train and spiced it up with some of his favorite CeramicSpeed bits.

derek's GT-X

I decided to go the drop-bar route and built mine with a Shimano GRX Di2 system. Both of us chose to build the rest with Enve cockpits.  We also both went with Enve’s ultra durable AG25 wheels paired with René Herse Fleecer Ridge 700x55c tires.

Baptism by Fire

After a quick shakedown on some local singletrack, it was time to gear up for our overnight adventure. With Mark and Aaron coming out from Colorado, Brennan was keen to show them the best of what Marin County has to offer, all packed into two days. Unlike the others, this would be my first time doing any bikepacking or touring, so despite being familiar with the routes, I really didn’t know what to expect.

mosaic GT-X bikes lined up


On day one, we met early at AC HQ for some last minute prep. We rolled a couple miles down the road for a hearty breakfast stop before the journey really began. After fueling up, we pointed our loaded bikes towards Mt. Tam to climb the entirety of RailRoad Grade, a nine mile fire road climb with a relatively gentle pitch, but a number of rough and rocky sections. If you remember back to our hill climb challenge earlier this year, I analyzed a good portion of this climb pretty thoroughly, coming to the conclusion that the 42c tires I was running at the time were the golden ticket. By running even bigger tires on the GT-X, the supple rubber soaked up the chunder even further. It’s tough to say if the added weight would be worth it in an all out speed effort, but for an all day adventure, that extra smoothness was much appreciated. 

GT-X day 1 part 1

At the East Peak lookout, we stopped to take in the view and chatted with another bike tourer heading south on a trip from the PNW. We refilled our bottles and zig-zagged our way on a mix of road and dirt to get out to the Bolinas Ridge Trail, a ten mile ridgeline packed with punchy rollers and diverse terrain spanning root covered forest loam to rocky and rutted hard pack. This passage is well known for getting bone-rattling rough at times, but the GT-X took the chunky terrain in stride. The big 55c tires ate up the bumpy sections and the bike felt effortlessly stable, even while fully loaded. 

gtx day 1 part 2

Once we completed the Bolinas Ridge Trail, we spun up the road to our lunch stop in Pt. Reyes Station. Loading up for snacks at the market, we ran into another fellow bike tourist on the tail end of summer long trip. We marveled at how well refined his packing set up was and he shared some stories from his journey roaming the country. All of us come from the racing scene where we’re quite used to building camaraderie with other riders at races and events, but it was great to experience that same kind of connection under a totally different context.

lunch stop

Refueled, we headed back out to finish our journey for the day. As one does, Brennan decided to plan the most grueling sections of the day’s route for the end of the ride. We rolled out to Inverness to take on the dirt route up Mt. Vision via the Perth fire road. It’s a three mile climb and a brutal one at that, often hitting the 18% grade mark. And when riding bikes loaded down with an extra 30lbs of gear it can easily leave you questioning your life decisions. Luckily, the effort is well worth it because on the other side of the climb we were treated to a ripping single track descent. The trail is pretty smooth, well maintained, and we often ride it on road bikes, however riding it on the GT-X was really something else. With all that stability and grip, it was easy to think we were out shredding on mountain bikes! But unlike on a MTB, the next graded fire road traverse and the rollercoaster-like road descent down Limantour road really showcased the extreme versatility of these machines. On tires like the René Herse Fleecer Ridge, that roll nearly as fast on the road as a slick, but also feature a massive contact patch gripping the tarmac, it felt as if we were cornering at speed on a moto. I was thoroughly amazed at how fast and fun this loaded down touring dirt bike railed down the Limantour Road descent!

GT-X day 1 part 4

With the Mt. Vision loop complete, we stopped once more at the Pt. Reyes market to grab supplies for dinner and make our way to the campsite before dark. The only thing standing in our way was another three mile/1,100 ft dirt climb. Exhausted and hungry, we pedaled our way up and over, reaching the top just in time to catch the sun setting over the ocean and with enough time to drop down to the site and set up camp before everything went dark.

gtx day 1 end

Get the route for Day One →

GTX day 1 route

Day Two

There really is nothing like waking up on a half deflated air mat after an 8,300 ft day of climbing to get you motivated to do it all over again, especially when you’re faced with a three mile wall of a climb before you can get to your first cup of coffee. But as the sun rose on day two, we packed up our gear and took on the burly climb to get over to town for coffee and breakfast. Once again we stopped back into Pt. Reyes Station, this time for coffee and pastries from the legendary Bovine Bakery before making our way back South.

 gtx day 2 pt 1

We spun for a few miles on the road into the Samuel P Taylor State Park before making our next big ascent of the day—the steep and loose trail up to San Geronimo Ridge from the Ink Wells, climbing ~1000ft in just two miles. If you rode our 2020 OSBR Ride, then you might remember this as an incredibly fun and ripping descent halfway through the ride. Taking it in reverse though is far from the same experience. After finally getting to the top, we traversed the rocky San Geronimo Ridge. This zone is exactly where a bike like the GT-X shines. A series of rolling pitches littered in rocks, that puts your strength, skill, and line choice to the test. Maybe people prefer this section on a mountain bike, but with mountain bike-sized tires, our adventurous gravel bikes were more than up for the challenge. Navigating our way through the chunky ridgeline, we dropped down into the more tame fire roads of the watershed territory, eventually making our way back into civilization for lunch.

 gtx day 2 pt 2

After stuffing our faces at a local burger joint, we headed straight back up yet another steep climb. This time on pavement, up Goodhill Road, a challenging pitch, but well worth it to get up to the beautiful (and surprisingly flat) dirt stretches of Crown and South Marin Fire Roads. This is a truly pleasant four mile stretch of dirt that connects to the lower slopes of Blithedale ridge. Greeted with a view of the San Francisco skyline in the distance, we were in the home stretch of our two-day trek.

 gtx day 2 pt 3

We easily could have just ridden straight back to AC on the road from there, but with our visitors in town, it would have been bad form to pass up showing them our backyard playground in the headlands. Cutting in through Tennessee Valley, we climbed Old Springs, descended down Miwok, and then cruised up Coastal to the Hawk Hill overlook to treat our friends to the iconic view over the San Francisco Bay. Hard to think of a better way to end the trip than soaking up the sunset from up above the Golden Gate Bridge!

gtx day 2 pt 4

Get the route for Day Two →

GTX Day 2 route

So How About That Bike?

So after 137 miles and 14,246 feet in two days carrying 30 pounds of gear, how did I feel about the GT-X? I know a lot of people like to make a point about how they “never had to think about it” being the gold standard approval, but the reality is, while riding the GT-X out on that trip (and on all of my rides since) I thought about it a lot. When climbing, I thought about how effortlessly it gripped on loose and rooty terrain. On rocky and washboardy sections, I thought about how smooth and composed it was. When railing singletrack, I thought about how agile yet in control it was. On the road, I was constantly thinking about how easily it sailed along despite its massive 2.2” tires. And even when I was at my lowest of lows suffering up hills and in the heat, I focused my attention on how solid the bike was performing to take my mind off how poorly my legs were.

 gt-X conclusion

Now, I don’t mean all this to say that this is some magic wonder bike that can easily replace your road, mountain, touring, and/or existing gravel bike. All these thoughts of how great it was riding were prefaced with the caveat for a loaded down adventure bike with massive tires. But, I will stand by just how impressively versatile it is, and ultimately how exceptionally fun it is, especially when pushed to the extreme use cases.

Sure, I wouldn’t reach for my GT-X if I was heading out to race a crit, but it certainly could be an excellent tool for a number of the more rowdy gravel race courses. It wouldn’t be my choice for ripping out full speed hot laps on my favorite downhill mountain bike trails, but at the same time, it could be a lot of fun to test my skills trying to pick my way through those same trails on a rigid, high-posted, drop bar bike. I couldn’t tell you exactly how, but Mosaic really did find a way of nailing some truly impressive performance on a gravel bike that can damn near survive anything. If my house was burning down and could only rescue one bike to ride for the foreseeable future, well, there’s a very good chance my GT-X would be it.

derek and GTX

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