Warning: There is more bike-nerd minutia here than most will ever want to know or have use for. Proceed if you must.
The past few years have seen what can only be described as tumult in terms of bicycle wheel and tire size options.
Where 26" was for seeming millennia the undisputed choice, what we eventually realized is that one "choice" was no such thing at all.
Today, adults can readily choose from 26", 27.5", or 29" wheels (properly 559, 584, and 622mm) shod with tires from 2" up to and over 5".
In essence, you can tune the width and length of your contact patch to a very fine degree. It is a good time, a good age, for cyclists.
For the last ~decade my go-to bike has been a 6 x 6" travel sled with slack HTA, steep STA, relatively high BB, and very short chainstays, shod with 29 x 2.5" tires run tubeless on 35mm rims.
Yes, that's a bit of alphabet soup but for those that know what they like and how to get there, what it translates to is "modern trailbike that happened to adapt 29" wheels much sooner than most".
For better than a decade this was *it*:
We collectively arrived at that package as a result of a confluence of factors, chief among them the concurrent arrival of 29 x 2.5" tubeless tires, SRAM's 1x drivetrains, and tapered steerer 150 and 160mm travel forks. Once there it felt like this shining "aha!" moment, where a decade worth of imagineering, prodding, and fiddling all coalesced and somehow came out yet better than we could have hoped.
Yeah. For years and years that was "it".
But change continues to happen whether you're asking for it or not. Surly released the 29+ idea on the wider world, and eventually the wider world responded with B+. Then fat-for-dirt bikes started to appear, and suddenly the waters were muddied and some sifting and settling needed to happen.
It didn't take many test rides to realize that there is emphatically *something* to the idea of wider tires run at lower pressures. You can feel this immediately going from a road bike at ~100psi to an MTB at 25psi. So when I stepped off of my LunchBox at 20psi and onto a plus bike at 10, my eyes lit up and I knew that this was my new path forward. "Doing more with less" became the new mantra, where more refers to the difficulty of the trails I lean toward, and less points squarely at the psi in my tires.
This time last year I convinced Devin Lenz to build a fat tire full suspension bike, partially as a recovery vehicle following a surgery I needed. At that time a 26 x 4" rim and tire made the most sense, and Devin dialed the geometry around that combo. He did, as he always does, an amazing job balancing all of the little details such that it rode like a much lighter, much more nimble bike than appearances or prejudices would lead one to believe.
Coming off a decade on 29 x 2.5's, the shorter, fatter wheels were ultimately nothing but a disappointment. No matter how dialed the geometry on Devin's creation, no matter how good the suspension felt, short wheels and tires simply can't do what taller ones can. Or, as my high school basketball coach phrased it when cutting me from the team, "You have solid skills, but I can't teach height".
So although I rode that bike for several months, I became increasingly frustrated at having to work so hard to get the bike up to speed, or through a rough section, or, most noticeably, up and over ledges, each of which wanted to hook the rear wheel and stop it, stop *us*, in our tracks. The most succinct way I can think of to describe the sensation of riding the 26" version of that bike was "It doesn't have to be this hard".
Clearly there is more to any bike than how tall the tires are. What it boiled down to, for me, was that dialed geometry and suspension kinematics were only 2 pieces of the puzzle, and without optimizing the 3rd -- tire size -- things just weren't coming together.
Next I laced a set of B+ wheels and took them for 2 rides. Two. On the first one I couldn't believe how much *worse* the bike felt than it did with the 26 fat tires on it. I chalked that up to a gravity squall sort of day, took a day off, then went back out for another ride. And it was every bit as bad -- the wheels/tires just kept getting hooked on every ledge along the way, or falling (as Wes Williams loved to say) into 28" holes. Honestly, for the way I like to ride and the places I get to ride, B+ seemed like the worst of all possible worlds.
I know, I know -- I could always go back to 26" x 1.9" if I really wanted to be frustrated.
But I don't! I ride bikes for fun, for joy, and neither 26 x 4" nor B+ were bringing that aspect to my rides. Thus I sold that bike and turned my experimentations back toward 29+.
For better than a year I've intensely experimented with different rim widths, tire sizes, tread patterns, rubber compounds, and tire pressures, not to mention different geometry and travel on the test sleds.
I'd venture a guess that I have more experience on a greater diversity of 29+ configurations than anyone else on the planet at this moment. 4, 5, and 6" of travel on each end, achieved with a plethora of different forks and dampers. 2.8", 2.9", 3.0", and 3.2" tires, run on 30, 35, 40, 45, and 50mm rims at pressures ranging from single digits to the middle teens. It was 29+ that taught me to think of psi in terms of 1/4 pound changes. If you're already in the ballpark, adding or dumping a *whole* pound is excessive.
My conclusion is that there is no more efficient human-powered vehicle than 29+ for converting energy output into forward motion. The rougher the terrain the more this is true. For all-day rides, for bikepacking, for expeditioning, 29+ wins the day, day after day after day.
The catch? Keeping in mind that I am Princess and the Pea incarnate, I found 29+ to be just a bit too tall for my favorite, most ridiculously difficult tech trails. The kinds of trails that are so slow, so tight, so diabolical that a trials rider has a distinct advantage over the rest of us regular schlubs. 29+ just felt a titch cumbersome in those situations.
So where does one go if 29+ is too tall, 26 fat and B+ are too short, and 29" is too harsh?
Straight to the shrink to have my head examined. I *do* keep reminding myself that if I don't like what's in front of me now, I can always go back to 26 x 1.9 for perspective.
Thankfully I haven't had to do that.
Sometime last winter I became aware of 27.5 x 4" rims and tires being made by Bontrager. I sidestepped the knee jerk "another standard" reactions and sourced a set to fiddle with on our expeditionary fatbikes. Jeny and I both rode and loved them on a spring desert tour, and it was there that my mind started racing toward...
Pictured above is the first test mule that Devin sent once I got him to understand what I was after.
What was I after? Goldilocks, of course! I wanted something that wasn't too big, wasn't too small, that felt just right.
Specifically, the 27.5 x 4" wheel/tire combo (think and say "B Fat" for pete's sake) has the same diameter, to the millimeter, of a 29 x 2.5" combo. Which should, in theory, give me the rollover characteristics I'm after without being too tall. But the tires are fat enough that I can run them in the high single digits, giving me the suppleness, comfort, and traction I desire at the same time.
Thus far it certainly seems so.
I rode the bike above all summer long, from low desert chunk to high alpine buff, in configurations ranging from 5 and 5" to 6 and 6", fiddling with angles every way I could.
When I got it to where I was just riding it, day after blissful day, and couldn't think of any further way to improve it, I asked Devin for the 'production version', with a few subtle geometry tweaks.
I am cognizant that needs, desires, abilities, and locations can all change, and with any of them my preferred bike will too. Having said that, and looking forward to where I live and hope to eventually retire, I feel like this is my 'climax vehicle'.
It isn't the lightest, nor fastest, nor most technologically infused bike out there. Keeping in mind that every bike is a compromise, this is the most capable, with fewest drawbacks, of any bike I've yet owned. Combining modern trail geometry (66* HTA, 16.7" CS) with 6 x 6" of supple, supportive, tunable suspension and light, durable, tubeless tires that are the right blend of tall and wide was the path forward.
I would be remiss in not pointing out that Fox built this fork with what can only be described as a freakish amount of tire clearance. Without this fork, and the Bontrager B Fat tires, and Devin Lenz's willingness to see what lies down every intriguing path, I'd still be yammering into my porridge and tugging at my ear hairs trying to figure it out.
I *did* build myself a 29" wheelset for this bike and shod it with a set of light 2.6" tires. Ostensibly these are for high-alpine days where the trail is smooth enough that I can tolerate the relatively high 20-ish psi's that are needed, and am after a lighter, faster, more efficient overall package for that day.
In reality I'll only use that wheelset a handful of times per year, and the rest of the time they'll hang there, representative of a mental bridge that I am as-yet unwilling to cross.
Thanks for checking in. Don't hesitate with questions.