Move Over 29er… First Look At 31-Inch ‘Monster’ Mountain Bike Wheels

As the winter snows give way to spring flowers, fat bikes remain relevant tools for ripping singletrack but with limitations due to the weight and low pressure of those big monster tires. A simple but costly modification can turn some fat bikes into summertime trail rippers known as 29+. Our contributor Steve Graepel swapped out his fat tires for 29+ on his Salsa Mukluk, a modification that creates a huge 31-inch outer diameter. This is his experience.

Last year, Surly released the beastly Krampus, a monster trail bike engineered around a 3” tire paired with the brand’s 50mm Rabbit Hole rims. Neither fat nor a 29er, the Krampus was an ode to speed in the golden age of fat. Surly crowned it a “29+” bike.

Keenly interested in riding my Salsa Mukluk into summer, I was already browsing a second set of wheels for my fat bike. I knew it could accommodate 29” (a fat tire sitting on a 26” fat rim reaches 29” size), but I was all-in when Salsa blogged that the Mukluk could fit a 29+.

*While it fits, the crew at Salsa does not endorse pairing of a 29+ wheelset with its Mukluk; the bike’s geometry is not formally designed to match the height offset.

A little haggling and a lot of eBay got me the parts to lace a pair of 29-plus in time for spring. A few hundred miles of riding so far has revealed a lot of smiles.

At 50mm, Surly’s Rabbit Hole rim is barely half the width of many fat rims. But it’s enough to box out Surly’s 3” Knard tire, purchasing some decent real estate on the ground.

Seated on the rim, the Knard tire reaches a monstrous 31 inches in diameter. Mounted to the bike, the wheels are definitely beastly, but not as aggressive looking as a 4” tire. It looks more like a 29er on steroids.

I noticed the weight savings immediately off the stand. On the scales, it cut 2.5lbs. All complete, my bike weighed (still a heavyish) 28lbs — a little less than a stock Krampus.

The wheels cleared all the rear angles, with the tightest fit at the bottom bracket triangle (where the chainstays meet the bottom bracket). I had to slide the alternator dropouts all the way back to accommodate the girth. The rest of the frame had plenty of clearance to run a full mud-pie tire.

I’m running the Mukluk as a 1x, which is a great match with the wheels. There’s plenty of room for the chain to play even in the highest gear. The tires are filled just under 40 pounds (compared to 10psi or so in my fat tires). This stiffens the ride but translates power directly to the ground. I’ve noticed a lot less vertical bounce in the saddle.

My first ride out was up a local test piece, a sandy climb to the Boise ridge appropriately called Hard Guy. The oversized carousel-wheels rolled over obstacles and chewed up the terrain, spitting sand into my gritty smile.

As I approached the crux, I shifted into low gear, sat back in the saddle and watched the bike climb like an angel beneath me. It made short work of a sandy berm that I’ve previously spent an entire season walking.

On the way down, it quickly became apparent the real gain was speed. The bigger wheels and smaller footprint, which are run at higher pressure, yields a lot less resistance. The bike can go from zero to hell-yeah in a few pedal strokes.

All that inertia creates a bike you need to drive, less it drives you. And the wheels don’t take the hits like a fat tire. Its old school, like you’re riding into 1999. End of the day, it was a physical two hours; I felt beat in the arms and upper body.

A pair of wheels is an expensive purchase. It can be more than 1/2 the cost of a bike. Tires, rims, tape, spokes, hubs … cassette …all told, it can be upwards of $1000 for just a decent set.

And it should be noted that Surly’s Knards are the only 3” tires for 29+. They are offered in two versions: 27 and 120tpi (folding). These tires are thinner (and rumored to be more fragile) than most fat tires. When chatting a trip up with a friend who builds wheels for a living, he flipped “I think you should definitely use ‘em … the time you spend fixing flats I can use to shoot more pics!”

I’ve yet to get any flats, but I’d still be cautious if you are venturing off the map and into the sharp end of the void … bring a patch kit or consider a burly traditional 29 tire.

Is this setup a quiver killer? There probably is no such thing in my house. I like bikes too much. As I wrapped this story up, a friend shared a link to Surly’s soon to be released 26+ bike. And the fat wheels keep on turning….

Sean McCoy

Editorial Director Sean McCoy is a life-long outdoorsman who grew up hunting and fishing central Wisconsin forests and lakes. He joined GearJunkie after a 10-year stint as a newspaperman in the Caribbean, where he learned sailing and wooden-boat repair. Based in GearJunkie's Denver office, McCoy is an avid trail runner, camper, hunter, angler, mountain biker, skier, and beer tester.