Any time a new standard is introduced in mountain biking, it’s bound to ruffle some feathers, and with the recent introduction of plus-size tires, this larger rubber is shaping up to be mountain biking’s newest divisive topic. Pairing a three-inch-wide tire with a 40mm- to 50mm-wide rim isn’t exactly uncharted territory, seeing as this approach was tried and abandoned by downhillers in the “Y2K” era. But the concept was resurrected by Surly a few years back, and it’s quickly gaining traction. These wider rims and larger tires, which strike a balance between the wheels found on a traditional mountain bike and a true fat bike, have been quickly adopted by a growing number of bike manufacturers, and the circumstances of their arrival demand a discussion about the benefits and limitations of bikes bearing the plus-size moniker.
Plus-sized bikes are being positioned for riders who are focused on riding recreationally, rather than competing, with the possible exception being off-road ultra-endurance racers (think Tour Divide). Unlike a true fat bike, these bikes aren’t designed for snow or sand, rather they’re typically designed for mountain bike trails or mixed-surface touring.
For plenty of riders, the sure-footedness of the larger rubber will be easily appreciated. The added volume allows for lower pressures, which encourages the tire to conform to the terrain. The added cushion afforded by the larger tires goes a long way toward smoothing out trails, without the “underinflated beach ball” feel of a true fat bike. In situations where grip is hard to find, be it mud, sand, or cambers, the bigger tires minimize the amount of body English necessary to keep the bike on a line. The upshot is that those whose skills aren’t razor-sharp will likely have an easier time cleaning technical sections of trail.
There’s no getting around it—the plus-size platform is heavy. The very lightest 27 Plus tires are about 900g, with constructions comparable to a lightweight ~500g 27.5 tire of a traditional size. Plus-size tires with reinforced casings can push above the 1200g mark, which makes them heavier than a dual ply downhill tire. That’s important for those of us who are prone to pinch flats, and accordingly, tend to prefer tires with some degree of reinforced casing. Despite the added tire volume, plus-size tires are no more resistant to flatting than traditional options, and additionally, the tire profile can leave the tire’s sidewall more exposed to damage from rocks. Finally, the added tire volume can make the bike feel “bouncy” at higher speeds, and although the phenomenon is not nearly as pronounced as it is on a true fat bike, that “bounce” can make it difficult to hold a precise line on fast, rough trails.
While “plus size” refers to tires nominally three inches wide, the platform is available in multiple wheel diameters. Early trends indicate that 27 Plus options will be available in greater numbers, which is due partially to the fact that most 27 Plus tires have an outside diameter that’s similar to a traditional 29-inch wheel. The upshot is that they’re easier to adapt to existing platforms, with some riders already converting their 29ers to 27 Plus (many, but not all, 29-inch bikes are 27 Plus-compatible). Additionally, they promise an acceptable fit for shorter riders, and free up more room for incorporation into full suspension frame designs. 29 Plus wheels are typically over 30″ in diameter, meaning that they’ll likely remain limited to use on hardtails, and will be more popular with taller riders.
Through a fortuitous series of events, I spent the first half of the past season aboard a Surly ECR, a rigid, 29 Plus rig designed for adventure touring. Admittedly, with my background in downhill racing and my greater-than-average number of flats per season, I’m far from the ideal candidate for a plus-size bike, and objectively speaking, I was riding this bike in a way that’s inconsistent with its intended purpose. That said, I had a ton of fun aboard the lumbering beast. Weighing in at over 30 pounds, it’s not a spirited climber, but it has enough gear range to get up pretty much anything, and the surefootedness helped it to make short work of technical sections of trail. As far as rigid bikes go, the weight and stability make it an absolute missile when pointed down a hill, although the lack of suspension and lightweight (~1,000g) tires contributed to an astounding number of pinch flats. But when the tires held air, it was no slower on many trails than my full suspension trail bike, which was a complete surprise.
My time aboard the ECR was also a useful primer for riding a few more mid-fat models throughout the year. I found a few constants across models and wheel sizes, first being that while 15 psi seems to be the sweet spot for traction, I had to run over 20 psi to avoid pinch flatting or burping tubeless tires in corners. In other words, I had to run 30% more pressure than is typically recommended, at which point the tires were simply overinflated and didn’t grip especially well. However, running more pressure did reduce the “bouncy” sensation, and helped all of the bikes hold a line at speed. All of this should be taken with a grain of salt, because as previously mentioned, my riding style is obviously ill-suited to this tire platform. The other constant that I found was that while these bikes will certainly go fast, they feel more at home riding at a moderate pace than they do when trying to get through technical sections as fast as possible.
If you love riding your fat bike on dirt, you absolutely owe it to yourself to try one of these mid-fat machines. They’re also going to be an appealing option for beginners, as well as those who typically ride trails that can be described as slow and technical, rather than wide-open and fast. That said, they’re not for everyone, and while nobody will stop you from racing a plus size bike, I struggle to imagine any situation in which they’d provide a competitive advantage. I’d be shocked if plus size bikes see any high-level success in XC racing, Enduro, or any other widely recognized race format. However, lamenting a plus size bike’s drawbacks for racing is really missing the point—these machines are intended to make riding easier and more fun. Personally, I don’t see the need for a plus size bike in my quiver. Then again, I can see where plenty of riders might find that plus size tires are exactly what they’ve been looking for.