When the founders of our sport put the first mountain bikes together in the late 1970s, they were often called “klunkers” or “fat-tire” bikes. Back then, most road bikers seemed to turn up their noses at the new fat-tire bikes. Bigger tires were heavier, and that was the worst thing in the world in the eyes of most road bike riders, who would normally do anything they could to make their bikes lighter. Since the bigger tires did offer advantages in the dirt, the added weight of the bigger tires was assumed to be a necessary evil of off-road cycling. Even in the early years of mountain biking, the most highly regarded off-road bikes were often the ones that were the lightest. In general, the more money you spent on an early mountain bike, the less your bike would weigh. In the world of cycling, everyone knows that lighter bikes are easier to pedal uphill, but there’s a new school of thought involved when it comes to tire sizes. Maybe those bigger tires, with their improved ride quality and traction, could actually be faster overall. It’s a question we field here at Mountain Bike Action often: What’s the deal with plus-sized tires—should I be using them? We set out to perform a quasi-scientific experiment to see if the yaysayers or naysayers are right when it comes to riding those big, fat, plus-sized tires.
Over the last 10 years or so, fat bikes have become one of the fastest-growing segments of the mountain bike market. Originally developed for racing in the snow in Alaska’s famous Iditabike event, most of those early fat bikes offered 3- to 4-inch- wide tires. In the last few years, those tire widths have grown to as much as 5 inches. What’s more, in the last six years or so, the bikes have been catching on more and more in the lower 48 states. Riders in Colorado, Wisconsin and Michigan started riding them in the snow, then transitioned to riding them in the dirt as the snow melted, and they liked that too. The riding was still fun no matter what size wheels were on the dirt.
The rising popularity of fat bikes led to a growing acceptance of bigger tires in the mountain bike market. That led to the development of plus-sized tires and bikes. The first plus-sized bikes on the market offered 29+ wheels and tires. The 29+ tires were noticeably bigger than normal 29-inch mountain bike tires and close to 3 inches wide.
While many people think that bicycle tire sizes reflect the wheel’s rim size (as with motorcycles), the bicycle tire size actually represents the outside diameter of the tire when mounted and inflated on the rim. Of course, the measurement is not very precise. It’s more of an approximation. If you pump more air into a tire, its overall diameter will grow a measurable amount, sometimes as much as 1/2-inch or more by the time you put the maximum allowable pressure in the tire. There’s a reason why they call bicycle tires “balloon tires.”
Since fat bike tires have now grown to as much as 5 inches wide, plus-sized bikes offer an appealing compromise. While 5-inch tires might be great in the snow, they are overkill on a normal mountain bike trail. The benefits of increased traction are generally thought to be negated by the increase in weight. Still, a 2.75- to 3-inch-wide, plus-sized tire might offer a nice middle ground between the increased traction of wider treads and the reduced weight of narrower tires.
IS THERE ONE BEST ALL-AROUND TIRE?
To tell the truth, we aren’t sure. Our skinniest, lightest test riders tend to think that plus-sized tires are unnecessarily heavy, while our riders who weigh a bit more tend to like them. We also have plenty of outliers, though. Everyone from novices to professional riders has sung the praises of plus-sized tires. We even saw a few of these things raced at this year’s Downieville Classic, an enduro race that’s grueling and super competitive.
If you switch from regular tires to plus-sized tires, you can run lower tire pressures and get the same amount of pinch-flat protection you’d get with normal tires at higher pressures. The plus-sized tires generally offer a smoother ride than normal tires, which helps explain why so many riders like them once they try them. That lower pressure delivers a more supple ride quality with improved traction, which helps explain why so many riders are intrigued by them.
A DECORATED PRO WEIGHS IN—ARE FAT TIRES FASTER?
We contacted Ned Overend at Specialized to see what he thought about the potential advantages of the wider treads. Ned tests both tires and bikes for Specialized. Ned was the men’s 1990 cross-country mountain bike world champion, as well as the USA’s 2015 fat bike national champion. If anyone would know about something like this, he would. Ned said that the testers at Specialized had found that tires with lower pressures actually roll more efficiently over bumps and rough terrain than tires with higher pressures. It was something Specialized had discovered when testing rolling resistance in its lab. The higher-pressure tires would bounce up and back when hitting bumps, while the tires with lower pressures would wrap around the bumps and roll over them. As a result, the softer tires actually offered less rolling resistance when going over bumps, according to lab measurements.
That got us thinking: was it possible that fat bike tires were actually faster than regular tires on mountain bike trails? We decided to do some tests and see what we could find out.
FIRST TEST THE MATCH-UP:
To get an idea of what kind of differences we might be looking at, we had our editors do timed runs up and down a short but technical trail we love to do hot laps on. It’s a quick but steep climb with a technical descent. In the interest of science, we limited the variables as much as possible. The climbs were timed with breaks between efforts to ensure the riders would be rested. We also instituted a coasting-only policy on the descents to ensure that one of the editors wouldn’t be hot-dogging on the pedals.
Fat bike climb: 30.83 seconds, 30.03 seconds, 28.61 seconds = 89.47 seconds (total)
27.5 Full-suspension bike climb: 26.11 seconds, 29.90 seconds, 27.00 seconds = 83.01 seconds (total)
The 30-pound trail bike with the 27.5×2.35-inch tires won the climbing challenge when compared to the 30-pound fat bike with 26×4.0-inch tires.
Fat bike descent: 13.41 seconds, 14.08 seconds, 13.51 seconds = 40.99 seconds (total)
27.5 Full-suspension bike descent: 14.25 seconds, 13.78 seconds, 13.93 seconds = 41.96 seconds (total)
The fat bike narrowly won the descending contest.
We were blown away by the results of this little micro experiment. We were shocked that a hardtail fat bike could actually hang with a dialed full-suspension carbon fiber bike with plenty of travel. We felt obligated to investigate further.
THE CASTAIC LOOP SHOWDOWN
For our next test, we decided to use only one bike—a Pivot Les hardtail with two different wheelsets. One was set up with 27.5+ tires and the other with conventional 29-inch tires. Chris Cocalis of Pivot was kind enough to send us the two wheelsets for our comparison. The Pivot Les was a bike that we had recently tested for the magazine. The bike wasn’t exactly “stock,” though; it had a Lauf fork on the front, which we were also testing this month. One wheelset was built up with 27.5×2.8 tires. The other wheelset had 29×2.2-inch tires. Chris picked out the two wheelsets because they were very close to the same weight. We could test them on the same frame and find out just how quickly our riders could complete the Castaic cross-country course with the two different wheelsets.
For this experiment, we selected a track our riders were very familiar with—a 3.1-mile course with 360 feet of climbing per lap that’s used for the PedalFest race series here in SoCal. We employed three test riders of varying skill sets: an XC specialist, an all-mountain junkie and an enduro national champion. The course was a mix of hardpack climbs and descents peppered with some technical sections. While not the most challenging course, the trail offered us a controlled environment to put the tires through their paces against a stopwatch. Each rider took two laps on the course on the 29×2.2-inch tires, then rested about an hour and took two more laps using the 27.5×2.8-inch tires. We timed every lap and got their comments afterward.
Our XC specialist completed his comparison first. He could do his two laps on the 29er tires in 31 minutes, 45.93 seconds. When he switched to the 27.5+ tires, he took almost 4.5 minutes longer to go the same distance. He had used a heart rate monitor so he could try to keep his heart rate the same in the two runs. As it turns out, the temperature had gone from about 70 degrees during his first run to almost 80 degrees by his next run, so that could have affected things, but he said that he did his best to put out similar efforts in both runs.
Our all-mountain trail rider rode next. He completed his two laps on the 29er tires in a total combined time that was 9.53 seconds slower than our XC guy’s run on the 29-inch tires. The two were closely matched here, but when our all-mountain guy switched to the 27.5+ tires, it took him about 7 seconds longer to do his first lap of the course on the plus-size tires. Unfortunately, he went for a stylish jump in his second lap on the 27.5+ bike wheel setup and crashed hard when one of the grips twisted in his hand as he got airborne. He wasn’t hurt, but his time on that lap was affected so much that we threw it out. In any event, our all-mountain guy’s one good lap time on the 27.5+ tires (15:57.92) was less than 1/4-second off his average time on the 29er tires (15:57.73). For our all-mountain guy, the two tire sizes produced very similar results.
ENDURO RACER’S LAPS
Our enduro specialist rode last and demonstrated why he’s been dominating the PedalFest races’ Pro class. He was nearly 2 minutes per lap faster than our other two riders on the 29×2.2-inch tires, and he was also incredibly consistent. Our enduro specialist did two laps on the 29er wheels, and they were only about 2 seconds different. His first lap was 13 minutes, 56.32 seconds; his second was 13 minutes, 54.41 seconds for a total of 27 minutes, 50.73 seconds on the 29-inch tires. When he had rested up from that effort, he rode the 27.5+ tires on the course, and he was almost exactly 45-seconds-a-lap slower than he was on the 29-inch tires.
Our XC guy had this to say: “The fat tires didn’t feel that much more stable. The weights are very much in the same realm, but when you ride the fatter tires, as soon as you hit a climb, you feel yourself slow down. I would prefer the skinny tires any day.”
Our enduro racer had this to say: “I felt an increase in traction and stability with the fatter tires in the loose corners. The fatter tires worked better in the sandy sections of the course, too. The big tires reduced the chatter of the small bumps pretty substantially. The plus-sized tires made it feel like I was putting out more effort to get equivalent lap times. It felt like it spun out easier on the wider tires. It was a weird concept, and I didn’t quite understand how that could be. I was frustrated that it spun out more easily.”
Our all-mountain guy had the same experience as our enduro racer, with the fatter tires spinning out a little more easily on the climbs than the narrower tires. One of our earlier testers mentioned that the same thing happened to him with the fat bike in our previous test a few days before. None of us were sure what might cause that to happen. Our all-mountain guy mentioned that he had heard from car experts that wider tires will make cars hydroplane more easily than narrower tires, and he wondered if there might be some parallel with wider tires on dirt. In any event, our riders were surprised that they had encountered the spinning-out problem with the wider tires.
Based on testing on our dry, mostly hard-packed cross-country loop, we’d have to say that the 29-inch tires worked better than the 27.5+ tires. It was interesting, though, that our all-mountain rider’s one lap on the plus-sized tires was almost exactly the same as his time on the 29-inch tires, but both the XC specialist’s and enduro racer’s times were noticeably slower on the plus-sized tires. They both said the plus-sized tires felt significantly slower. When we asked them if they might have done better if the plus-sized tires had higher pressures than the 15 psi they were running, they discounted the idea and said that the tires seemed too bouncy at 15 psi, so they wouldn’t want to increase the pressure.
From our limited testing, as imprecise as it was, we would guess that most riders would not improve their times on the hard-packed Castaic cross-country course by switching to 27.5+ tires. How the tires would fare on other courses is anybody’s guess, however. On the right trails, with softer conditions than the dry and dusty hardpack of SoCal, the plus-sized tires might offer more of an advantage and would likely hold their own against the 29ers better, especially considering that the 27.5+ tires did seem to offer a distinct advantage in the loose, sandy sections of our test course. The wider tires also offered an advantage on the descents, according to our test riders, who said they could rail the turns faster on the plus-sized tires, thanks to improved grip in the corners.
A HISTORICAL PERSPECTIVE
It’s a well-known fact that many pros like to run wider tires up front and narrower tires in the rear. It makes sense. If a front tire washes out in a corner, the rider will usually crash. If the rear tire slides out a bit in a corner, a good rider can usually keep going and avoid a crash. In fact, that kind of looseness in the rear end can actually allow a rider to complete a sharper turn.
We probably shouldn’t try to draw too many conclusions from our test. It was far from perfect. The riders’ efforts could easily vary from one run to the next without them realizing it. What’s more, the results were so close in some of our tests that the time differences were very minor.
Most riders will probably need to try out the different tire choices for themselves to find out which one works best on their trails. As more than one rider has pointed out to us in the past, “When somebody starts winning World Cup races on plus-sized tires, we’ll believe they’re faster. Until then, we’ll stick with the ones we have now.”
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