The Lowdown on the Rollers and What You Should Get

Summary

Except maybe tires, nothing affects how your mountain bike moves, maintains momentum, and agility than wheel size. All else being equal, the larger the diameter the more momentum, top end speed, and smoother the ride; the smaller the diameter the more acceleration, agility, and strength.

A Little History of Mountain Bike Wheel Sizes

If you are counting from the first production mountain bikes, the 1979 Lawwill Pro Cruiser or the 1981 Specialized Stumpjumper, for almost 30 years the standard wheel diameter for mountain bikes was 26-inches. There were forays into 24-inches with dirt jumpers, trials bikes, and long travel DH bikes (I owned a mullet 24/26 Big Hit in 2003), but the floodgates opened up around 2010. Manufacturers started experimenting with 29” and 27.5” (also called 650B) wheelsets. Although 26” mountain bike wheels largely disappeared from the mainstream in 2015, there’s been a push/pull since then between 29ers and 27.5 as a standard, with 29ers being in the lead thanks to racing use cases. For trail mountain bikes (the do everything bike), a new buyer can depend on either size being considered a standard, and a safe bet for support for years to come, however for medium and larger frames 29 is the most common.

1. For enduro racing largely the 29er is standard.
2. For downhill racing specific bikes, 27.5 with several 29ers starting to crop up.
3. For XC, 29ers are the only option.
4. For downhill/park bikes meant for pure fun, 27.5 is popular and making a comeback in new models, or a mullet setup: 27.5 in the back, 29er in the front, errr.. a reverse mullet.

You can still find 24/26-inch wheels for dirt jumping, and on junior bikes. Many fatbikes for snow use 26 inch wheels in order to fit 5” treads, but are often swappable for other wheel sizes with narrower tires. Again, race specific fatbikes for groomed courses tend towards the larger 27.5 with 4” tires.

Advantages and Disadvantages of 26, 27.5, and 29ers

As with everything there are tradeoffs, and you will need to decide the type of mountain biking you are interested in doing, and the feel of the ride you enjoy the most. The majority of new mountain bikes that are coming from a bike shop are going to come with 27.5 or 29-inch wheels, except for some specialty use cases.

Rolling Resistance Decreases as Diameter Increases

• Just like with a 4x4 the bigger the overall diameter of the wheel, the lower the angle of the wheel to rolling over obstacles.
• The larger diameter will also “bridge” over depressions in the terrain, like potholes, washboard, dips, holes, and gaps between roots or rocks.
• This can be felt by getting “hung-up” less when going over rock gardens or root clusters.
• Besides maintaining momentum, larger diameters may ease climbing up rougher terrain. This depends on the gear ratio between wheel size and drive train though. Think of climbing up and over obstacles like building a ramp, the longer the ramp the lower the angle and easier the climb. A larger diameter wheel mimics the ramp, decreasing the climbing angle on a root, rock, step up.

Wheel Weight

This one is mostly self-explanatory, bigger wheel = more weight. If you are talking equivalent tire widths, there usually isn’t a lot of difference between sizes, maybe 100-200g for a wheelset, but this is unsprung rolling mass so you’ll notice weight differences more here than on your frame.

I feel the decreased rolling resistance more than makes up for a bit more weight when climbing or descending. Obviously, cross-country racing bikes have long gone the way of 29ers, where every gram counts.

Speed and Momentum are easier to Maintain as Diameter Increases

This was the most notable improvement as wheel diameters increased. If you were riding on a 26” mountain bike with people on larger wheels you will notice how much more you are pedaling than them. In rolling terrain a 29er will tend to just coast down and back up the next rise while you're pedaling your heart out on a 26. The larger diameter gives you a lower rolling resistance with obstacles like rocks and roots, the bigger wheel won’t drop as much between bumps.

On the flip side, it will be harder to accelerate/decelerate those larger diameter wheels. Think as you hit a tight switchback and the sprint out of it. It’s just harder to get a bigger hoop turning than a small one. You probably won’t actively notice it until looking at your times on tighter routes though. At the same time it’s hard to exert brake forces on larger diameters, aka harder to stop. This can be offset with modern 4-piston brakes and bigger disks.

A Note About Gear Ratios

Keep in mind that your wheels/tires are part of your gearing. All else being equal, a larger wheel diameter creates higher geared pedaling and smaller creates lower geared pedaling. Higher will increase your top speed with lower making climbing easier. This is usually slight and can be mitigated by your drivetrain gears of course, but it is something to consider if looking at bike models that allow you to switch wheel sizes (Pivot Switchblade, Salsa Spearfish) or offer different wheel size configurations on the same model (Norco Sight).

I will actually switch to my 27.5"x3” plus wheelset from a 29"x2.4" on days I’m feeling tired and have a lot of climbing planned. My gear ratio is still lower on the smaller diameter (easier pedaling), and my grip is much better for techy climbing sections.

Agility Decreases as Diameter Increases*

Sort of. Large diameter mountain bike wheels aren’t necessarily less agile by themselves, but they have the side effect of lengthening your bike’s wheelbase. The longer the bike the wider it turns. Although longer bikes have become popular, and are faster for straight line, rough routes, they tend slower for tighter terrain. This is because a long bike is slower in tight corners or twisty situations. Some may argue this point, but there’s evidence on the EWS (Enduro World Series) circuit of this. Top riders’ rigs often choose shorter frame sizes for their heights than what bike company marketing departments think we need for speed. EWS routes often have stages where agility is speed. Fortunately, modern frame sizing is more focused on length instead of height, so you can decide if you want long and bomber or short and nimble regardless of wheels. If you like a more playful feel, you can downsize your frame if switching to a 29er from a 27.5.

It also seems harder to “flick” a 29er around on the trail vs a 27.5, but there’s lots of other factors to consider like suspension travel and geometry. Pay attention to the wheelbase and bottom bracket drop. Shorter wheelbases are going to be more agile, bigger BB drops will lower your center of gravity and make corners feel less tipsy. Manufacturers and many riders have trended back to smaller 27.5s for agility and 29ers for all out racing. I would suggest trying on the terrain you enjoy before you buy especially if agility is important to you. The first bike I could hop over or across obstacles and nail tight switchbacks was eye-opening vs my monster truck 29er or DH rig.

Tire Fit Becomes More Restrictive

There are advantages to running a bigger volume tire (i.e. wider). Whether for better loose rock speed/grip/cushion or for snow/sand on a fatbike, you may want to fit a wider tire than what the “bro rider” who only rides tame, buffed-out hardpack. You also may want more clearance for mud if you live in a wetter region. As wheels get bigger it gets harder to fit a bigger tire obviously. Several mountain bike manufacturers have gotten around this by tweaking their frames and embracing wider hub sizing like super-boost (157mm). However, when buying a bike, be aware that you may not be able to fit an aggressive 2.5+ wide tire on some 29ers, especially on any XC oriented frames. This is less common with bikes on the enduro end of the spectrum vs XC, but watch for it if that is important to you. Even on fatbikes not all 27.5 frames are created equal with some popular brands not fitting over a 4” width with a 27.5" wheel on all their models. You'll still need a 26" to fit a 5" wide tire for better flotation.

About Buying Used Mountain Bikes and Wheel Sizes

When buying a new mountain bike, mostly your only choice will be a 27.5 or 29er. When buying used, it will get trickier because you’ll have more options. There are a few cases where a bike with 26 inch wheels will be fine and early 29ers probably should be avoided.

Early 29ers

Personally from my experience and the people I ride with, I wouldn’t touch a 29er that’s older than 2014 for an all around trail bike. I might consider one (and have) for XC riding and as a bikepacking rig, although with caution. Here’s why:

1. Geometry - It took several years for manufacturers to nail the geometry for the bigger wheel size. Your axles are going to be higher with big wheels and to accommodate that your bottom bracket needs to be lower to keep a lower weight position. The bottom bracket is where most of your weight connects and is centered over on the bike when in techy terrain off the saddle. In the saddle there are other geometry considerations that have only been addressed relatively recently ( steeper seat tube angles). These are improvements on all wheel sizes, but 29ers are an awful experience without them. By around 2014 there were several 29ers that didn’t feel like a 26 frame with hula-hoops strapped on. Out of the bigger names, Yeti and Trek really led the pack around 2017-2018 tweaking geos and bringing our current style of big wheel enduro sleds and progressive trail bikes.
2. Wheel Durability - The common hub width when 29ers first started appearing was 135mm. There is just not enough space between hub flanges to create a laterally strong 29er wheel. The 142mm standard came out around 2010, and this was much better, but you probably will want at least to find something with Boost spacing (148mm) which came out shortly after. Even if I bought an older XC bike I would toss the wheels for some modern 30mm wide hoops, and look for frames with the Boost standard width at a minimum. Pre-boost the hubs couldn’t stand up to the torsion the larger diameter leveraged on the spokes. Truing wheels and fixing spokes was a constant experience with the early 29ers vs 26” wheels. Not for the novice bike owner.
3. Frame Stiffness - Directly related to the wheel issue, before 142mm+ hubs and 12mm rear axles the frames were not stiff enough to handle the bigger wheels and give a good ride. The frames were wet noodles in rough terrain or when cornering hard with 29ers. 142mm was barely passable, it wasn’t until boost (148mm) and super-boost (152mm) that frames/hubs were rigid enough to really show what 29ers are capable of in tough terrain.
4. Tire Fitment - Tire width is oddly controversial, and we’ll tackle it in a separate post. However, for the purpose of buying used bikes you should be aware of the limitations. Many older 29ers that are on the 142mm hub width or narrower won’t be able to fit a modern enduro tire/wheel combo. You should still be able to fit up to a 2.3/2.4” wide tire, which is decent but versus a 26 or 27.5 from the same era that can fit a 2.5/2.6 it can be limiting for serious terrain. Older 29er frames often were built around standards that were for 26” wheels. This is also why I made the caveat of the XC/bikepacking use case. Although I prefer 29x2.6” tires for bikepacking, many modern XC bikes still only fit up to a 2.4 at best. So might as well save the $$$ and buy a used XC bike if you are not looking for the latest UCI marketed bike.

Park Riding and Downhill

There are cases where buying used when a 27.5 or even a 26 inch wheel will be fine and early 29ers probably should be avoided entirely. One of these is buying an older used bike for aggressive (most) bike parks, downhill (DH), and certainly for jumping.

Since around 2017 or so, the newer 29er enduro/all-mountain bikes make for great park bikes with strong wheels, stiff frames and slack geos, but their advantages for racing don’t necessarily lend themselves to the best park bikes, especially when considering spending your cash. The smaller diameter wheels are stronger, which is important for the abuse of jumping and all day laps on a ski hill, and can be more agile for stunts in the park. For non-racing the speed of the bigger wheels usually doesn’t matter in a bike park setting. So if you are on a tight budget for a park bike you can save cash by buying a smaller wheeled mountain rig and not fret on what you’re missing out with the latest and greatest.

Only the latest of 29er full-suspension bikes have caught up in suspension travel to their smaller wheeled cousins. The latest 29er Specialized Enduro is still at 170mm of travel versus my 180mm 2014 26er, but that's close enough. You really need to think about what trails you ride the most and the ride feel you enjoy the most and how new of a bike you can afford. Despite marketing efforts a bigger wheel doesn’t make up for suspension travel after a point (about 10-15mm).

Downhill bike designs didn’t embrace bigger wheel sizes until later (thanks largely to Greg Minnaar) , so buying used you’re going to run into more 26ers and 27.5, with many new DH bikes still being in the 27.5 camp over 29ers. For a park I would still choose a 26er and definitely a 27.5 DH rig over a 29er enduro, especially if buying an older used bike. DH to DH, I can’t say, unfortunately I haven’t had the chance to ride a 29er DH bike.

Even if you are buying a brand new park/jib/gravity bike, some companies have moved focus to the smaller 27.5s and longer travel, the Yeti SB165, or the Norco Shore for example.

The Best Mountain Bike/Wheel Size of them All

The best bike or wheel size is the one you already own. The next best one is the one that you’re comfortable paying for. I still have my long travel Specialized Enduro EVO 26er and it still beats my best times on my 27.5 and 29er bikes for many trails in my area. 180mm of coilover travel with a shorter wheelbase still kills it in rocky/switchbacked techy trails, regardless of wheel size. They excel at different things. It wasn’t until the last couple of years that 29ers got the amount of suspension travel my old 26er mountain bike has. If I rode the park a lot more I would still own my weirdo 26”/24” mullet Big Hit DH bike as well. Yes, I enjoy my other bikes, especially for rolling hills, or longer rides on milder terrain, but it’s hard to say they’re worth $4-8K over a bike you already own.