Roll into bike shop this weekend looking for a new ride and you’ll see not only a huge range of mountainbike types but a selection of wheels sizes too. Whats the deal? Which one will suit you?

It really does seem a lot more complicated than ever before. Many purchases now end up like this though - cameras, phones, cars etc – there’s more brands and choice to confuse you. Having a broad selection to choose from is often a good thing and that’s the best way to look at wheels sizes in the mountainbike world. There are pros and cons to all of them and we'll run through the options.

Not so long back the majority of mountainbikes were rolling on 26 inch wheels – they were there from the very early days and settled into being the main wheel size for production bikes. It only just ended up this way though, because with early 80s experimentation with larger sized rims (700c – now referred to as 29" in the off road world) many saw advantages in these bigger wheels and it was only issues with supply that halted further developments. So 26" became the norm and most riders ended up being happy with them from downhill through to trail bikes, hardtails to dirt jump bikes. Although there was no other real world choice, few riders looked for anything else. They just worked.

Fast forward to 2016 and we are bombarded with choice; 27.5"/650B, 29" and now 'Plus' sized tyres (and in particular 27+/B+) are all options with good old 26" being a rare sight on new bikes.

Here's a run through of the options.

_MG_2792 Transition Smuggler
_MG_2792 Transition Smuggler

29"

Larger 29 inch wheels sat in the background for a long time, with Gary Fisher (now part of Trek bikes) one of the key players in moving this wheel size onto production bikes and gaining awareness of them. They were mainly only XC bikes in the early days – mostly hardtails too – and when full suspension frames were built with 29 inch wheels they were often flexy when pushed hard. Big wheels, with steep geometry and long stems meant that they weren’t initially on the agenda for us here at Dirt. Times have moved on and this week we see the launch of the Trek Slash, a long travel 29 inch wheel enduro machine – a hugely capable bike and an indicator of just how this wheel size has evolved. The Slash sits alongside the Specialized Enduro 29 in the elite club of hard hitting big wheeled machines. There are plenty of 29ers in the 2016 Dirt 100 too - the Canyon Spectral and Transition Smuggler are favourites of ours.

So when would we choose a 29er and why?

The larger 29 inch wheel size initially looks appealing to a taller rider and here at Dirt many of us are around six feet in height so there's no question that a bigger bike will look in proportion with this size wheel and be a good fit for us. This was (and still is) an issue for many, who feel that this wheel size simply 'looks ungainly' and think this will translate into how the bike rides - especially for a shorter rider. Year on year we've seen huge developments to both geometry, sizing and layout to 29ers and now, when equipped with a suitably short stem (not often standard issue on production bikes), these comments are really not valid. Sure, they'll always feels well proportioned for a taller rider but don't discount them if you are of average height.

Dynamically, this wheel size is fast - everywhere. Get on a well dialled, up to date 29er trail bike and you'll be surprised at the rate you can cover ground. The larger diameter wheel rolls well, carries speed to your advantage and gives great traction year round - a huge benefit in a wet climate. With wider rims now available (giving better support to high volume tyres), this added grip and increased rollover can be a huge advantage whether pressing on in fast singletrack, dropping into sketchy off-camber drops or railing corners. Stability is nothing short of exceptional. They just keep on giving.

What are the compromises? What might you not like?

Well, early 29 inch wheels weren't very stiff and not the toughest especially for the harder rider and those of a heavier weight. The bigger wheels are a touch heavier anyway, so bulking them up will take the zip away though. New wider, lighter rim designs, with wider Boost hub spacings (110mm F, 148mm R) builds a stronger, stiffer and more supportive wheel. Arguably they are still not as stiff and tough as a smaller wheel, so for harder enduro racing and high mountain use many will stick to a 27.5" or 26". For DH use, we've seen forward thing brands playing with 29 inch wheels but nothing has yet made production.

Dynamically, you need a few rides before you settle in and adjust to the different timing required with the bigger wheels - we did - read our feature from Feb 2012. You'll find a 29er can carry speed and you can be into the approach of a corner before you're ready, and it's this that often becomes an issue for riders who are making a quick test ride on a big wheeler and then making a judgement call on what wheel size to buy. The extra stability this wheel size gives, along with the increased wheelbase is often translated into lack of agility.

Tyre options were initially a problem, with 29er's history being in the XC world we were seeing fragile fast rolling tyres that weren't man enough for the job in hand if fitted to a capable trail bike. As with wheel construction and frame geometry, time has marched on and now we are seeing plenty of options including 29 inch versions of Dirt 100 favourites such as the Maxxis High Roller.

There are very few who try this wheel size, learn to understand it and then move away from it. For us at Dirt, on most trail bikes up to 150mm of travel, we'll choose a 29er.

_MG_7916 YT Capra
_MG_7916 YT Capra

27.5"/650B

27.5" wheels (or 650B as they are also known) started appearing on production bikes approximately five years ago and looked at first like a niche option that would stay that way. Sitting somewhere between the established 26" and the larger ‘wagon wheeler’ 29ers this size was being quietly pioneered and championed by Kirk Pacenti for many years before that. We’ve now seen this wheel size become common place and knock the 26" wheel almost off the showroom floor. We had our first 27.5" test bike in October 2012.

Are these wheels bang in the middle of the 26" standard and the wagon wheel 29" size? Are they a happy medium? Well nearly but not quite...

Have a look at actual rim diameter (without a tyre) of these three wheel sizes:

26" = 559mm

27.5" = 584mm

29" = 622mm

Why 27.5"? Who are they for and what are they good at?

As you can see, the 27.5" size is just a touch smaller than the mid way point (590.5mm) between the 26" and 29". This gives a look and feel that is nearer to that of the smaller wheel and for a lot of riders a result that is not a world apart from what they're used to.

'I can't get on with 29ers...they don't go around corners' are words often heard - and it's here where the 27.5" scores. It builds a strong wheel, that has some of the increased roll-over and grip that a 29er gives with that nimble and familiar feel of the established 26" size. The dynamics are reassuringly familiar, with little if no adjustment time needed and we think this is what convinces many riders. It's a natural progression - a quick turning, agile wheel size with an added dose of rollover - updated but not controversial.

Tyre choice is huge, with trail, enduro, DH and mud options a plenty. We might be only a few years in with 27.5" but tyre manufacturers have not held back with development.

Is this the best all round wheel size to go for if there's any doubt in your mind? It depends on the bike and the amount of travel really. For harder hitting 160mm+ travel enduro bikes it's a given - this wheel size sits well with geometry and the forks that are currently best in class for this application. The sizing and layout of these longer travel bikes match the dynamics and increased strength that this mid-size wheel offers and the whole package adds up to great results for most riders. The YT Capra, Orange Alpine and Radon Swoop are good 27.5" examples and highly rated by us here at Dirt.

For DH use we're starting to see a move to this larger wheel size on new production bikes and frames but there are plenty who are still on 26" wheels for park and gravity use.

What's not to like with 27.5" wheels?

If you're coming from 26" wheels and not experienced a fast rolling modern 29er then there's very little, if anything, to dislike with the 27.5" size. It feels like a natural progression, a move forward, without compromises.

If you've spent time on 29ers, especially on short to mid travel trail bikes, you'll agree that they result in a very rapid and capable combination. It's on these bikes, with 110mm to 130mm of travel that we feel for all but the shortest of riders that the 29 inch wheel gives a better result. Here, the smaller 27.5" size with the shorter travel may feel 'nimble' and a touch more playful but we feel they offer few other advantages. The smaller wheels may be a touch stiffer and potentially stronger too but for these shorter travel, lighter duty trail bikes the bigger wheels give us a more pleasing result. It's as simple as that...

specialized 6fattie14
specialized 6fattie14

27.5 PLUS/B+

We're not talking 'fat bikes' here... not the 4-5" balloon tyred monsters initially designed for exploring the wastelands and most remote corners of the globe. Those tyres sit (in most cases) on 26" wheels with very wide rims. The 'Plus' sized wheels systems are available in both 27.5" and 29" and are usually running 2.8" to 3.0" tyre widths. We're looking at 27.5 Plus, a size that we've tested recently and although in its early years is really started to show promise for certain applications and riding conditions. We even picked one in our 2016 Dirt 100 selection.

What's the reasoning behind this Plus sizing?

If you take a 27.5" wheel, add some width to the rim (30,35 and 40mm widths being common) and a larger volume 2.8-3.0" tyre, you're not only adding extra width and volume but also a good degree of height too. Enough height that with a 3" tyre the overall diameter of a 27.5 Plus wheel is just shy of a 29er. So, we've got increased volume and a wheel that has a lot more rollover capability, almost that of a 29 inch wheel.

A dedicated frame and fork is needed though - and this is where the new Boost hub spacing standards come into play. With a 110mm hub spacing at the front and 148mm at the rear (initially developed for 29ers, with Trek playing a big part in this), frame and forks can be designed to give the clearances needed for both the larger tyres and compatibility with various chainsets. It's evolved nicely and single ring transmissions work well here.

Why would you go 27Plus?

The large volume tyres offer some advantages - increased footprint, with more traction and stability. Careful pressure set-up is needed (20psi max - and that's for a heavy rider) but the results can give impressive results with these tyres giving great cushioning as they can damp small bump vibration that your bike's suspension may not dealing with, especially if you have your settings on the firmer side. There's less room for error here, so these tyres go hand in hand with a rider keen to get their bike dialled.

These large volume tyres have less aggressive tread than your usual gravity tyres and carry speed well - get them set up tubeless, ditching the 300g inner tubes and reducing rolling resistance. A cushioned ride that soaks up the trail very well.

Surely there's some downsides with 27Plus?

Yes, they're not perfect for every condition. To get these large volume tyres down to a realistic weight the carcass of most Plus tyre choices has been lightened up and therefore compromising both durability and sidewall support. With tyres weighing in around the kilo mark (or more) we need to see tougher tyres to withstand the abuse faster and rougher bike par and enduro tracks can dish out. Trail bike territory in most cases will be fine for all but the hardest riders, but step into nastier terrain and sidewalls will be vulnerable and these tyres don't come cheap; £45 upwards a go in most cases.

Mud and Plus tyres are not always a good mix. On familiar terrain, and with the correct pressure set ups (on both tyres and suspension) you'll learn to work with this tyre size but we need to see proper mud tyre options for Plus tyres as well as more robust casings. No doubt we'll see developments here as this is an area where many riders will have concerns about - especially if it's there only bike.

We're also seeing some variations in rim width and we have settled on 35mm being the choice for us. When running 40mm widths we found the bike 'stands up' in some corners and the 30mm (such as those on the test Stumpjumper 6Fattie) a touch narrow and now on a standard width on may bikes with a 2.3" tyre.

Easton Haven Wheelset
Easton Haven Wheelset

TESTING, TESTING...

If you're coming from years on a 26" wheeled bike there's a strong chance your next bike will have larger wheels, along with improved geometry, better sizing and fewer gears. It's all progress but there's a lot to take in.

We'd suggest you get out and test a few bikes with different wheel sizes before you make a decision - and not just for a quick thirty minute loop. Shops have test days, brands will have demo weekends or you may find a hire centre that has one of your short listed bikes available. Get the set up right, be open minded about your choices but also be realistic about the bike's intended usage and your riding style. There's plenty of choice and we feel that's a good thing.

Read our buyer's guides to buying a TRAIL bikes, ENDURO bikes and DOWNHILL bikes.

Also check out:

Specialized Enduro 29 - Six ways - Three wheel sizes.