Feature

Supersize Me!

By Dan Joyce, What Mountain Bike

What's the big idea with 29in wheels on mountain bikes? Dan Joyce explains

Why size does matter

Big wheels are big news.

After a slow-burn start, 29ers - mountain bikes with now seem to be on every other stand at bike shows. It's not a sudden freaky-bike fad. Bikes with bigger wheels have simply arrived.

Know this: 26in is not mountain biking's magic number. When the pioneers were building clunkers, the bikes happened to have 26in wheels. That, and the lack of suitable tyres for bigger rims back then, is why we're on 26in wheels today.

In a different world it could have been 700C, the 27in diameter road standard that becomes 28.5in - let's call it 29in! - when you put a mountain bike tyre on it. Or even 650B, which is between 26in and 700C.

Big wheels good

Bigger wheels roll better, even on smooth tarmac, and better still on rough surfaces. A bump hits a larger wheel at a more acute angle so the bigger wheel climbs more easily, converting less forward momentum into upward momentum. Small bumps feel smaller and it takes a big bump to balk the wheel.

Over small bumps, a 29er is more comfortable and carries its speed better. It's like the smoother flow you get riding with suspension versus riding rigid, or with fatter tyres. On climbs, the bigger wheel climbs over roots and rocks rather than being knocked off line.

A bigger wheel is also better on soft surfaces like mud and sand. It doesn't sink in as much so you're not having to churn through as deep a rut to keep going. And the 29er's longer contact patch gives it better traction too, as more tread blocks engage with the surface.

Big wheels bad

A big wheel is heavier because there's more of it - more rim, more tyre, longer spokes. This makes bigger wheels marginally harder to accelerate. A heavier wheel has more inertia, slowing down steering response. A larger wheel also increases trail - the distance between the front wheel's contact patch on the ground and a line to the ground through the bike's steering axis.

Trail is a crucial component of bicycle steering. More trail gives a steadier bike with greater tendency to go in a straight line; less trail gives a bike with a more immediate steering response. You can change trail by altering the head angle (steeper = less trail, shallower = more trail) and/or changing the fork offset (less offset = more trail, more offset = less trail). That's what current 29ers do to stop them handling like barges.

Smaller wheels are stronger than large ones - by about 10 per cent for a 26in wheel over a 29er. They're laterally stiffer too, because of the shorter spokes. You can overcome these differences by having 36 spokes instead of 32, by using a tougher rim, or by using an oversized bolt-through axle.

Function and form

Sticking 29er wheels in a 26in-wheel frame would jack you up in the air another inch and a half. To avoid this, 29ers drop the bottom bracket height relative to the wheel axles, so both it and the saddle are at the same height as on a 26er. To prevent the handlebar being 3in higher, 29ers use a flat or low-rise bar and a shorter head tube.

At the back end, the chain-stay brace may be omitted to tuck the back wheel in closer and stop the wheelbase from getting to long. At the front end, especially on smaller frames, manufacturers need to avoid toe-overlap, which is where your leading foot can hit the tyre during a turn. Just as 24in wheels are a better fit for smaller children than 26in - due to stand-over, steering and toe-overlap issues - so 26in wheels are a better fit for smaller adults. How small?

If you can't fit a small-sized 29er frame - 16in, about 5ft 7in - take that as your cue. The taller you are, the fewer reasons there are not to use a 29er. The proportions will look and more importantly feel right.

Make mine 29

Current 29ers best suit anywhere where there's plenty of cycling in your biking, because you'll go slightly faster or further for the same effort. They're great for trail centres too, but if freeriding, dirt jumping or trials riding are your bag, stick with 26in wheels.

For racing, it can only be a matter of time before 29ers make a big impact in cross-country and enduro events. For general use, it's ultimately down to fashion. But just don't knock 'em until you've had a proper ride on one.

Have you got that in large?

Choice isn't what it is in 26in components, but it's improving.

Wheels: 700C is not new. Any robust touring bike rim will be fine for a 29er, especially if it's 'suitable for tandems'. A wide rim - internal width 19mm or more - better suits fat 29er tyres.

Tyres: Pick from Kenda Nevegal, Schwalbe Little Albert, Bontrager Jones ACX, Panaracer Rampage, WTB ExiWolf, Maxxis Ignitor, Continental Vapor, Halo Choirmaster. And that's just for starters.

Inner tubes: Tubes will stretch a bit. A 26in inner tube will fit in a 29er tyre.

Forks: Most are short travel, as you don't need as much on a big wheel and it's harder to fit it in without jacking your bars. Choose from: RockShox Reba Race, White Brothers Fluid or Magic, Pace RC29 S100 or S80, Fox F29 RLC, or Maverick SC32 or use SUB Anti-Drive.

Drivetrain: The bigger wheel makes every gear ratio about 10 percent higher. Look for bigger rear sprockets - up to 34T.

User Comments

There are 3 comments on this post

Showing 1 - 3 of 3 comments

  • I wish they would standardise on 700c for all adult bikes.

  • I remember a few years ago, MBR did a feature that said 25" wheels would be the best way of compromising between 26" and 24". That should be the future, not 29".

    29" bikes are sluggish up to speed, a pain in rocks and twisty stuff, heavier and less fun. You can't loft them in the air half as easily as a bike with the right sized wheels (wave that photo of Jeff Jones at me all you want, some people can ride owt), and they'll never be as fast on a whole ride as a 26" wheel bike it'd bet.

    In the US, there are a lot of trails that suit them- if I lived there, I'd have one. The woods in New Jersey are ideal for example. However, in the UK, there's only a handful of places where you can get the most out of them. Folk riding them over here are just into the idea of buying a new bike because the bike companies told them to.

  • Allow me to present a dissenting view.

    In a nutshell, the one thing 29er advocates have right is that the wheels will, in theory, roll faster. And if we were talking about smooth tyres on smooth surfaces, they'd be right.

    Unfortunately, they try and have it both ways. "The 29er's longer contact patch gives it better traction too, as more tread blocks engage with the surface." If that's the case, then those same extra tread blocks are going to slow you down.

    That probably explains why the 29in size hasn't taken over the world of cross-country racing. If there were any significant difference in the way the bikes roll, and therefore in their speed and efficiency, riders would be winning world cup races on 29ers.

    29er advocates will doubtless say that the racing world is very conservative and that's true to a point. But there are times when an improvement is unavoidable, and sweeps through the racing world like a tsunami. We saw this in time-trialling in the '80s. Aero bars were such an advantage that as soon as they became legal for road time trials everyone started using them.

    29ers have been legal for cross-country racing for several years now, sine the UCI dropped its silly restriction on mountain bike wheel size. If they offer the performance advantage their advocates claim, they'd have taken over years ago.

    Since the performance advantage appears to be nonexistent, what are we left with? "They fit taller riders better." But that's not an inherent function of the wheel size, it's a function of the way the 29ers must have longer chainstays to fit the wheel in at all.

    That's a good thing. To balance the weight distribution, big frames should be longer in both directions. They're often not, as manufacturers stick with one chainstay length through the range.

    But there's nothing to stop 26-inch bikes having longer stays too, just as you can build a bike with very small wheels to fit an adult (Brompton folding bikes and Alex Moulton's wonderful suspension road bikes come to mind). Fit is about the relative position of saddle, bars, pedals and tyre contact patches, not wheel size.

    We're therefore left with extra weight, less choice of tyres, shorter suspension travel and a bike variety that doesn't fit the 10 percent or so of the male population under 5ft 7in or the 90 percent of women under that height. Given all that downside, it's very hard to see what the fuss is about.

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