Supersize Me!

A 29er primer

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.

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