So we have a new technological debate in the world of bike racing, this time sparked by a third wheel size being thrown into the ring after Nino Schurter's excellent win at the Pietermaritzburg World Cup race last weekend. There has been fierce debate on wheel sizes for racing and riding alike since Gary Fisher first started pioneering alternative sizes in the 2000s, and if anything the debate appears to be intensifying rather than settling down.
So let's be clear about a few things. Firstly, let's dispense with the impression that "big wheels" are new - they are not. The engineering argument behind the now franky ridiculous-looking "Ordinary" (what most of us think of when we hear the term "penny farthing") was that the larger wheel would provide a smoother ride for the user on the rough and cobbled roads. So big wheels have been used on rough surfaces since the 1860s.
The historical curiousity that resulted in so many modern-era bicycles having 559mm ISO wheels (what we think of as 26") was that the pioneering band who met on Marin Mountain in California in the late 1970s to ride bicycles down offroad dirt trails did so on old Schwinn clunker bikes from the 1930s. These had wide frame clearances to accommodate their larger "ballooner" tyres, and very laid-back frame angles that allowed extra stability when careening downhill on a foolishly unsuitable bike. The tracks that these guys rode down the mountain bore very little resemblance to what we would consider an mtb trail today - they are wide, open double-track with gentle turns and rough terrain. This is what we would consider perfect 29er territory today, where there is little need for acceleration and deceleration, and the larger wheels would smooth out the lumps and bumps a bit, but the opportunity to build a "better" bike was lost in simply having fun!
Cometh the hour, cometh the bike - as the sport became more and more technical, and moved away from the three hour epic hillclimbs and fast descents that were so common during the 90s (see for example http://www.youtube.com/watch?v=QTdMEZkZyTI ), and became more about a shorter 1h45m sprint over much harder, more technically demanding courses (i'm thinking more of this: http://www.youtube.com/watch?v=E1HteyJiOcs ) these 26" wheels suddenly came into their own; they are lighter and have a lower moment of inertia, and so require less force to accelerate compared to their larger-diameter cousins, perfect for twisty, tight fast-slow racing. But of course, no XC race can contain *just* technical features, and so of course there will always be fast sections where a larger wheel would win out.
The point I am trying (and possibly failing) to make, is that in terms of perceived benefits, changing wheel size is always going to be a compromise. If you can imagine the same rider is capable of winning a world cup race, where the differences are smallest between athletes, on any size of bike, it suggest the optimisation curve is very shallow indeed. As a former design engineer, i know what i would have been encouraged to do in this situation. I would have been encouraged to look for quick wins, but not waste time trying to infinitely refine because it makes little difference and costs a lot of time and money. So there are two explanations for the fact that the bike industry has pursued this apparent hiding to nowhere - either because they are led by their representative athletes (like Scott with their 650b prototype for Schurter), and are prepared to sink potentially millions of Euros to help them get the "marginal gains" to win. Or, the more cynical view would be that the industry has isolated a new way of encouraging us to buy a second bike, especially if we are easily-led racers, and with that realisation they are prepared to make an investment.
I am aware that i have been jaded by my experiences of working in industry to some degree, but it seems highly unlikely that manufacturers would expect to see a return of ~1M Euros from having their riders win world cups, it would perhaps improve their selling power a bit, but actually the top-end race models comprise a tiny component of overall sales. And most people don't buy new race bikes all the time. So it makes sense to believe that they think this is a great way to sell us a second race bike - if we all do it, and they're ahead of the curve, that's a lot of revenue!
I am prepared to believe that there are small gains to be made by particular bikes on particular courses, but i would imagine that they pale into insignificance ccompared to the effect of the riders themselves. I don't imagine that if Absalon had been on a 29er at the Dalby World Cup last year he'd have roasted Kulhavy, or even that he'd have finished in a different place in the final results. I can see the sense in bigger wheels for bigger people, but ultimately you should ride what you're comfortable with and what allows you to have a good position. For me, that's a 26er - i'm too short to get the position i like on a 29er, and the way i like to descend wouldn't suit a 9er anyway. The "its always faster" fanboys have missed a crucial point in their testing - the psychological placebo effect, that if you think something will be faster, you will sub-consciously ride to make it so. That doesn't work for me, as a physicist and engineer, i know it's not faster, it's just a compromise which puts a different emphasis on straight line and cornering speed, so i may as well stick with what i like!