Big wheel, small wheel
Most road bikes have 700c wheels, but would they ride better if the wheels were bigger or smaller?
Cyclists are a picky bunch. We question handlebar width, saddle height, crank length, pedal position and the microscopic composite lay up of our carbon frames. But the central part of the entire miraculous process of our movement seems to go unquestioned: the size of our wheels. We all accept that 700c is just the size that road bike wheels must be, but is it really the best size for performance?
The story of how 700c came to be the standard wheel size is a strange one. Firstly, a 700c wheel has a diameter of 622mm and is so named because, apparently, 622mm was close enough to 700 for the name to stick. The only other pretender to the throne was 650c, which actually boasts a diameter of 571mm.
‘The 650c lived a pretty short life, mainly in the triathlon group,’ says Darren Snyder, product manager for Bontrager wheels. ‘There were multiple reasons why the 650 lost out to the conventional 700.’ In practical terms, it was largely because tyre manufacturers didn’t take an interest, but science was also to blame. ‘The theory was that 650c would accelerate faster.
But there’s a trade off to any gains of that type. The 650c accelerated fast but it had less conserved momentum because it was smaller in mass, meaning it won’t hold speed so well.’ But there’s more to wheel size than that. A small wheel may theoretically have less friction, but will have a harsher relationship with the road. A smaller wheel could also be more aero and stiffer, but will make serious handling and comfort sacrifices in the process.
As Snyder suggests, a bigger wheel has a larger mass, if we assume the material and composition is similar, but more complicated physics also play a part in momentum. Stephen Beck, professor of mechanical engineering at Sheffield University, says, ‘It comes down to “the second moment of area”. That means momentum is influenced by the area of an object as much as by its weight. The second moment is calculated by the area of the rims times the square of the distance from the hub.’ So a larger wheel will have greater momentum even if it’s the same weight as a smaller wheel.
Having established that a bigger wheel will accelerate slower but slow down less, could a different size actually affect a bike’s constant speed? According to Beck, no: ‘Newton’s first law of motion says that if something’s going in a straight line at a constant speed, it doesn’t need any energy putting into it, so a wheel travelling at 40kmh will use the same energy regardless of the number of revolutions per minute, in purely mechanical terms.’ But it does get a
little more complex: ‘Bigger wheels run more smoothly so you get relatively get less friction.’
Friction is another debate altogether. Snyder argues, ‘A larger wheel grips the road with a different contact patch profile – longer and more ovalised – which could actually reduce friction, despite having more contact area. We see this with thin versus fat tyres, where a thinner tyre at higher pressure is bouncing
off the ground and not letting the tyre absorb energy. It’s creating a ground reaction that’s pushing the rider up away from it, versus
letting it roll over and absorb it.’
There’s another reason why larger wheels are smoother over the road. Beck says, ‘As part of my general engineering I’m very interested in non-dimensionalising things. So when you think about the ratio of the size of the wheel to the size of the obstruction [the roughness of the road], as the wheel gets bigger the obstruction appears to get smaller and it’s a question of scale. So clearly the larger the wheel, the smoother the ride.’
So why not make wheels larger still? Well, for one thing there’s the issue of rigidity. A smaller wheel will be stiffer with the same composition because the rim is closer to the hub – a shorter spoke will naturally be stiffer (think of breaking raw spaghetti strands). ‘If you’ve got a bigger wheel it will by necessity
be more flexible if it’s the same cross section and shape as the smaller wheel,’ says Beck.
What’s more, a bigger wheel means a bigger frame. A bigger frame will be less stiff because of the same principles of scale, as well as potentially being less aerodynamic. ‘If you’ve got a bigger wheel on the bike, the rear triangle will need to be bigger, and your front forks will need more rake, so the whole thing has to get bigger. As it gets bigger it becomes more flexible,’ Beck says.
All things considered, in real-world conditions the benefits of the current size of wheel may outweigh the losses in acceleration, but there are situations where a smaller wheel could win out. ‘If you’re looking at something like a track bike there’s no reason why you shouldn’t use smaller wheels, because for a smaller wheel on a flat surface you may get less friction due to the smaller contact surface,’ Beck says. ‘You would also be able to reach a slightly lower aerodynamic position, plus you’d make a marginal gain in the aerodynamics of a slightly smaller wheel. That could be quite interesting…’
For road bikes, however, bigger remains better for comfort and handling. That’s evidenced in the current migration of mountain bike wheels toward the road bike standard of 700c with ‘29 inch’ (in reality 622mm) wheels rather than ’26 inch’ (559mm). But that’s not to say that for the speed freaks
out there a smaller wheel could, in the right circumstances, be quicker.
As with many scientific questions, real life can complicate the issue. Beck says, ‘The problem is that nothing is a simple as it looks. There’s a debate – there are advantages and disadvantages to various things. If you change one thing, unforeseen disadvantages may arise. But the debate is far from closed.’