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Rim weight vs Hub weight

Science lightweight hubs
James Witts
11 May 2015

We put the great wheel debate to rest by getting out the science textbooks.

There’s a staggering array of wheels on the market, most of which proclaim to make you faster and more efficient. Many make a big deal about how light they are, but rarely do they explain where on the wheel the majority of the mass is located: the hub or the rim?

That got us thinking. If you had two wheels of the same overall weight and design, but one had a more weight at the hub and the other had more weight at the rim, which would make you faster over the course of an average ride? It’s time to get the old physics textbooks out again.

Let’s start with inertia. The moment of inertia works on the principle that mass further away from the centre of rotation is harder to rotate than mass close to the centre of rotation. In cycling, of course, the former means the rim, the latter the hub. Newton’s Second Law of motion, when related to rotating objects, says α = t/i where α is rotational acceleration, t is net torque and i is moment of inertia. In other words, the higher the inertia, the slower the acceleration for the same torque.

Marco Arkesteijn, sport and exercise scientist at Aberystwyth University, provides another way of understanding inertia: ‘Consider a figure skater rotating on the spot with their arms spread out. They rotate around the mid-point of the body – the part that’s stationary. This is their centre of rotation. By tucking their hands into their body they will increase the speed of rotation. What they are doing is decreasing the mass that is furthest from the centre of rotation, which lowers the inertia. As the energy in the system is constant, it follows that their angular velocity increases.’

In short, it seems lower weight at the rim equals faster accelerations because it requires less energy to reach any given speed. You can see the advantage of this when a puncheur like Philippe Gilbert sprints up a short climb – a scenario that’s been verified out on the road. Mavic’s product manager, Maxime Brunand, conducted an experiment where he added 50g to two sets of wheels – at the rim on one set, at the hub on the other – and had a rider power up to 500 watts on a 10% gradient and see how long it took them to reach 20kmh. ‘The wheel with added rim weight took five times longer to reach 20kmh than when the same rim was used on the flat,’ he says. ‘Using the wheels with weight added at the hub, it took only four times as long to reach its equivalent horizontal speed. Essentially, on hills, inertia becomes more important.’ The French team also observed that on the flat ‘it was easier to maintain speed’ using the wheels with extra weight at the rim.

The flywheel effect

Science lightweight rims

That brings us onto the flywheel effect. Could a heavier rim carry momentum for a rider once up to speed, just as a locomotive wheel carries momentum for a steam engine? Ondrej Sosenka broke the Hour record in 2005, covering 49.7km. The Czech rider was one of the biggest riders to ever race professionally, tipping the scales at 90kg and measuring two metres tall. His record-breaking bike weighed 9.8kg including a 3.2kg rear wheel – the reason being, argued Sosenka, that while a heavier wheel took longer to reach maximum speed, once there it was easier to keep there. On the flipside, Eddy Merckx went to great pains to keep his whole bike as light as possible when racing on the track.

So who’s right? ‘In some cases a slightly heavier wheel at the rim may result in a faster time, such as over a flat course,’ says Paul Lew, director of innovation at Reynolds Wheels. ‘It may also benefit a cyclist who exerts most of their energy on the pedal downstroke. The heavier wheel may help fill in the missing force in the rearward and upward portion of the cadence by carrying the momentum of the wheel through to the downstroke.’

So is it worth Sir Brad loading his rim with lead when attempting the Hour record in June? And what about for normal riders out on the road? ‘While it’s true a slightly heavier wheel can result in a faster time over a completely flat course, this is the exception, not the rule,’ says Lew. On most road rides the added weight will prove an impediment rather than a benefit: ‘Ultimately, the momentum generated by a wheel used for the purpose of creating a flywheel effect to boost velocity comes at a cost to the cyclist. The cyclist will realise less return and benefit than the cost of the effort. The cost will not outweigh the gain.’

What about smaller wheels?

It seems the flywheel effect of heavy rims only pays dividends in very specific circumstances, whereas reducing the weight of rims can have real benefits whenever climbing or accelerating. And because inertia increases the further away the mass is from the hub, is there an argument for us all using smaller 650c wheels instead of the usual 700c? It was a trend seen in triathlon in the early to mid-1990s and research has shown 8% weight-savings using such a wheel.

‘I think any advantage is negated by several factors, including comfort,’ says Arkesteijn. ‘The buzz from the road is more intense because of the smaller radius.’ A 650c wheel also has to rotate more than the 700c (510 rotations per kilometre compared to 475 approx) meaning more friction. ‘A bigger rider will feel that buzz even more intensely because of their added weight,’ continues Arkesteijn. What none of this takes into account, of course, is aerodynamics. A heavier rim may offer advantages if that weight is used to create a better shape to carve through the air. ‘If you’re a big unit and can hold your speed up a hill, a heavier rim is less of a disadvantage to you because you’ll enjoy a greater aerodynamic effect,’ says Jonathan Day of Strada Handbuilt Wheels. ‘However, if you’re a skinny 62kg and you excel at climbing, you want something light at the rim to maximise your speed up the hill.’

Ultimately though, most people don’t have a range of wheels to choose from to suit every type of ride and road condition. So, except in very specific circumstances and perfect conditions, lighter rims are generally better for maximising your speed and enjoyment. And despite the lung-tearing pain and streams of sweat, that’s what cycling is all about.

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