Logic suggests that if you want something to slice through the air quickly, you make it thin and sharp – like Concorde. It was shaped like a dart while other passenger planes were bulbous, and as a result it could cross the Atlantic in under three hours. Much the same thinking went into early generations of deep section aero wheels: thin, deep V-sections tapered to a sharp edge, which gave the impression of slicing through the air with maximum efficiency. It made intuitive sense, but times have changed.
The cutting edge of wheel design now is, well, less cutting and more blunt. Edges have been softened and rims widened to the extent we are now being told that fat, rounded rim profiles are the best wind-cheating shape for all-round performance. So what has happened?
Hed winds
The original proponent of the wider rim shape was Hed Wheels, with its founder, the late Steve Hed, driving much of the thinking in the 1980s. When Hed launched its chubby, wide-profiled aluminium alloy Ardennes wheelset in the mid-2000s, recommending it be paired with 25mm tyres rather than the ubiquitous 23mm, many expressed disbelief that this could be a quicker set-up. At the time, technical details were sketchy. It seemed as though Hed was seeking better tyre stability for more cornering control, along with decreasing the chance of pinch flats on rough terrain, yet the initial research in the 80s had also shown that wider rims could be faster aerodynamically. Then, when the Hed patent was up in 2009, the door was opened for a wave of innovation.
Michael Hall, director of advanced development at wheel manufacturer Zipp, says, ‘For years the industry chased overall drag efficiency [the fastest results in wind-tunnel testing] with little regard to how well those components performed on sub-optimal, real-world days. In the real world, riders have to deal with everything their environment throws at them. For our Firecrest wheels, launched in 2010, we shifted our focus and the resulting products sought to be more stable and predictable in any wind direction than that of our previous generations.’
Kevin Quan, director of engineering at Knight Composites, goes into more detail: ‘The way we design is from the trailing edge, meaning the rear half of the wheel.’ That might sound counterintuitive given it’s the tyre and leading edge of the rim that hits the wind first, but Quan says, ‘Our research shows that when the wind hits a tyre at a yaw angle [any angle other than straight on] it’s going to detach [lose its smooth flow over the rim], so the aerodynamic shape at the front of the rim isn’t doing a lot – it’s almost always stalled.’ In other words, there’s no point in prioritising an aerodynamic shape at the front.
Aero benefits
To understand how a wider rim can improve aerodynamics rather than increase drag, we need to consider the fact that the air we are riding in doesn’t behave in a consistent manner. Even on a calm day, the air is a swirling, complex mess. What aerodynamic science has come to realise is that optimal fluid dynamics – the way air interacts with the shapes and surface textures it comes into contact with – boils down to reducing friction.
When it comes to airflow, there are three broad categories. The first category is ‘laminar’ airflow. This is the most desirable state for low frictional losses and it refers to the air moving in smooth, straight or curving lines. When it encounters a moving object, laminar airflow separates, slides around the object, then resumes its flow on the other side with minimal fuss.
If you compare tyres with different widths but identical specs – same compound, profile and inflation pressure – in terms of rolling resistance wider is faster.
The second state is ‘turbulent’. As the name suggests it refers to muddled air that is far from smooth flowing, although it can have elements of both ‘laminar’ and ‘stalled’ air within it. The reasons for the turbulence can be many: perhaps it’s a windy day, or you’re following another rider closely, or there are cars and lorries passing by. These sub-optimal conditions are sometimes called ‘dirty’ air, and it’s the most common state we ride in.
The third condition is ‘stalled’. This is when the air is no longer flowing, but is eddying in different directions at once. This state causes the greatest amount of friction and as such has the greatest effect on slowing a rider down.
What all this means is that while it’s great to have a wheel and tyre combination that works well in laminar flow, as you get head-on in a wind-tunnel, what is of greater benefit in real-world scenarios are wheels and tyres that work well in turbulent air. The most successful modern designs actually aim to take air that is turbulent and reduce its drag – to clean up the dirty air. This is one of the reasons why thin, sharp wheel rims are being replaced by wider, rounder rims – the new designs are simply faster at cutting through the messy air that riders encounter on most real-world rides. But there is another major reason why rims are getting wider, which is rolling resistance.
Contact sport
The shift to wider rims is partly the result of a simultaneous move towards wider tyres. Where 23mm tyres used to be the norm, more riders and manufacturers are opting instead for 25mm, and occasionally even wider.
‘Continental’s research shows a 25c tyre has 10-15% less rolling resistance than a 23c tyre,’ says Quan. ‘Continental showed that if you have a larger tyre the contact patch, instead of growing longer, becomes shorter but wider, so the actual surface area on the road remains the same at the same pressures.’
This is backed up by tyre manufacturer Schwalbe’s findings. Product manager Marcus Hachmeyer says, ‘If you compare tyres with different widths but identical specs – same compound, profile and inflation pressure – in terms of rolling resistance wider is faster. If you imagine your bike and rider parked on a sheet of glass and that you were looking up from underneath at where the tyre meets the glass, you’d see two distinctly different shapes. On a narrow tyre the shape would be long and thin, an oval. On a wider tyre that contact patch would be shorter and fatter, more of a circle, and in that way fewer of the threads that make up the side wall and help create rolling resistance are used at any given moment, and friction is less.’
That’s all very well, but why not simply fit wider tyres to narrow rims? When the rim is narrow, the tyre forms a ‘lightbulb’ shape when viewed in profile – pinched where it hooks into the rim, and bulbous away from the rim. With a wider internal rim, the tyre forms more of an inverted ‘U’ shape, which helps create a rounder contact patch with the road and subsequently less rolling resistance.
The internal width of road wheel rims – the distance between the two hooked flanges that seat the tyre bead – has until recently been around 14mm. On the first crop of wider rims, that space grew to more like 16mm, and now manufacturers are taking them wider again. Bontrager’s latest Aeolus TLR D3 range, which was launched earlier this year, expanded that width from its previous D3 at 17.5mm to a massive 19.5mm, a substantial increase in percentage terms. A word of warning comes from Michel Lethenet of wheel manufacturer Mavic, though. ‘Both elements [tyre and rim] ideally need to be designed to match perfectly to enhance the system. If not, it makes no sense with the rising inertia, rotating weight and aero drag to just use a wider tyre. Plus there’s the safety aspect to think about if you consider the opposite scenario – a narrow tyre used on an excessively wide rim. This may have a high risk of the tyre not seating correctly and potentially blowing off.’
Safety with something as vital as tyres is paramount, and Quan adds, ‘Currently 17-18mm [internal rim-bed width], seems fine, but any wider, say up to 20mm, and we’re getting into uncharted territory. At the moment we haven’t seen any adverse effects, but it’s not yet really been seen in the mainstream.’
Handle’s Messiah
Just to prove wheels are perhaps the most complex of problems for engineers to tackle, there’s another vital consideration in the design brief: handling.
‘It’s a hugely important factor,’ says Simon Smart, technical director of Smart Aero Technology and designer of Enve Composites’ aero wheel system (Enve SES). ‘If we go back seven years, athletes would come to the wind-tunnel and we’d identify the fastest wheelset for them. But we found that often in the real world the wheels were slower. This wasn’t because the wind-tunnel was wrong, it was simply because the riders couldn’t keep a straight line when racing because the wheels lacked stability.’
Part of going fast is being able to maintain control, so if a wheel lacks stability in crosswinds or turbulent air, the result is loss of confidence to go fast and performance inevitably suffers. ‘For me, riding stability was the big thing missing from wheel performance, and I knew that if we could develop a more stable front wheel then even if it proved slightly slower in the wind-tunnel I knew it would be faster in the real world,’ says Smart. ‘That’s why I embarked on the development programme with Enve, with handling as the number one priority.’
All of this points to the fact that the wheel and tyre must work together as a complete package for an optimal solution, not only in aerodynamics but also in stability at speed, predictable handling and lower rolling resistance. In light of this, will we see wheel manufacturers working more closely with tyre manufacturers in the future?
In the case of Bontrager, they are already one and the same. Ray Hanstein, Bontrager’s wheel product manager, says, ‘Our wheel and tyre engineers are considered the same. Wheels and tyres are so entwined that you cannot develop one to its potential without having an intimate understanding of the other. These guys work in the same room, ride together, take lunch together.’ It’s a similar story at Mavic and Zipp, which make their own tyres as well as wheels, so can create precisely paired products. The big question left to answer is: have we reached the pinnacle? Smart says, ‘Designing rims is quite a challenge, but exciting. In the last five years frame designs have changed a fair bit, and it’s things like allowing for wider tyres that has given us the freedom to explore even wider rims. As with most things, there’s a point of diminishing returns, but I don’t think we’ve peaked yet.’
Ultimately, wider tyres and correspondingly wide wheel profiles are the way the industry is heading, and the right choice for riders if you want maximum gains in the kind of conditions we all experience day to day, with predictable handling to boot. The science backs it up, so it might be time to reject the narrow view and go wide.