Reinventing The Wheel

Road bike wheels may look the same as ever, but the move to disc brakes has changed everything, as Cyclist discovers

There’s more to the wheel than simply going round – and cycling’s move to disc brakes has opened up a whole new world of possibilities

Rim brakes have served us well for more than a century, but it seems their time is coming to an end. Road bikes are following in the tyre treads of every other mode of transport and adopting disc brake technology. It is, in the words of Campagnolo’s communication director Lorenzo Taxis, ‘one of the biggest win-wins in the history of the bicycle’. The consumer gets better performance, while the manufacturers get to redevelop almost every one of their products.

Accommodating the switch from rim to disc brakes requires distinct changes to several areas of a bike, yet arguably no component has been more acutely affected than the wheel. Its design has needed not merely adjusting but totally changing, and for the past couple of years brands have been scrambling to adapt to the market’s new direction.

‘Less weight at the rim will be felt far more keenly than anywhere else’

Yet within chaos lies opportunity. As the dust settles, R&D teams have found themselves freed from the shackles of rim brake wheel design. It might still be round, but the disc brake wheel is now a very different beast to its predecessor.

From the outside in

The wheel rim is where the most dramatic changes are taking place, yet the early days of the transition saw some brands cutting corners just to get a disc brake wheel to market in a timely fashion.

‘Some manufacturers simply repurposed their rims,’ says Jake Pantone, Enve’s vice-president of product. ‘They just laced it to a disc brake hub and moved the stickers up to cover the brake track.’

According to Pantone, doing this negated essentially every opportunity the move to discs was supposed to

open up: ‘The brake track is the biggest difference between rim and disc wheels,’ he says. ‘Rim brake rims not only had to be light and stiff, they had to feature flat, even brake tracks and deal with heat build-up. It took us eight years to nail that rim design. I’d say we got it right just in time for discs to come along and make the technology obsolete.’

Disc brake rims don’t need to deal with such things, which means they can be lighter.

‘Before, we needed to use heatresistant resins and carbon fibres, as well as add in extra material purely as a heat sink for the friction generated by rim brakes,’ Pantone says. ‘Disc brakes sidestep all of that, so immediately we were able to shed around 40g per rim.’

While that might not sound like a huge amount, it equates to a decrease in overall weight of around 10%, and improvements of that magnitude are largely unheard of elsewhere in the hyper-refined environment of modern bike design.

What’s more, that’s a decrease in a rotating mass. ‘Less weight at the rim will be felt far more keenly than the equivalent weight dropped anywhere else on a bike,’ says Bastien Donzé, global product manager at Zipp. ‘A light rim reduces the wheel’s moment of inertia, which means the bike will feel more responsive and livelier.’

The other main restriction was rim width. ‘The clearance of the calliper meant rim brake rims were limited to about 27mm wide,’ says Luisa Grappone, engineering and

product manager at Hunt. ‘You always had to start a design with those boundaries and work from there. With disc brakes there are no such restrictions. We can play with any shape we want and don’t have to worry about set starting points.’

Inherently lighter designs with no space restrictions mean there are no penalties for going wider, so rim width has become a bit of an arms race of late. Three years ago 17mm was considered a contemporary internal rim width. Now 19mm is conservative, and the most extreme designs stretch that to 25mm – almost the width of a traditional rim brake design’s external dimensions – with external widths of up to 35mm.

Grappone suggests tyres have been the main driver behind this expansion. ‘The body of evidence suggesting that wider tyres – 28mm or 30mm – are better for performance is unequivocal,’ she says. ‘They can be run at lower pressures so that they create more comfort and grip with less rolling resistance, but rim brakes prohibited their use because of clearance issues.’

Donzé agrees: ‘Last year CanyonSram rode all of the spring Classics on our 303 Firecrest Disc wheels with 30mm tubeless tyres inflated to about 55psi. The feedback we got was awesome – they loved it.’

It’s important to note that tyres this size need wheels wide enough to support them properly, hence the increase in rim width. A traditional internal rim width of 17mm would pinch a wide tyre’s beads together and create a rim/tyre cross-section similar in appearance to a lightbulb.

Think of the difference in effort required to unbalance someone with their feet together compared to them adopting a wide stance. The wider a rim’s internal width, the more stable

‘The evidence suggesting that wider tyres are better for performance is unequivocal’

a tyre’s sidewalls are and the better its riding characteristics.

Enve claims it has recorded savings of up to 8 watts with a 28mm tyre on the 25mm internal-width 4.5 AR wheel compared to the same tyre on a 19mm internal-width 4.5 SES.

Fatter but faster

In a revelation that flies in the face of conventional thinking, brands have found that wider rims and tyres, despite possessing a bigger frontal area, can be made more aerodynamically efficient than something narrower.

‘Frontal area will always be king at 0° of yaw [a direct headwind],’ says Pantone. ‘But the wider the yaw angle gets, the bigger advantage wider wheels have. They curve a tyre’s radius less aggressively, which smoothes airflow over the system.’

This effect has led several brands to claim shallower, wider wheels provide the same aerodynamic advantages as deeper, narrower wheelsets, not to mention better handling in crosswinds. So could we see rims getting shallower generally in future? Grappone doesn’t think that is unreasonable.

‘Potentially we could,’ she says. ‘It’s true that wider rims at 45-48mm deep are producing drag numbers similar to narrower 60mm rims. That isn’t to say deeper rims are worse, though – the benefits of increased width can be taken through to make deep wheels even more aero.’

Stefan Riehle, product manager at DT Swiss, doesn’t see the death of

60mm-plus wheels coming anytime soon: ‘Rim depth still depends on application – deeper will still be better when outright speed is the goal, in a time-trial for example.’

Less is more

Despite historically having a tough time gaining traction in road cycling, the latest developments in the wheel sector also mean the case for going tubeless has never been stronger.

‘The lower air pressure used in wider tyres means the risk of puncturing an inner tube is higher,’ says Riehle. DT Swiss has been a proponent of road tubeless for several years now and all of its wheelsets

Brands have to manufacture their tubelessready wheels to incredibly tight tolerances

come set up tubeless-ready. ‘Going tubeless counteracts this effect and further benefits rolling resistance.’

‘This is another problem sidestepped by disc brakes,’ says Pantone. ‘Tubeless requires very stringent dimensional tolerances. Rims that are braked on and heated up change those tolerances and undermine the safety of the technology. Now with tubeless that isn’t an issue.’

Grappone, however, admits that it can be tough persuading people to adopt tubeless: ‘We recently met with a pro team we are sponsoring. Even the mechanics, the guys who are supposed to be on top of all the latest technology, had concerns about tubeless: how it performs, how to set it up, what happens to the tyre if it punctures mid-race. The difficulty for us is educating people, but with the latest wheels sending out a strong marketing message as to the technology’s benefits I don’t see it taking long for its use to become far more common.’

To get tubeless tyres to sit perfectly within a wheel rim and therefore

provide an airtight seal, brands have to manufacture their tubelessready wheels to incredibly tight tolerances, and many are discovering that the best way to do this is to use a hookless design. This means that the protruding walls of the wheel rim forgo the usual bead hook that retains the tyre.

‘It is definitely better from a manufacturing point of view,’ says Grappone. ‘From a manufacturing perspective, making that bloody hook is a nightmare. Hookless also makes a rim lighter, but due to issues with safety we aren’t featuring it across many of our wheelsets just yet.’

The issue comes with the disparity in fit between different wheels and tyres, which can affect tyre security at high pressures. Many brands – Enve, Zipp and DT Swiss included – are now developing their own ‘approved’ lists of compatible tyres that they have tested and know to pair safely with their wheels.

‘You can design according to

ETRTO or ISO standards,’ Grappone adds, ‘so officially rims can be safe, but we think that if there was ever an issue with a tyre blowing off a rim, the finger of blame is always first pointed at the wheel manufacturer and not the tyre maker, so for the moment that is a risk we want to avoid.’

Elsewhere, though, the technology is viewed more favourably. ‘It’s just the right way to do things,’ says Pantone. ‘We do still make rims with bead hooks, but they necessitate the use of “soft” tooling that needs to be removed after the carbon is cured and recycled after each use. With hookless we can make the rim stronger and lighter, and the manufacturing process is more repeatable because we can use “hard” tooling – a machined metal tool that can be exact every time – so the consumer gets a reliable, safe tubeless setup.’

Core of the issue

Modern rims may be receiving much of the attention but changes elsewhere in a wheel have been no less drastic. Spoke counts have gone up and lacing patterns have changed to cope with braking torque now originating at

Hubs were bulked up, but have been refined down to nearer the weight of rim brake hubs

the hub. Radial lacing can’t be used to build a disc brake front wheel – crosslacing is mandatory.

Hubs were precautionarily bulked up too, but have since been refined back down to sit nearer the weight of rim brake hubs, meaning the early reputation of disc brake wheelsets being heavy is now outdated. Riehle claims the rim and disc variants of DT Swiss’s ARC wheelset are now less than 25g apart.

However, the developmental ceiling looks likely to be a lot lower for these sub-components. Pantone reveals that Enve has explored the advantages to be gained from pouring resources into hub development and says there isn’t enough progressional scope for it to be worth it. Carbon disc brake hubs are unlikely to become common, for example.

‘Much like in a stem, the forces acting on a hub are particularly complex, so the carbon would have to be so bulky that aluminium just makes more sense,’ says Grappone.

So it is at the rim that we can expect to see further developments, although they will inevitably slow down now that the major leaps have been taken. That may not be such a bad thing, though. The experts we spoke to all concur that we have arrived at a sweet spot of light weight, aerodynamics and comfort in wheel design that hasn’t ever been achieved before.

Wheels are often credited as the most effective upgrade a rider can make to their bike. It seems this piece of advice has never been more true. Sam Challis is tech editor at Cyclist, which is the second most effective upgrade a rider can make