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Reinventing the wheel: Halo’s carbon Carbaura RC wheelset

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10 Sep 2018
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There’s no denying that carbon fibre wheels do two things well – they go quickly and look awesome. There was a time when those two perks cost a great deal, making them the preserve of pro cyclists and the fabulously wealthy.

The early generations of carbon wheels cost thousands, weren’t particularly effective under braking, while the material challenges of a clincher rim meant gluing on tubular tyres was the only reasonable option.

Fortunately, technology has come a long way, and while old and modern carbon deep section wheels may cut a similar silhouette, they are very different below the surface.

As a case in point, we’re taking a look at the anatomy of the Halo Carbaura RC, from the carbon fibre to the brake pads and the tyre system to the spoke pattern, to show how carbon wheels can now be the workhorse wheel of the everyman.

Braking tradition

When carbon wheels first arrived, there was no doubt they were good at going quickly, but they weren’t so great when it came to slowing down.

Early generations of carbon rims were plagued by overheating problems caused by carbon being less effective than aluminium at conducting heat away from the braking surface.

With this came the increased risk of delamination of the resin exterior of the rim wall, which at best compromised braking performance and at worst could result in the occasional rim failure.

It was such an issue that many wheel providers would only build tubular carbon rims, which allowed for additional carbon to be used in reinforcing the brake track, and many manufacturers sided for carbon-aluminium hybrid wheels, with a hefty weight penalty.

Fast-forward to today and Halo’s Carbaura RC wheelset showcases all the latest manufacturing methods that have been deployed to ensure that braking is not only safe, but predictable and powerful too.

‘The rims are UD finish but we’ve used a 1.5k weave on the braking surface. The reason for that is that we did a bit of lab testing and the 1.5k weave dissipated heat more efficiently,‘ says product design engineer Ryan Griffiths.

Crucially, brake pad technology has also developed – meaning that the levels of friction and adhesion are carefully balanced to offer sufficient braking force while not generating too much heat to delaminate the wheel or force the tyre to explode.

‘We’ve been working with carbon for years now and we’re actually starting to understand the relationship of the brake pad and the carbon rim, rather than before when we were just using a standard brake block and wondering why the lacquer was melting and the heat not dissipating,’ Griffiths adds.

Testing times

As wheel performance is so critical to overall ride quality as well as safety, Halo conducts in-depth scientific testing on its wheels, to ensure that measurable braking criteria is met.

For example, the Carbaura RC wheels were loaded with 35 kilogram-force (roughly equivalent to the weight of a 70kg rider), spun at 25kmh for 15 minutes while 3 kilogram-force of braking was applied – the equivalent of dragging your brakes out on the road.

In Halo’s tests, this braking force saw the rim width increase by 0.14mm due to the heat build-up generated by the braking force, but crucially the rim showed no signs of mechanical failure and the tyre did not blow from overheating.

The real-world benefits of such testing comes into its own on a long alpine descent, where dragging the brakes in challenging conditions used to be a nervous experience on a carbon clincher wheel.

Giving riders that confidence in challenging winds and difficult terrain has been the carrot for wheel developers for years. It’s not only about braking force, but also the stability of a wheel in unpredictable or strong winds – which has come a long way from the days that a side gust could nearly unseat a rider.

The shape of the future

‘Our older models had an V-shaped profile, which meant they were relatively fast at low yaw angles. But as the yaw angle increases the drag also increases significantly,’ says Griffiths.

It’s a familiar story across numerous wheel brands, where older generations (including this one pictured) had a thinner frontal area and a sharper profile, in the belief that it was faster.

The sharp and flat profile of the wheel not only reduced efficiency in crosswinds, but meant the wheel was easily caught by a gust, disturbing the handling of the bike.

‘We live near the Fens, so we do a lot of testing in crosswinds,’ explains Griffiths. ‘Thankfully, these rims are still very controllable in crosswinds.’ The key to that control is a wide U-shaped profile that creates less side force.

While stability makes a big difference to the usability of the wheel, with the use of more calculated profiles of tyre and rim together, a wheel can be wider and shallower and still be aerodynamically quicker.

‘The new shape offers the aerodynamic advantages of a deep section rim, but at a lower rim depth, giving a lower mass and better acceleration [these gain 29 seconds over 40km compared to Halo’s Devaura, a modern, wide, 30mm deep aluminium rim],’ says Griffiths.

Another substantial gain from a wider profile is the effect on the tyre bed of the rim. With the blunter shape of the rim, Halo has afforded a 19mm internal rim width. That creates a wider tyre profile, and therefore a wider, more efficient contact patch.

That brings us to one of the most important developments of recent years – the tyres themselves.

Tubeless tyres

There’s no doubting that tubular tyres glued to a carbon rim work for the pro peloton, but anyone who’s punctured on a training session on a set of tubular tyres will know that walking home isn’t much fun.

On the other hand, clincher rims and tyres make the whole experience more practical, but some of the rolling resistance gains and pure ride feel from tubular tyres is lost.

It’s only recently that brands have embraced tubeless tyres, which use sealant rather than inner tubes to hold in the air, finally giving us the best of both worlds.

‘It’s important that we make the swap to tubeless as simple as possible, as so many riders are now converting,’ says Griffiths.

As such, the wheels come with tubeless tape, fitted with the option of tubeless valves and soon to be released sealant available from Halo. The tyres themselves are the only purchase necessary from another brand to go fully tubeless.

‘I think once you realise how much better the puncture resistance is with tubeless tyres, it’s hard to go back,’ Griffiths argues.

‘If you get a hole you get a little bit of latex sealant over your shoe but it seals itself. So I’m completely won over but I understand it’s still a bit alien to a lot of traditional road riders.’

While the newest innovations have allowed improvements to be made in some places, traditional designs have actually proven more effective in others.

‘J-bend spokes make for a cleaner build,’ says Griffiths by way of example. A J-bend hooks into a hub flange, rather than a ‘straight pull’, which is held internally within the hubs. It’s a small but significant difference.

‘Chris King has gone the way of just doing J-bends, we suspect on account of spokes pulling out from a hub,’ Griffiths explains. ‘You just can’t do that with a J-bend because it’s hooked in.’

Halo’s own touch is an irregular spoke pattern. In order to increase stiffness without adding weight, Halo adds 16 spokes to the drive-side of the rear wheel but only 8 to the opposite side, where many wheels will side with 12 spokes on each side.

This reduces the difference in spoke tension between the drive and non-drive side spokes, making them far closer to equal. ‘The 16-8 spoking pattern actually influences the wheel’s stiffness more than any developments in the stiffness of carbon rims,’ says Griffiths.

So a modern carbon wheel offers more speed and stability, more stiffness, improved braking and therefore safety, along with fewer punctures.

As much as things have changed, however, one thing has remained a constant: deep section carbon rims simply look slick. And for all its other attributes, the Carbaura RCs are as slick as any.