The case for aerodynamic wheels

Let’s get the basics of aerodynamics nailed down right from the off. As Simon Smart, founder and aerodynamic guru of Smart Aero Technology, says, the forces that slow cyclists down on a flat road come down to two key components: rolling resistance and aerodynamic drag.

“At very low speeds the tire rolling resistance is most significant, but as speed increases aerodynamic drag quickly becomes the dominant force,” he says. “If we take a rider in an average road position with reasonable tyres, we can expect there to be a 50/50 split in tyre and aero resistive force at 12.5mph.”

Even at that relatively low speed, aerodynamics still have a crucial part to play – and, from there, Smart points out the ratio leans towards aero drag much more heavily. “At a more realistic speed on the flat of 20mph, for example, 75 per cent of your effort will be tasked with overcoming aerodynamic drag,” he says.

A deeper aero rim will therefore have a positive impact in reducing the aerodynamic drag the whole bike-rider package presents to the wind. Given that aerodynamic drag is the primary resistance the riders is required to overcome, it follows that an aerodynamic wheelset offers the genuine potential for ‘free speed’. Add other aero gains into the mix and the argument begins to look compelling, says Smart.

The case for lightweight wheels

While Enve are arguably most famous for their aero-optimised wheelsets, Smart also recognises the benefits of going more towards the lightweight end of the spectrum – particularly when it comes to climbing and acceleration. Bike manufacturers will often spec machines with relatively heavy wheels, so many riders will look to upgrade those hoops for something lighter – and it can have a genuine impact on the ‘feel’ of the bike

“When accelerating and climbing, weight comes into play,” Smart says. “For our average 75kg rider on a ten per cent climb, around 90 per cent of effort will be consumed in overcoming gravity.

“To sustain 10mph on this gradient, this 75kg rider would need to produce roughly 415 watts. Each kilogram in weight equates to a saving of around five watts of effort,” he adds.

However, it’s also worth noting it’s very rare that a wheelset marketed as lightweight versus an equivalent aero option will offer a saving anywhere near a kilogram. For example, if you compare Enve’s SES 2.2 climbing wheel with the SES 7.8 aero-specific wheels, there’s actually only 332g of claimed weight difference between the two (1,110g against 1,442g respectively).

While the reduced rotational weight of a lightweight wheelset has a greater effect, pound-for-pound, than static weight in terms of resistance, it also remains true that much bigger differences can be made within the rider themselves, as long as a rider doesn’t adversely reduce their power output by losing weight.

Ultimately, while weight trumps aerodynamics when climbing, if you’re riding on largely flat roads, rolling terrain or even a mountainous course with long valley roads, aerodynamics may well be more important over the course of the ride.  It’s about balancing the demands of your riding with your choice of wheelset, but we’ll come on to that.

When it comes to lightweight wheels specifically, and as Smart alluded to when talking about aero wheels, while relatively affordable lightweight hoops are available, you will soon reach a point when you have to dig deep to make further gains. “The cost tends to go up exponentially as you look to shave off more weight to save watts,” says Smart.

The right wheel for the job

Right now, you’re probably thinking ‘this is all well and good, but I’m not a pro’. The same idea applies, however, and you need to carefully consider the type of riding you do to narrow down your options.

“We are left with the fundamental question of where you do most of your riding,” says Smart. “If you only climb, then you’re best advised to go lightweight. However, if you intend to descend quickly down the other side or ride rolling and flat terrain then a mid-depth wheel is a no-brainer.”

“We find that the aero benefits from increasing the rim depth between 30-50mm will far outweigh the small increase in weight and, invariably, the wheels are more stable too.”

Most wheel manufacturers will offer a range of rim depths to suit the needs of all riders. We saw that with DT Swiss’ new range of ARC aero wheels, available in 48mm, 62mm and 80mm depths, and, returning to Shimano’s Dura-Ace line-up, Hillsdon says there are three options to cater for different riders – C24, C40 (selected for the 2017 RCUK 100) and C60s, all of which have specific characteristics designed to best suit the type of riding you do.

“Which wheel is best for you fully depends on the type of rider you are and the kind of riding you’re doing”

“Which wheel is best for you fully depends on the type of rider you are and the kind of riding you’re doing,” Hillsdon says. “Are you just training or are you looking to win a race? Are you a lightweight climber or a punchy rouleur with a diesel engine? How’s the weather, are you likely to suffer in the crosswinds? What’s your bike handling like? What does your typical route profile and the terrain look like?”

“All these questions can have an impact and when choosing which wheel to opt for you should think about all these factors with the goal of getting you from A to B as fast as possible.”

Alloy or carbon rims?

Whichever way you choose to go in terms of aero versus lightweight performance, you’ll then be faced with actually choosing a wheelset. One of the main questions that remains is whether to opt for an alloy or carbon rim construction, which can have a significant impact on the look and performance of the wheel. While advances in aluminium mean alloy represents excellent value for riders who want a set of lightweight wheels which won’t break the bank, high-end aero wheels are typically made from carbon fibre.

“Alloy rims present great value, and certainly HED have done an excellent job of making an alloy rim work well aerodynamically with a carbon fairing,” says Smart. “Until recently, alloy rims have had better braking performance. It’s only some of the latest carbon rims that are able to match alloy braking performance.

Smart says carbon fibre comes into its own in not only enabling engineers to manipulate the rim to boost aerodynamics, but to do so without adversely affecting other areas which impact on performance.

“While the aerodynamic advantage of a carbon wheel may not always be huge, it’s the whole package that brings the carbon aero wheel into a world of its own,” he says. “Reduced weight, increased stiffness whilst retaining ride quality and responsiveness are all factors that will transform any bike.”

Hillsdon also throws the strength of the wheel into the mix. Shimano’s Dura-Ace clincher wheels are constructed as a carbon laminate around an alloy core – the intention being to increase strength and heat resistance. Aluminium has it’s drawbacks, though, and Shimano’s race-ready tubular wheels use a full-carbon construction.

“Gram for gram, carbon is lighter than alloy but it’s not as strong, so an alloy wheel is going to be more resilient than a carbon wheel, but potentially heavier too,” he says. “As a result there comes a point where the increase in material needed to make an aero rim is negated by the weight of making it from alloy materials.

“Dura-Ace tubular wheels [used by the likes of Team Sky], on the other hand, have a full carbon construction for out-and-out, gram-for-gram aero performance.”

Taking the plunge

When it comes to upgrading your wheelset, the most fundamental issue to consider is the kind of riding you’re going to be doing on your new hoops.

That will give you a starting point to make a shortlist, but you also need to consider other factors including how often and when you plan to use the wheels, typical weather conditions, your bike handling skills and general strengths as a rider, plus a multitude of other design considerations, from rim width to tubeless compatibility. Naturally, budget will be a key consideration, too, as well as the wheels you’re upgrading from. For example, if that means a relatively heavy set of down-spec wheels on an off-the-shelf bike, then reducing the weight of your machine may be more of a priority.

Ultimately, as our conversations with Smart and Hillsdon have confirmed, both lightweight and aero wheels have their advantages – it’s why both Enve and Shimano offer a range of wheels to suit. However, as Smart stressed at the start, aerodynamics have a huge part to play for any cyclist.

And, unless you have specific demands for your wheelset (for example, you’re a lightweight climber heading to the hills or a time triallist looking for every aero advantage), or the option of choosing an individual set of hoops each time you ride – and, chances are, that’s unlikely – then it’s about balance. While a mid-section wheel may be seen as a compromise, we’ve learned that medium depth rims are considered closer to a “sweetspot” by offering an aerodynamic advantage while keeping weight under control, and are favoured by many pros as a result.

“It’s about finding the exact balance between watts lost and gained in different circumstances and different types riding. But don’t worry, mid-section is anything but middle of the road,” says Shimano’s Hillsdon. “You only need to take a look at the pro peloton in an average stage and you’ll see how many riders choose this to get the best of both worlds.”