Aero bike versus road bike - what's better for you?

A cyclist spends $7K on an aero frame upgrade.

They take their new pride and joy for a spin through the Adelaide hills to test out the watt savings and see how many PBs they can set.

Halfway up the old freeway, they’re overtaken by a roadie cruising past on a 10-year-old alloy frame.

Wait, what just happened?

It’s the classic aero bike vs road bike dilemma: to upgrade to a carbon streamlined machine, or keep rolling along on your old road bike.

But are aero bikes worth it for most riders?

The Aero Marketing Machine

To understand what an aero bike is, all you have to do is look at the marketing.

From frame design and integration to helmet shapes and rim profiles, companies making aerodynamic tech promise you one thing: Product X will make you go faster.

This is where marginal gains emerged as a concept.

What is marginal gains in cycling?

Marginal gains is a term popularised by Sir Dave Brailsford, who was made head of British Cycling in 2002.

Over its 76 year history, British Cycling had only ever won one gold medal. There was clearly a job to be done. Brailsford was fascinated by process-improvement techniques, and decided to apply it to the BC squad. The theory was if you broke down everything about riding a bike—components, frame, sleep, nutrition—and increased each individual aspect by just 1%, all of those small 1% gains would add up to a huge advantage.

This is marginal gains cycling in a nutshell.

Brailsford and his team applied the theory to everything they could lay their hands on, even down to the team’s pillow type. As a result, the BC team won 60% of the cycling gold medals at the 2008 Olympics, and went on to set multiple Olympic and world records at the 2012 Games. British riders Bradley Wiggins and Chris Froome added to the cache by giving team BC five Tour De France victories in six years.

Clearly, marginal gains works, right?

Well, not necessarily (and even Brailsford agrees).

How the bike industry sells marginal gains

A.k.a. wind tunnel watts savings that sound impressive

How many watts does an aero bike save?

Let’s look at what some of the bigger brands say.

• Cannondale: “At 48km/h (30mph), SystemSix saves you over 50 watts.”
• Cervélo: “The new S5 is 6.3 Watts faster than its predecessor…(and) at least 5 Watts faster than the bikes of our competitors.”
• Giant: “The new Propel Advanced SL model improves aerodynamic drag by 6.21 watts. This equates to a savings of 27 seconds over 40km at 40kph."
• Trek: “The new Madone is 19 watts faster than the previous generation at 45kph. That’s a savings of approximately 60 seconds per hour."

As you can see, companies love to advertise aero frame watt savings.

If you’re racing, bunch riding, training or just love going fast, statistics like these are hard to resist, especially if there’s only a minute between you and your nearest competitor. This new bike could be the secret sauce that gets you on the podium.

But do aero bikes really make a difference once you’re outside the wind tunnel?

The Inconvenient Physics of Aerodynamics

At what speed do aerodynamics matter on a bike?

At 48kph.

When you’re diving into aerodynamics in cycling, you’ll start seeing this number pop up repeatedly, such as in Cannondale's claim about the SystemSix.

What’s so special about aero bike benefits at 48kph?

Whenever you’re riding your bike, the air around you is resisting, pushing you back. This is aerodynamic drag. Aerodynamic drag isn’t consistent, either: when you go faster, the air pushes back much harder.

When you're spinning along at under 20kph, most of your effort is going into just rolling the bike forward.

When you're riding at 30-35kph, over half of your effort is spent just pushing the air aside.

When you're riding at 48kph (the magic number!), almost all of your effort is channelled into fighting the air, and this is where aerodynamics really matter.

This isn’t to say that aerodynamics don’t matter at lower speeds, such as 25-35kph; it’s just that they’re much less noticeable. At 48kph, though, even small aerodynamic tweaks can make a real difference in watts savings.

However.

This also means that to reap the full benefits of aerodynamics, you need to be riding at 48kph, and maintain it.

How fast is 48kph in real world terms? Well, the average speed of the pro riders in the Tour De France is 43kph, and that's including all those mountain climbs. At this level, aerodynamic gains genuinely show themselves.

Needless to say, most cyclists don’t ride at this speed, let alone for long periods of time.

What happens to aerodynamics when you’re not riding at 48kph

Let’s take the Cannondale SystemSix again.

At 48kph, Cannondale claims a 50W saving. Over a 40km ride, saving 50 watts means you could shave around one and a half minutes off your time, assuming you manage to sustain that 48kph speed.

Drop your pace down to 30-35kph, which is more realistic for a club racer, and your watt saving drops dramatically—closer to 20% if you're lucky, and this means even less time saved off your 40km ride.

If we then look at an average speed of 25kph, which is normal for a regular recreational rider in the hills, the savings are marginal at best. (Yes, they're called marginal gains...but these are really, really marginal.)

So there are savings at all speeds to various degrees, but clearly they don't make the staggering difference that the marketing claims they do.

Do slower riders benefit more from aerodynamics?

Some people claim that slower riders benefit more from aero bikes because they spend more time out on the road.

A cyclist going at 35kph takes around 70 minutes to ride 40kms. Another cyclist going at 48kph will take around 50 minutes to do 40kms. The argument is that because the 35kph rider is spending more time on the bike to cover the same distance, they’re therefore gaining more incremental aero benefits overall.

Not quite.

Aerodynamics is about making you go faster for the same effort. If your new aero bike takes your average speed from 35.0kph to 35.5kph and your output didn’t change, that's aerodynamics at work.

The fact that a slower rider takes longer doesn’t magically make their aero gains bigger; they're just exposed to those gains for more minutes.

It’s like saying that you get twice as fit using an ebike because you can ride it twice as far, ignoring that the reason you can go twice as far is because you’re working half as hard.

Or, for another analogy, if I eat this entire cake, but really slowly, it’ll have fewer calories.

These ideas look good on paper, but they’re not how physics—or reality—works.

Wind tunnel testing versus the real world

Sometimes companies use testing tricks.

Like all data, wind tunnel data manipulation is possible. For aerodynamics, there are several common ways to make a frame look much faster on paper than it actually is in real life:

• Testing the bike with far narrower tyres than riders would actually use.
• Testing the bike only at a 0 degree yaw, thereby completely eliminating any crosswind interference.
• Testing the bike without a rider.
• Dropping the seat height to reduce how much seat post is exposed.

It's also expensive to rent a proper wind tunnel (assuming you’re not a multi-million dollar brand with your own in-house lab). Add in the fact that tests have to be done at multiple speeds and repeated to ensure reliability, and a smaller frame manufacturer may not have the tens of thousands of dollars required to throw at these facilities.

What does a small company use instead?

CFD, or Conceptional Fluid Dynamics, which is computer modelling. CFD which will tell you in theory how aerodynamic something is. It’s theory only, though, and, like wind tunnel testing, can be skewed or manipulated.

So what does wind tunnel testing miss?

An aerodynamic frame always assumes the wind will only be coming at you from directly head-on, never sideways or even at a slight angle. Regardless of how impressive the data is, wind tunnel tests don’t necessarily translate to what you experience riding out in the real world, such as dealing with these crosswinds. (And hills. And traffic lights. And other road users.)

While a wind tunnel can actually test under crosswind conditions (though they don't, because it doesn't give good figures), it can't test with swirling or multidirectional winds, which is something that all bike riders deal with, whether we want to or not.

Crosswinds also don’t mix well with the flat and elongated tube shapes typical to aero frame designs. In a crosswind, an aero frame will act more like a sail than a watt-saving machine. Pete once owned a Kestrel 4000. Although the marketing material for this bike did state that a small amount of lift was created at a 12 degree yaw angle, he was fortunate that it never shot off into the sky due to a mildly angled wind. It was, however, terrifying in any wind, and was rigid as hell (which is why it was fast).

The bike is not the main source of drag

a.k.a. Aerodynamic Bike Position vs Frame Design

There's a larger problem to the promised benefits of aerodynamics that's often not discussed.

What’s the biggest source of drag on a bike?

The rider.

Your position and body shape account for 70-80% of your total riding drag. Even if you’re built like a greyhound, your torso and limbs aren’t designed to cut through the air like an arrow. Add in the movement of your legs, causing air turbulence, and you’re the drag, not your bike.

The other 20-30% of drag is everything else: the frame, wheels, handlebars, bottles, etc. Out of this 20-30%, the actual frame is usually closer to 5-10% of the total drag. That’s a lot less than some aero companies would like you to think.

Why are aerodynamics promoted as the cure-all for speed issues?

Back in 1996, cosmetic company Philosophy launched a moisturiser that broke multiple sales records. The name of the product? Hope In A Jar.

This simple concept is pretty much the basis of aerodynamic marketing: apply X and you will be a better rider, no extra effort required. For anyone looking to gain a competitive edge, it’s a promise that’s hard to resist.

“X watts at Y kph!” also sounds science-based and measurable, and therefore more credible. When given the choice between training harder (which requires time, effort and sacrifice) and simply purchasing a bike (which promises the same result, minus the work), many riders will choose the latter.

After all, we’re human: the path of least resistance is hardwired into us via evolution. We will always be tempted by an easy win. And it’s also nicer to believe that the reason you’re not hitting your PBs is your bike, not… well… you.

Human nature is also why it’s hard to fight against the psychology of aerodynamic marketing.

Why aero bikes can feel faster, even if they aren’t

Cyclists often report feeling faster on an aero bike, even when the data doesn’t back it up.

There are multiple factors at play here, such as the placebo effect (which is incredibly powerful) and the sunk cost fallacy, characterised by a reluctance to give up on something you’ve spent time/effort/money on, even if giving up or finding an alternative would be more advantageous.

There’s also the motivational boost that comes with having a shiny new toy. Nothing beats New Bike Feeling, especially if it comes with the promise that it will make you the rider that you want to be.

Additionally, bike riders love gadgets and gear, so for those chasing elite performance, the concept of marginal gains—not to mention a super-slick-looking carbon machine—is incredibly attractive.

From a bike shop's point of view, it’s also a lot easier to sell the concept of how much faster this frame or wheelset will make you than going through the alternatives, such as the nuances of bike fit or tyre choice.

Finally, we can’t overlook the fact that aerodynamic frames and components sit in a premium price bracket. The sale of a top-end bike can make a shop hundreds or thousands of dollars more than a mid-level offering. Every shop has to make money, and aerodynamics can be an easy sell to the right rider.

After all, why spend hours on your hamstring flexibility when a superbike will shave seconds off your time all by itself?

What you may not know about aerodynamic bikes

So is an aerodynamic bike worth the money?

Besides the fact that you may not be getting the speed gains that you think you will, there are two main drawbacks to a bike that’s been designed for aerodynamics:

1. They can be uncomfortable to ride.
2. They’re expensive to service.

Aero frames are designed with stiff carbon layups to increase power transfer, which is why they feel fast. This stiffness, however, means that they can ride harshly, especially on rough roads.

Are aero bikes good for long distance rides?

Unfortunately, not really. An aero frame won’t absorb bumps and road chatter like an endurance frame will.

Also, not every rider can physically cope with the geometry of an aero bike. Bikes in this category tend to have longer reaches and lower cockpits, which is great if you’re super-flexible but not so great if your neck and shoulders don’t like being in a slammed position. (With these factors in mind, you can really appreciate what Ironman entrants go through.)

Now, the service side of things.

Aero frames can look amazing—all those sleek, hidden cables!—but they can make even simple maintenance a nightmare.

Take the concept of changing a headset bearing.

On a regular bike, you drop the forks out, clean and grease the cups, fit new bearings, pop the forks back, and voila!

On an aero bike, you *takes a deep breath* try to drop the forks out (this simple act can be quite tricky and time-consuming in itself), split the brake lines, remove the gear cables and casings, clean and grease the cups, fit new bearings, reconnect and bleed the brakes, and then have to put everything back together.

What takes 30 minutes on a regular bike can take 2-3 hours on an aero bike with fully internal routing.

And that’s time you have to pay for.

Many riders aren’t aware of this aspect when buying a fancy new aero bike, and the increased service costs can come as a shock, especially on top of the bike sticker price.

How do you actually improve aerodynamics on a bike?

The best ways to save watts cycling without buying a new frame are quite simple.

They're also a lot cheaper (and sometimes free!) than forking out for a new rig.

1. Improve your body position

This is the number one thing you can do to save watts.

Poor rider position will immediately wipe out any aero component advantage. If you’re sitting up on the tops, you may as well be a parachute.

Let’s take a rider holding ~250 watts at ~35kph. ~200 of those 250 watts is the rider overcoming their own body drag. ~50 watts is everything else, including the bike.

That rider can:

1. buy a more aerodynamic frame to shave 10% off the bike drag, and save 5 watts, or
2. switch from an upright position on the tops of the bars to a more aerodynamic position (hands on hoods, forearms horizontal, elbows tucked), and save 35-46 watts.

The answer here is pretty obvious.

2. Get a bike fitting

Being comfortable on the bike will lead to greater efficiencies—and, thus, speed—because you'll be in a position that works best for your body.

The fitter may also advise you on how to further improve your position; most commonly this will be focused on achieving greater flexibility. (See the previous point about positioning.)

3. Change your clothes

Does lycra make you faster when cycling? If your jersey is flapping in the wind, that’s costing you watts, and more than you’d think. A baggy jersey can add 20-50 watts of drag to your ride.

Switching to a tight-fitting outfit is an effortless way to fix this. You don’t need to go the full skinsuit, but at least to ditch the loose club cut if you’re serious about going faster.

Interestingly, pro mountain bikers historically haven't had much of a choice when it comes to cycling clothing aerodynamics. In 2008, the UCI banned all ‘tight-fitting clothing’ for MTB racing, and this ban was only lifted in 2023.

4. Improve your fitness

Switching to an aero frame can reduce the effort it takes to ride at a particular speed. But increasing your fitness means that you can put out more power, regardless of the speed you’re riding at, the terrain you’re riding on or even what bike you’re riding. Raising your functional threshold power from 220 watts to 250 watts gives you a 30 watt increase everywhere, all the time. Over 40 kilometres, that watt increase translates to a 4-6 minute faster time, far more than even the most aero of aero bikes.

So if you’re tossing up whether to upgrade your bike or your fitness, the data says fitness every time.

And no-one has ever regretted getting fitter, regardless of their race ambitions.

5. Spend money where it counts

Forget the frame: when it comes to bike parts, your choice of wheels will make the biggest difference to your speed.

Your choice of wheelset should be based on where you mainly ride - in the hills or on the flats?

If you ride mainly in the hills, how steep and long are the climbs?

Do you ride in all conditions, or only on a nice, calm day?

Wheels for moderate climbs and/or people who ride in any conditions should have a rim depth of somewhere between 25-40mm, whereas someone who only rides on the flats in calm weather can benefit from 50-60mm deep rims.

A person who really likes super-steep climbs should look for the lightest wheels they can get, which will usually be of a shallower profile.

When do aerodynamics actually matter?

Aerodynamics definitely have their place in cycling. Pro riding careers can be made or broken on the back of these kinds of incremental advantages. Entire teams and companies base their careers and livelihoods on it.

If you’re a serious competitive racer, a time trialer, triathlete, or someone who can happily sit on a 48kph ride pace, then an aero frame might be just what you need to nab that podium position or set a new PB. Hell, you may just really like the look of one and have some money to burn – it’s all good!

While road bike aero gains do exist, for most riders, improving fitness, position, clothing and wheel choices will deliver more speed than any aero frame upgrade.

And you may not have to spend a single cent to do it.

(Assuming your bike came with the perfect wheels for you, that is.)

Need your headset bearings replaced? Even if it's an aero bike, that's what we're here for.