Race day. It’s 5am and your alarm is going ballistic. You sit up and rub your eyes, then pause for a moment and listen. A howling wind is blowing outside. A sinister grin creeps onto your face.
You eat breakfast, pack the car and drive down to the race site, the wind buffeting your car. As you prise your bike from the roof racks, other competitors walk past with shocked expressions as they see the full disc wheel attached to your steed. As you set up transition the competitor next to you says, “Man, you must be crazy to ride a disc in this wind.” You turn to him and smile knowingly, then quietly gather your gear and head off towards the start.
I’m constantly amazed at the anxiety, apprehension and downright fear people express to me about riding a time trial bike in the wind, particularly with deep-dish race wheels. In my experience, many people go to extraordinary lengths to psych themselves out, based on a lack of understanding of how their equipment is designed to work, proper riding technique and aerodynamics. The outcome is poor results and a mindset that ‘I just don’t race well in the wind’.
This post aims to provide some practical tools for racing fast and safely in windy conditions, as well as dispel many of the myths surrounding equipment and race wheels. While much of the content applies to other riding situations, please note that this information is targeted towards triathletes and time trial cyclists riding in safe conditions on reasonable roads and/or wide shoulders. Poor/narrow roads, traffic and other cyclists in close proximity are hazards and must be taken into account. If in doubt, stay home and ride the trainer.
Wind and cycling – some basic science
Put very simply, ‘wind’ is the movement of air from an area of high pressure to an area of low pressure. Various things influence its speed and direction across the Earth’s surface, including (but not limited to) landscape features (mountains, trees, buildings, lakes, oceans), gradients of temperature, pressure and moisture, and the Coriolis Effect.
Now for some light high school physics revision – vectors (I know you all loved those!). The area of land containing a race course can be thought of as a two-dimensional vector space. Within that space, the wind moving through the area can be represented by a vector – it moves in a certain direction at a certain speed. A cyclist riding along the race course could also be represented by another vector, traveling at a different speed on a different path. As the two collide, the net angle of incidence (known as the ‘yaw angle’) and the net force of the wind on the cyclist will be a combination of the speed and direction of both the wind and the cyclist.
A yaw angle of 0⁰ is where the net wind force meets the cyclist directly head-on, with higher yaw angles indicating a net wind force coming in at an angle. HED Cycling has a great little tool which allows you to calculate the net wind speed and angle on the rider (the ‘apparent’ wind). I encourage you to have a play with it to get an understanding of how changing different variables affects the wind force as experienced by the rider. In completely still conditions, a rider traveling at 30km/h will experience an apparent wind force of 30km/h at a yaw angle of 0⁰.
An important observation to make is that a change in the wind or rider speed will influence the net yaw angle, as well as the apparent wind force. Given any constant set of (not head-on) wind conditions, the faster a cyclist travels in a given direction, the smaller the yaw angle. This means that fast riders tend to experience the wind more ‘head on’ (at a higher force), while slower riders will experience it more ‘side on’ (with less force) under the same conditions. Similarly, given a constant rider speed/direction and wind direction (again, not head-on), the faster the wind speed, the larger the yaw angle. This reflects the increasing influence of the wind’s energy over the cyclist’s energy.
To state the obvious, in order to push a cyclist off their line at high yaw angles, the airflow needs to actually interact with the bike/rider ‘package’. The lower the bike/rider’s drag, the less interaction occurs. The mass of the bike/rider also plays a small part in resisting the force of the wind (inertia). The result is the sideways ‘push’ we associate with a crosswind moving us off our line. There are other ways in which wind affects the handling of the bike however, which I will describe later.
Weathering the storm
Many cyclists’ first instinct when faced with windy conditions is to ‘fight’ the wind. Watch any windy triathlon and you will see scores of riders sitting up on the hoods or bullhorns, tensing up and gritting their teeth as they try to ‘smash through’ the wind. Not only are they going slower and wasting energy, they’re also doing untold damage to their self confidence! If you try to fight the wind, it will win every time.
With this in mind, the first step in limiting the wind’s influence over you is to get aero. Assuming you have a good fit/position on your bike (if you don’t, come talk to me!), you need to stay down on your aerobars and get as small as you can. Whatever you do, resist the urge to sit up and fight!
Next, try to maintain a fairly high cadence into the wind (at least 90rpm, preferably higher), and don’t go crazy on the power output – keep it fairly similar to non-windy conditions. Yes you’ll temporarily go marginally slower into the wind than if you try to smash through it, but you’ll avoid wrecking your legs so you’ll have more juice in the tank to power past everyone when you turn around and pick up the tail wind.
Riding ‘with’ the wind
As every cyclist and triathlete knows, hanging onto your bike in a gusty crosswind can be a handful. However, it doesn’t have to be if you know how to ride ‘with’ the wind, a technique I call ‘buffering’. To do this, it’s important to have a good understanding of what the wind is doing and the space you have available to occupy on the road.
Let’s assume we’re racing a triathlon in Australia, which means we need to keep to the left of the left hand lane unless overtaking (to avoid a blocking penalty). That means we have half a car lane of lateral space to work with, which in Australia is approximately 1.5 metres. Now let’s assume we have a wind yaw angle of 15⁰ on the left of the bike, meaning we’re experiencing winds that want to push us towards the middle of the lane.
Rather than trying to fight to keep your line on the left of the road, make use of the 1.5m buffer you have. Keep to the left of the lane, and when the wind blows, let it push you. Rather than fighting to stay within 10cm of lateral space, use the whole 1.5m at your disposal to absorb the gust, and concentrate on staying relaxed.
Similarly, with the wind coming in on the right side of the bike, position your bike towards the centre of the lane and buffer to the left when the wind blows.
It’s possible to comfortably absorb some enormous gusts within a 1-1.5m buffer zone. Knowing this, and making full use of all the lateral space you have, will save you lots of physical and emotional energy, which you can use to make time on your frazzled and burnt-out competitors.
The deal with wheels
The conventional wisdom is that deep-dish and disc wheels are a handful to control on a windy day, and this perception is magnified when races such as the Ironman World Championship ban disc wheels ‘for safety reasons’. There have been a million articles written about the pros and cons of various wheels which I don’t intend to replicate, however I will try to explain how different wheels affect your handling – and your bike split – in the wind.
Even in the most aerodynamic position, the rider’s body contributes the vast majority of the drag of the bike/rider package. Wheels make a noticeable but small contribution, but rest assured if it’s super windy, you’re likely to be blown around no matter what wheels you ride!
As mentioned earlier, a sideways ‘push’ isn’t the only way wind impacts on bike handling. Many people experience twitchy steering while riding in the wind. This is a result of a complex interaction between the wind and the front wheel, resulting in a twisting force on the the steering axis, and is the most dangerous aspect of windy conditions.
As the wind interacts with the leading and trailing edges of the front wheel at a large yaw angle, the trailing edge (usually) will produce more drag due mainly to the different profile it presents to the wind at the angle of incidence. This difference in drag between the leading and trailing edges creates a twisting force on the axle, which is transferred through the forks to the handlebars. This upsets the overall balance of the bike. Thus, the front wheel has a much greater influence on bike handling than the rear wheel in the wind. The below picture (with thanks to Bike Rumor and Knight Composites) demonstrates this effect.
Deep-dish race wheels come in various ‘depths’, from shallow 30mm deep rims up to 100mm+ deep. Within those options, different brands offer different rim ‘profiles’ or cross-sectional shapes, ranging from traditional ‘Deep-V’ to modern U-shape profiles, which seek to reduce the drag difference between the leading and trailing edges of the wheel (including the tyre) and thus make the wheel handle better. One of the most effective things you can do to improve your bike’s handling in the wind is to use a front wheel with a similar drag profile for the trailing and leading edges.
In addition to deep-dish wheels, there are also full rear disc wheels (these also come in a variety of different ‘profiles’, including flat, toroidal, lenticular, etc., designed to work best in different yaw angles) and moulded carbon wheels with 3-5 spokes such as the HED 3 or Mavic iO.
Each rim profile will perform differently in different yaw angles in terms of total drag, and will also be influenced by the particular tyre that is mounted and how the air flows around that tyre, as well as the number (less is faster) and type (bladed is faster than round) of spokes the wheel has.
As a general rule, most well-shaped wheels will perform similarly at small yaw angles. This is because everything is neatly tucked behind the leading edge of the front tyre, out of the airflow. However as the yaw angle increases, the depth and shape of the wheel becomes more important.
As you can see in this graph (with thanks to Flo Cycling) and at this link from HED Cycling, as yaw angle increases, drag decreases markedly up to around 15-25⁰, with the deeper wheels exhibiting the greatest reduction in drag. The Flo Disc and Flo 90 (a 90mm deep U-shaped rim) wheels’ drag numbers even drop below zero above around 12⁰ yaw.
To explain simply, this is because a greater interaction between the rim and the wind, combined with the tuned profile of the rim, allows the wheel to harness energy from the wind – in other words, the wind is pushing the bike forwards, like a sail propels a boat. The deeper the rim, the greater the energy harnessed. This is why the athlete in my earlier scenario was so excited – disc and deep-dish wheels are seriously fast in high-yaw wind conditions.
In contrast to regular spoked deep-dish wheels, 3, 4 and 5 spoke carbon wheels tend to outperform all others in very small (below 5⁰) or very large (above 17⁰) yaw angles, but often aren’t quite as fast in moderate yaw angles. A tri-spoke front wheel is an asset in very still conditions or when racing on a loop-course where you are likely to experience a wide variety of yaw angles. Obviously individual wheels have subtle differences in performance in different conditions.
Once the yaw angle gets seriously high, most wheels will ‘stall’ and increase in drag. However, these conditions are quite unusual or short lived. Modern wheel shapes are usually tuned to work best in the yaw angles most commonly experienced.
One last point regarding disc wheels. Rear discs have a bad reputation for being hard to handle, however this is contrary to the physics. Discs are very good at smoothing out turbulent airflow as it flows off the back of the rider, leaving less turbulence in your wake. Turbulence manifests from a mixing of low and high pressure air, and objects affected by turbulence tend to get buffeted around somewhat randomly and unpredictably (anyone who has experienced turbulence on an aircraft will understand what I mean). In these conditions, a disc has the effect of stabilising the bike, so the ‘push’ you experience from the wind is controlled and predictable. An open spoked wheel, by contrast, whips up the already-turbulent air and will move around somewhat less predictably, particularly in gusty winds.
But I ride really slow, surely the benefits of race wheels are negligible for me?
Actually, they’re even more beneficial. While drag is indeed lower at lower speeds and the drag savings will be less, the time savings are actually much greater due to the increased time you are out on course. Flo Cycling have a great blog post about this very topic which explains how this works far better than I can. In summary, getting aero will make you faster, regardless of how much or little power you put out.
Summing up
There’s no reason to be afraid of racing in the wind. With a basic understanding of aerodynamics, sensible equipment choices, a few skills up your sleeve, and a bit of confidence, you too can wake up on race morning with a grin when you hear those trees a-swayin’. Just don’t let anyone in on the secret. 😉
Guy Jones Triathlon 