A drop in temperature translates to a reduction in speeds, Cyclist investigates
There’s no doubt that, as a rule, we ride faster when the weather is warm compared to when it’s cold – a quick check of Strava KOM times will confirm it. The question that was debated in the Cyclist office one chilly afternoon recently was: why?
The answers proffered included air density (it’s thicker and creates more aerodynamic resistance in colder temperatures), tyre compounds (tyres may roll slower when they are cold), and the human body (muscles don’t work as well in the cold). But this was all conjecture, so we decided to dig a bit deeper to get the full story of why the cold kills speed.
Body of evidence
Let’s start with the human body. The easy assumption is that your muscles simply don’t work as well when they’re cold, much like a car engine. However, Dr James Pearson, biomechanics researcher at Cardiff Metropolitan University, begs to differ.
‘Even though it’s cold outside, as soon as you start to exercise every muscle contraction you make generates heat and it’s very difficult to imagine a scenario whereby you have a cold muscle temperature during exercise – it would need to be extremely cold. Your muscles are simply producing too much heat during exercise to become cold.’ What’s more, says Pearson, low temperatures actually have a role in enhancing performance.
‘It’s easier for your body to avoid excessive increases in temperature when it’s cold. Recent studies looking at pre-cooling, such as putting on an ice jacket or sitting in an ice bath before exercise, show that your time to exhaustion over a long period of cycling is increased.’ So cooling your muscles could actually help reduce overheating and strain over a long period of time.
But cold air can be more difficult for the body to take in than warm air, as anyone who has had a coughing fit on an icy day can testify. ‘Cold air is difficult to process at first, and asthmatics suffer from that quite a lot,’ Pearson says. ‘If we go from a warm room to a cold room we can get a respiratory problem where our airways don’t like the change in temperature.’ But when our airways are accustomed to the cooler air, our lungs should be firing with all their normal efficiency. ‘Generally a pre-ride warm-up will help your lungs to tolerate slightly cooler air,’ Pearson says. ‘But once that initial adjustment has been made there shouldn’t be all that much difference in overall performance.’
So physiologically it looks like the cold isn’t a hindrance to our endurance performance. But when the winter weather sweeps in, it throws a few other things out of joint.
At reasonable speeds, aerodynamic drag accounts for around 70%-90% of the total resistance a cyclist has to overcome, so a 5% difference in air density makes for approximately a 3% difference in resistive force
Round in circles
Next, let’s look at rubber. When it comes to tyres, warmer is generally considered to be better, hence why F1 teams warm up the tyres on their cars before a race to achieve maximum grip. But for cycling it’s not quite as simple as that.
Wolf VormWalde, tyre product manager at Specialized, says, ‘In principle, when rubber gets warmer it gets more supple, more flexible and more elastic. If a tyre has less flexibility it has a higher rolling resistance, because the tyre has to roll over the road, which is never perfectly smooth, so the tyre contact patch must deform to compensate for imperfections. A colder stiffer rubber takes more energy to deform and thus the rolling resistance increases.’ But changes in tyre tech may be cutting the potential losses.
‘When you look at the various tyres on the market, there are significant differences in the technology on offer,’ VormWalde says. ‘Some brands use not only natural rubber and carbon black compounds but use synthetic rubber and silica compounds. So for instance our Gripton compound [used in the S-Works Turbo tyre] works better at lower temperatures. Natural compounds get harder and stiffer as they get cold, whereas silica compounds maintain their suppleness.’ So a switch to a more synthetic tyre may soften the winter blows.
Rubber compounds aside, however, how much of a penalty in rolling resistance are we to expect in the cold? ‘We have done tests on a summer’s day where we’ve seen a temperature change [upwards] of 8°C through the morning, and that has resulted in a rolling resistance change [reduction] of roughly 5%,’ VormWalde says. Assuming that rolling resistance accounts for only 10-20% of overall resistance, a change of 8°C would make less than 1% difference in overall resistance. Significant on a 70km TT perhaps, but marginal on a Sunday ride.
If physiological impairment and tyre adhesion make little difference, why do we seem to ride slower in the winter? The answer possibly lies in the air itself. A basic principle of gas is that as it cools it becomes denser. That impacts aerodynamics: if there is more air to push out of the way, there is more resistance. But how much of a role does that really play?
Lessons from the track
Martin Bridgwood, track cycling manager for the velodrome at the 2012 London Olympics, explains that temperature within the arena was a key consideration, with 29°C at trackside chosen as optimal for speed.
‘We went to 29°C because athletes say that the warmer they are, the more their muscles relax,’ Bridgwood says. ‘But the scientists explained that it also helped with aerodynamics and hence record breaking times.’
The maintenance of a high temperature was clearly a high priority and it was no easy task. ‘The problem was for spectators in the highest seats during competition. The actual temperature at the top of the stands was close to 35°C, and our health and safety people at LOCOG were quite worried about it,’ Bridgwood recalls.
But how much difference will a change in temperature actually make? Well, for that we have to resort to a few of our own calculations. The formula for drag force is as follows: FD=½p x CD x A x VA2 (½ air density x coefficient of drag x frontal area x air velocity squared – spelling out CAV is just a coincidence).
Next we need to know how much air density changes through the seasons. The average British temperature varies from around 6.9°C in the winter to 20.9°C in summer. Roughly, that results in an air density of around 1.27kg per metre cubed in winter and 1.20kg per metre cubed in summer – that’s about a 5% difference. As air density is a multiplier in the drag force equation, a 5% rise in air density makes for a 5% increase in air resistance.
At reasonable riding speeds, aerodynamic drag accounts for around 70%-90% of the total resistance a cyclist has to overcome, so a 5% difference in air density makes for approximately a 3% difference in resistive force. All in all, that translates to a perceptible 1kmh or 2kmh difference in speed, which is significant enough to affect your ride times.
Winter riding is slower, then, and if you must pin down a culprit, air density is the main offender, with rolling resistance being given a caution for aiding and abetting. But there could be one other factor we haven’t addressed: no one really likes to ride in the cold.