Cycling at the speed of light

Suppose you are in a Zwift race that comes down to a sprint finish. How long does it take for your avatar to respond to your heroic effort in the final dash for the line? Could a time lag cost you the race?

Consider the steps involved. First the ANT+ signal travels from your power meter to your device (i.e. computer or phone) then it goes to your router and on to Zwift’s server somewhere on the cloud. At some point your watts per kilo are converted into a velocity, taking account of your previous speed, the gradient, rolling resistance, drafting and any PowerUps in play. This calculation can be performed pretty much instantaneously compared with signal transmission time.

The ANT+ signal travels at the speed of light to your device, which is likely to be very close by, so there is little to be gained as long as there is a clear line of sight. The next step, to the router, can be slower, especially if you are relying on a wireless signal from your garage, while running a raft of other applications on your device (best to shut these down). Serious e-gamers often use a direct wired link to the router. It also helps if you have a super-fast high bandwidth internet connection. However, the time taken for the signal to travel from your router to Zwift’s gaming server, called latency, typically introduces the longest delay, especially if it has to go halfway around the world.

We don’t know the precise location of Zwift’s server, but let’s suppose it is in San Francisco. You can check the latency from your location to other parts of the world on web sites like this one. When I looked, the latency from London to San Francisco was 136ms (milliseconds) and from Cape Town it was 281ms.

In the past, banks have moved their trading desks as close as possible to exchanges, in order to obtain prices nanoseconds earlier than their rivals. As a general rule for interactive online gaming, you need a latency of less than 100ms for acceptable gameplay and over 150ms can become frustrating. But we are not talking about playing DOTA, so how do these figures apply to Zwift?

Zwift not DOTA

Let’s go back to our sprint finish, where the bunch is riding at 60kph. This equates to 16.7 metres per second, which is just a bit less than one bike length every 100ms. However, your ability to overtake your rival depends on your relative speed, not the absolute figure. Imagine a situation where you make a Herculean effort to increase your speed to 18 metres per second (64.8kph), drawing level with the leader’s rear wheel with 30 metres to go. To win the race, you have to make up a bike length, say 1.8m, travelling at a measly 1.3m/s faster than the leader. Who will cross the line first?

If you have 30m to go and the leader is a bike length ahead, he only has 28.2m left, taking 1.69 seconds. But at your higher speed you will cover 30m in 1.67 seconds, so you win by about half a wheel. However, if your avatar had responded to your acceleration with a 100ms lag, you would certainly have lost the race. If you experience this level of latency, a slower rider could beat you, just because he is located closer to the gaming server. The speed of your avatar really is limited by the speed of light.

However, sometimes it can feel like a zPower rider is overtaking you at an appreciable proportion of the speed of light. If this really were the case and Zwift wanted to represent the avatar correctly, what would it look like?

E. A. Cryer-Jenkins and P. D. Stevenson Gamow’s cyclist: a new look at relativistic measurements for a binocular observer Published:03 June 2020

The physicist George Gamov posed this question back in 1938. He highlighted the effect of relativistic length contraction, predicted by Einstein’s theory of special relativity. In fact, the avatar would change colour, due to the Doppler shift, and light intensity would fluctuate. These effects would be further be complicated by our binocular vision, causing an unnerving blurring effect. This is helpfully explained in detail by physicists in a recent scientific paper. Surprisingly, there are practical applications for this work that may help interpret data gathered by spacecraft passing objects at very high speeds.

Time to be aerodynamic

The Covid-19 epidemic provided a huge boost to the Zwift streaming service. Confined by a global lockdown, cyclists freed themselves from the boredom of pedalling on a static turbo trainer by logging into one of a broadening range of online virtual worlds. Zwift racing has become particularly popular. While it is relatively straightforward to simulate variations in gradient and even the effects of drafting, it is not possible for riders to demonstrate superior bike handling skills. Nor can racers benefit from adopting a superior aerodynamic position on the bike, in fact this may prove to be a disadvantage.

Setting aside e-doping suspicions, such as riders understating their weights, in the artificial world of a Zwift race, the outcome largely comes down the the ability to sustain a high level of power (watts per kilo). The engagingly competitive nature of simulated races encourages everyone to push their limits. However, since Zwift offers no penalty against maintaining a non-aerodynamic body position on your trainer, it is quite possible that regular Zwifters might become habituated to riding in position that is far from optimal for the road.

Fresh aerodynamics

Once out in the fresh air again, many riders may have noticed improvements in the levels of power they are able to sustain, thanks to the high levels of exertion required to compete on Zwift. But in the real world, when it comes to beating other riders in a race or a time trial, the principle force a rider has to overcome is aerodynamic drag, not electromagnetic resistance.

Maximum speed is attained by adopting a riding position that provides the optimal tradeoff between the ability to generate power and a low level of aerodynamic drag. Drag depends on a rider’s CdA, which represents the drag coefficient multiplied by frontal area. Since power rises with the cube of velocity, there comes a point where it is better to compromise on power in order to reduce frontal area. This is the key to time trialing and successful breakaways.

When the race season begins, skilful and more aerodynamic racers will be able to benefit from drafting in the huge wind shadow created by Zwift diesels, while offering back much less assistance when they pull through. So after prolonged training on Zwift, racers and time trialists really need to focus on improving their aerodynamics

There are various ways to reduce drag, starting withs some basics as described in an earlier blog. Post ride analysis can be performed using Golden Cheetah, BestBikeSplit or MyWindSock. There is also a range of devices that claim to offer real time measurement of CdA. These have been primarily targeted at the TT/triathlon market, but there’s no doubt that these could be incredibly useful for both training or even, perhaps, a race breakaway. Cycling Weekly recently reviewed the Notio device, but, while useful, these tools remain expensive and a bit clunky.

Whatever you choose to do, stay safe and stay aero.