The way a time gap is measured is the time taken between the last rider of the breakaway bunch and the first rider in the peloton or the next bunch down the road. You could calculate this based on distance and relative speeds, but simply using a stopwatch is the best tool to measure the gaps.
You would automatically think that “GPS” be the most obvious solution, however a GPS device such as your Garmin is only a receiver with no transmitter. It receives signals from numerous satellites to display your coordinates, speed, distance, etc. However, it would would need a massive battery and transmitter to relay information to race officials in order to calculate the information necessary to figure out time gaps between a breakaway bunch and the peloton. So how do television crews and race officials get this information?
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The way the Commisaires at the World Tour level (such as the Tour de France, Paris-Roubaix, Giro d’Italia, etc) calculate official time gaps on the road by using the timing motorbike. The bike will simply drive up to the breakaway bunch, stop, and start the stopwatch until the peloton catches up. They will then relay the gap back to the Commisaire who communicate via race radio to the team cars (who will then relay onto the riders if they choose). The race motorbike will then write the gap down on the chalkboard and show it to the riders in the race.
If the breakaway’s gap is rather large (up to the Race Official’s discretion), Commisaire Car #2 will drive up to the break and measure the gap by doing time checks using a static landmark (intersection, building etc) in coordination with Commisaire Car #1 who is back in the peloton. “Start your clock when you hit the black spotted cow on the side of the road!” However, by the time the gap is relayed it could already be inaccurate.
When watching a live television feed you’ll often see the time gap on the screen appearing to change in real time. This is not the same measurement used by Race Officials.
In major races there are always two or more television camera motorbikes. One will always go with the lead break and one will stay with the peloton. Each of the motorbikes with the cameraman has a rather large transponder which is used to calculate the time gaps based on speed and distance for the television production. Note that it is really showing the distance and speed of two motorbikes and not the riders. Wildly fluctuating time gaps usually indicate that one motorbike is changing its position in one of the bunches. Usually this time gap is often more precise than the Race Official’s calculations as it does not have a time delay. Sometimes you’ll see the chalkboard saying the gap is 4.20 but on television it’s 4.10.
Read more about how the cycling television broadcasts work here.
There are sometimes other solutions which calculate time gaps for applications like real-time race trackers. The Jayco Herald Sun Tour implemented a live race tracker with the help of a company called Flaik. They used a device which has a GPS receiver, but more critically also have a Telstra SIM in each transponder to be able to track the riders. The reason for the SIM card is so that the device can transmit the rider’s GPS coordinates to the mobile base stations and received by Flaik’s tracker application. Each rider would put the device in their back pocket so that the tracker could relay their location back to the online application. This is completely separate from the electronic timing transponders that determine the rider’s placings at the finish line and the Race Officials still rely on their own calculations for measuring time gaps.
SRM will sometimes attach devices to riders’ bikes to help them relay power and heartrate data back to their applications so this data can be analysed or tracked. These have nothing to do with calculating time gaps for Race Officials or television broadcast however.
There you have it. Measuring time gaps isn’t as magical and hi-tech as you might think, but that fact alone is quite astonishing. With all the technology available to us I find it amazing that each rider doesn’t have a device which lets us track every piece of their race data while watching it on television. The Tour de France live race tracker is excellent, but it’s only once a year and there’s more to bike racing than the Tour! If you want to see a cool timing innovation, check out this animation from Port Adelaide’s Cyclocross series.
Thank you to Matt Keenan, Gennie Sheer, and Michael Hands for helping me with all the information I needed with this post.