Lap Timing Part 1

One of the most useful tools to measure your progress on track is a lap timer.

A lap timer enables you to “see” how changes in your riding style, race line, tyre choice, gearing or suspension setup affect your lap times.

Normally these devices cost from a few to many hundred of pounds (or dollars) depending on which make you buy and the features that it offers.

How would you feel if I told you that you could potentially have all this lap timing capability for free?

Pretty excited, I’d imagine…

With the advent of smart phones, this is a reality because a small group of Finnish software developers created a lap timer called RaceChrono for mobile phones. It runs on Symbian (Nokia), Windows (Mobile) and Android devices. Sadly, there isn’t a version of RaceChrono that runs on iPhones so those users might want to consider getting a “cheap” Android phone to use instead.

If you have one of these phones that can run RaceChrono app then you can get a fully featured lap timer… for free. Yes.. you read that right! All you need do is to ensure is that your phone incorporates a GPS receiver or has bluetooth capability built-in to allow it to connect to an external bluetooth GPS receiver. Without some form of GPS, RaceChrono cannot work.

How it works
RaceChrono has details of over 400 tracks around the world including different track layouts for the same circuit. The track at Brands Hatch for example has two layouts; the Indy circuit or the full GP circuit.

Each track has a start/finish line recorded as well as the location of either one or two additional split time markers.

As you ride around the track, RaceChrono will use the GPS receiver to determine where you are on track (up to 10 times a second). In this way, RaceChrono is able to determine when you pass the start/finish line or any of the other split timing markers. As you do this repeatedly during your track session, RaceChrono will mark off each lap and calculate your lap and split times automatically.

In order to use RaceChrono for your track day, just choose your circuit (and track layout) from the list. If the track doesn’t exist in the list then you always have the option of adding your own track and defining the start/finish line and any additional split timing markers that you want or need.

Just before you start your session, switch the app on to record, slip the phone into your pocket or attach it securely to your bike, and you’re good to go. There is no need for any external track side timing equipment for the app to work – just clear line of sight to the satellites in the sky.

Accuracy
In fact RaceChrono works so well, that anecdotal evidence suggests that it is accurate to within a few hundreths of a second compared to the times reported by official track side timing equipment.

But, that accuracy comes at a (small) cost…

The internal GPS units built into most phones, while often very accurate, cannot sustain a high poll rate. This means that they often cannot report a position fix more frequently than once a second. When you’re travelling at 100mph (160kmh) that equates to a distance between two fixes of around 48 yards (44 metres). A GPS unit that can operate at 10Hz (polled 10 times a second) will get a fix every 4 metres when travelling at 100mph. The faster the GPS can be polled, the more accurate your lap times will be.

A further issue affecting accuracy is that the phone internal GPS receivers normally don’t have a differential capability (DGPS) built into them. If they did, it would allow them to compensate for the selective inaccuracy that the US military build into the GPS network.

You can overcome both this issues by using RaceChrono in conjunction with an external Bluetooth GPS receiver. Two popular ones are the Garmin GLO and the QStarz 818 series – these typically cost between £50-90 (US$90-150) depending on the unit.

In the case of the QStarz 818XT (which I have), the DGPS capability is disabled at 10Hz, but can be enabled by running the unit at 5Hz instead. 5Hz means a position fix every 1/5 of a second and is perfectly good enough for accurate lap times, especially since the position accuracy is enhanced by the DGPS capability being enabled.

Although you might be wondering whether to bother with the hassle and cost of an external receiver, it is worth noting that in addition to providing inherently better accuracy, you can also mount these units so that they will have a better view of the sky compared to a phone in your pocket. Popular mounting points include on/under the tail or under the fairing bubble. All you need is some gaffer/duct tape to stick the unit in place.

Relying on the internal GPS of a phone stuffed into your pocket when your body is shielding much of the sky could lead to poorer quality position fixes at certain points on the track and as a result less accurate lap and split times.

Now that we’ve covered the preliminaries, I’ll explain how the app works in more detail in Part 2 of this article.

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