Someday mobile phones will work perfectly as cycling GPS devices. However, the current state of phone-to-bar mounting systems, battery life, weather resistance, and tracking apps all mean that a dedicated GPS unit -- specifically a Garmin 800 or above -- is, by far, the right way to go.
As always, we're specifically talking about long hours of mixed-terrain riding in all weather conditions.
Here are the fundamental shortcomings of using your mobile phone instead of a Garmin:
Crash and Damage: When do you need your phone most? Probably it's when you have an emergency, maybe a crash. If your phone is on a bar mount, it's very exposed in a crash. And the phone-to-bar mounts that are worth anything are not designed to move in a crash. For a number of reasons, including the bar mount design, your Garmin is very unlikely to get damaged in a crash.
Battery Life: If you're looking to do an all-day ride, your phone is definitely the wrong tool for the GPS job. There's two aspects to this; 1) Many phones can't run a full day without getting near a dead battery. This is not a chance we're interested in taking on a ride. We are strong believers that your phone is for emergencies and that phone has to be ready for any emergency. If you're using your phone for GPS and the battery starts running low, now what? There have been plenty of rides where we have to turn our phones off just to conserve battery and we don't even use them for GPS tracking. And, 2) Having your phone constantly run GPS tracking for you burns your battery really quickly; we guess it's around 50% faster burn-time compared to when your phone is not tracking.
Rain Damage: Most phones don't like getting wet. If you're riding in an eight-hour rainstorm, a Garmin unit will be fine. Almost every phone on the market will have problems -- major problems. Covering a phone in a plastic baggy or using one of the shielded bar mount systems will not protect the phone for eight hours of rain. I wouldn't trust it for an hour of rain. Don't risk losing your phone's capabilities -- particularity if you're caught in a rainstorm; keep your phone functioning for emergencies.
Cold Weather Issues: If you're riding in temperatures below 25 degrees, keep your know in your pocket. If you use your phone for GPS tracking on your handlebars, the battery will quickly die and you will be frustrated. Here are some details about why you don't want to use your phone in cold weather.
Backup Plan: We always like to have a backup plan for every piece of electronics. We make sure we have redundancy for GPS tracking. In fact, we ride with two Garmins for expedition riding. The phone is the triple backup in case we have trauma with two Garmins. Ensuring we can figure out where we are by GPS is one of the most basic safety tools we use.
Emergency: The reason we bring a phone on adventure rides is for emergencies. We won't do anything that could make the phone a brick in the case of an emergency. That means not draining the battery unnecessarily, not exposing it to harsh environment, and not risking damaging it.
As always, not all of these reasons are equally important, and some of them may not be important at all to you. We list all these reason because every rider has different priorities and every situation is different.
Also, we know that at some point all these issues will be solved and using a mobile phone will be a very viable option. Someday it will be a better option than a dedicated GPS unit. However, we're still at least a few years away from this; probably more.
Disclaimer: We're not great fans of Garmin units -- they have their challenges -- but every other system we've used has had a lot more issues. We wish there was some competition, but there's not. Mobile phones are now competition for Garmin for some kinds of riding, but not for the kind of adventures we like.
Do you use your phone for GPS tracking? What's your experience?