I see many riders with a Garmin Edge or another dedicated bike computer, and then an iPhone in their jersey pocket. Are there certain reasons that we can't use the iPhone as the bike computer?

Nope. The only limitation may be size, battery life, and exposure to the elements. Go for it.

The most common reasons are battery life of your phone and lack of ant+. There are some phones with ant+ now but not iphoiP as far as I know. I used to have a Garmin but now I'm using my Android phone with a mount and I have no complaint . Bike computer apps use GPS so they can drain battery so if you are the type that is going on 4-6+ hour rides that may be a concern . My rides are usually under 2 hrs and my phone gets great battery life so I don't see that as an issue for myself anymore. There are plenty of Bluetooth accessories so that ant+ isn't an issue. I use Bluetooth cadence, speed and heart rate with no problem. Some people claim ant+ is more reliable and less prone to interference but I think the latest Bluetooth protocol gets rid of most of those issues .

Bigger, heavier, more fragile, more expensive, worse battery life, and doesn't talk to my PM.

What do you want it to do? If you want constant display of data - speed, cadence, HR, distance - then you will quickly grow tired of having to touch the screen every few minutes to have it show you that information. You will then also quickly drain the battery during your ride by ensuring a constant display. You will also tire of having this large device mounted to your handlebars, and become angered should you wreck and have it crushed or fly off.

But, if you just want to stick it in your pocket to track your ride for review later, then no need for a computer.

I haven't used an iphone for bike computer replacement but I've tested 4 apps on my Android phone and they all display info while I'm riding just the same as my Garmin did. Screen doesn't go to sleep, there is no need to touch the screen during ride to show me any information. I'd assume the apps like Strava, Map My Ride, Wahoo tracker, bike computer all work the same way on iphone The size of the phone is a consideration if you have one of the bigger iphones. I have an android phone with a 5.5" screen and it fits perfectly on my stem but that depends again on size of the phone, size of your stem etc. I have a $20 mount that is shock proof rubber and there is no way the phone is going to fly off. I've been over all kinds of bumps and quick stops and fast descents and it stays put. There are more expensive mounts like Quadlock that are even more substantial. With the correct mounting solution the chance of your phone flying off your bike is about the same as your bike computer flying off your bike and I've never experienced either. If you wreck your bike you will have more problems then just your phone but if you wreck you could break your bike computer and you could break your phone if it's in your jersey pocket as well. If you are worried about that you should probably pay for insurance on your phone. I touched upon battery life and that is a concern depending on your phone and how long your usually ride. I won't argue with anyone that wants a dedicated bike computer but the fact is that any smart phone can provide all of the functionality of a bike computer and more and some of us don't feel the need to spend $250+ on redundant tech when we are carrying our phone anyway

I use my iPhone with the Panobike app. Third party bluetooth sensors work with the Panobike, I have speed/cadence and HRM. You will need one of the phone holders to use your phone as computer though. I use this one: https://www.morpheus-labs.com/

If you're doing 1-2 hour rides in your local area, the phone will do the job just fine. Start adding sensors to the bike, and going on longer rides, and a dedicated computer is a better solution. I did ~5 hours today, and my phone went through about 30% of it's battery doing nothing but playing Pandora. Ride apps like Strava devour battery.

To be fair, everything devours iPhone batteries. Thanks Apple for planned obsolescence.

I never said iPhone. Buuuuuttt... I get on average, 3 years out of each iPhone. People "upgrading" annually are doing so out of want, not out of need.

My wife's iPhone 6 battery only lasted about 3 hours on a full charge after 1.5 years. "Lucky" for us Apple replaced the battery for $30 since they know that their iOS upgrades caused battery issues

coincidentally, new stories about this today:

https://www.cnet.com/news/apples-bat...with-ios-12-1/

https://www.forbes.com/sites/ewanspe...ttle-problems/

​​​​​​​lol that's 3 limitations

iPhone data is also very inaccurate and all over..the data on bike computer is more accurate as sensors are built for specific purpose.

i also hate operating touchscreen while cycling..buttons are better as u dont have to look at them to operate them.

It's because the GPS in phones doesn't do a great job of plotting points at speed. Standing still, it can place you pretty well-- at 20mph, the margin of error is like 10 meters. The Strava app can also be a bit... wonky.

I have Wahoo Bluetooth speed, cadence and hr monitor and they work great with my phone and it's just as accurate as my Garmin 520 was

​​​​​​​I like to give bonus answers.

One issue with the phone (that won't apply to the OP given where he lives) is that in the cold, the batter can drain really quickly. I've had my phone go from 50% down to 5% on a 1/2 hour commute in winter (about -12 degrees that day). Once the phone warmed up, it went back up to something like 35%, but had I been going for a longer ride, I would have missed recording a bunch of it. I don't doubt a cycle computer would potentially experience a similar issue, but it's easier to keep that fully charged as you're not using it for other stuff all the time. Plus, the longer battery life in general will counter this somewhat.

following

I have never, EVER ridden with someone who was using the phone app and had them end up with less distance or elevation. Last time out, the person using the phone app had the most distance, and I knew for an absolute fact I had ridden about 1 mile further, as I had ridden ahead and looped back several times, and made several parking lot loops waiting for others to catch up. Now I can't 100% say it's the phone or the GPS-- as the apps like Strava and RWGPS basically use their own math.

Phone GPS latency also helps guys gobble up short-segment KOMs-- the ones on Strava where their average speed is higher than their max speed.

Battery life, potential damage to the phone, and not wanting something that large on my handlebars are my reasons for not using my phone as a bike computer.

I have a basic GPS bike computer that cost ~$50. It's significantly more accurate than the Strava app I used to run on my Android (results are fairly consistent with the Garmins that my riding companions use), and can pair with heart rate and cadence sensors. It just doesn't have the mapping features that one gets with the higher-end Wahoo or Garmin devices.

My Android phone is not much bigger then a Garmin 1030, weighs about 30g more, pairs with my Wahoo speed, cadence and HR monitors and has GPS/Maps/turn by turn and I already own it.

no reason you can't. lots of reasons why you (me? some people?) prefer not.