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I wonder if GPS smart watches are better alternatives to some basic Garmin devices without navigation. They are better for the price and you can record the heart beat rate data in sync without the need of a heart rate belt. Their battery life also extends to weeks whereas Garmin devices batteries are limited to hours. The only thing that wonders me is if these smart watches are as accurate as Garmin devices and if we can see gradient data while climbing.

Can you share your experiences if you have used both devices?

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  • 8
    As a con for watches/ pro for devices, I've seen anecdotal comments that optical heart rate monitors are not as accurate as chest strap monitors.
    – DWGKNZ
    Dec 8 '21 at 16:23
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    I moved from a watch (TomTom) to a Garmin device. While GPS tracking appeared to be accurate across both, I felt a watch to exposed while mountain biking. I've used the map function, but never the navigation on the device.
    – DWGKNZ
    Dec 8 '21 at 16:24
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    @DWGKNZ the accuracy issue is generally accepted, not just anecdotal. That said, optical HRM is often accurate enough despite this.
    – Weiwen Ng
    Dec 8 '21 at 18:27
  • @WeiwenNg Not doubting you, but do you have good references for "the accuracy issue is generally accepted", and if so, is it current? Technology is always changing... Dec 10 '21 at 2:17
  • Is there a particular reason why you excluded the 3rd category ("Garmin watch") from your question? Also, it may be wiser to ask a very concrete question, maybe in multiple actual questions here on SE, to give the responders a bit of a focus on what to actually write...
    – AnoE
    Dec 10 '21 at 8:42
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I wouldn’t recommend using a fitness watch on the bike. At least for me it got uncomfortable pretty quickly because your wrist is bent while on the bike. In addition all the hard vibrations make the watch shake against your wrist. Because your wrists are bent the built-in heart rate monitor of many watches won’t work properly. To read data from a watch’s display you have to look down on the watch and turn your wrist. A bike computer can always be directly in front of you.

The weeks of battery life you’ve stated are without GNSS. With GNSS and always-on display most devices (watches, smartphones, bike computers, outdoor navigation devices) are limited to 8 – 20 hours. Most bike computers have a transflective display which is perfectly readable even in direct sunlight. The bigger display can also show much more information.

Many fitness/smart watches and fitness trackers can’t connect to a power meter, cadence sensor or speed sensor you might have installed on your bike.

In my opinion the question becomes much more difficult when you compare smartphones with dedicated bike computers (especially since most people already have a smartphone and many people bring them on a ride regardless of other devices). If you already have a smartphone (preferably water proof, with a bright display and good battery life) you merely need a mount to attach it to the bike.

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    Good points. Some considerations: there are bike mounts for watches, and the pricier smart watches should be able to accept ANT+ and Bluetooth connections. They are more $$, though. Also, the OP seemed to be inclined more towards basics, and they may not care about peripherals (yet?). Last, GPS is commonly understood to be nav satellites, but good catch that GNSS includes the North American GPS network and all the other sat nav networks
    – Weiwen Ng
    Dec 8 '21 at 17:23
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    I think the smartphone v dedicated GPS unit needs to be a question in itself. There are a large number of pros and cons on both sides of that conversation.
    – DWGKNZ
    Dec 8 '21 at 18:08
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    I know this sounds like "you're holding it wrong" but the watch should be strapped above ulna for the HR sensor to work. If it's interfering with your wrist movement it's way too low.
    – ojs
    Dec 8 '21 at 18:24
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    Another minor consideration: if the OP wants HR, smartphones would need to be paired to some sort of Hr sensor. Now, we could maybe convince them that they don’t absolutely need HR, but they did state that in the question.
    – Weiwen Ng
    Dec 8 '21 at 23:53
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    as @WeiwenNg says you can strap the watch to the bike (there are generic mounts, and I had one for my analog heart rate monitor)
    – Chris H
    Dec 9 '21 at 10:32
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The question overlooks some important points or gets them wrong.

  • The cheapest Garmin, the 130, does have navigation.
  • Some exercise-oriented smart watches will operate for weeks, but a chest band will run for a year or so on a coin cell.
  • Using GPS on a smart watch burns through the battery much more quickly, so that there's little or no battery-life advantage compared to a bike computer. (Garmin's Enduro watch, which has the longest runtime I've seen, can get 50 days without GPS on, 70 hours with.)
  • The smart watches that are roughly comparable to a bike computer in their features may cost double or triple what a Garmin 130 costs. A chest band costs about $50, by comparison.
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Problem with Optical HR is it is unreliable, not inaccurate. For HR accuracy within a few percent won't make a lot of difference to most people, but if your watch locks to the wrong thing, you get silly HR readings you know are wrong. This screws your training session and season averages. Optical sensors often have trouble tracking quick changes to HR, so work well enough for long endurance rides but fail miserably with high rep intervals. Also they are prone to light entering the gap between the wrist and watch, so need to be worn quite tight. Even then, on a MTB, this can be a real problem. However a vast majority (if not all) Garmin watches support BLE and ANT+ for chest strap HR sensors.

Smart watches when running GPS have battery life measured in hours - 6 to 8 is typical. When running optical HR and ANT+, battery life is reduced. I have seen 13 Days from my Garmin, but in typical use is charged a couple of times a week (Practically after each GPS use). A watch connected to a bike speed sensor can provide speed and distance data without the GPS and will run for days.

GPS accuracy is as good as any other GPS (The limit these days is the GPS system, not the device). However some watches, especially those at the lower end or not sport focused reduce processing and logging rates to save battery. Nearly Garmin watches are good enough for this.

Navigation on the small screen of a watch is difficult. You need to keep your expectations low, and a dedicated bike computer with a bigger screen makes a huge difference.

For on-ride information, a watch is not great. For post ride - provided the watch connected to on bike sensors (Cadence and speed, power meter), and a chest strap, its as good as a bike computer.

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  • I have seen a big difference between the reliability of the sensors in older watches (TomTom Runner 3) and more recent watches (Garmin Forerunner 55). The new ones are very reliable. My heart strap has been accumulating dust this year.
    – Vladimir F
    Dec 9 '21 at 8:39
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    Chest straps aren't all that reliable either, after a while, and poor contact can lead to unreasonable or zero readings. The best HRM I've used (borrowed) was an upper-arm optical one (Bluetooth IRC) but it was expensive and chunky
    – Chris H
    Dec 9 '21 at 10:44
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    Actually even Garmin's cheapest watches now have quite reliable HR. I have an older expensive one which has reliable HR on a bike (but not when running or otherwise shaking the arm too much) and a new cheap one which is very lightweight and thus stays nicely put in the right location even when worn quite loose and was always accurate so far.
    – Nobody
    Dec 9 '21 at 17:19
  • Optical HR is very dependent on you own physical makeup and activity. Vein location, skin tones, blood flow (reduces in cold conditions), along with light interfering with the sensor, and manufacturer make/model/software. Variations between brands and models add to the mix, so on balance, it is best described as unreliable then follow on with the PHD thesis required to explain the limitations and variations. (HR monitor are significantly more reliable in hard conditions, but as indicated by @ChrisH, still not 100%. )
    – mattnz
    Dec 9 '21 at 21:08
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One point - the watch is on your wrist, compared to a computer on your handlebars and a phone is in your pocket..

Only one of those can be seen without letting go of the handlebars, briefly for the watch and quite awkwardly for a phone.

Therefore the watch is less safe than the computer.

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    As I commented elsewhere, there are bike mounts for watches if the OP is determined to go that route. Naturally, they will preclude use of the optical HRM.
    – Weiwen Ng
    Dec 8 '21 at 18:25
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    And of course there are many ways to mount a phone where it can be seen hands-free. I can't recommend using a phone without a mount even for audio turn-by-turn. I've tried it and had to ride so cautiously (because I could only hold on properly with one hand or had to fiddle with putting the phone away) that the ride was tediously slow. I still left a colleague behind though.
    – Chris H
    Dec 9 '21 at 10:47
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    @ChrisH Phone screens are very difficult to see in bright sunlight compared with bike computers, which use transflective screens. You need to run them at very high brightness which results in extremely low battery, and they are still much harder to read. I do have a phone mounted, but I wouldn't be without my Garmin for the always on data I want to see (power, heart rate, speed, cadence, etc)
    – Ivan McA
    Dec 9 '21 at 15:49
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    @IvanMcA I use my phone. I can navigate for 17 hours on a charge, on a sunny day with the right settings (screen dims after a few minutes until I tap it, also mostly on aeroplane mode). I find it far easier to read than a Garmin I borrowed (830?) with a much clearer map and text, and everything I need on one screen. I managed a week without power except my dynamo and a couple of big battery packs (one still 3/4 full at the end), so it even works for touring .
    – Chris H
    Dec 9 '21 at 16:03
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    ... me 2 years ago. I now carry a backup phone, turned off in my jersey pocket, on long isolated stuff. That was instead of going to a dedicated navigation unit
    – Chris H
    Dec 9 '21 at 16:08
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I have been useing a sports watch on a bike for many years. It is absolutely fine if recording your ride is what you are mainly after - and that is certainly my case. And regularly checking the time, distance and maybe the current speed from time to time.

I use it for all my other outdoor sports, cross country skiing, hiking, running.

One cannot check the watch in every ride situation, especially off-road. It is not always safe or even convenient. But in my kind of riding, one can normally fine a moment when it is safe pretty soon.

For continuous monitoring you would be better served by a head unit that you can always view. I do have a cheap mapping gps unit, but only carry it when expecting problems with navigation.

The situations when changing the modes of the watch is safe or convenient are even less common than for just viewing the watch. Be prepared to have the watch display in one mode all the time so set it up accordingly.

As for the heart strap issue: You can use the hearth strap eith either if you so wish. I certainly did so, when I wanted, that is not an issue at all. But with my new watch, I do not see the need to use my heart strap at all, the HR sensor is very reliable (unlike on the old TomTom with a first generation sensor).

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  • What is your smart watch make/model?
    – Ender
    Dec 9 '21 at 9:30
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    Garmin Forenunner 55, the cheapest line, missing some cycling features.
    – Vladimir F
    Dec 9 '21 at 10:29
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    Notably, it does NOT show the gradient during climbing.
    – Vladimir F
    Dec 10 '21 at 7:42
2

Per our FAQ, we technically don't allow most opinion-based questions. I'll instead focus on things to consider if you're choosing between a basic head unit/GPS bike computer (many of which aren't made by Garmin!) and a smart watch.

Some smart watches are all-rounders (the Apple Watches are one obvious example) and some are more sport-focused. We are all assuming that you’re interested more in a sport-focused wearable (my term, not sure if it’s a generally accepted one). The sport-focused wearables are the ones with the long battery life, although others have stated that GPS or other satellite navigation systems and the optical HRM will drain the battery life a lot faster than wearing the watch on standby. To some extent, this will depend on the unit’s power saving algorithms. Some wearables may have lower refresh rates than others, or the option to refresh at a lower rate.

This has been stated already, but to phrase it differently: a wearable is fine for logging your ride afterwards, but if you have it on your wrist, it won’t be as good as a bike computer. You may be fine with this. There are bike mounts for wearables, but to state the obvious, you will not be able to use the optical HRM if you mount a wearable on the bike.

You might reconsider how useful heart rate data are. As an estimate of calories burnt, heart rate is very imprecise. As a means of pacing interval training, heart rate is useful and a lot cheaper than power, but it wasn’t clear that you wanted to do this. As a side note, a lot of general purpose smartwatches can’t natively broadcast heart rate data to a bike computer. The Apple Watch is one of these. There may be software workarounds available, but these watches may not be designed to do this, and Apple may be particularly bad about this functionality. I believe that many Garmin wearables are able to broadcast HR data, but consider that Garmin is a sports and navigation company, whereas Apple is a computer manufacturer (and they like to tightly control their software).

If you’re involved in multiple activity types, that could skew your preference in favor of a wearable for the obvious reason that bike computers are specialist units.

One factor that nobody has addressed is your preference for gradient data. I am not sure how to check this, and I did look. In theory, any wearable with an activity profile for outdoor cycling might have gradient as a selectable field. However, I checked the manufacturer pages for a few major wearables, and I couldn’t see any specifics about the cycling profiles. This could be because relatively few serious cyclists use wearables as a primary computer. In theory, I suspect that any GPS-equipped computer could be programmed to use GPS functionality to estimate gradient, including a watch. I don’t know which ones have been programmed thusly. (It’s generally accepted that if you want accurate elevation and gradient data from your head unit, it should have a barometric altimeter, which not all do. I am not sure how prevalent this capability is among wearables, but I’d guess it’s not common, and especially among more entry level wearables.) As with heart rate, you could consider if accurate, real-time gradient info is a critical feature. If you upload your ride to Strava, you will be able to check gradient after the fact.

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  • I had asked about gradient data in my question. I also think only the top end sports-oriented watches may have this data available on the screen. I have a Chinese brand GPS cycling computer with gradient data function but it's inaccurate most of the times. Maybe my area is not well recognized or documented by the satellites outside the main roads.
    – Ender
    Dec 9 '21 at 9:44
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    @Ender if the computer gets altitude from the GPS satellites, it shouldn't matter if you're on road or off, in a well mapped area or you've found a section of the Earth that's never been mapped. The display should be able to tell you where you are and how high you are, even if it can't draw a known map around you.
    – FreeMan
    Dec 9 '21 at 15:04
  • @Ender if the computer gets altitude only from GPS satellites, it is going to be inaccurate enough that gradients calculated with it will look completely crazy. A GPS/barometer hybrid approach is better but still not infallible.
    – ojs
    Dec 10 '21 at 10:12
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The watch is only good on a bicycle when you wear short sleeves. Otherwise it is usually gets obstructed by the clothes on your hand. If not outright below the glove, then the jacket is covering it enough.

Mobile phone has another problem - the display may not be viewable in direct sunlight.

As a result, probably the dedicated bicycle computer is the best option if you want to see something during the ride. If not (like just make logs for Strava), I see no reason why your smartphone you likely have anyway could not do.

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  • There's this trick that Scuba divers have known for a long time: wear watch over sleeve. If you want HR, use a separate sensor.
    – ojs
    Dec 13 '21 at 8:13
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The answer is a bit different is you speak about generalities, or if you want to purchase new devices and weight in the different options. The first one has already been addressed in the other answers, so my answer will be focused on the second one.

If you consider purchasing new devices, it is important to consider the additional specs required by cycling, especially if you like to ride more than 2/3 hours. Unlike what you write, if the GNSS is activated, the autonomy of a sport watch is counted in hours, and that varies greatly depending on the model you choose: entry level models only last a couple of hours, and high-end ones 10 hours.

Also, sport watches are also less integrated to the smartphone ecosystem and less convenient to use for non-sport tasks (especially if you are on the Apple side), so you have to see if you're interested in giving up some every day conveniences, just "to have one device".

Given what you write, if the features set you need is covered by entry-level cycling GPS+sensors, taking the cycling GPS has much more pros than cons compared to a smartwatch.

On the pro side for the cycling GPS:

  • you have more smartwatch options (and smartphone options),
  • a screen that is always on (and more legible under direct sun light) and in your field of view,
  • a device that is designed to be operated when cycling,
  • it can cost less (especially if you factor that you'll put less pressure on the watch battery, which means it will last longer).

On the con side:

  • more devices/accounts to manage.

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