GPS vs Speed/Cadence Sensor

Neverlandland

Hi Guys,

I'm thinking about getting a new computer. It will be used 80% of the time for cycling and 20% running. The item I was looking at was the Garmin Edge 500 (black) and I was going to buy it without the extras for now, which I can get at a later stage. I had a look at one in the shop yesterday, and they're tiny and could easily be slipped into the pocket for a run.

Now, I am wondering whether it would be better to go with a watch, such as the FR60 + the speed sensor. The only big differences between the 2 (other than cost) seem to be the lack of altitude readings and GPS.

Would a well calibrated speed censor be just as good as a GPS though? I don't know how often GPS computers take readings, but I imagine the censor gives a very good indication of how far you've gone also?

Does anyone here have any experience of using either of these, if so I would be interested in hearing your opinions?

Thanks

smacl

Autonomous GPS (i.e. GPS with no corrections) is not particularly accurate, e.g. absolute accuracy of plus or minus 30 metres in plan and 50 in elevation for a reasonable receiver, relative accuracy somewhat better but prone to big spikes and various interference factors and outages). Differential GPS is better, where you are receiving a correction signal, and the accuracy is based on the type and quality of correction, with good systems delivering 1 - 2 meters. GPS is ok for average speeds, but not good at measuring instantaneous or current speed.

Methods counting cadence or bike wheel revolutions are much better at measuring speed and shorter distances if they are well calibrated, but can suffer from a scale error and are hence less reliable than GPS for measuring longer distances. So for example, if I select a standard a wheel diameter on my bike computer, but this is slightly off if I have thicker tyres, the distance travelled is slightly less than the actual distance travelled. This error accumulates over longer distances, and is much more of a problem with running as you stride length varies. It's also important to remember distances measured using this method are slope distances (e.g. amount of tarmac covered), whereas GPS is giving plan distances (e.g. the distance you'd get by scaling off a map). Plan distances will be shorter where you are on hilly ground.

Good kit, such as the high end Garmin GPS range, can use multiple sensors (GPS, cadence, altimeters, tilt sensors, etc...) and combine the data to get the best results, but you pay for this accuracy.

I tend to use cadence / bike measuring wheel based stuff where I'm only interested in speed and distance covered, and I tend to re-calibrate based on maps every so often. GPS scores where you want navigation and background mapping, and want to overlay where you've been on a map after the event. Great if you like exploring the back roads, or go hiking, which I regularly do. For running, I use a Timex HRM with datalogger, and look at the distance covered on a map later.

Neverlandland

That's very helpful Smacl, thanks. Do you connect the bike sensor to your Timex watch, or do you have a separate computer for your bike?

I was considering just buying a cheap watch for the running that can record split times, and I can then worry about average speed when I get home. With the distance I'd be running (10k) it wouldn't be hard to memorise target split times. I think I would be more interested in an average/instantaneous speed feature on the bike rather than when running.

I think it's nearly a case of trying to justify the money for the edge 500, when a watch can do nearly everything that I need (albeit with a bit less accuracy), but be a hundred euro cheaper.

Neverlandland

Just to ask, those with Smartphones, how would the cycling apps compare with something like a Garmin cycling computer?

smacl

Neverlandland wrote: »

That's very helpful Smacl, thanks. Do you connect the bike sensor to your Timex watch, or do you have a separate computer for your bike?

I have a watch that links to a heart rate monitor and small data recorder for running. The heart rate monitor gives me a good idea of effort I'm putting in, and graphing it later you can actually see the hills on route as increased heart rate. I also use this on the bike occasionally, but more often don't bother.

For the bike, I have a cheap Cateye computer for speed, distance, and time (€16), which is a great piece of kit for the money. There have been similar ones for sale at less than €20 in Aldi and Lidl over the week. I also have a much older Garmin GPS, bought for hiking originally, that I can download routes and show limited background mapping. I bring this on long solo cycles if there is any risk of getting lost, and it has been a lifesaver on one or two occasions (e.g. been on the bike all day, losing light, don't know quite where I am and should I go left or right at the next junction). Very good for long point to point journeys on unknown roads.

ashleey

Get the garmin edge 500 for cycling and you won't be disappointed. You can use altitude corrections after upload to garmin connect for accuracy. Then you don't need to worry about wheel magnets etc.