Cadence sensor vs GPS

Gabbo

I'm currently using both, of course. Problem is, without my cadence sensor it takes me longer to clock a mile and the gps records me cycling slower than when I'm using my cadence sensor. Either GPS' are inaccurate or I've incorrectly installed my cadence sensor. But the whole point of this post is to get answers as to which device is lying!

Anyone experienced the same issue?

DHA987S

Is the wheel size correct in the cadence sensor (assume its a Garmin, you set it in options). GPS needs to be able to "see" and if you ride under tree cover/tunnels/buildings it will lose sight of the satellites and this causes the route to jump. The combination of the two sources is the most accurate you will get from a small bike based GPS.

antfly

Garmins don't combine them, if you are using the sensor it uses that for speed and distance, just make sure it's calibrated, auto is good enough. Your GPS is probably accurate.

sungod

a consumer-grade gps, especially one cut-down to fit a bike or smartphone, is poor at determining distance travelled or speed unless you only travel in a straight line on flat ground (no doppler, poor altitude accuracy, low sample rates, and the effects of trees/buildings on signal)

on a twisty route, a basic cycle computer calculating speed/distance (calibrated to match wheel circumference) will be much more accurate than a bike gps

antfly

I agree, a well calibrated and set up computer is always better, which is why Garmin uses it to override GPS, but when I download my rides {always hilly} it gives GPS and sensor readings and they are always almost exactly the same so I think you are underestimating Garmin GPS.

turnerjohn

Gabbo wrote:

I'm currently using both, of course. Problem is, without my cadence sensor it takes me longer to clock a mile and the gps records me cycling slower than when I'm using my cadence sensor. Either GPS' are inaccurate or I've incorrectly installed my cadence sensor. But the whole point of this post is to get answers as to which device is lying!

Anyone experienced the same issue?

I'm the other way round...when I had a wireless computer I ride fast up hill then my Garmin....I did wonder if Garmin speed takes into account % incline as you travel further actual then viewed directly from above aka a sat ?!

sungod

antfly wrote:

I agree, a well calibrated and set up computer is always better, which is why Garmin uses it to override GPS, but when I download my rides {always hilly} it gives GPS and sensor readings and they are always almost exactly the same so I think you are underestimating Garmin GPS.

it's heavily dependent on route/conditions, and some gps units are better than others, i've got a garmin too, but i only use it for navigation, for training i use the cycle computer

there's a running-oriented article here, there was quite a variation between different gps units...

http://www.nytimes.com/2011/12/20/healt ... =all&_r=1&

antfly

Running is different from cycling.

rolf_f

antfly wrote:

I agree, a well calibrated and set up computer is always better, which is why Garmin uses it to override GPS, but when I download my rides {always hilly} it gives GPS and sensor readings and they are always almost exactly the same so I think you are underestimating Garmin GPS.

Plus one - I find the differences to be so small as to be negligible. Unless you record distance to a tens of feet accuracy.

Gabbo

antfly wrote:

Garmins don't combine them, if you are using the sensor it uses that for speed and distance, just make sure it's calibrated, auto is good enough. Your GPS is probably accurate.

I use Garmin Edge 500

Sounds stupid but I'm not sure how to calibrate them. The device is physically set up and synced to my Garmin, but as for calibrating I have no idea. Will check on it later if there are certain options.

Thanks

p1tse

Also cost to consider

sungod

antfly wrote:

Running is different from cycling.

to post such an irrelevant comment indicates that either you are trolling or that you do not understand the various sources of error

neither makes you look good

MattC59

Gabbo wrote:

antfly wrote:

Garmins don't combine them, if you are using the sensor it uses that for speed and distance, just make sure it's calibrated, auto is good enough. Your GPS is probably accurate.

I use Garmin Edge 500

Sounds stupid but I'm not sure how to calibrate them. The device is physically set up and synced to my Garmin, but as for calibrating I have no idea. Will check on it later if there are certain options.

Thanks

You need to measure your wheel / tyre circumference. Put a chalk mark on your tyre, roll it along the ground starting with the chalk mark level with a fixed mark on the ground. When the wheel has moved one rotation and the chalk mark is back at the ground, mark that point on the ground. Measure between the marks. You can then enter this into the Garmin in the 'bike settings' menu.

I found that the difference between the auto value and measured value was several cm. worth measuring and inputting yourself IMO.

antfly

MattC59 wrote:

Gabbo wrote:

antfly wrote:

Garmins don't combine them, if you are using the sensor it uses that for speed and distance, just make sure it's calibrated, auto is good enough. Your GPS is probably accurate.

I use Garmin Edge 500

Sounds stupid but I'm not sure how to calibrate them. The device is physically set up and synced to my Garmin, but as for calibrating I have no idea. Will check on it later if there are certain options.

Thanks

You need to measure your wheel / tyre circumference. Put a chalk mark on your tyre, roll it along the ground starting with the chalk mark level with a fixed mark on the ground. When the wheel has moved one rotation and the chalk mark is back at the ground, mark that point on the ground. Measure between the marks. You can then enter this into the Garmin in the 'bike settings' menu.

I found that the difference between the auto value and measured value was several cm. worth measuring and inputting yourself IMO.

There's no point in doing the roll method if you are not sitting on the bike while you roll it, easier and better just to put in the quoted figures for the tyre size in use.