Garmin 500
 

Am I correct that when using this computer on the road you do NOT need the speed/cadance sensor to track speed (because this is monitored via GPS)?

If I were to use this computer on the trainer indoors, I assume the speed sensor would be needed if I wanted to track speed.

Thanks.

Use a sensor. You don't NEED it to track speed when outdoors, but the GPS-only calculations are not nearly as accurate compared to when you use the sensor. The sensor is necessary if you are using a trainer or rollers to give you some speed-like measurements.

No you do not need the speed sensor whenever the GPS can "see" the satellites. The GPS will give you location within a few feet and speed within some pretty precise limits. Of course while you are going up or down hills the speed shown on the unit will be a bit off. But, over the entire ride is will be more than accurate enough for just about any purpose. Speed sensors have their own inaccuracies that render them less than precise as well.

I would think a cadence sensor would be much more useful for indoor riding since you aren't actually going anywhere and a good workout is the goal.

Here's a relevant article in the New York Times that talks about running with GPS. Basically, at slow speeds and over tortuous routes, GPS errors add up to reduce the accuracy. Can't use a wheel sensor running, but you can on a bike.

http://www.nytimes.com/2011/12/20/he...EXCITE&ei=5043

The foundation of the article is mis-directed at best. It concentrates on how the GPS is "wrong" and that differenct GPS units are "wrong" to different extents. In fact, what is happening, and which is briefly mentioned, is that the GPS is being used in a wrong way. GPS navigation is extremely accurate and is used in aviation, shipping, military guidance, surveying, etc. But, like all tools must be used correctly. By the way Google Maps is not a suitable reference if a person wants To The Foot accuracy.

In that regard instead of following the time honored Forum practice of arguing the point I suggest that if you are really interested spend some time researching in the areas I mentioned. There is a wealth of material available.

In the end I'll bet you come to the conclusion that for cycling speed and location the 500 gives perfectly adequate data.

People don't tend to criticize wheel speed sensor accuracy because it is seldom evaluated. But, once through the setup and you'll see it has its' own problems.

Me? I've found that over the typical 30-60 mile routes I ride there is an insignificant difference between my 705 GPS input reading and the reading from the wheel speed sensor input.

Exactly. The correct way to use it on a bike is to supplement it with a wheel sensor to decrease it's degree of wrongness.

Here is how gps works. Taken from a website: A GPS receiver's job is to locate four or more of these satellites, figure out the distanc*e to each, and use this information to deduce its own location. This operation is based on a simple mathematical principle called trilateration. So if you drop or have bad signal you suffer erros in data. So just do what Looigi says and supplement with sensors. If you don't care that much don't put the sensors on. I ride with a buddy who never put the sensors on his bike.

I never bother with the wheel sensor. GPS is plenty good enough for me. It's easier to swap the unit to different bikes, or swap different wheels on the bike without having to mess around with wheel sensors.

Me too. Once I realized just how little difference there was as related to a recreational rider I stopped worrying about the sensor. If it is a recreational ride GPS is plenty good. If it is training it still is plenty accurate. If it is a competition the course should be surveyed with something more precise than either the GPS or the speed sensor that has its' own inaccuracies.

But, I can remember those bad old days when I, too, got all twisted around the axle with pseudo precision and tracking everything to the knats behind. Then I realized what I was doing was akin to the carpenter who measured with a micrometer, marked with a crayon and cut with an axe.

You are correct. I use my Garmin 500 on my bikes for commuting (on a cyclocross or a folding bike) without sensors. On my road bikes though I have sensors so it can also measure cadence. As others have noted, there might be a slight increase in speed accuracy when using sensors.