redryder74

Hi,

I recently bought a Cateye Wireless bike computer and was shocked to see that the distance estimates were 4 to 8km less than that reported by my iphone apps. I tested Runkeeper and MapMyRide, Runkeeper recorded a distance about 8km more than the computer, while on another day MapMyride recorded 4 km more than the computer.

The computer was installed by the LBS, and it was set to my correct tire size (I checked). Which should I believe? I'm training for distance rather than speed.

Spld cyclist

Was the wheel size set according to a measurement of circumference or by the chart in the computer's instructions? Carefully measuring circumference by rolling the wheel over a surface and measuring the distance for one full revolution is generally far more accurate than using the table. If the computer is programmed based on actual circumference, it should be more accurate than your GPS.

MinnMan

What Spid said. Things like tire inflation and deformation under load can have a non-negligible effect on distance/rotation.

You didn't say if the 4-8 km difference is over a ride of 20 km (in which case something would be seriously wrong, and tire inflation, etc. can't be the problem), or 80 km.

Also, I haven't used MapMyRide for years, but it used to suck. I don't know about Runkeeper. If they both consistently agree with each other, then your Cateye needs some kind of recalibration.

pdlamb

I'd have expected the Cateye to read higher than the GPS, since it measures each wheel revolution and most GPS units only take a reading every 100 yards (or more). You can get a lot of squirrely swerving that'll ratched up the distance the computer counts.

To further what Spid and MinnMan said, calibrate the computer yourself. If you've got a good road nearby with decent mile markers, that's probably the best. Note the "decent" qualifier; most U.S. highways have well-measured distances, although they're not always the best to bicycle on. Smaller roads are sometimes good, sometimes laughable. (I remember one Missouri state road that had four mile markers within two miles.)

redryder74

The 4-8km difference was over a ride of 25km. During the weekend I did a calibration check using google maps. I found a 400m stretch of road, cycled down it and the Cateye reported 0.39km. The Runkeeper app reported 560m, and Mapmyride 500m. Looks like the problem isn't with the Cateye. The GPS signal on my iphone was shown as "good", so it's weird that the apps are so off.

GeorgeBMac

Yes... On the IPhone 5S (released last September) they added a second processor just to help apps like that -- but I haven't seen any detailed or specific explanation on exactly WHAT they are trying to improve. Perhaps GPS accuracy is it?

I do know that I keep seeing notices from Strava users about speed inaccuracies (which are probably a related problem.) When I use Strava, I do see quite a number of inexplicable spikes in the speed.

MinnMan

You don't see these kinds of problems with Garmin devices. I can see that an IPhone owner would not want to invest in another GPS device, but if it's a big deal to you, consider....

BigAura

Your bike computer should be the most accurate way to measure the distance you have ridden. Obviously you'll need to set wheel circumference correctly as the others have pointed out.

Since your numbers seem way out of line I'd be sure that your wireless "connection" on the computer is not being blocked or interfered with. Be sure the fork transmitter is as close as possible to the computer. You may want to, at least temporarily, move the mounts to be very close. You could be getting interference from communication towers or wi-fi hot-spots that you pass on your ride. Visually Monitor your computer speed display, and be sure its never zero, when moving.

redryder74

Using google maps as a guide, my bike computer is having a 2.5% error (reporting 390m vs 400m). To me that's pretty good, but I'm new to cycling so I could wrong. Is this acceptable?

I was just taken by surprise that the iphone GPS was so off, as I assumed it would be accurate. The apps were giving me 25% to 40% error during my 400m test. After doing my test, I'm just going to stick with my bike computer.

BigAura

A couple of percent difference is totally reasonable. Thanks for the clarification. As stated earlier, a properly calibrated and functioning bike computer is the most accurate way to measure your cycling distance. Best of luck, enjoy the ride.