Weakbikr

Hey guys, I have a cateye strada wireless computer on my bike. I have 700x23 tires the default settings in the computer for tire size is 2096 which says in the manual is a 700x23c tire. Now I rode a quick loop through my neighborhood and here are the results. Cateye says distance 4.76 miles and avg speed 15.6mph. Strava says distance 4.8 and average speed of 14.5. So which one is right? Something seems way off. I wonder if I have my computer set up incorrectly but when I check it has the right circumferance in there.

canklecat

That's a typical range. If I use three different GPS activities apps I get four different readings. But I'd consider the results you got to be within an acceptable range.

GPS is remarkably accurate but cannot be as precise as mechanical measurement. There are variations due to GPS sync and other factors.

When in doubt, add another GPS activity app. Try Wahoo Fitness, a good free app that can upload to Strava. Running Wahoo Fitness and Strava simultaneously you'll get slightly differing results. Not worth worrying about.

Also, the on-bike computer will continue recording as you wheel the bike indoors. There's no auto-pause/resume. It's physical motion. It can be "paused" only by lifting the wheel or moving the sensor away from the magnet.

Strava, Wahoo Fitness and other apps use different thresholds to trigger auto-pause/resume. If I walk quickly enough indoors while wheeling the bike, it resumes recording and factors a slow speed into my overall average.

Usually after a ride I crop out approximately 400-800 yards from the beginning and end of a ride for Strava. That's because I'm often meandering around getting warmed up, stopping to chat with neighbors, etc. Sometimes I coast around in circles while chattering with neighbors. So those bits aren't useful workout info.

Weakbikr

Ok so which would you say is most accurate? Reason I ask is my best time on that route per strava is 15.1 avg I know I had to have gone faster today because I was hammering up the hills at 18 to 20 mph where I'd usually not and spin at around 12mph or so. Just kinda confused.

canklecat

I always regard whatever says I'm faster and ride farther as being the most accurate.

FWIW, Cyclemeter for iPhone always showed my times as faster than Strava. Never compared it with a bike computer. I didn't get one until recently, just a $12 generic wireless speedometer to save a little weight on the front end compared with the iPhone and mount. Smartphone goes in my jersey pocket and I usually don't even look at it until I get home.

And my cheapo bike computer usually agrees with Strava, Wahoo Fitness and others. Not much real difference between 'em.

Weakbikr

Typically my bike computer and strava are pretty close, mileage is usually within a tenth and mph is usually within 2 to 3 tenths but it's never been that far apart. Oh well may have just been a fluke. I'll keep tracking it to see what happens.

Branko D

If the wheel size is correct, the difference in speed between Strava GPS tracking on the phone and cycle computer boils down to the following:
- Strava doesn't take gradients into account when calculating speed. The actual distances on a gradient and speeds on a gradient are slightly greater than what Strava says.
- Strava speeds can be higher than cycle computer speeds if you spent a fair chunk of time on red lights, because Strava doesn't take that into account when calculating average speed. Your cheap non-GPS cycle computer doesn't care though.
- While post-ride it doesn't matter because it's going to recalculate it correctly, watching the speedometer during the ride if you're riding anywhere which impedes GPS signals (just trees will do that), it's going to show a significantly lower than real speed, or even zero (eg. in tunnels). Strava will also do funny things when you go through tunnels, like conclude you went over the mountain if there's a segment like that. Whooops.
- If you brake hard enough the wheel doesn't have proper traction anymore, or jump, or do anything where the front wheel isn't gripping to the ground properly - there's going to be a small difference on that account. Not going to be significant, though.

Some differences are normal and just an artifact of using a different method of measuring speed.

CliffordK

I like to unroll a tape, and measure the actual roll-out of my wheel to calibrate the cycle computer.

In your case, 4.76 and 4.8 miles are likely identical with respect to the accuracy of the devices. A longer ride would be a bit more telling. So, any differences in average speed would be due to different calculations of time. So, time elapsed between the point where Strava was turned on or turned off, or any stops during the ride.

One thing you might try is to crop your ride on Strava... Just skim a couple of yards of riding off of each end of the ride.

On the Analysis page in Strava, you can also choose to display by time rather than display by distance, and any pauses should show up.

Weakbikr

how do you do that?

CliffordK

Basic tape measure. Centimeters are best, but you can convert from inches.

Set the valve down at 0

Roll forward (maybe with a little weight on the bike???) 1 or 2 revolutions, and measure to where the valve is down again.

Use that number for your computer calibration.

Weakbikr

sorry I wasn't clear, i meant how do you crop your ride in strava.

bluehills3149

Rather than type it all out here, google "crop your ride in strava" and you can chop off the start or end of a ride (handy when you forget to end a ride and then take your bike home in the car/train)

Weakbikr

Will do, thanks.