Strava Speed vs. Bike Computer Speed

I’m now able to irritate a lot of my friends because I use Strava. The record-keeping is useful, and the segments — while they do prompt one to speed up when one might not want to — provide a nice progress report on one’s ability over certain parts of rides. But there’s one thing I’ve noticed: My speeds according to Strava are different from the speeds registered by my bike computer.

screen

Now, I know I have el cheapo computer. However, I’ve been pretty satisfied with it. I measured the circumference of the front wheel by hand, and found that I need to enter 19 more millimeters than the 2096 commonly used for 23mm tires. My tires are 25mm, so 2115 mm is less than the suggested measurement for that size. Anyhow, when I compare my readouts to known distances on some of my more frequent rides, they are accurate to a level I find acceptable.

So I was puzzled when I noticed a difference between the stats in Strava and the stats on el cheapo. After the first couple of rides with both, Strava settled in and registered total distances for rides within one-tenth of a mile of my old computer. Even this morning, when I did a longer ride, the two were only four-tenths apart, and that was because I neglected to start my old computer at the beginning of the ride. I lost the data for about four blocks of riding.

But on Friday’s ride, there was a real disconnect. The problem area is maximum speed. For one downhill leg I do, the old computer clocked 31.9 mph. Strava had me at 36.4! That’s too much of a difference. Wonder what causes it? I also know from experience that the old computer’s speed was more likely to be the accurate of the two. I’ve ridden this stretch once or twice a week for the last several years, and I know what to expect. I also know what 36 mph feels like on my bike, and I didn’t have that feeling this time.

Therefore, while Strava is fun and useful, I have learned to be skeptical of it sometimes. I’ll continue to enter my ride stats on my spreadsheet (Yes, I know) according to my computer, and use Strava to gauge improvement.

 

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13 thoughts on “Strava Speed vs. Bike Computer Speed

  1. It has to do with the length of coast of England. Really. http://en.wikipedia.org/wiki/How_Long_Is_the_Coast_of_Britain%3F_Statistical_Self-Similarity_and_Fractional_Dimension
    Read that. Good. Now, your GPS samples less often than your computer, much like the diagrams in the article. It’s sure you didn’t go as far as you think you did. Plus your front wheel, due to tiny course corrections, actually travels farther than you (or your back wheel) do.
    Because of sample rates, your top speed was likely not consistent between samples, so looked lower than your computer reported. But don’t worry, someday your GPS will hiccup and insist you were going much faster than you really were going.

    • besides your enthrallment with Strava, I have to giggle about your use of precision

      what is the difference between 31.9 mph and 32 mph?

        • that is an entirely different issue and nicely highlights what appears to be the insanity of Strava….this concern over the accuracy (or lack thereof) of the speed of one particular segment

          I think this comes from the same illness that makes drivers frustrated that they can’t go the speed limit, not realizing that in an urban environment, the particular speed on a given segment makes very little difference in the total trip time

          you were going either 32 mph or 36…doesn’t it really matter which?

  2. I agree with the cell tower theory. The computer is measuring based on the sensor on the wheel, the cell phone GPS is not true GPS (does not communicate with an actual satellite), but rather triangulation based on the nearest 3 towers. Usually only accurate to within 3 meters (15 feet) or so. I would assume the computer to be MUCH more accurate.

  3. I agree that a correctly calibrated cycle computer will be more accurate than cell phone GPS, but it is “real GPS”. I have an app which identifies available satellites and maps the ones being used; which on a recent test was 7. Even then, the indicated accuracy was only ~20ft. Remember reading somewhere that the GPS is more accurate with wifi off so that only satellites are used for location. This might also help battery life on the road. Just my 2 cents to add to the discussion.

    • Thank you for saying this so I didn’t have to. Often people confuse Cell tower triangulation with GPS. The reality is that most modern smartphones have honest-to-god talks-to-satelites-in-space GPS, only old flip phones and other “Dumb phones” rely on Triangulation and they don’t provide mapping functionality.

      So while the GPS modules in cellphones may not be as finely calibrated as, say, Garmin, they are certainly no less “real” in the critical technical details.

  4. Pingback: How Do I Know I’m Improving? | Bike Noob

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