Distance/Speed Accuracy: Map vs. GPS vs. E-Bike Display: Serious Discussion Needed

alphacarina

Member
Region
USA
@alphacarina points out that a measurement from the wheel rotation is measuring how far the wheel traveled. To me, that’s the distance I’m interested in since it corresponds most directly to effort. I’m not really interested in how far two places are apart by way of roads or trails, I’m interested in how far I (and the motor) moved, wiggles and backtracks and wrong turns included.
Use the back wheel to measure everything - It travels less distance than the front and does it in more of a straight line. It's also much closer to the distance you actually traveled

Don
 

theemartymac

Well-Known Member
Any recreational Pilots in the house? For more accurate GPS, it would be interesting to see if you could enhance results with a Bad Elf, or Dual unit. Pilots have been using GPS dongles for years to make very accurate course tracking and plotting with an ipad or iphone?
 

mschwett

Active Member
Region
USA
Any recreational Pilots in the house? For more accurate GPS, it would be interesting to see if you could enhance results with a Bad Elf, or Dual unit. Pilots have been using GPS dongles for years to make very accurate course tracking and plotting with an ipad or iphone?
not a pilot of manned aircraft, but i fly and build some big drones, and even without rtk (which requires a known theater) the combination of multiple antenna, inertial sensors, and barometric sensors and much much much more frequent logging is very accurate. not really sure there’d be much point adding all that hardware (although obviously it’s VERY light) to a bike.
 

K PierreR

Well-Known Member
Region
USA
Guys, I've got to tell you, as a very pragmatic retired engineer, that I keep track of all the stats on an e bike because I helps with future planning and ideas but, I am much more concerned with precision than I am with accuracy.
I use the GPS in my phone along with Gaia GPS app. It might be both precise and accurate but I use it for navigation and it gets me there. I don't use it for distance or elevation but note each. I do use the GPS for start and stop times as the bike does not differentiate. The bike readout is fairly precise and I use it for distance. It is not accurate and reads slightly high, I compensate in a post but not for my own tracking information. I use a watt meter for both watt hours and voltage. It is fairly precise but not accurate at all. I compensate for reporting purposes. I have an accurate hand held volt meter.
How well do I really do? A man with two watches never knows what time it really is. I am satisfied with the numbers I come up with and don't sweat the fine details. They are unimportant to the final outcome. And the use of the numbers and ride is what I care about.
 
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Catalyzt

Active Member
Region
USA
Keep in mind that any measurement using the bike's wheel is going to give you the distance the wheel has traveled, rather than the distance you have actually traveled. None of us ride in a perfectly straight line - We wobble a bit as we pedal, left to right, so the tire actually goes farther than it would if we were able to stick to a perfectly straight line. I'm pretty sure even with a properly calibrated wheel, the distance it shows would be greater than if you drove the same route in a properly calibrated 4 wheeled vehicle. Personally, I would trust my GPS speed and distance measurement over anything generated by the bike itself

Don
I think I may have seen this effect for the first time today. On the the fastest downhill segment of Griffith Park Drive last week, Strava & bike computer agreed that top speed was exactly 35.4 MPH. Yesterday, however, on the same stretch of road, Strava & bike computer disagreed: Strava said 35.3 and bike computer said 36.8.

What was the difference? I think it was wobble-- as I accelerated past about 35.5, I was pedaling so hard and at such a high cadence that my balance was slightly off, and I picked up a side-to-side wobble I hadn't experienced the previous run, when I had a much more stable line. I think Strava may have correctly guessed my straight-line speed, but the bike computer was closer to my ground speed.

I wonder, does the Shimano bike computer get telemetry from the front wheel or rear? My line probably wobbled a bit overall, but the front wheel wobbled more. Could also be a random sampling error, but this run felt faster.
 

mschwett

Active Member
Region
USA
I think I may have seen this effect for the first time today. On the the fastest downhill segment of Griffith Park Drive last week, Strava & bike computer agreed that top speed was exactly 35.4 MPH. Yesterday, however, on the same stretch of road, Strava & bike computer disagreed: Strava said 35.3 and bike computer said 36.8.

What was the difference? I think it was wobble-- as I accelerated past about 35.5, I was pedaling so hard and at such a high cadence that my balance was slightly off, and I picked up a side-to-side wobble I hadn't experienced the previous run, when I had a much more stable line. I think Strava may have correctly guessed my straight-line speed, but the bike computer was closer to my ground speed.

I wonder, does the Shimano bike computer get telemetry from the front wheel or rear? My line probably wobbled a bit overall, but the front wheel wobbled more. Could also be a random sampling error, but this run felt faster.
isn't it pretty much only front hub drive bikes that have a speed sensor up front? it's definitely in the back on all the specialized mid-drive bikes. in the bike in your profile pic, all the electrical wires seem to go into the frame, and just the brake goes to the front rotor. you can probably confirm from a closer look that the speed sensor is in the back.
 

mschwett

Active Member
Region
USA
for those who want to scratch their heads on this a little more, here's the same ride from two different data sources, processed in three different ways (lol.)

the mission control data uses some combination of the phone's GPS and other sensors plus rear wheel speed data and the power meter in the crank. the ridewithGPS data is only from the phone, no connection whatsoever to the bike. the strava data is an upload of the mission control data, but clearly strava is reprocessing it differently, e.g. not relying on the wheel speed data for "maximum speed" which is really substantially different than both the mission control and rideWithGPS conclusion!

3209-hh7.jpg


As an aside, this was my personal best for least battery usage per mile and per vertical foot! At this rate, with the range extender, I could do 155 miles and 10,000 vertical feet on a battery if my body could continue the same output for 9 hours. (which it can't LOL)