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

[alphacarina]

@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]

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]

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]

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.

 

[Catalyzt]

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]

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]

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!

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)

 

[Hornet II]

Is not the real issue, how do we get our ebike systems to accurately assess our true speed so that we can get assistance all the way up to the legal limit of the bike's capabilities rather than coming up short because the bike is reading faster than actual speed?

That is my main complaint too Alaskan. I paid a lot for a 28mph cut off and I get only 26.5 mph.
I have verified distance on many measured tracks. Garmin is correct ebike off. Every mile.

 

[Roamers]

Just found this string and only skimmed, I apologize if this has already been mentioned. As I skimmed, most comments related to bike computer speed showing faster than gps speed. Unless you have the tires pumped up so high there is no drop, your effective tire circumference is going to be smaller than published, therefore rotating more times over a given distance; thus showing faster.

No offense intended, but why is such precision in distance and speed that important?

Personally, bike, boat or auto I follow gps over onboard odo/speedo.

 

[retiredNH]

Agree with Roamers. While I understand those that want to eek out a bit more speed before limits kick in, I'm not sympathetic. Probably it's because I'm of that age where speed is no longer a thrill. It's a threat...
But it is too bad that companies like Bosch allow such a limited range of adjustment, such that I can't even enter the true circumference of Trek 8s OEM tires!

 

[6zfshdb]

Personally, bike, boat or auto I follow gps over onboard odo/speedo.

I do the same but for convenience only. GPS speed/distance is less accurate than a properly calibrated ground effect sensor.
Post #4 explains why.

 

[Mr. Coffee]

Honestly I think that sweating this very much isn't worth the trouble. As long as you are comparing rotten apples with rotten apples and not with plastic oranges you aren't going to get too confused. So compare GPS distances to GPS distances and on the average you won't get in too deep.

If you want to get seriously geeky about this, though, there are additional sources of error when using GPS. Typically distances are calculated between GPS fixes using haversines. However, haversines assume a spherical earth and so introduce a small but consistent error (on the order of 0.5%) into all distances. You can improve on this by using vincenty's formula.

 

[Roamers]

I do the same but for convenience only. GPS speed/distance is less accurate than a properly calibrated ground effect sensor.
Post #4 explains why.

Agree, but "properly calibrated" is the important piece that is often tough to maintain. As anal as I am with many measurement instances, boat, bike and auto odo/speedo just isn’t if top importance for me.

 

[Alaskan]

Agree with Roamers. While I understand those that want to eek out a bit more speed before limits kick in, I'm not sympathetic. Probably it's because I'm of that age where speed is no longer a thrill. It's a threat...
But it is too bad that companies like Bosch allow such a limited range of adjustment, such that I can't even enter the true circumference of Trek 8s OEM tires!

I had the same problem with my Trek Allant 9.9S. The local Trek shop where I purchased the bike could not expand the correction range to accommodate the OEM tires either. However there is one guy in the Waterloo, WI, Trek HQ who holds the codes to be able to expand the adjustment range to the correct levels. The local shop had to coordinate with him to be available when they had my bike connected to their system. He went in, put in his super secret password and the needed adjustments were made. Case closed.

 

[Roamers]

That is my main complaint too Alaskan. I paid a lot for a 28mph cut off and I get only 26.5 mph.
I have verified distance on many measured tracks. Garmin is correct ebike off. Every mile.

Haven't been into the system, but can you put in a much larger circumfrance tire size and trick the motor into giving more than 28? Then use gps for closer to actual.

Edit: might have that backwards. Have to think more.

 

[fooferdoggie]

Haven't been into the system, but can you put in a much larger circumfrance tire size and trick the motor into giving more than 28? Then use gps for closer to actual.

Edit: might have that backwards. Have to think more.

you do you have to make the wheel smaller to make it look like faster.

 

[Hornet II]

Just found this string and only skimmed, I apologize if this has already been mentioned. As I skimmed, most comments related to bike computer speed showing faster than gps speed. Unless you have the tires pumped up so high there is no drop, your effective tire circumference is going to be smaller than published, therefore rotating more times over a given distance; thus showing faster.

No offense intended, but why is such precision in distance and speed that important?

Personally, bike, boat or auto I follow gps over onboard odo/speedo.

Roamers, it makes the power assist cut off 1.5 mph too soon.

 

[retiredNH]

Roamers, it makes the power assist cut off 1.5 mph too soon.

So why not start a class action law suit against Bosch and the bike manufacturer for how you've been cheated?
Or just move on with life...

 

[dodgeman]

I am a retired land surveyor and used GPS for surveying for years. It was the high quality stuff costing a lot of money. The stuff in your phone is still quite good but is generally is considered 10 meter accuracy, although it often does better.

One thing I learned as a surveyor is your sky view. Even the expensive stuff I surveyed with didn’t work if it couldn’t see the sky. For example tree cover. Your phones gps will work better on your handle bars better than buried in your pocket or in a bag.

 

[Roamers]

The Engineer in me says I should check mine. The retired recreational rider says never mind. If I tell my wife what I'm doing, she'll roll her eyes. Since GPS is considered less accurate, is it good or bad if they match.

This is too much stress! Maybe I'll jump in the van and drive to the wildlife refuge and commune with the water fowl and cranes.