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Road Cycling “It is by riding a bicycle that you learn the contours of a country best, since you have to sweat up the hills and coast down them. Thus you remember them as they actually are, while in a motor car only a high hill impresses you, and you have no such accurate remembrance of country you have driven through as you gain by riding a bicycle.” -- Ernest Hemingway

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Old 06-29-14, 12:26 AM   #1
TaylorC
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Top Speed Downhill / Strava

Tried to search the idea, came up short.
I'm new to road biking. Strava claims I hit 61.5 mph down an 8% grade. Almost twice as fast as I've ever been. Is that realistic? How accurate is Strava on a phone?
Not trying to start a pissing contest, yes it was faster than I've ever been and probably will ever try to go again, just wondering if the consensus is that the app is accurate.
It would explain the look on my friend's face when he caught up with me, if nothing else.
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Old 06-29-14, 01:08 AM   #2
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Strava is very inaccurate with top speed. GPS only doesn't do a good job of calculating top speed. My buddy has a normal bike computer + strava on his iphone. Strava top speed always reads high. He'll get like 55mph... when in reality it would only be 45mph.
In addition, i've noticed some other inflated numbers.
Also his total distance reads higher.
During the ride... his total elevation also reads higher. It'll read 20-30% more elevation during the ride. it'll get corrected by strava later after uploading to the server. But even after the GPS map correction, it'll still read more total elevation than my Garmin.

GPS only isn't accurate enough. Need to pair it with a wheel sensor if you want better accuracy.

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Old 06-29-14, 01:14 AM   #3
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So the bike computer is more reliable?
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Old 06-29-14, 01:21 AM   #4
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a normal bike computer with a wheel sensor is more accurate.
Strava on GPS only will give you 95% of what data you want. You just can't trust anything like Top speed / total Elevation / total distance.
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Old 06-29-14, 02:04 AM   #5
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Errors happen. I was losing my signal on my GPS (Soleus) and right before I lost it, I got some kind of crazy high speed in the 60s. I was going downhill at the time, but probably not even in the 40s.
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Old 06-29-14, 03:34 AM   #6
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Originally Posted by TaylorC View Post
Tried to search the idea, came up short.
I'm new to road biking. Strava claims I hit 61.5 mph down an 8% grade. Almost twice as fast as I've ever been. Is that realistic? How accurate is Strava on a phone?
Not trying to start a pissing contest, yes it was faster than I've ever been and probably will ever try to go again, just wondering if the consensus is that the app is accurate.
It would explain the look on my friend's face when he caught up with me, if nothing else.
When it comes to speed, Strava is the most inaccurate program, EVER...at least the free version is. My wife and I ride the same paths at the same time, side by side most of the time, and "somehow" I'll wind up doing not only higher elevation, but also higher speeds, or vice versa. I have a trip computer I've used as comparision, and the difference is off by at least 2-3 mph on Strava.

Just last week I rode a completely flat trail and found myself keeping a steady pace of between 14-16 mph over the course of about 22 miles. Strava recorded me at 11.8 mph. "11.8 mph" tends to be the magic number with Strava, because it always tells me I'm averaging 11.8 mph.

I've found the only accuracy Strava records are distances, which is fine for me, as I only really concern myself with how many miles I'd be logging over a season.

While it is possible you hit 60+ mph, you would certainly notice a substantial change in the stability of your bike, and you would probably be scared as hell before you got anywhere near 60 mph and brake. 60 mph on a bicycle is just...that's scary-fast. While everyone is different, once I hit about 30-35 mph on a decline, I get a little bit nervous...
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Old 06-29-14, 08:04 AM   #7
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All the speed errors I've ever got have been when my Garmin was only tracking speed from satellite and not the sensor.

To your question....it's absolutely unrealistic in your case. If 61 mph is twice as fast as you've ever gone on a bike...you'd know without a doubt from using your god-given senses if you were really going anywhere near that speed. The transition from 30-40 is very noticeable.
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Old 06-29-14, 08:14 AM   #8
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In the mid 80s I had an Avocet 30 computer on my race bike, with a wheel sensor and crank sensor, and topped out in a multi-bike draft on a 9.1% grade and 53F/13R at over 100 cadence (before I quit looking) I was seeing 59.6 mph. I think it was fairly accurate, because I was pretty obsessive about the setup of the computer and sensors. Anyway, it was fast. I wouldn't necessarily want to repeat that experience at my current age, but it was exhilarating at the time.
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Old 06-29-14, 08:16 AM   #9
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I wonder if there aren't other things going on here, primarily because I don't experience the inaccuracies with Strava that I'm hearing here.

I always run Strava in conjunction with a Garmin Edge 500 unit with GSC-10 speed/cadence sensor, and the data I pull down from each almost always matches. In fact, it's so close, I rarely bother to upload the Garmin data to Strava anymore, and just go with the direct-from-phone Strava recording.

I also ride the same roads, the same routes, on other bikes equipped with different measurement devices-- including a Powertap hub, a Cateye Mity 3, a Knog NERD, and a Specialized Speedzone-- and don't notice any variation, or reason to suspect variation, from what Strava on my iPhone5 records.

That, and I've been riding these same roads for almost 30 years, and there's no way in hell that Strava is 10mph-20mph higher than actual. No way.

Also, most phone GPS chipsets are the same performance spec-wise, and record at 1sec intervals, so variation from device to device is minimal, while open-sky location accuracy is quite high as well, so again, little opportunity to record the wild variation y'all are talking about. Right?

I mean, even if the location is off by 10 feet in one recording out of 10 (90% accuracy, 1 point/sec), isn't that only a calculated speed gain first, if the point is pushed forward towards the following location point, and then a max of like 6.5mph if it's the full 10 feet ahead of actual location (shortening the distance traveled in that 1sec)? I've looked at GPS tracks, and it seems just as likely that the deviation would be further from the next point, so I don't know how GPS accuracy can explain this.

So, that leads me to think that perhaps you guys are riding in conditions where the GPS signal is significantly degraded, or, you're positioning the phone somewhere where the reception is compromised. What I'm talking about here is heavy tree coverage over the road, and a phone buried in a seat pack with a bunch of crap around it.

Try moving your phone, removed from it's case, to a plastic bag in the center back pocket of your jersey, and see if that improves the accuracy. It certainly should, and a handlebar mount even better yet.
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Old 06-29-14, 08:17 AM   #10
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So the bike computer is more reliable?
Yes, particularly a wired bike computer.
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Old 06-29-14, 08:39 AM   #11
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To your question....it's absolutely unrealistic in your case. If 61 mph is twice as fast as you've ever gone on a bike...you'd know without a doubt from using your god-given senses if you were really going anywhere near that speed.
Yes, I agree with that...and I'm pretty sure that an 8% grade isn't enough to yank someone along at that speed!

A few weeks ago I hit 60.4mph coming down Valley Mission Rd. on the south end of Lake Monroe (IN), and while the entire segment averaged 8-something% over the .7mi segment length, the upper portion was considerably steeper, bettering 23% grade in spots, and I was rolling pretty fast into the start of the segment.
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Old 06-29-14, 08:50 AM   #12
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I hit 96mph once...according to Strava...lmao

Strava is woefully inaccurate. regarding climbing...Strava "Giveth and taketh away..."

I've synced a ride that indicated, say 6500 ft, and 2 hours later 1800 ft will be trimmed off the total.
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Old 06-29-14, 08:55 AM   #13
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On seven hill repeats I stopped and turned around at exactly the same spot, but Strava tracks showed something like a 30 foot range for that end point. Strava calculates that I moved maybe 20 extra feet in the time between readings, so it displayed 20 mph at that point even though I was at a dead stop. In the middle of a downhill it would have added 20 mph to the speed somewhere. And then subtracted speed on the next reading.

So while the average speed with GPS is pretty accurate, the high and low can be way off.
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Old 06-29-14, 08:57 AM   #14
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Strava is woefully inaccurate
That statement is woefully inaccurate.
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Old 06-29-14, 08:59 AM   #15
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So, that leads me to think that perhaps you guys are riding in conditions where the GPS signal is significantly degraded, or, you're positioning the phone somewhere where the reception is compromised. What I'm talking about here is heavy tree coverage over the road, and a phone buried in a seat pack with a bunch of crap around it.

Try moving your phone, removed from it's case, to a plastic bag in the center back pocket of your jersey, and see if that improves the accuracy. It certainly should, and a handlebar mount even better yet.
Agree with regards to signal, LOS, and all that, when I get into the Palisades area where I ride I'm sure the steep ridgelines and dense tree coverage play a part...but a bike bag is fairly minimal, I can't see how that would impact it significantly. I used to carry my iphone in my jersey until a couple of days ago, it went flying out of my pocket and smashed somehow!
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Old 06-29-14, 09:00 AM   #16
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Strava calculates that I moved maybe 20 extra feet in the time between readings, so it displayed 20 mph at that point even though I was at a dead stop.
I'm not following you here...?
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Old 06-29-14, 09:03 AM   #17
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Agree with regards to signal, LOS, and all that, when I get into the Palisades area where I ride I'm sure the steep ridgelines and dense tree coverage play a part...but a bike bag is fairly minimal, I can't see how that would impact it significantly. I used to carry my iphone in my jersey until a couple of days ago, it went flying out of my pocket and smashed somehow!
Yeah, that's all an issue...and an argument for a dedicated unit like a Garmin, Cycleops, or whatever.

But as for the saddle bag, it is significant, and when GPS folks talk about "clear view of the sky," they're not kidding. I've also seen some of those threads where people talk about all the stuff they've got in their bags, and put it all together, and you've got a significant impediment to accuracy.
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Old 06-29-14, 09:11 AM   #18
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Yeah, that's all an issue...and an argument for a dedicated unit like a Garmin, Cycleops, or whatever.

But as for the saddle bag, it is significant, and when GPS folks talk about "clear view of the sky," they're not kidding. I've also seen some of those threads where people talk about all the stuff they've got in their bags, and put it all together, and you've got a significant impediment to accuracy.



The garden variety stuff found in a saddle bag ain't gonna impede RF whatsoever, unless somehow you've managed to unwittingly construct the equivalent of a Faraday cage. My phone goes in my glovebox in my car, buried at the bottom and driving GPS works hunky-dory. It's all LOS/application related.

Most everyone in my riding locale can point to similiar, common and what is recognized as typical Strava reporting inaccuracies, jersey pocket or otherwise.
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Old 06-29-14, 09:11 AM   #19
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I'm not following you here...?
GPS was off by say 30 feet for an instant reading, so the Strava app thinks I traveled an extra 30+ feet in the time between position checks which are one second apart. It would calculate speed erroneously as 30 fps, or 20 mph when it was actually zero or a 2-3 mph turning around.

Strave should calculate the high speed using a moving average or gaussian smoothing of several consecutive instantaneous position readings, but it appears to use the highest instant speed between two positions.
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Old 06-29-14, 09:22 AM   #20
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GPS was off by say 30 feet for an instant reading, so the Strava app thinks I traveled an extra 30+ feet in the time between position checks which are one second apart. It would calculate speed erroneously as 30 fps, or 20 mph when it was actually zero or a 2-3 mph turning around.

Strave should calculate the high speed using a moving average or gaussian smoothing of several consecutive instantaneous position readings, but it appears to use the highest instant speed between two positions.
Oh, you'd said 20ft before, which is why the numbers didn't make sense.

As for averaging, yes, I can see where the issue is, particularly if you get a lot of erroneous readings on a very short, very slow ride. However, when you average in a 1sec reading over the course of 1 hour ride, I'm not seeing a statistical problem, and even in a worst case scenario, I don't see how Strava over reports speed on the order of the claims that have been made in this thread...unless, the GPS loses reception, which is not a Strava problem, but a hardware/human error one.
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Old 06-29-14, 09:36 AM   #21
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Oh, you'd said 20ft before, which is why the numbers didn't make sense.

As for averaging, yes, I can see where the issue is, particularly if you get a lot of erroneous readings on a very short, very slow ride. However, when you average in a 1sec reading over the course of 1 hour ride, I'm not seeing a statistical problem, and even in a worst case scenario, I don't see how Strava over reports speed on the order of the claims that have been made in this thread...unless, the GPS loses reception, which is not a Strava problem, but a hardware/human error one.

It's not a problem there, because the position errors will tend to average out over the longer term. It's only the instantaneous speed readings that will really have a problem.

Trees and even atmospheric conditions will throw a given reading off. Even then, even with everything perfect, without augmentation the civilian gps-only device is accurate to at best 3 meters, worst case 8 meters. Which means we simply can't expect a phone GPS to provide an accurate instantaneous speed reading but it can be can closer if the software is sophisticated enough.
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Old 06-29-14, 10:04 AM   #22
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The garden variety stuff found in a saddle bag ain't gonna impede RF whatsoever, unless somehow you've managed to unwittingly construct the equivalent of a Faraday cage. My phone goes in my glovebox in my car, buried at the bottom and driving GPS works hunky-dory. It's all LOS/application related.

Most everyone in my riding locale can point to similiar, common and what is recognized as typical Strava reporting inaccuracies, jersey pocket or otherwise.
Well, every GPS manual I've ever seen says go outside, get a clear view of the sky, and as a student of GIS it was standard operating procedure to keep GPS receivers/antennas on top of bags and out in the open. And I'm talking pro level stuff, not consumer grade devices, though the same SOP applied.

I don't understand how you can claim it's LOS/app related, when satellite connectivity has always, and remains in my experience, to be the bugaboo, although since the GNSS (i.e. GPS+GLONASS) I understand accuracy has gotten even better (assuming clear view of the sky again).

As for riding, I've never heard any complaints from anyone in my club on this matter, and there are lots of Strava users, so forgive me for questioning your explanation, just because it's quite different from what I know, what I was taught as a masters student in GIS, and what I've experienced.

I really think the problems people have, or think they have, with Strava derive from environmental, hardware (Androit GPS was an issue a couple of years ago) and user issues, rather than Strava problems.
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Old 06-29-14, 10:33 AM   #23
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It's not a problem there, because the position errors will tend to average out over the longer term. It's only the instantaneous speed readings that will really have a problem.

Trees and even atmospheric conditions will throw a given reading off. Even then, even with everything perfect, without augmentation the civilian gps-only device is accurate to at best 3 meters, worst case 8 meters. Which means we simply can't expect a phone GPS to provide an accurate instantaneous speed reading but it can be can closer if the software is sophisticated enough.
I understand what you're saying, but I do not understand how that explains why Strava is responsible for the wild reading inaccuracies we're hearing reported here.
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Old 06-29-14, 10:55 AM   #24
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Well, every GPS manual I've ever seen says go outside, get a clear view of the sky, and as a student of GIS it was standard operating procedure to keep GPS receivers/antennas on top of bags and out in the open. And I'm talking pro level stuff, not consumer grade devices, though the same SOP applied.

I don't understand how you can claim it's LOS/app related, when satellite connectivity has always, and remains in my experience, to be the bugaboo, although since the GNSS (i.e. GPS+GLONASS) I understand accuracy has gotten even better (assuming clear view of the sky again).

As for riding, I've never heard any complaints from anyone in my club on this matter, and there are lots of Strava users, so forgive me for questioning your explanation, just because it's quite different from what I know, what I was taught as a masters student in GIS, and what I've experienced.

I really think the problems people have, or think they have, with Strava derive from environmental, hardware (Androit GPS was an issue a couple of years ago) and user issues, rather than Strava problems.
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Old 06-29-14, 11:23 AM   #25
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I understand what you're saying, but I do not understand how that explains why Strava is responsible for the wild reading inaccuracies we're hearing reported here.
Mainly because Strava is the reporting mechanism, people will say "Strava gives me this error" when "GPS gives me this error" might be more accurate.

Strava can't completely escape the blame however. If you look at some Strava data files you'll see some big jumps in speed from one point to the next, and then a big drop in speed at the next point. Estimated power, same thing and for the same reason. You'll see a big momentary spike in power that you know didn't happen, followed by a zero.

If you look at raw data from the magnetic bike computer sensors you'll see something similar though not as pronounced. The reed switches will open or close a small time interval away from when the magnet actually passes, so the raw data will show an instantaneous high speed followed by a lower one, or vice versa. This is in addition to debouncing, different from from and after the switch signal has been debounced. Yet even the cheapest bike computers can compensate for it (the variance will completely cancel out with successive closures). Strava though doesn't appear to have a good handle on that, so I lay part of the blame at Strava's feet.

To be fair, an anomalous reading at the end of a motion vector would be more difficult to recognize than one during continuous motion, so my 20 mph differential wouldn't occur during the downhill coast, and I've never actually seen that magnitude of an anomaly during continuous motion so I think that Strava is doing some sort of sanity check on the GPS readings. But I can see how some sequence of GPS inaccuracies could conceivably add 20 mph to, say, a high speed of 40 mph downhill, and escape whatever filter Strava is using. In that case I have to blame Strava for not catching and compensating for known hardware inaccuracies.
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