GPS vs Sensor for speed
 

I've just bought the BRYTON Ryder 30, which is a great unit, awesome screen and features.

In the box, it came with 1 sensor and a heart rate monitor, so if you want to track cadence, you are defaulted to use the GPS for speed.

I've always had a magnet sensor telling me my speed, so it feels a little different but have nothing to go from

Has anyone had any experience on the performance and accuracy of the GPS units vs a standard sensor? I'm considering buying another sensor for speed, but it's another $60, so if the GPS is accurate there may not be any need for it.

Thanks in advance.

I once tested both simultaneously on a ride. The GPS was a Garmin Forerunner and the computer was a cateye. There were some minor differences in speed and distance, but they are so insignificant in my opinion that they don't even merit discussion. I wouldn't hesitate to use GPS. Think about it...No wires to break, No sensors, No wheel size needs to be programmed. It is the ultimate wireless.

The only real issue was reception. In cities with tall building, and I mean a lot of buildings, reception could be lost. Additionally under really, really, heavy tree cover you could also lose the signal. But it was a rare occurrance. This test was in 1997, and I undrstand the antennas in use now are even better.

I still use the forerunner on my Bike Friday. The lack of sensors and wires make packing it up for a trip easy.

The accuracy of a speedometer/odometer, wired or wireless, which derives its readings from magnetic sensors mounted on fork and wheel should depend upon the circumference of the wheel, which is entered by the user when the unit is installed on the bike.

Read your instruction manual; the GPS probablyuses the GPS exclusively for location, distance, speed, etc., everything but cadence, and only uses the sensor if the GPS signal is lost. That is they way that the Garmin works. The GPS calibrates the sensor unlike a non-GPS computer where the user does this manually. Your unit may work the same way.

Incorrect. Garmin uses primarily the wheel sensor. You can set the diameter or let it auto calibrate it. If it loses the sensor, it starts using GPS for distance and speed. GPS position is noisy over short distances and clips corners if you ride a tortuous path. That's why they have the wheel sensor.

I don't bother with the wheel sensor on my Garmins. The gps is plenty good enough.

Thanks for your feedback guys. I'll stay with the GPS for speed and distance and save the $$. I live in a pretty open city (Perth, Western Australia), so it's all good.

I appreciate your feedback on this.

I took the unit out for a ride yesterday and was very happy with the results. Bryton Rider 30 is looking like a top unit so far. The screen size and customisation on what is where is awesome.

I use the cadence sensor and it's proving to be a big help to push me a little harder.

I decided I needed the speed sensor for trail riding MTB because there is no way that the GPS can track the tight turns. Also, when you climb and descend over a small hill, the GPS will only see the horizontal movement component and not the up and down. Two of my friends went for the same 20 mile trail ride and their distances were over one mile apart. This is both due to GPS error and auto-pause errors. The wheel sensor ends the auto-pause errors.

Is one mile off over 20 miles good enough? No, not for me. No way. I use Strava and I don't want the GPS to say I went 19.0 MPH when I went 20.0.

For a road bike I think it is much less important - though a GPS will still clip corners when you turn, so I just got one for my road bike.

This.

What phone/gps are you using? Newer, more current iterations of hardware and software are pretty accurate, including barometric elevation detection.

I ride a lot of "metric" centuries (doing another tomorrow) and most participants will be using a GPS type of device. I use a Garmin 800 because it also saves the rides to my computer and now I also use a HRM, so the ride data is all there (except for cadence, which will be next).
Anyway, at the end of those rides, the GPS's vary in ride length, climbs and ave speed. The wheel magnet ones, are all very VERY close. I will add that we are in the rural country and maybe hills, trees and other things might help cause the GPS to be harder to track and throw it off a little.
For me, a GPS is plenty accurate enough and if it loses a mile of so, I'm okay with that. I'm not a professional athlete or have plans to be one at 58 years old. I just like have a fairly good account of my activities.

It is impossible. GPS is +- 10 feet at best when it is working optimally.

Garmin 520 with 1-second recording interval and GPS+GLONASS.

This 25.9 mile ride at 7.6 mph average (with a Garmin speed sensor that I measured to the mm and it just so happened to auto-calibrate to the exact same value which I let it do when I was riding on a straight road):

https://www.strava.com/activities/681888403

When I took the Garmin file, stripped out the speed-sensor data, and let Strava calculate the distance using only GPS+GLONASS, it came out to 24.1 miles and since the distance was recorded as less than it really was but the time was the same, the average speed in Strava became 6.9 MPH.

I for one would rather have Strava say 7.6 than 6.9 for the same ride.

What exactly is it I said you are stating is impossible?

I have the Garmin speed sensor and I read on the manual, it use this to calculate speed. If I dont have this, then it use the GPS to calculate the speed, ie the mph?

Incorrect. If you read the Garmin manual, it says if speed sensor is lost or not use, it will use the GPS to calculate the speed, ie, mph.

Garmin Edge 305 used GPS for everything but cadence(used the GSC10 cadence feature) unless GPS signal becomes too weak and then used the speed sensor until GPS signal resumes. I tried to copy the manual text but was unable to. Were you talking about a different model Garmin?


Think about it, you could use a GPS enabled Garmin Edge 305 without a GSC10 sensor and cadence would be the only bike data not displayed. You also had the option of manually entering the wheel size or letting the GPS automatically calculate it. BTW, I used a refurbished Garmin Edge 305 for five years and read the manual extensively.

I'm pretty sure you have this backwards. I owned a 305 as well.

In every GPS based bike computer I've ever owned, the GPS is the fallback when the speed sensor is absent. Garmin units will use the GPS to calibrate the wheel circumference and thereafter use the more accurate speed sensor.

The speed sensor gives more accurate speed and distance than does the GPS because of the sampling rates and the inherent accuracy of +/- 4 to 7.8m for GPS when at it's most accurate. The speed sensor will be accurate within some fraction of a wheel circumference (or the time to make that rotation) which is much less (and therefore more accurate) than the 1 second sampling plus between 4 to 7.8m accuracy of the GPS.


J.

Manual here:

https://www.gpscity.com/pdfs/manuals...-EN-MANUAL.pdf

Page 76, speed sensor is only used when GPS is turned off.

For Edge 820 it says that GPS is used for speed if no sensors are paired.

https://www8.garmin.com/manuals/webh...E6557007B.html

The newer Garmin models (knowing for a fact that my 520 does) will use the sensor for speed, only defaulting to GPS if no sensor is found. The 520 also constantly auto-calibrates the speed sensor on the fly, so it's about as accurate as one could hope for.

The speed sensor is worth every penny if you ride anywhere with heavy tree cover-- on several rides in the local mountains, I've had the 520 completely lose satellite lock for several minutes, so without a sensor, speed would've dropped to zero.

My post was to clarify the 305 argument/debate.

I was surprised to see this. Explains some of the wonkiness of the 305 I owned which was one of the reasons I upgraded.

Here's the quote from the Edge 1000 manual:

Presumably, since the Edge 1000 (and the 820) are much newer than the 305, their GPS capability is more advanced. So it's surprising that Garmin made a big about face in how they treat speed sensors vs GPS.

J.

I have been wondering about this too, especially I don't find a place to set the wheel size for the speed sensor. still no clue for this.

They probably mean if there is no "speed" or "speed/cadence" sensor.

A "speed/cadence" sensor is just a "speed" sensor with cadence added. That is, as far a speed is concerned, they are the same thing.

Small/cheap GPS can't detect fast changes in speed.

The speed sensor is still, even for the newer units, much better for speed.

I agree and I guess that was sort of my point. Garmin made a big about face on how they treat speed. In the 305, the GPS seems to have preferred and the default where a speed sensor was not really necessary unless you were indoors. In the subsequent units, Garmin seems to have shown a strong preference for the speed sensor over the GPS sensor and only going to the GPS sensor in the event that no other sensor is available. I agree with you that that is the way it should be and Garmin came around to that in between the 305 and the later units at some point.

J.