Speed Difference

pauls

Finding my (electric) wheels
Jul 20, 2020
6
1
Brighton
I have a Carrera Crossfuse with a Bosch Purion display plus a Livall Bluetooth helmet which pairs with the Livall app. When I use the bike, I see a different speed on the Bosch display (which presumably picks up speed from the sensor mounted on the rear wheel) to the speed displayed on the app (which presumably uses GPS). Does anyone else see the same thing happening? Which speed is correct?
 

sjpt

Esteemed Pedelecer
Jun 8, 2018
1,668
1,100
Unless you have a poor GPS its speed should be pretty accurate. However, a small jitter error on GPS can lead to grossly distorted speed reading (like being stationary and being told you are going at 3mph). When I had a mid-range LG (?LG-G3?) the GPS was pretty poor; my cheapo Numo 20 is much better for GPS (partly why I chose it).

The Purion display speed could be wrong if the wheel size is wrong (even fitting different tyres changes it a bit), otherwise it should be accurate too. You haven't said how big or how consistent the difference is.
 

sjpt

Esteemed Pedelecer
Jun 8, 2018
1,668
1,100
To check the GPS
1) stay still for a while and see what speed it registers
2) record a track while riding a straight road, and then look at it really zoomed in to see how straight the recorded track is.

There have been a few complaints about incorrect speed reading on the Purion: it looks as if you can't even query the wheel size setting, let alone change it.
 
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Bikes4two

Pedelecer
Feb 21, 2020
46
15
The bike is new so the wheel size should be correct. The difference is about 1-2 mph.
Should be but probably a generalisation based on average tyre, tyre pressure, tyre profile ......
And what speed are you seeing this difference at?
GPS requires a bit of time to do the calculation - such a small error is splitting hairs imho.
 

soundwave

Esteemed Pedelecer
May 23, 2015
10,539
4,337
if you want the right speed then use something else as the bosch purion display can be out 1-2mph either way same as the old one.

a dealer can dial it in with the wheel size if i remember as i was able to do this with the intuvia display but not a option with purion.
 

Bikes4two

Pedelecer
Feb 21, 2020
46
15
I would think the most accurate device for measuring your speed is something like one of the Cateye cycle meters in which you can enter the exact (to within a cm) circumference of your cycle tyre. Update rates are very fast too.
 

GSV3MiaC

Pedelecer
Jun 6, 2020
36
11
I would think the most accurate device for measuring your speed is something like one of the Cateye cycle meters in which you can enter the exact (to within a cm) circumference of your cycle tyre. Update rates are very fast too.
I'd rather believe a garmin GPS, but it has to be in non power save mode, have a lock on at least 8 satellites, and take the average over a minute or so. Oh, and you have to be under 600mph (?) or something, else you need a military version.

Under proper conditions it gets the right answer regardless of wheel size, tyre spin, or ebike electronic optimism. I use an ancient gpsmap 62.
 

pauls

Finding my (electric) wheels
Jul 20, 2020
6
1
Brighton
Should be but probably a generalisation based on average tyre, tyre pressure, tyre profile ......
And what speed are you seeing this difference at?
GPS requires a bit of time to do the calculation - such a small error is splitting hairs imho.
I notice the difference at about 9-10mph. At that speed, the difference is about 10% so the trip and range could also be 10% out.
 

GSV3MiaC

Pedelecer
Jun 6, 2020
36
11
Uncertainty principal applies to any instantaneous measurement.. If you count wheel revs per unit of time you have a +/- 1 rev error, and if you time one rev you maybe have speed changing every second or two, which would be confusing as heck. You really need to worry about the average over a reasonable period or distance..
 
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Bobajob

Pedelecer
Nov 1, 2019
231
112
Cornwall
I ride somet
Easiest way to check is over distance. The further you go the more the difference becomes. I found that every bosch bike I have owned shows around 1mph above.
I sometimes cycle with a friend, him Bosch me Yamaha. At the end of the ride he’s always completed abut 1-2miles further than me comparing odometers.
 

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