Fat Tires, wheel diameter, and speed differential - an observation and suggestion.

antboy

Member
I get more used to my e-bike, I'm also getting more geeky, and made a bit of an observation...

I've got a 20x4" fat tire bike (the Spark Mini), and over time I've noticed that trip travels didn't quite match up to either calculations on Google maps, or using Strava, and the on-bike trip counter was always a bit lower, even though they were started/stopped at the same time.

I know there's always some level of variance when relying on GPS, and there's also the issue of tire pressure, surface being ridden etc, but overall the trip counter is about 90% the distance of Strava. I've done a comparison on 9 rides now, but here's the highest, lowest and average difference.

HIGHEST DIFFERENTIAL
Strava - 20.8 km
Bafang - 18.6 km
diff - .894

LOWEST DIFFERENTIAL
Strava - 25.3 km
Bafang - 23.6 km
diff - .925

AVERAGE OVER 9 TRIPS
Strava - 257.4 km
Bafang - 235.5 km
diff - .915

NOTES:

1) For a few of these trips, I got geeky enough to plot them out after the fact on Google Maps, and the number always is very close to Strava's estimate, which makes sense.
2) Whether it was a more or less hilly ride didn't seem to make much of a difference on the disparity.
3) My partner has the same bike, and is using Strava as well. When we've ridden together, our Strava trips are VERY close, though her on-bike trip counter usually has an extra 0.1 km or sometimes 0.2 km per 10 km, give or take, but that's negligible and could be down to PSI and other factors.

SOLUTION:

I'm changing my wheel diameter from 20" to 22" on the controller.

The differential between 20" and 22" is .909, which means I should get 99.3% accurate speed/distance in the future. :)

UPDATE:

After setting my wheel diameter to 22", I've done an 11 km ride, and a 25 km ride. The two numbers are much closer now, with the on bike reporting just over 1% more distance.

TRIP 1
Strava - 11.4 km
Bafang - 11.6 km
diff - 1.017

TRIP 2
Strava - 25.1 km
Bafang - 24.7 km
diff - 1.016
 
Last edited:

antboy

Member
One month follow up.

If you have a fat tire bike, this is well worth testing, because you might actually be going slower than what your display is saying.

Both bikes now have wheel diameter set to 22" and the on-bike trip counters are only 1% (give or take) off from what Strava, Google, and Fitbit all report.

More importantly, this will also give you a more accurate speedometer.