Wheel Circumference Calibration….

[oceanskipper]

My wheels/tyres have a circumference of 2330 m according to the manufacturer’s website and I have set this in the configuration for the bike. However the speed reading when my motor cuts out is 16.7 MPH. If I adjust the circumference to the minimum allowed, the speed reading when the motor cuts out is around 15.5 so correct as far as the display goes but it begs the question why when I put the correct circumference in is it so far out?

 

[Mikerb]

2330 sounds right for a 2.3 tyre on a 29 inch wheel. 29 inch tyre ETRTO ( bead diameter) is 622mm and 2.3 inch tyre = 59mm....( x2=118) .
622+118=740..........x3.142= 2325..............( near enough!!)
My Bosch powered bike is on 27.5 and it also cuts out at 16.5mph.
I think it just allows a little margin.

ps interesting that the diameter of a 29 inch wheel is in fact only 24.5 inches!! Wheel sizing is a joke!!

 

[JZA]

My wheels/tyres have a circumference of 2330 m according to the manufacturer’s website and I have set this in the configuration for the bike. However the speed reading when my motor cuts out is 16.7 MPH. If I adjust the circumference to the minimum allowed, the speed reading when the motor cuts out is around 15.5 so correct as far as the display goes but it begs the question why when I put the correct circumference in is it so far out?

As I understand it, the wheel circumference that we can input doesn’t change the assistance settings, it only changes the displayed speed, range and odometer readings. The manufacturer will calibrate the bike/motor to cut out at 15Mph with the minimum tyre size for your wheel size I.e. they calculate how many RPM the wheel would be doing at 15Mph with the smaller tyres and that is the wheel rotational speed at which the motor cuts out regardless of what is displayed on the screen. At the end of the day, the system only has a wheel magnet to measure rear wheel RPM and that is what is used for the assistance cutout and is not user adjustable. We only get to adjust the speedometer calibration which is nice because if you run a larger tyre, you get a tad bit more top speed (same wheel rotational speed with a larger circumference tyre means higher speed). Bike manufacturers are doing us a favour by calibrating the bikes with the smaller tyre size.
Now I have a question for you. My ebike came with 29x2.6 tyres but is calibrated to a wheel circumference of 2305 mm which is that of a 29x2.2” tyre. A 29x2.6” tyre would be 2369 mm. Should I calibrate my bike to 2369mm or should I allow for a little bit of tyre squish that would reduce the effective tyre radius by maybe 5mm?

 

[oceanskipper]

The calibration does only affect the speed reading but even with the calibration set to the lowest possible circumference mine still reads about 15.9mph when it cuts out so it’s very inaccurate. I have two sets of 29” wheels and the results are the same no matter which I am using.
To answer your question in my experience it matters not one jot what circumference you set so just put it at the lowest unless you want to feel like you are going faster than you are in which case use the max allowable which will still only read 16.7mph when the motor cuts out.

 

[JP-NZ]

As I understand it, the wheel circumference that we can input doesn’t change the assistance settings, it only changes the displayed speed, range and odometer readings. The manufacturer will calibrate the bike/motor to cut out at 15Mph with the minimum tyre size for your wheel size I.e. they calculate how many RPM the wheel would be doing at 15Mph with the smaller tyres and that is the wheel rotational speed at which the motor cuts out regardless of what is displayed on the screen. At the end of the day, the system only has a wheel magnet to measure rear wheel RPM and that is what is used for the assistance cutout and is not user adjustable. We only get to adjust the speedometer calibration which is nice because if you run a larger tyre, you get a tad bit more top speed (same wheel rotational speed with a larger circumference tyre means higher speed). Bike manufacturers are doing us a favour by calibrating the bikes with the smaller tyre size.
Now I have a question for you. My ebike came with 29x2.6 tyres but is calibrated to a wheel circumference of 2305 mm which is that of a 29x2.2” tyre. A 29x2.6” tyre would be 2369 mm. Should I calibrate my bike to 2369mm or should I allow for a little bit of tyre squish that would reduce the effective tyre radius by maybe 5mm?

View attachment 92175

Thanks for posting this chart, this morning I took my Ebb to work (I normally don't) and whilst riding I took my phone out (strava) and the speed on the bike was a lot higher 30.3kph vs 28.8kph on strava (GPS). When I first got my bike I measured the roll out of the 29x2.6 at 2330mm but now according to that chart I see it should be 2369mm.

I have changed it to 2369 within the flow app so will see how this changes closer to the correct GPS speed. Thank you

 

[Husky430]

To be 100% sure, rotate your wheel until the valve is at the bottom, chalk a mark on the floor, roll bike forward until the valve is at the bottom again, chalk and measure between the two marks. That to me is the easiest and most accurate way to measure wheel circumference. Just make sure you do the back wheel where the magnet/sensor is.

 

[JP-NZ]

To be 100% sure, rotate your wheel until the valve is at the bottom, chalk a mark on the floor, roll bike forward until the valve is at the bottom again, chalk and measure between the two marks. That to me is the easiest and most accurate way to measure wheel circumference. Just make sure you do the back wheel where the magnet/sensor is.

Thats what i did at 2330 but obviously its wrong

 

[wenna]

Thats what i did at 2330 but obviously its wrong

If you want it as accurate as possible I guess you need to sit on the bike, with the correct tyre pressure, when you roll it to measure the distance. I have not tried this so I don´t know how much difference it results in.

 

[JP-NZ]

If you want it as accurate as possible I guess you need to sit on the bike, with the correct tyre pressure, when you roll it to measure the distance. I have not tried this so I don´t know how much difference it results in.

I'm about to ride home from work so will be able to see how it compares to the GPS speed.

 

[RustyIron]

If you want it as accurate as possible I guess you need to sit on the bike, with the correct tyre pressure, when you roll it to measure the distance. I have not tried this so I don´t know how much difference it results in.

Yup, weight the bike, mark the floor with chalk, and roll her for one rotation.