Intuvia Speedometer

[AlanDB]

I am a e-bike newbie, so please be gentle if this is a stupid question.

I mounted my handheld GPS (a Lowrance Endura Sierra) on my new Gazelle Arroyo today and went out for a short 6 mile test ride. I had the GPS trip meter set up to show speed, elevation and distance. The speedometer on the Intuvia display consistently showed about 1 to 2 MPH faster than the GPS speed. My average riding speed was about 14 MPH, so a 1 to 2 MPH difference in the speedometers is pretty significant. My experience with GPS devices is that they are pretty accurate on speed/distance measurements, so this concerned me that the GPS and Intuvia speedometers did not match more closely.

My first thought is that the LBS didn't set the wheel circumference correctly in the Intuvia setup. It is set at 2205. To measure the actual circumference, I made a mark on the tire where it was perpendicular to the garage floor, then rolled the bike forward until the mark was again perpendicular, and measured the distance between the two points. It measured 87 1/8" which calculates out to just under 2213.

My thinking is that with weight on the bike, the compression of the tire would cause some reduction in the effective circumference, but even if 8mm is significant, it seems like it is in the wrong direction for the speed discrepancy I am seeing. Wouldn't increasing the circumference setting actually increase the displayed speed for the same number of wheel revolutions in a given time?

Please tell me if my methodology is flawed, or if I am expecting too much from the Intuvia speedometer.

I do have another GPS (a Garmin Oregon) so I intend to repeat this experiment to see if I get the same results with the Garmin.

 

[Carterk]

Sorry, not an answer- but I’m convinced my computer (Brose, on a 2018 Raleigh Redux) is exaggerating my speed and distance as well. So I’m curious to see what people have to say...

 

[Deleted member 18083]

Please tell me if my methodology is flawed.

Alan ...

Bike computers calculate distance and speed from a tyre’s “circumference” (actually the tyre's perimeter) which, for your bike, is 2213mm.

Cicumference = 2213mm; therefore, radius = 352mm.

Let’s assume that a reasonably heavy rider squashes the tyre by 7mm. The distance from the centre of the wheel to where the tyre contacts the road is now 345mm.

Here’s my point: 345mm is NOT the tyre’s new radius. This is because the tyre is no longer a circle. As soon as the tyre ceases to be a circle (by having a flat section), we must stop using equations involving π, radius, diameter and circumference.

The distance around your tyre remains 2213mm (insignificant fractions of a millimetre ignored) and is not altered by having, for example, 50mm of curved tread changed into 50mm of flat tread.

... David

 

[AlanDB]

Thanks for the explanation David. That pretty much confirms that my method of measuring circumference was sound.

I have calculated that with the setting of 2205 mm, 14 MPH would be 170.3 RPM for the wheel. If the setting was changed to 2213, 14 MPH is 169.7 RPM. I am thinking that would not make a significant difference in the speedometer reading. And again, assuming the Intuvia speedometer is already reading high, that change would be in the wrong direction.

So I am still trying to figure out why the Intuvia speedometer is so far off from the GPS speedometer.

 

[Va. Bch. Electric Bike Center]

Damn, this means I'm not as fast as I thought I was...

 

[Sweetwater]

Thanks for the explanation David. That pretty much confirms that my method of measuring circumference was sound.

I have calculated that with the setting of 2205 mm, 14 MPH would be 170.3 RPM for the wheel. If the setting was changed to 2213, 14 MPH is 169.7 RPM. I am thinking that would not make a significant difference in the speedometer reading. And again, assuming the Intuvia speedometer is already reading high, that change would be in the wrong direction.

So I am still trying to figure out why the Intuvia speedometer is so far off from the GPS speedometer.

I don't think the GPS speed is an accurate measurement at any specific point in time. I understand that it's an average from your unit and the satellite feed which is not instantaneous. Contact your GPS manufacturer to verify your unit. I'm ok with a physical measurement rather than a triangulated measurement from a satellite above the atmosphere interpreted by another calculation in the receiver.

 

[Mr. Coffee]

Your methodology is not flawed.

My guess is that what you are seeing has much more to do with how a GPS computes your speed than to any systematic error on the part of either the Intuvia or your GPS.

One experiment is to find a very straight section of road and ride your bike very consistently straight and at a consistent speed. Both the GPS and the Intuvia should end up very close.

My guess is that your GPS is taking position samples every few seconds, calculating the distance between those positions, and then dividing by the sample time to calculate your speed. The glaring flaw in that approach is that your GPS assumes that you are moving in a straight line during the sample period. So if the sample time is fairly large (e.g. 5-10 seconds) and the road is fairly curvy, the GPS will consistently understate your speed.

 

[Over50]

On the few occasions I've hooked up my Garmin Virb for my commute, I also suspected the Bosch Intuvia was slightly exaggerating my speed. I never came to a solid conclusion since the camera requires the user to synchronize the video files with the GPS files and I never has assurance that I was performing that synchronization exactly. But from what I could tell by comparing my video clips which captured the Intuvia display, the Intuvia speed was about 1 mph faster vs the GPS speed.