What size is my wheel?

[Cybersnow]

I am riding my new Zen Photon Ultra and the speed and distance on my bike. The bike computer was set at the factory for a 26” wheel. The original specs on the bike said 27.5”. When I measure the diameter of the wheel from the floor to the top of the tire it is about 27.5” but the wheel rim is about 25”. I am comparing the reading on my phone which uses GPS which is supposed to be pretty accurate but my wife’s phone and mine can be totally different as there is a margin of error in standard GPS. Should I reset my bike computer to 27.5” or leave it alone?

 

[fooferdoggie]

GPS is only so accurate. the best way is to mark the wheel on the tire facing down. then sitting on your bike roll it till that line is straight down and measure that that will give you the real wheel size. but PSI and weight can change it.

 

[Jeremy McCreary]

I am riding my new Zen Photon Ultra and the speed and distance on my bike. The bike computer was set at the factory for a 26” wheel. The original specs on the bike said 27.5”. When I measure the diameter of the wheel from the floor to the top of the tire it is about 27.5” but the wheel rim is about 25”. I am comparing the reading on my phone which uses GPS which is supposed to be pretty accurate but my wife’s phone and mine can be totally different as there is a margin of error in standard GPS. Should I reset my bike computer to 27.5” or leave it alone?

If you aren't looking to game the max assist speed — say, to get assist above 28 mph — I'd correct the wheel diameter. That way, the display will show a good approximation of actual ground speed.

From @PedalUma , I learned to measure actual wheel circumference and divide by pi to get actual diameter. Put the rear wheel valve stem at bottom dead center and mark the underlying spot on the floor. Then roll the bike straight forward till the valve comes back to BDC and mark again. The actual circumference is the distance between marks.

 

[Cybersnow]

Thanks, I went out to the barn to try that and received a note from Zen saying it is a 27.5. Dont know why the set the computer to 26. But then there are several about the bike that make little or no sense.

 

[fooferdoggie]

Thanks, I went out to the barn to try that and received a note from Zen saying it is a 27.5. Dont know why the set the computer to 26. But then there are several about the bike that make little or no sense.

but the rim is 27.5 not the wheel.

 

[PCeBiker]

I am comparing the reading on my phone which uses GPS which is supposed to be pretty accurate but my wife’s phone and mine can be totally different as there is a margin of error in standard GPS.

GPS is only so accurate.

I used a Speedometer app on my phone to check my speed.

The trick was to find a straight flat smooth road with a clear view of the sky and no buildings, trees or clouds blocking the view and then ride top speed to help reduce the error to negligible.

I had 26-32 satellites tracking me at the time.

Remember that a 26" rim can have a 4" tire or a 2" tire installed, so that changes everything.

Find the setting that most accurately shows your speed if you can't dial it in exactly.

Choosing wheel diameter would make a lot more sense.

I've got 26", 27.5", and a 29" rims that all have Very close to the same wheel diameter with the tires installed.

 

[il Munduato]

You want to match the computer to the markings on the tire.
Some of the more sophisticated computers do allow for you to input tire circumference but for the most part the difference is negligible unless you are riding for NASA

 

[PCeBiker]

,..but for the most part the difference is negligible unless you are riding for NASA

Yeah,..
I don't care if my odometer is off by 10 cm for every km.
It's close enough for me.

Changing the air pressure probably affects the speedometer reading more.

 

[Jeremy McCreary]

You want to match the computer to the markings on the tire.
Some of the more sophisticated computers do allow for you to input tire circumference but for the most part the difference is negligible unless you are riding for NASA

Wait, you can ride for NASA? Choice of planet or moon?

My rear wheel, nominally 27.5" diameter per the specs and the markings on the somewhat worn tire, measures 27.2" actual at 35 psi. This is at the tread surface, not the rim.

Don't recall the details at the moment, but the display and RideWithGPS differ slightly on ground speed with the display set for 27.5". Plan A is to enter the 27.2" measurement and see what happens to the discrepancy.

 

[PCeBiker]

,.. Plan A is to enter the 27.2" measurement and see what happens to the discrepancy.

Just enter whatever you have to to get the reading accurate.
My KT controller allows an adjustment from 0-255 to adjust the speedometer reading.

87 got my speedo dialed in perfectly.

 

[PCeBiker]

From @PedalUma , I learned to measure actual wheel circumference and divide by pi to get actual diameter.

That's like the story of the army guys trying to measure the height of a flag pole.
All kinds of math and cosine a measuring tape and a surveyors scope.

The gardener walked over to the flag pole, pulled it out of the ground, laid it down and measured it.

 

[PCeBiker]

From @PedalUma , I learned to measure actual wheel circumference and divide by pi to get actual diameter.

If you are sitting on your ebike with larger tires at a lower pressure, then you create a flat spot on the tire at the road.

That flat spot along with the same flat spot at the top of the tire actually becomes the diameter of the wheel.

 

[Nvreloader]

You might want to check and see if you can enter MM measurements for the tires diameters/circumference , it should be better for accuracy. I had to go to 31.5" to get any accuracy. ymmv

 

[Jeremy McCreary]

If you are sitting on your ebike with larger tires at a lower pressure, then you create a flat spot on the tire at the road.

That flat spot along with the same flat spot at the top of the tire actually becomes the diameter of the wheel.

All that really counts for the ground speed from wheel speed calculation done in the display is the radius from the axle centerline to the tire contact patch. That's half the effective diameter given by the @PedalUma method — ideally with the wheel under normal riding load. I didn't bother with that last bit.

 

[PCeBiker]

That's half the effective diameter given by the @PedalUma method —

I can't see it being half?

I can remove my 26"X4" tire from the rim, and I still haven't reduced my wheel diameter by half.

I think something is wrong with our math?

Somebody got a free piece of PI

 

[Jeremy McCreary]

You're correct, D = 2 R = C / pi. Didn't mean to imply differently, just wrote sloppily. My physics-trained brain automatically favors radius over diameter in thinking about rotational problems. Diameter is sometimes more practical, though, and most people do the opposite.

 

[PCeBiker]

Diameter is sometimes more practical, though, and most people do the opposite.

Apparently the math is a lot simpler using diameter as well.

I remember watching a 5 minute video a few years ago where the math was done to calculate circumference, area, and volume using diameter vs. radius.

I hadn't taken Calculus since 1983, but I remembered enough to see that the number crunching was A Lot simpler using diameter instead of radius.

Even going into quadratic space and further, the math was simpler.

 

[Stefan Mikes]

@Cybersnow:
Just read the marking on the tyre sidewall.

Just to clarify the things: all 27.5" wheels have the rim diameter of 584 mm (you should find this value on the tyre sidewall; I expect it to read 60-584). 60 mm is 2.4", which is the tyre width. The 27.5" is just a nominal value. The actual wheel diameter for 27.5x2.4" (60-584) is approximately 704 mm (or 27.71"): now you know where the 27.5" was taken from!

The Wheel Circumference for the 60-584 tyre (according to size tables) is 2,213 mm (87.13"). Hopefully, your bike computer can accept the WhC value?

 

[il Munduato]

Wait, you can ride for NASA? Choice of planet or moon?

My rear wheel, nominally 27.5" diameter per the specs and the markings on the somewhat worn tire, measures 27.2" actual at 35 psi. This is at the tread surface, not the rim.

Don't recall the details at the moment, but the display and RideWithGPS differ slightly on ground speed with the display set for 27.5". Plan A is to enter the 27.2" measurement and see what happens to the discrepancy.

Yes... I chose the moon as it's reachable with my frequent flyer miles and that I already have a few craters mapped out to ride.
And not to take anything away from Mr. Pedaluma but those instructions are included with every bike computer I've ever seen since... Oh I don't know... Forever? Imo they are the easiest and most accurate way if you are so inclined to be as dead on balls as possible.
And don't get me wrong... I'm all for accuracy, but in the end it's all anecdotal and as I get older this kind of stuff matters less and less. I never adjust my speed by display readout but by what I'm feeling and seeing and knowing that you are doing 18.0mph or 18.1mph or if you've traveled 30 mi or 29.8 mi doesn't change the price of a beer.
I input the tire size and call it a day.... After all we are talking about <1%

 

[Slaphappygamer]

I have a 27.5” rim. I got my tape measure and placed across the tire. It measured 28”. I changed my display to match the actual tire size and there was no change in the displayed speed.