What size is my wheel?

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? ...
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%
For the record, no one claimed that @PedalUma invented the method. Just giving credit to the EBR member who told me about it.

Bought my last (very simple) bike computer with my last bike (an MTB) ca. 1985. Whatever the instructions said is long gone, and I didn't care much about numbers back then. Probably just entered the nominal tire size and hit the trails.

Of course, the accuracy in question is largely meaningless in the saddle. My current display is still set for nominal tire size. Did the 2-minute measurement mainly out of curiosity.
 
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.


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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.
To each is own. When you get to rigid body dynamics (the physics of rotating bodies), diameter makes the equations and the thinking messier.
 
,.. 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 like to ride by feel and adjust my speed depending how I'm feeling that day.
Some days I'm feeling off and I need to slow down and be more careful or I'm going to end up getting hurt.

I only really care about my speed because I'm not allowed to go over 32 kph, and my new e-bike will go 42 kph with a fully charged battery.
I used to obsessively watch my power output and voltage to make sure that I always had enough juice to make it home, but now I carry two batteries so I only keep an eye on the trip meter.

I find myself riding faster now and using quite a bit more power, so now I'm pulling over to check my motor, controller and battery temperature to make sure nothing is getting too warm.

I'd like to ditch the display all together and just ride by feel, so now I like to guess my speed before looking at my display to verify my feelings.

To make things exciting, I ride no-hands then see how long I can close my eyes and ride blind before running off the road. 😂

You don't need to go fast to have fun.
There's plenty of exciting ways to risk your safety going slower. 😂
 
I think I mentioned that you make a chalk mark on the bottom of the tire and on the the sidewalk. Roll the bike until that spot is down again and mark that spot on the sidewalk. That distance is the actual circumfernce.
 
I think I mentioned that you make a chalk mark on the bottom of the tire and on the the sidewalk. Roll the bike until that spot is down again and mark that spot on the sidewalk. That distance is the actual circumfernce.
I already posted that come on man wake up 😅 😁
 
By international agreement, the inch is exactly 25.4 mm. So the conversion is no more or less precise than the measurement itself.
This is not personally to you, Jeremy but I can see there is still a lot of confusion regarding the tyre and rim size.

By the European Tire and Rim Technical Organization (ETRTO) all rim and tyre dimensions have been standardized and can be found on the sidewall of the tyre.
The ETRTO size is expressed by nn-xxx where
xxx is the internal rim diameter in mm
nn is the tyre width (basically also the height) in mm.

All 28 and 29" rims have the ETRTO diameter of 622 mm.
All 27.5" rims have the ETRTO diameter of 584 mm.
All 26" rims are 559 mm in diameter
and so on.

The tyre width might or might not be converted from inch. A screaming example could be the 2" tyre. 2" is exactly 50.8 mm.
  • Schwalbe makes a 50-622 tyre and names it 29x2"
  • Specialized makes a 51-622 rubber and names ir 29x2"
Are these two tyres identical in size? Certainly not.

Only the ETRTO size is the true tyre and rim size. All the 26, 27.5 or 28 or 29" rim sizes are of historical meaning and do not reflect anything really. Also, the inch tyre width is incorrect. For instance, a 2.4" tyre is not 60.96 mm but just 60 mm wide... Like in marking 60-584 (27.5x2.4"), where the inch size is just a guidance but not measurement.
 
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.
Reminds me of my uncle, who counted his cattle by counting the legs, then dividing by four.
 
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.
Edison's men were working on the light bulb and going through all the comclicated calculation trying to figure out the volume of the glob was when Edison walked over and said just fill the damn thing with water and measure the water.
 
Edison's men were working on the light bulb and going through all the comclicated calculation trying to figure out the volume of the glob was when Edison walked over and said just fill the damn thing with water and measure the water.
... resulting in all being electrocuted.
 
That's not 100% true.
Distilled water does not conduct electricity.

I removed the globe from an LED light bulb and submerged the electronics in distilled water,..


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I worked for over 20 minutes,
I stuck my finger in the water.
No shock, then I got bored and poured the water out.

I refilled it with tap water and plugged it in again.
It sizzled, bubbled and burned out in less than 30 seconds.


LED strips are fine too if you coat them with silicone.

View attachment 178162
Is too... They hadn't yet invented distilled water 1879 🚬 🤔
 
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.
My first e-bike had 20 x 3.3 tires. I didn't know what wheel size to tell my display. Rolling would have been inaccurate unless I used a carpenter's level to measure when a mark on the sidewall was directly below the axle. I simply measured from the center the axle to the pavement. I used mm because they're simpler to read and write than 32nds. It was 290mm.

Next, I lashed a brake lever to prevent rolling and adjusted a rope on each side to hold it vertical. I put 100 pounds on the rack, over the axle. At 30 psi, the tire would have to add 3.3 square inches. If the axle height lost 3mm, that would make the odometer read 1% higher. The change in height was much smaller. Since then, I've read even if it had dropped 1%, the effect on the odometer would have been less because even if the radius is reduced, the tread length remains the same.

Twice 290 is a diameter of 580mm, or 22.83 inches. I chose 23, which would read 0.74% high. Close enough.

To test it I used a large-scale map of a circuit I ride. I broke down into a dozen or so straight segments, added up the mm, and used the scale to calculate the feet. Some days my odometer agreed exactly with the calculated distance, but the odometer could be several percent low. That bike used a geared Bafang motor. I figured the revolution sensor was erratic.

Another bike has 27.5 x 2.00 inch tires and a direct drive motor. Its odometer, too, could read right or several percent low.

Twenty years ago, I twice measured a walking track near here: 528 yards, or exactly 0.3 mile. There's a No Bicycles sign, but nobody walks there because lack of maintenance has left a rutted surface with clumps of grass. I tested both bikes there. I'd put a marker on the track at the bottom bracket, ride until 0.3 came up, put down another marker, measure, pick up the first marker, and ride until 0.6 came up. Etc.

It takes an effort to pedal no faster than 5 miles an hour. At that speed, both odometers were incredibly accurate. As I I rode a little faster, both odometers read low, perhaps more than 15%. As I rode still faster, both turned around and read high. On my fastest laps, I maintained 10 to 15 mph. The odometers read 60% high.

The wheel revolution sensor in a motor seems to be a magnet and a hall-effect transistor. One of them is on a holder than can bounce. Depending on the magnitude and frequency, this can result in undercounting or doublecounting.

The performance of these odometers seems to depend on how well I avoid or slow for violent pavement ripples, which can be invisible in daylight. Another kind of of wheel sensor is external to the motor, using a hall-effect transistor on the chain stay to detect the passage of a magnet on a spoke. I wonder if that's more reliable.
 
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My first e-bike had 20 x 3.3 tires. I didn't know what wheel size to tell my display. Rolling would have been inaccurate unless I used a carpenter's level to measure when a mark on the sidewall was directly below the axle. I simply measured from the center the axle to the pavement. I used mm because they're simpler to read and write than 32nds. It was 290mm.
simpler? that's a nightmare. then I just used mile markers to set it the rest of the way.
 
simpler? that's a nightmare. then I just used mile markers to set it the rest of the way.
Measuring from the center of the axle to the ground gave me precisely the information I needed. That's easier than trying to roll exactly one revolution. The rest was checking, which showed that sensor error can be a much bigger problem than errors in diameter.

The only mile markers around here are on the interstate, and they're placed where it's convenient.
 
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Sure they did.

Distillation was invented in 3000 BC.

I'm sure they knew could boil water through an alcohol still.


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You can boil 🦕 and 🌿 fuels or any organic mass through a still too and get cleaner burning vapors coming out,..
Ooohhhh the water bong effect.
But the Edison lackey didn't have a time machine to go back to Mesopotamia?
 
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