Calling any maths wizards out there!!

[Mike D.]

My Bafang-equipped bike enables the system to be setup for 26" wheel size or27". The problem is, my wheels are 26.5"...

Question: If I set up for 26", is there a formula that would enable finding out the accurate trip distance using the one displayed?

 

[steve_sordy]

Depends how accurate you want to be. You could work from the declared wheel diameter (worst), measured wheel diameter (close), or do a roll test with you on the bike (best).

You can compare the ratio of diameters or rolling circumference (best).

Roll the 26" wheeled bike across flat ground and measure how far one turn is (Approx 82"). Use the tyre valve as the marker. Do the same with the other wheel (assume 27.5", therefore approx 86"). Divide the big measurement by the little one to get 86/82 = 1.049.

Rolling is important because tyres are so different that they can increase or reduce the nominal wheel diameter. It is better to be sat on the bike when you roll, because your weight distorts the tyre.

Anyway, once you have the correct rolling circumference for each bike and the correct factor, then you can apply it to your measured trip distance. If you are riding the 27.5 wheeled bike and have the trip computer set up for a 26er, then multiply the recorded distance by the factor, in the case of my example 1.049. Or just add 5%

If you can't be arsed to do the rolling, then just use the measured diameter. 27.5/26 = 1.058. ie Just add 6%

My mate and I had 2.2" wide Conti Trail Kings on our 26ers; the wheels measured 26.75" diameter. My mate bought a 27.5 wheeled bike that had 2.25" Nobby Nicks. His wheels measured 27.25". No wonder he couldn't tell the difference!

For us the difference by diameter was 27.25/26.75 = 1.019 or just add 2%.

 

[Zimmerframe]

26 x pie = a fat bastard ?

What kind of pie's help you calculate wheel sizes ?

 

[Mike D.]

Well, that was a real "D'OH!" moment.

So I measured the diameter from the tyre surface. 27". No need for a formula. ?

 

[Tim1023]

Well, that was a real "D'OH!" moment.

So I measured the diameter from the tyre surface. 27". No need for a formula. ?

Apparently there's an expression in French that seems to fit here:

"That might work in practice, but it would never work in theory."

 

[Mike D.]

Rene Descartes walks in to a bar.

The barman says "Pint, Rene?"

"I think not" says Descartes, and promptly dissappears.

 

[Tim1023]

Rene Descartes walks in to a bar.

The barman says "Pint, Rene?"

"I think not" says Descartes, and promptly dissappears.

It's from a few years back, but you'd probably appreciate this lot: How many surrealists does it take to screw in a light bulb? A fish:

 

[steve_sordy]

Well, that was a real "D'OH!" moment.

So I measured the diameter from the tyre surface. 27". No need for a formula. ?

I did wonder. But some people just "don't do maths." Probably had a dreadful maths teacher at school, took against maths, fell behind and because maths is all building blocks, never caught up. Which is very sad.
For me it was music, at age 11-12, my music teacher at school, was a 6' tall, 18-stone puce-faced psychopath with a permanent five o'clock shadow (think Bluto from the Popeye cartoons). You could smell the fear in his classroom. I believe that he got off on terrifying small boys, he so frightened one lad that he wet himself in class! Such were the times that the lad got a merciless ribbing from his classmates rather than the teacher being carted off to jail. The good old days I suppose, almost 60 years ago now.

 

[Manc44]

Multiply the trip distance by 1.019230769230769

(because 26 x that = 26.5)

The difference is only 0.19 miles (or 1015.38 feet, or 309.49 meters) every 10 miles.