Why I don't trust mechanical speedometers.

OK, I am basing this solely on the cheap Taiwanese speedometer that I got from Spooky Tooth. If you check the picture below you will see that 200 rpm equals 21 mph so I'll start from there:

200 rev/min = (21 mi/hr) / (60 min/hr)

converting hours to minutes gives:

200 rev/min = 0.35 mi/min

Thus:

200 rev = 0.35 mi

Doing some unit conversions:

200 rev = (0.35 mi) * (5280 ft/mi)
= (1848 ft) * (12 in/ft)
= 22,176 in

Now divide both sides by 200 :

1 rev = 110.88 in

One revolution is equal to the circumference of the wheel and the circumference of a circle is equal to pi times the diameter so the diameter is equal to the circumference divided by pi:

diameter = (110.88 in) / (3.14159)
= 35.29 inches

So unless you have a mighty large front wheel these speedometers are useless! Likely they are a kids toy designed to make them think they are going a lot faster than they actually are.

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the speed in mph works fine on those speedometers,its the rpms that wont be accurate unless youre just peddaling because the rpms on our motors will be alot higher than those numbers.

Those units are made for exercise bikes. The RPM figures relate to pedaling speed I think.

LOL! Those equations with that speedometer in mind...

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The rpm would have to be for the wheel since that is the only thing it can measure. If you can't trust one scale you can't trust the other. It definitely is not a "one size fits all" device since there is no way to calibrate it to the bike. I stand by my conclusion, it's just a kids toy!

Humor me, I'm currently unemployed, stuck at home, and bored silly!

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If it can't be broken I'll find a way!

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I'm not lost, I'm exploring!

its probally for a 16" or 20" kids bike.

I also got one of those speedometers from another source (see EBAY) and that meter seemed to work until I got past thirty M.P.H. then it pegged. After that it stopped working. Before that I tried cheep digital speedometers but they never worked (I think the vibration killed them.)

Does anybody out there know a speedometer that might work?

Mike Frye

if the circumference is pi X wheel diameter, then that's 26" X 3.14= 81.64 inches, or 6.8 (approx) feet per wheel revolution

6.8 X 200 wheel revolutions is 13,600 feet, per minute

since there are 5, 280 feet in a mile, that equals just over 2.5 miles in a minute

which of course is over 30 miles an hour- so no, it appears that the revs scale at least does not match the speed scale

the accuracy of the speed scale could be determined on a pre-measured course at a constant rate of speed, 60 mph for one minute,

so say, 20 mph which would take 3 minutes to cover a mile.

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Ok Fin,

Humor...I can manage that!

Wang chung say...faster you go, needle go up...Go slow, needle go down! We no calibrate...pedal faster...see what happen!!! We have correct product!

Ahh...you install motor...this make difference...front wheel faster with motor! Instrument for pedal use only!

You miss use product...we are not talk to you.

Jim

I just measured out my wheel travel, and it came out to just about 81 inches exatly per erevolution

My diameter is just slightly less than 26 inches, with the smooth 26 X1.75 tires I put on-

the 2.25 cruiser tires would be a little greater diameter and slightly more than 81 inches of travel

these speedos always claim BOTH 26 and 27 inch speed, with no apparent change of calibration, so I'd be curious as to which wheelsize they are more acurate on- I suspect 26"

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