Freewheel speed

[erdurbin]

Should the freewheel speed of the 26" front wheel kit be exactly 24mph (36kmh)? I am trying to calibrate my speedometer that I purchased on ebay, 9 bucks. Right now, I have it calibrated at freewheel full throttle, 24.0 mph.

[OneEye]

According to the motor performance chart (http://www.goldenmotor.com/e-Bike-DIY/hbs-36.pdf) at 0 torque the motor speed is ~310 RPM.  For a 26" wheel (26" tire diameter, ~81.7" circumference) that should translate to something very close to 24mph.

My math...
81.7 (inches/rev) * 310 (rev/min) * 60 (min/hr) / 12 (inches/foot) / 5280 (feet/mile) = 23.98 (miles/hour)

Given the lack of precision in reading the graph, actual wheel circumference in motion, and variation between the motor you have and the one they performed the measurements on, this sounds close enough.  How precise did you want your speedometer to be anyway?

-Mike

[erdurbin]

I just wanted to make sure it was right. Some other kits claim 28mph, but only do 22. I am a perfectionist, sorry. I will post pics of the bike when I finish it.

[OneEye]

Sorry, I wasn't trying to be sarcastic.  I was just trying to note how my back-of-envelope math will not necessarily match up to real world measurements from calibrated or uncalibrated devices (although they may be curiously close at times).

Oh, and a 26" tire may only be 25-5/8", so that might mean only 23.6 mph...

-Mike

[myelectricbike]

Hey, this question give me a great idea which I will post shortly as another topic.

My 48 volt setup will do almost thirty but you have to be careful in relying upon a speedometer that is triggered by a magnet on the wheel. Many pedometers I have adopted are just not fast enough and reed switches are great for doors and windows but subject to vibration as well as air gap when put on a wheel. Even Hall effect sensors can be fooled if the air gap is not consistent throughout the rotation or starts off too great.