Speed

Will a bike generator give the proper rotational speed for the alternator? I have measured alternators that have a single V-belt pulley with diameter between 2.5" and 2.8". Wide 6 ribbed belt alternators I have seen use pulley sizes between 2.2" and 2.5" diameter. Taking my car as an example the diameter of the alternator is 2.6" the engine pulley is 7.25" diameter. Assume a minimum idle engine speed of 800 RPM is enough to generate reasonable current to maintain some amount of minimum charging to keep the alternator light off.

The perimeter of a pulley is proportional to the diameter (Pie times the diameter). Thus the minimum rotational speed of an alternator in revolutions/min (RPM) will be

(7.25"/2.6")*(800 RPM) = 2230 RPM

as a targeted minimum speed. To get full rated charging amperage one would need to run the engine at 2000 RPM, per amperage output testing. To do this the Alternator in my case would need to run

(7.25"/2.6")*(2000 RPM) = 5,577 RPM

for full charging amperage. Other alternators may be designed to run optimally at other speeds.

A typical kids bike with 20" tires has a rim of about 16" diameter and a 2.5:1 ratio between the back wheel and the pedal rotation of one turn. A typical adult 10 speed bike with 26" tires has a rim of about 23.5" diameter and a 3.75:1 to 1.37:1 ratio between the back wheel and the pedal rotation of one turn. A typical exercise bicycle with 19.5" tires has a rim of about 17" diameter and a 2.5:1 ratio between the back wheel and the pedal rotation of one turn. My test show an optimum pedal rotational speed in the range of about 60 RPM. Note: One can do a maximum of 110-120 RPM for short bursts but it is a strain.

Now if one mounts the alternator with a 6 to 8 ribbed pulley directly against the hard rubber tire of the exercise bicycle. This could be spring loaded. May need to remove the fan on the alternator to miss the tire. May need to slightly grind the edge of the tire down to fit the grove if a 6 ribbed pulley is used. An 8 ribbed pulley should work with out modification in most cases. Grind the sharp edge of the ridges of the pulley down so it doesn't cut the tire. This will also make it a bit smaller in diameter. For a 2.2" diameter multi-ribbed pulley modified in this way the diameter of where the belt travels is about 2.0". The rotational speed of the generator will be about

(19.5"/2.0")*2.5*60RPM = 1,462 RPM

This a little low to expect much output power.

 

Most alternator shafts are .5" or .625" diameter. As a second test one could try the bare shaft on the hard rubber tire. Assuming a .625" shaft this would give the rotational speed of the generator to be about

(19.5"/.625")*2.5*60RPM = 4,680 RPM

This is more like it, however it probably will wear the tire too quickly.

Offered by Mike.