gear ratios

a license from the motor vehicle department for a custom vehicle is place to start - it usually defines a lot of what you may do -

once you have the brake light requirement, the brake sizing requirements, etc., you can get the weight

then you can see if your drive train can take the output torque of the engine - you are a third horsepower on a good day - when you put ten men's leg power into your gears all at once, you may find that the level of force plus the requirements from the DMV lead you toward a nice used moped.

seriously - been there...

Dont worry, I went through and checked around. In maine (i just found this out last night) as long as it WAS a bicycle and you put an engine on it, it is still a bicycle. They call them toy viehicles. I just have to go along with all bicycle alws and cannot go over 20MPH. I need no liscene or anything.

It doesnt really matter anyway, I was planning of using it offroad. I just cant seem to find A way to get good enough reduction

You may have noticed that add-on motors for bicycles typically drive a tire by friction. The small driving roller solves the reduction problem.

Driving the tire surface solves a cascade of other potential problems, all of which relate to capacity limitations. A modern bicycle is a highly evolved machine. There is no huge factor of safety hidden within it; every part is just exactly as strong as it has to be, for human power.

That includes the frame. Hang an engine on it anywhere, and use it offroad, and it _will_ break.

If will not break while resting in the garage; it will break while someone is riding it. A frame fracture is a serious failure.

The rider will be hurt. Probably not badly enough to cause instant death. Possibly badly enough to cause lifetime paralysis.

Look for a nice dirt bike. Even the ones with very small engines, like you should buy, are way heavier than a bicycle. The weight of the engine is not the reason.

-Mike-

Here's a hint. Once upon a time, strap on cycle motors were popular. Their drive shaft bore directly on the tires outer surface (from above) The drive shaft was carborundum coated, and about 1 or 2 inches in diameter. The clutch lifted the motor bodily away from the tire. Using a 26 inch tire, the effective gear ratio would have been somewhere between 13:1 and 26:1

Brian Whatcott

Yes, but then tires wear much faster, and wet weather is a problem.

I see, but I am not going to be jumping or anything, just something to bomb around on, I said 30mph as a street speed. On flat level ground, and I probably will never go that far anyway.

The whole purpose of this project is to have fun building something _my_self_ and buying a dirtbike, really defeats the whole purpose of what I want to do.

All the safety consideration listed so far are correct. As with the other posters, I suggest you reconsider the project, but your question was basically a math problem.

You haven't supplied enough information to solve the problem.

You need to know the outside diameter of the inflated tire as well. Once you know that, here are some relationships that will help you solve the problem.

Let D = outside diameter of the inflated tire. Therefore the circumference or your tire = PI*D So the bike travels PI*D inches per revolution of the wheel.

You will need to convert the 25 mi/hr to in/min for this problem.

25 miles/hour = 26400 in/min (prove this to yourself).

Let w1 = speed of the rear wheel at 25 miles/hour w1=26400/(PI*D)

The fact that you have included both the front and rear chain wheel specs indicates you will be driving the pedal crank shaft with your motor (where are your legs going to be?).

To find out how fast the pedal crank shaft has to go when the bike is travelling 25 mi/hr: Let N1 = rear wheel sprocket teeth Let w1= rear wheel rotational speed Let N2 = front sprocket teeth Let w2 = pedal crank shaft rotational speed

N1*w1=N2*w2 (convince yourself that this is true)

if w1=26400/(PI*D) at 25 mi/hr then w2=(N1/N2)*26400/(PI*D)

Once you have the pedal crank shaft speed you can figure the gear ratio you are seeking.

Let w3 = motor speed (6000rpm!?!) Let N3 = teeth on motor drive sprocket Let N4 = teeth on pedal crank shaft driven sprocket (the regular chain wheel listed above) recall that w2 = pedal crank shaft speed

w3*N3=w2*N4

I suspect when you get your answer, you will find that the sprocket required for the pedal crank shaft driven sprocket will be quite large. Too large to consider mounting the engine where you would expect a motorcycle engine to sit. You will probably find that you need at least a two stage reduction via a jack shaft to get the sprocket sizes reasonable. This among other things (like Rube Goldberg drive arragements and screaming chains flying by your legs) will probably lead to the friction wheel drive being more practical. This would allow you to bring the bike up to speed by pedal power and then engage the friction wheel drive against the tire and then stop pedalling. It would also relieve the engine of the high torque requirements necessary for starting.

Need a good project for your bike? Try looking up heinzman hub motors...they are electric motors that are specifically designed to replace your current hub and provide motive force to the driving wheel...there are also electric battery packs and controllers specifically for the application....on the other hand a freind of mine used a weedeater motor and a friction drive on his front wheel, the weedeater already comes with an accellerator triger that can be mounted on the handlebars...

have fun!