New to mb and could use some help.

[Chester]

Hello gents, I would like to inquire about your automobile bikes. I have a mountain bike with 26 inch tires and an engine that red lines at 3600rpm and I would like the bike to have a top speed of 20 mph. OK with that data my question is would it be possible to have a sprocket attached to the engine shaft then have a chain run to a sprocket attached to the back wheel? I guess what I'm asking is if I did that would the sprockets be a normal size or would one of them have to be huge? Sorry if the question is hard to understand I'm new at this and it hurt just to think of what to say.

 

[bluegoatwoods]

Welcome aboard.

The drive train you've described is about identical to the typical Chinese two stroke engines that a lot of us ride.

Most of them have a 9 tooth sprocket up front and a 44 tooth in the rear. Most of these bikes will go faster than 20 mph. But these engines also run faster than 3600 rpm, I think.

so these two things might cancel each other out, giving you just about 20 mph. Perhaps.

I can't tell you where you can find these sprockets other than by buying an engine kit.
But they're not very expensive at all and you'll have all the parts you need for an install.

Good luck. You'll have fun once you start riding.

 

[HoughMade]

You can't forget that those 2 stroke engines have a reduction of about 4:1 built in.

With a typical small engine like a Briggs, Tecumseh or Honda and a 3600 rpm running speed, a front sprocket that is 9 teeth will need a rear sprocket of 100 teeth if there is no other reduction. This is much larger than in commonly available with the largest standard sprocket being 56 teeth.

My smaller Honda engine goes to 7200 rpm and with a 3.7:1 gearbox and 11 tooth engine sprocket to 48 tooth wheel sprocket, the top speed is in the 30s. With a reduction of 2:1, a 10 tooth engine sprocket to a 50 or 56 tooth wheel sprocket will get you close to what you want.

A friction drive would be easier- a 2" roller would give a top speed a bit over 20 mph at 3600 rpm.

Hear is a link to a thread where the ratio calculator I used is found:

http://www.motoredbikes.com/showthread.php?t=18102

 

[bluegoatwoods]

You're right. I had forgotten about that 4:1 reduction.

But maybe the engine in question has something similar?

 

[Chester]

You can't forget that those 2 stroke engines have a reduction of about 4:1 built in.

With a typical small engine like a Briggs, Tecumseh or Honda and a 3600 rpm running speed, a front sprocket that is 9 teeth will need a rear sprocket of 100 teeth if there is no other reduction. This is much larger than in commonly available with the largest standard sprocket being 56 teeth.

My smaller Honda engine goes to 7200 rpm and with a 3.7:1 gearbox and 11 tooth engine sprocket to 48 tooth wheel sprocket, the top speed is in the 30s. With a reduction of 2:1, a 10 tooth engine sprocket to a 50 or 56 tooth wheel sprocket will get you close to what you want.

A friction drive would be easier- a 2" roller would give a top speed a bit over 20 mph at 3600 rpm.

Hear is a link to a thread where the ratio calculator I used is found:

http://www.motoredbikes.com/showthread.php?t=18102

Ah yes that calculator helps a lot. So a reduction of 2:1 with a 8 front sprocket attached to a 50 back would get me up to 22.3mph. Ok so my question now is what kind of a duehickey do I need to get the initial 2:1 reduction?

 

[Chester]

You're right. I had forgotten about that 4:1 reduction.

But maybe the engine in question has something similar?

I believe the engine rpm and shaft rpm both come in at 3600rpm. Here's the engine I'm speaking of http://www.kohlerengines.com/onlinecatalog/productdetail.jsp?engnID=5260.

 

[bluegoatwoods]

Yes. I'm not an expert, but my quick glance would say that the output is the crankshaft.
No reduction there.

Pretty sleek little engine, though, isn't it?

 

[skyl4rk]

Circumference of 26" wheel: Diameter * pi
81.6" = 26" * 3.14 (note that actual diameter may differ due to tire size)
6.8 feet = 81.6" / 12 (convert to feet)
One wheel revolution is equal to 6.8 feet.

20 mph conversion to feet per minute
0.333 miles per minute = 20 mph / 60 minutes
1758 feet per minute = 0.333 mpm * 5280 feet per mile
Desired top speed of 20 mph = 1758 fpm

Wheel revolutions at desired top speed
Speed in feet per minute divided by wheel circumference
259 revolutions per minute = 1758 feet per minute / 6.8 feet

The question is what reduction ratio is required to run the wheel at 259 rpm when the motor is going 3600 rpm.
13.9 = 3600 rpm / 259 rpm (reduction ratio, no dimension)

If you use a motor sprocket with 10 teeth, the wheel sprocket would have to be 139 teeth.
139 teeth = 10 teeth * 13.9

If you use a wheel sprocket of 165 teeth, the motor sprocket would have to be about 12 teeth.
12 teeth = 165 teeth / 13.9

Because a large sprocket like this is expensive and not very practical, most people use a jackshaft or reduction gear of some kind.

For practice, try repeating the calculation at the idle speed of the motor, which will be roughly 1000 rpm. How fast does the bike go?

 

[Chester]

For practice, try repeating the calculation at the idle speed of the motor, which will be roughly 1000 rpm. How fast does the bike go?

OK did you want me to redo the calculation with the idle speed of the motor to actually get practice? Or did you want me to do redo it and see that at idle the bike would be going approximately 6mph? Because if it's the latter I could see where that would be a problem. If the clutch kicked in somewhere around the idle speed I would be popping a wheelie every time the clutch activated wouldn't I? Maybe I see what your trying to show me, but if that wasn't your point is what I just wrote about popping a wheelie a concern or is there some way around that that I can't see? Sorry if my newbieness is showing.:-/

 

[biken stins]

have you checked the dementions of engine to see if it will fit. if you do a bit of searching you will find a jack shaft setup. look up some threads on gear ratio. do you really want to run 3600 rpm doing 20 as a top speed. given any thought do 20 at a lower rpm might be easier on the bike. might think about a clutch,10t front 40t rear. half the fun of these machines is tinkering on them to make them the way we want. I think the pro CS4 would be a grand engine and easier on the bike at 20mph top speed.good luck !