deXtrous' Journal Of His First Project!

[Alaskavan]

DeX, I'd advise listening to Lou. I've never found an error in his calcs.

 

[deXtrous]

Took a closer look at your jackshaft. It looks as if you have about 40 teeth on the input sprocket on the jackshaft, and 8 on the motor? The first stage would then be about 5:1

The total gear ratio, to get 30MPH at 8000 RPM with a 26 inch tire is 20.6:1

Since you already have approximately 5 to 1, you would need another 4.1:1 reduction between the jackshaft and the rear axle.

In order to be able to coast downhill, you really should have a freewheel on either the rear axle (LH threads) or on the output of the jackshaft (RH threads.)

If it's on the jackshaft, about the smallest standard freewheel you can get is 16T, so, the axle sprocket would need to be about 66 teeth.

Ref the attached screen shot.

Thanks loquin! The pocket bike sprocket is a 54t and the pinnion is a 6t. This gives me a 9:1 ratio. The pocket bike wheel is about 7 inches in diameter. My bike wheel is 22. Lets say 21". This means it has to travels 21" instead of 7". This would mean a secondary ratio of 3:1, if I wanted to keep the same speeds as the pocket bike. Your measurements ended up being quite similar in terms of ratios, which is good!

So, even with the accurate measurments, do you suggest I increase the gearing to 4:1 to allow more low end, or keep 3:1, the same as the pocket bike?

You've been a great help and I really hate asking stupid questions, but how the heck does a freewheel engage and disengage? Say I were to put one on my jackshaft, to, as you said, allow myself to coast, how does it all work? I tried to search but I don't think anyone has asked such a silly question!

Once again thanks folks, you guys are much more help than the crew over at motorbicycling.com (but please don't tell them that! )

 

[deXtrous]

Ok scrap the previous question about the freewheel. I did a bit of reading and from what I understand it's not the freewheel sprocket that disconnects, it's the clutch, when you don't want to use it. Correct?

Therefore I could kill the motor when going down hill or when I just want to pedal, then when I need the motor just give a pull on the pull start and get going again? Is this what you're suggesting?

 

[deXtrous]

Scrap that question again. Figured it all out.

I just need to know where to get a couple sprockets now, one being a freewheel.

I got a pair on a BMX bike. a 16t freewheel/44t sprocket. Don't know if they're any good for the high rpm load though.

By the way, check this out, what do you think?

(cool program btw).

 

[augidog]

20" or 26" ?

i'm confused now...what size is that bicycle?

 

[deXtrous]

i'm confused now...what size is that bicycle?

I measured the diameter of the rear wheel and it was 21" [or maybe 22" if they're more common and I just measured wrong (couldn't get it flat due to hub and forks)].

 

[augidog]

ah. the calculator should say "tire" diameter for clarity, because that's the dimension you need to work with. "26" is close enuff...third time's a charm?

 

[deXtrous]

Ah ok! 26 inches it is then.