gear ratios for weed eater bike

[keatonx]

I'm building a rack mounted 16 inch weed eater bike with a 31cc reed valve MTD weed eater with tons of compression and power. I plan on driving this thing in the trails and maybe some small hills. I heard that the recommended gearing for 16 inch bikes is 14:1, but I only have the parts to have 4.5:1 on my chains and 3.5:1 in an old cordless drill gearbox i'm putting on my engine.
I'm not completely sure how to add these, but i think it'll be less than 14:1. Will the powerful engine, the fact i'm a really short and light kid, and the drilled out muffler with no spark arrestor make up for the messed up gearing?

 

[srdavo]

4.5 X 3.5 = 15.75

engine rpm 8000, at the above ratio, on 16" wheels will run almost 25mph.

Here's a great gear ratio calculator:
http://www.motoredbikes.com/showthread.php?t=20374

 

[keatonx]

Thanks! I thought you added, not multiplied, so I'm fine.

 

[5-7HEAVEN]

You'll love it more on the trails if you gear it 20:1 or more.

 

[wheelbender6]

Weed eater exhaust can be painfully loud. You may be able to use an expansion chamber from a goped. Simply tearing out all the restriction in your stock exhaust may add a bit of power but it will definitely attract law enforcement.

 

[keatonx]

I don't have any good gearboxes for higher gearing, but i have 2 gears that do 7:1. However, i can't weld, so i don't know how to position it. that would be around 30:1, which will probably be painfully slow.

 

[keatonx]

I'll try the 15:1 and see how it works.

 

[5-7HEAVEN]

I don't have any good gearboxes for higher gearing, but i have 2 gears that do 7:1. However, i can't weld, so i don't know how to position it. that would be around 30:1, which will probably be painfully slow.

If you're offroad, you will definitely appreciate the 30:1 gears. On the road, maybe 19:1.

What you COULD do is set up the jackshafts so you could gear them from 3.5:1 up to 8.57:1. That way, you could swap gears for offroad and street.

 

[keatonx]

How would I set this up? Anyways, wouldn't 30:1 be really slow?

 

[5-7HEAVEN]

Once you calculate the ratios and size sprockets you'll use at the jackshafts, it's a matter of removing and lengthening/shortening the chain(s) and swapping jackshaft sprockets to change gearing.

If you'll be climbing offroad hills, you'll need 30:1 gears or better or the engine will bog. While testing and tuning my motorized bike, I leave it in 1st gear, which is 32.46:1. With a running start, I fly up the hill. If I start in 2nd gear, which is 20-something:1, my bike will bog.

I have a shift kit with 8-speeds and shift kit. Top gear is about 14.32:1 @ 45mph.

How are you getting 4.5:1 on your chains?