I need some speed

Sounds like you're geared way too high... the 10 tooth going into the jack shaft and out to a 10 tooth will keep the ratio there the same so it's still effectively geared at 4.4:1 now if you go for. A 10 tooth off the clutch into a 30 tooth at the jack shaft, then to a 10 tooth out of the jack shaft then run the chain from the jack shaft into the 44T rear sprocket would put your new final ratio at 13.6:1 so the engine could accelerate the bike better and allow it to reach a higher top speed since it's not straining the engine just to take off....

Basically, with the 4.4:1 final ratio you got now would be similar to trying to take off in a car from 4th gear, then the wind resistance with the 3hp engine would account for the really low top speed, there's just not enough power from that engine to overcome the wind resistance so top speed suffers instead of increasing.

With the 3hp engine, I'm guessing somewhere around 16:1 for a final ratio would give a good combination of acceleration and speed.

You are currently geared 4.4:1. That is way too tall.

That 3hp briggs makes maximum power at 3500rpm and floats the valves at 6k, so 16:1 will NOT take you to 40. I suggest a ratio of 8 to 9. That would be a 19t on the jackshaft for 8.5:1. However, since Staton-inc does not stock 19t #41 sprockets, I suggest you....

....ditch the 10t JS output and replace it with an 11t as chain doesn't like drive sprockets smaller than 11t due to chordal effect. Then you can use a 21t for 8.4:1, and a 22t for 8.8:1.

I put approx 1000mi on my setup 8:1 single speed on my 3.5hp Briggs. Briefly tried a 6.42 overall, which was wayyy too tall but did get me to 55mph after about a half mile

Pictured is my bike with 2-speed setup, 12.2 1st gear and 7.7 high gear. It takes off hard and fast and then shifts into high gear around 25mph.

Did you make a 2 speed out of a set of centrifugal clutches and one way bearings or freewheel sprockets?
I think either that method or a cvt is about the only way to get decent acceleration as well as a good top speed with a low powered engine like anything under 5hp...
I had a Comet cvt on a go kart when I was a teen and that thing would easily do burnouts and donuts but still topped out at just over 55mph.
I've been interested in making a 2 speed automatic for a bicycle for some time but want to figure a way to get it as compact as possible so it doesn't interfere with normal riding. I saw one that msrfan built where he integrated the trans case into the seat post tube that really sparked my interest since it was good and out of the way. I like your setup with the frame stretch which gives the needed space too, it doesn't look out of place at all this way. What engine are you running?

Davezilla said:

Did you make a 2 speed out of a set of centrifugal clutches and one way bearings or freewheel sprockets?
I think either that method or a cvt is about the only way to get decent acceleration as well as a good top speed with a low powered engine like anything under 5hp...
I had a Comet cvt on a go kart when I was a teen and that thing would easily do burnouts and donuts but still topped out at just over 55mph.
I've been interested in making a 2 speed automatic for a bicycle for some time but want to figure a way to get it as compact as possible so it doesn't interfere with normal riding. I saw one that msrfan built where he integrated the trans case into the seat post tube that really sparked my interest since it was good and out of the way. I like your setup with the frame stretch which gives the needed space too, it doesn't look out of place at all this way. What engine are you running?

Click to expand...

One way bearing is the same as freewheel sprocket. Look a few threads older to 3 spd auto trans. Same as 2 speed without middle gear. Had it up to 54.5mph today on briggs 3.5hp at 7.64 ratio on 26" wheel. MSRFan 2-speed had a big influence. My trans is very similar and I'm preparing a post with full details. Going to do highway 9 tomorrow (look it up, buddy )

BTW the 3.5hp briggs does not want to push a bike faster than about 50mph unfortunately. The comet cvt will eat the power (see my "not satisfied" thread). 40mph tops. Cvts eat too much HP. 3.5hp is only good for just under 50 in my experience. Man if I had a 5hp........ Forget it.

Yup... the cvt really needs the 5hp engine because they do eat the power once in "top gear" and the Comet type are rather heavy, lots of rotating mass on the end of the crank for sure. I also noticed the newer ones eat up a lot more power than the one I had back in the early 80's.... when my son was younger my parts bought him a go kart with a 6.5hp Robin engine that was similar to the one I had when I was a kid and the old one I had was a Lot faster, both in acceleration and top speed.