Final gear ratio for standing start?

S

Stormsorter

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Final gear ratio for standing start with 79cc Pred?

Before I buy an engine I'd like to know what kind of final gear ratio I need to take off with just the engine's power. From a thread awhile back I remember reading about 36:1 with a cag engine. I doubt it, but would the 79cc Predator or 49cc HS need the same ratio?

My goal is to turn my Giant Stiletto into a shifting trike that can take off from a dead stop and still hit 25-30mph at a safe rpm for a stock 79cc Predator.
 
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Well after more digging on the forums it seems most with the 99cc Predator used 10:1-12:1 and mention never having to pedal, without running a shift kit. Sadly my cheap or free trike rearend is no longer available. My buddy decided he doesn't want to get rid of it anymore and I'm not sure I want to buy a brand new one.
 
I currently run a 9:1 with a 3.5hp/110cc Briggs and it takes off pretty good, top speed is close to 40mph. At one time I did use a 6:1, but it was difficult to take off on its own and top end was about 30ish because the engine wasn't able to make enough power to push it any faster with that gear ratio. Right now, I'm working on making one with a 15:1 or something close to that because people keep asking if they can ride my bike, and being it can go so fast, I don't want anyone else on it. With 12:1 and the engine governed down to 3600rpm, top speed should maybe be around 25 tops.
 
22.8 at 12:1 for my 24x3 tires, measured rollout was rounded to 25.5. Which is almost perfect if the cops try to give me crap about the motor size. I'll just disconnect the governor for the long outa town rides hehe.

Luckily there's an old Roadmaster factory that Pacific Cycles uses as a distribution center and Factory Store about 25 miles away from me. I can get a complete Meridian geared for $207 or single speed $187. They even told me they probably had few rear ends for $35 but didn't have time to look until Monday when the store part was closed. I just hope they got a 24" other wise I'll have to use the stock tires for a wile. If I have to re-lace the hubs its getting 4" tires and that's costly.
 
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