Best gear ratio for clutch and sprocket setup?
I find it highly unlikely that you will find an afforadable clutch with 14 teeth. As you have 3.5 HP Tec and 11.5 inch diameter tire I feel that the current 12/72 is your best bet unless you are riding on soft ground and do a lot of stopping and starting in which case it will probably be a bit too high geared. A jackshaft solves all the problems. If you wish to ride on hard blacktop and want max speed simply change one of the jackshaft sprockets. Same thing if you wish to climb hills and need a lower final ratio. If you save one clutch from being burned up, due to a ratio that's too tall, you have saved the price of the jackshaft.
you might be expecting too much from your minibike,as long as you have decent take off power leave it alone.changing the gear ratio will make it sluggish in the low end. that was the biggest complaint about minibikes in their day was the lack of power. A lot also depends on how you will use your bike and where you ride it. Originally they were intended to be driven around a small dirt lot or back yard with out much need for high speed.back in the day the average mini would top out at about 25 or 30 mph,which is plenty fast enough for what they are. Many guys think their mini bikes should be in the 40 to 50 mph range,they were never intended to go that fast and pushing one that far is dangerous
Absolutely true in my book! Shouldn't expect too much from a mini that was DESIGNED to supply back yard and vacant lot fun. The vintage minis were built to do mid twenties MPH. Note that a brand new DB [clone of an average 70s minibike] won't do 15 MPH! Sure there are a lot of exceptions built over the 50 or so year time span, however, the 3.5 HP and even 5 HP models were never intended to go fast. The 12/72 sprocket ratio is pretty common when 6" rims were supplied. Here is what the math says about this mini with 11.5 inch diameter wheels and useing 4000 as a rev limit and the above 12/72 sprockets. 22.8 MPH. As wind resistance goes up by the SQUARE of the velocity the HP requirement doesn't just double between 25 and 50. If a minibike type engine [sporting no expensive special parts] is restricted to 4000 RPM [typically the governer is set for 3600] and the tire diameter is 11.5 inches the gear ratio would have to be 2.738 to 1. In other words the rear sprocket would have to go from 72 teeth to 33 teeth!
I have a DB with a stock bypassed governer clone, a 12 tooth clutch and 66 tooth rear sprocket, it does'nt fly off the line but it's as fast or faster than many I've ridden! I'm making around 5000-5500 rpms as the governer is not working the way I have it set up. I have gone 42-43 mph top speed and it's perfect IMO. With an ungoverned 5.5 intec with an animal cam and 18# springs I'm getting roughly 6500rpms and went 45mph. An all around good setup! A torque converter can be setup so that it'll give ya both low and high end performance! Good luck!:thumbsup:
I just hate to hear people dog out the DB. It was my first minibike and it only had 2.5hp but I liked it, it was comfortable. I backed out the throttle spring, everthing else stock.. 27mph on a roadside speed checker.
(It was 3600rpm x 12t x 72t w/ a 14-15" diameter tire)
Anyways, a 14 x 72 setup might be asking a little too much from the 3.5. I run about that setup (12/60) on my 5hp Briggs and she loves it (the header and intake really added the pulling torque tho, if stock it would be a slug). I seriously think that a TAV would be your best option speed/torque wise, plus the smaller engine is putting less stress on the TAV belts etc.
(It was 3600rpm x 12t x 72t w/ a 14-15" diameter tire)
Anyways, a 14 x 72 setup might be asking a little too much from the 3.5. I run about that setup (12/60) on my 5hp Briggs and she loves it (the header and intake really added the pulling torque tho, if stock it would be a slug). I seriously think that a TAV would be your best option speed/torque wise, plus the smaller engine is putting less stress on the TAV belts etc.
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I gotta agree with you; maybe I did dog the db a little bet. But my point was that they, and the vast majority of vintage minis, were designed and manufactured to be slow. My db [yes I own one] has 12.5 inch diameter tires [not 14-15 inch] which means, if the 100cc motor is capable of suppling enough torque to attain the 3600 RPM and the 12/72 sprockets are used, my machine would do 22.3 MPH. I also agree wholeheartedly with old fat guy. Too many folks expect too much from a 3.5 HP mini. Sure, if it is to be a single purpose machine, top speed only for instance, then accepting the consequences of slug performance off the line and burnt-up clutches if 'normal' riding is also to be attempted. By the way, I like my DB.