4:1, 6:1 ??:1 that is a royal question

[Sphere]

.... BTW, you want a 6 to 1 reduction from engine to rear wheel for good performance.

Cris

If you recommend folks a final reduction of 6 I think you should also include your definition of "good performance"

[balboa_71]

Sphere,
Thank you for the question... The 6 to 1 reduction has always worked for me when calculating a total reduction from engine crankshaft speed to driven wheel rpm...simple as that. It's not perfect, but when folks post a question to the forum wanting to know numbers for reduction, that is all I can offer due to many unknowns with their bike build. It's only a starting point...might be too slow, might be too fast, but its a known value used by the industry. Many minibike manufactures, building single speed rides, since the 60's, have chosen a 6 to 1 ratio to work best for their customers needs. When I was building electric bicycles, I used the same formula for total reduction, and it was spot on.....

Thanks,
Cris

[realnutter]

balboa_71....

While a 6:1 ratio is the right order of magnitude, It seems quite a way off reality.....

I'll expand, keeping the numbers simple....

My bike has a rear wheel 2 feet across, so taking Pi as equal to 3, it is six feet around......

Now, if i want to do 60mph, thats 1 mile per minute.... so dividing a mile (5280 feet) by 6, i can conclude that my rear wheel needs to go round 880 times to do a mile..so must go round at 880rpm to do 1 mile in a minute....

But my engine only revs to 3600rpm..... so MY personal ratio is 3600:880...or near as damn it 4:1 !

If I was using little scooter wheels, half the size...they'd have to spin twice as fast at 60mph...so I'd have a ratio of 2:1!

If my engine revd to 7200rpm, I'd be looking at 8:1 (with the 24" wheel).....

Given that most of us are using industrial engines, and sensible wheels, the 4:1 figure is much more realistic.....

[Sphere]

Coincidentally, the final reduction of an Enfield gearbox is in the vicinity of 4ish, depending on the gearbox cog. I'm sure this won't work very well in hilly environment or against the wind, but if you want to ride 10 horses around on less than summery days you should prepare for that anyway.

[balboa_71]

Well I just ran some numbers on how my bike is set up. I'm actually running 4.8 to 1 rear ratio with a 15 tooth jackshaft sprocket and 72 tooth rear sprocket. So with my torque converter I'm running approx 14 to 1 in low to 4 to 1 in high range. I think 6 to 1 is a good starting point, but everyones ride and weight is going to be different. My bike with me on it weight 650 lbs, so 10 hp will just barely make it. I can get up to approx 55 mph with a fully broke in engine. Good performance to me is being able to keep up with street traffic.
So to answer the questions posted: 4 to 1 or 6 to 1, single speed minibikes and electric bikes do well with 6:1 total reduction ratio....It's a starting place, no more than that. As everyone here has found out, a stiffer ration approaching a 4:1 maybe more practical...who knows...it's just a matter of physics

Thanks,
Cris

[jeremy]

I just noticed this thread, and while it seems we've gone through it in past, there's recurrent confusion or disagreement about how to view it. So with my bike build despairingly idled from spending all my time on other people's things, I can at least talk about it by sharing the way I worked my powertrain's theoretical top-speed equation:

My new Hatz 1B30 engine (rated under 7 hp) has a max rpm of 3,600. My Comet 500 CVT tops out at an 'overdrive' ratio of 0.81:1, which means the jackshaft will be spinning at 4,444 rpm maximum--i.e. faster than the engine's crankshaft. I have chosen a final drive ratio of 1:5 for jackshaft-to-wheel (in my case 14-tooth to 70-tooth--I was advised away from small gears, e.g. 10-tooth, due to the strains they put on chain--hence the bigger rear gear). This ratio will spin the rear wheel (4,444 divided by 5) around 888 rpm at top engine rev's.

Here's where 'the rubber hits the road': my rear tire's center-rib has an outside diameter of 25 inches, which I multiplied by Pi (3.14159) and then divided by 12 to convert the sum to feet (6.545'). This is the street-length of one full wheel-revolution. When I multiplied this number by the 888 'rev's per minute' the wheel will be turning, and that sum by 60 to give the 'hour' value, I got 348,718 feet-per-hour. Finally I divided this by 5,280--a mile--and Voila! My small, underpowered bike has a theoretical top speed of 66 miles per hour. I'm guessing the whole package, including my 165 lbs, will weigh something over 400 lbs. We'll see what I get--based on others' real experiences, this 66 may be wishful thinking, but I'm banking on being substantially lighter all-up than many others. Who knows, I may have to find a gentle slope to realize this top speed...

If I find I can't reach max engine rpm's as-built, then I'll have the option to switch the 14-tooth jackshaft gear for a 13 (61 mph) or a 12 (56 mph)...

[pietenpol2002]

However that theoretical top speed is only achievable in a vacuum, not accounting for parasitic drag. With 6 hp, I wonder whether real world experience would suggest the 12 tooth?