Drive ratio check please

[PsychDoc]

Gear ratio check


Am I doing this right (or close enough)
Engine to tav2 or equivalent to rear sprocket

Engine rpm range roughly 2000-5000 keeping it conservative
The TAV2 starts at a 1:3 and progressively goes to 1:1- drives a 10 or 12 tooth sprocket.
26” rear tire

So say I have a 50 tooth rear
So the trans engages at around 2000- to a 1:3- 10 tooth- to the 50tooth rear equals a 15:1 total and 10.35mph
Max speed 5000rpm trans is now 1:1 overall ratio 1:5 and I’m at 77+mph
(Note to self- larger rear sprocket)


I’m using this page- https://electricscooterparts.com/motor-jackshaft-wheel-gear-ratio.html

 

[FNTPuck]

Gear ratio check


Am I doing this right (or close enough)
Engine to tav2 or equivalent to rear sprocket

Engine rpm range roughly 2000-5000 keeping it conservative
The TAV2 starts at a 1:3 and progressively goes to 1:1- drives a 10 or 12 tooth sprocket.
26” rear tire

So say I have a 50 tooth rear
So the trans engages at around 2000- to a 1:3- 10 tooth- to the 50tooth rear equals a 15:1 total and 10.35mph
Max speed 5000rpm trans is now 1:1 overall ratio 1:5 and I’m at 77+mph
(Note to self- larger rear sprocket)


I’m using this page- https://electricscooterparts.com/motor-jackshaft-wheel-gear-ratio.html

The catch with using a gearing calculator for a converter is that the "theoretical" speeds are based on the engine having enough power to pull that gearing. Once the converter goes close to that 1:1 gear, there is a ton of load on the engine and it needs a lot of power to keep accelerating. What usually happens, is the engine does not reach peak power when overgeared so you won't reach that redline and will stop accelerating where your power:weight says you should. IE, in your example with a stock ~2.5-3hp 4stroke you will peter out between 45 and 50mph and not rev past whatever rpm gives you that speed with your gearing.

In short: that theoretical 77mph is correct...if you have the ~10hp needed to get there.

 

[PsychDoc]

Thanks man- that’s what I thought. I think I’m going to find the largest rear sprocket I can reasonably get. Also the engine is rated at 4.6hp (don’t tell CA) I imagine it loses a touch being used, but I’ll free up the intake and exhaust so maybe 4+. 45-50 would be PLENTY

 

[CrazyDan]

4 hp is good for 55 mph if you tuck over your bars on a windless day on level ground according to a bicycle power calculator I like to use.

 

[FurryOnTheInside]

There's surely a limit to how low the gearing can be and still be useful. I think it would be fun to be geared high enough to go 77mph down hills.

I would not expect to be completely satisfied with the gearing first try. It would be an unlikely amount of luck to get it exactly right from theory and maths. Too many variables in the real world. Expect to buy at least a couple of different sizes of rear sprocket.

 

[FNTPuck]

Torque converter allows you to gear more for top end than a similar clutch setup without losing low end since you get additional gear multiplication at low speed/high load. Most start at 3:1 and slowly work to 1:1 or even .9:1 with a 10% overdrive.

Of course they also eat more power which offsets it some, and won't ever technically increase your speed vs a properly geared 1:1 clutch setup since you are still ultimately limited by power:weight:final gearing. The extra low end is NICE though!