What are good overall ratios?

[Tony01]

So I finally bit the budget bullet and ordered a CVT for my build. It's a 4-stroke Briggs & Stratton 3.5hp. Being in CA I'm shooting for a top speed of about 45mph with a cruise of 30mph, and it MUST be able to climb steep hills.

With the MaxTorque clutch my overall ratio was 6.67. The bike crawled to 40mph, and forget about grades more than 2%.

With the belt clutch my overall was 11.67. The motor happily revved to max ungoverned valve-floating RPM with an approximate speed of 26ish and showed me what was or wasn't bolted well enough.

I won't know what the valve-float RPM is until I get the cheap digi-tach in the mail next week.

Ordered the CVT, which is supposed to have a low ratio of 2.7 and high of .9, a 3x spread. Also got some sprockets, so I will have CVT to jackshaft 10:23, jackshaft to wheel 12:44 or 15:44 (two different sprockets to mess around with).

Overall 12t: 22.77 low and 7.59 high.
Overall 15t: 18.22 low and 6.27 high.

Will the low ratios allow me to climb super-steep hills? Are the high ratios too high?

 

[dtv5403]

18:22 is definitely plenty low enough to climb hills, however, 6.27 is very high for a 3.5hp engine. You can also change the final (rear) drive sprocket size to make different ratios as well. Its really up to you, but I would wait till you know your redline rpm before making a decision.

 

[FFV8]

Well,it is not rocket science. Assuming a 26" tire:

Your max rpm with the 11.67 ratio was 4072.

Your 40 mph power peak with the 6.67 ratio was 3448 rpm - just below the rated power rpm for that engine - 3600 rpm.

Since the max legal speed is 30 mph in CA, and you have hills to deal with, run the 12t pinion:

At 22.77:1 the drive begins upshifting at 12mph - Comet designed the CVT to hold about 3600 RPM under full load.

At 7.59:1 you are doing 36.7 MPH @ 3600 rpm. Fast enough to get a ticket and still not abusing the engine.

At 4072 rpm & 7.59:1 you max out at 41.5 MPH. Really too fast for bicycle tires & brakes anyway.

You wind up with a 30 MPH cruise around 3000 rpm. Good speed for that engine.

.

 

[Tony01]

Well,it is not rocket science. Assuming a 26" tire:

Your max rpm with the 11.67 ratio was 4072

................

At 22.77:1 the drive begins upshifting at 12mph - Comet designed the CVT to hold about 3600 RPM under full load.

Thanks for your reply. I did not know the Comet shifts at 3600. That means it would have a good hill climb window from 2300rpm engagement of 8mph to 3600rpm shift at 12mph.. Can't wait to try it.

The max speed when the valves floated was a guesstimate and could be higher (HOPING HAHA!) I did further research to find that upshift RPM can be adjusted, so if I make it later, figure how to run my motor at higher RPM and switch to 15t the bike could really fly.. 45mph at 3600 and 55mph at 4300