What size sprocket should I get?

[cdevidal]

If I wanted to go 25mph but pedal as though I am going 10mph what would my options be?

 

[rwaterman00]

too much work for me...I will stay an outlaw although I can understand having a goal as building one within legal limits.

 

[outatime88]

Have you considered a 4 stroke(HuaSheng 142f)? Much more reliable than a two stroke and a more realistic choice for commuting, although it is more expensive for a decent transmission.

 

[cdevidal]

Yes I have. In fact I used to have one, but never got it running. What size sprocket would you use for it, and if I wanted to exercise some but still get to work in enough time what would my options be?

 

[cdevidal]

I want to mention and I hope no one takes this personally. But, you are not going to get a workout on a gas bike. If you want to workout go electric and draw all the juice going to work (8 miles) and then do 50W or eco mode going home. This is a workout and a lot of commuting folks do this for a workout.

Ugh. I was under the impression I could boost the top end with pedals. Perhaps I would need a larger top end gear ratio to do that, but that’s not going to happen today.

Other option is as you said, just cut the power. Maybe a 250W mid drive.

Can a jack shaft kit allow me to boost the top end with pedals?

 

[rwaterman00]

lolol how fast do you need to go? full throttle gives me 40+mph if I use a 44 tooth, probably about 50mph if I would use a 36tooth . that's damn fast on a bicycle. in respect to real road riding... potholes and bad road brother... at 40mph a lil pothole will waffle a rim on a bike and put you on the pavement. I think bakaneko is right. if you want a workout get an electric bike that you can adjust your settings for leg work. on a motorized gas powered bike the leg work is to get you up to speed or pull out depending on the rear sprocket size.

 

[bakaneko]

Other option is as you said, just cut the power. Maybe a 250W mid drive.

A jack shaft will help with the top end but I do not think it is a significant increase with a properly geared and tuned motor direct drive kit from what I have seen.

Always go for more power within legal limits for the ebike. 250W is like average 12-15mph depending on rider size, weather, and road conditions. You might find especially going to work that you just want to get to work asap at 20 mph+. This is where a stronger motor will do the job. And, on the way home you can drop it to 100W or 8-11mph and pedal up to 15 mph for a workout. Also, get a good tire with low rolling resistance and puncture protection. I changed my rear tire to knobby tires (available), slime tube, and tire lines and literally lost 30-50W from a smooth tire.

I found 4-stroke kits to be more reliable and maintenance free if you are not mechanically inclined or just want to focus more on riding. 4-stroke kits only require oil changes along with the general bike maintenance while 2-strokes require gas mixing and a lot more up keep. Of course, 4-stroke will limit you to certain type of bikes with a bigger inframe to fit the motor.

 

[cdevidal]

lolol how fast do you need to go?

Fast enough to not spend a long time on the bike every day -- I have two jobs, one of them is at home -- but not so fast I cannot also exercise at the same time.

I currently pedal about 8-10mph. An eight mile round trip at that speed is about two hours. Too much time spent, and too tiring for this old, fat dude, I would more often not go and take the car if I had to only pedal, which I don't want, either.

Getting to 25mph means I can spend less than 40 minutes a day on the bike. There are few stop lights, so I would spend most of that time at cruising speed.

If the motor only adds ~17mph on the flats, that's perfect. So the challenge now is to find the right electric motor size (or gas motor sprocket size) for the weight, that lets me feel like I'm contributing, burning some calories, but not losing precious time.

Best option now seems to either be a medium-sized electric mid drive, or intentionally reduce the speed of a hub motor using a potentiometer or something, and a pedal assist sensor ring for the bottom bracket. I would want to reduce the speed in a way that is difficult to change while riding, to reduce temptation to be lazy and speed it up.

 

[cdevidal]

A jack shaft will help with the top end but I do not think it is a significant increase with a properly geared and tuned motor direct drive kit from what I have seen.

Well, I don't want more top end. I want to contribute on the top end. But it doesn't sound like a jack shaft would do that. I am inclined to go with an electric motor. That seems like a better solution.

20 years ago I had a ~250W friction drive kit with a lead acid. It seemed to only add about 3-5mph. Helped, but not tremendously so. I was taking the same types of flat roads, for about the same distance, and I weighed about the same -- a few pounds less, actually, both in backpack and in body -- and on skinnier road tires than my current steed. So I will seek a 500W kit and as you said, reduce the speed until it is just right, and go with pedal assist.

Alright thanks everyone!

 

[bakaneko]

Yeah, I think electric motor is a good fit but remember that it does take a good battery, which can be expensive. The speed will come with more exercise and getting your leg stronger. Spec out the build to see the full cost. Also, get good tires. Rolling resistance is no joke. It really does help a lot.