Reed valve with extended intake

[Fabian]

It's been a while since i have crunched the numbers, but i can comfortably pedal in conjunction with the engine at around 2,500 rpm using the 10:17. 9:48 gearing.

Like yourself, i would prefer a larger jackshaft sprocket size such as a 54 tooth attached to the freewheel bearing, which would allow pedal power to be placed into the system at higher engine rpm.
If a 54 tooth sprocket is used, you will then need to use a 40 tooth final drive sprocket to bring top gear road speed back to where it used to be with the 48 tooth sprocket.

I wished SickBikeParts would have these sprocket options available, or even if they could direct you to a website that stocks a much wider range of freewheel compatible sprockets.

 

[Ghost0]

We will look into a 21 tooth left side jackshaft sprocket. I think that will improve the pedal assist greatly using more common chainrings.

 

[Fabian]

Now that is a great option.

+1 for the 21 tooth sprocket and mark me down as the first customer!

 

[Fabian]

Is the option for a left hand side "belt drive" still on the table?

 

[FurryOnTheInside]

We will look into a 21 tooth left side jackshaft sprocket. I think that will improve the pedal assist greatly using more common chainrings.

Thanks for your response Ghost0
21T to replace the 17T though? Maybe I'm doing the maths wrong.. which I am known to do.. but that seems like it'd give you 4480RPM engine = 100RPM pedalling, WAY more than nessesary. Maybe you meant this to be used with the 10 tooth JS output sprocket? This makes more sense actually, as it would run more smoothly and wear more slowly than a 9 tooth.. and give an engine to pedals ratio of 4032RPM = 100RPM at the pedals (I think!)
Since pedal input becomes ineffective above about 100-105RPM "spinning out" and below about 60RPM "mashing" (and damages the riders knees!) it is important to not go too far with this.
Gear changes at 100RPM pedalling is fine for me as I'm a "spinner" but not a reallyy fast one (some cyclists like to pedal at 95-105RPM, they really are spinners) and I'd like to match that to 4100RPM at the engine if I follow Fabian's numbers (which I am taking as a standard since I must take someone's figures as standard until I build one of these!).
I had looked at the left side JS input sprocket and replacing the 17T alloy sprocket offered by SBP with a 19T but thought this too difficult to do myself, since 19T sprockets are not available and 53T bicycle chainrings are. I thought about emailing SBP about this but I'm not finished saving up to start a first build so didn't think there'd be much point.. so I was just going to use one of the spiders. I'm still assuming I can attach any standard road/mountain bike chainset to an SBP spider.

 

[FurryOnTheInside]

It's been a while since i have crunched the numbers, but i can comfortably pedal in conjunction with the engine at around 2,500 rpm using the 10:17. 9:48 gearing.

Like yourself, i would prefer a larger jackshaft sprocket size such as a 54 tooth attached to the freewheel bearing, which would allow pedal power to be placed into the system at higher engine rpm.
If a 54 tooth sprocket is used, you will then need to use a 40 tooth final drive sprocket to bring top gear road speed back to where it used to be with the 48 tooth sprocket.

I wished SickBikeParts would have these sprocket options available, or even if they could direct you to a website that stocks a much wider range of freewheel compatible sprockets.

Thanks for getting back to me Fabian. I've been using your numbers from various posts as my standard (since you post more numbers than anyone) and I'm planning on basically following most of your build (again due to the info that you post as well as some similar goals/values).
I had seen an earlier post of yours where you mentioned that you'd like to get a 50-something chainring but I thought the reason you'd not gone that route by now was due to your heavy haulage requirements:
As I understand it, your engine stalls at 1600RPM (44RPM at the pedals), so IF you want to haul uphill then you're able to help the engine when it's really struggling without mashing too hard and damaging your knee cartilage, hopefully keeping your engine speed over 2500RPM (69RPM pedalling). Well I thought that was your design, I thought you meant it to be like that!

If a 54T input-chainring is used then the bike might actually reach top speed! (I insist that top speed can ONLY be reached with both engine and rider producing their maximum power!) However, I wouldn't think of changing the output chainrings since I have not built a motor assisted bike yet, so from my point of view it would be all based on bicycle gearing. As I use a 48T on a road-hybrid for a top speed of 35.1mph, I'd actually think I'd need much larger than a 54T (combined/final) output chainring if I wanted a really high top speed with both myself and a 67cc engine working together, flat out.
Actually now as we're talking about it, that's one thing that worries me about the 21T JS input that Ghost0 just suggested... The final/combined output chainring doesn't really want to be larger than the input, as I think there'd be dimensional problems, getting it all to fit.

(wow we're good at keeping on topic here, huh? )

 

[Fabian]

It is much nicer to have pedal rpm at a lazy pace, especially for those with weak knees and just wanting to gently pedal along with slow pedal rpm.

The 21 tooth left hand side jackshaft sprocket is the answer, though if a belt drive option became available i would jump at the opportunity.
Having said that, you would need to definitely need a 40 tooth or maybe a 42 tooth final drive front sprocket to enable original engine rpms for the previous road speed with a 36 tooth sprocket and a 17 tooth jackshaft sprocket, whilst keeping the 9T-48T right side jackshaft sprockets.

It would give benefits all round: higher engine rpms with noticeably slower pedal rpms.
Improved hill climbing power over and above the original jackshaft gearing, whilst still using the 24 tooth front sprocket to enable good crawling speed to negotiate your way around tricky obstacles, and at the same time giving even greater pulling power.
The ability to really slam serious pedal power into the system when the engine is operating in it's maximum torque range of around 3,800 rpm.

 

[Fabian]

The final/combined output chainring doesn't really want to be larger than the input, as I think there'd be dimensional problems, getting it all to fit.

In some ways there are advantages to having a smaller diameter jackshaft sprocket, and a larger final drive sprocket, because you don't have to run the front derailleur right up against the inner face of the 48 tooth sprocket.

Either way, it's not a problem, because there is enough room to play with sprocket spacing if running a (front derailleur actuated) dual range ratio on the freewheel.

 

[FurryOnTheInside]

It is much nicer to have pedal rpm at a lazy pace, especially for those with weak knees and just wanting to gently pedal along with slow pedal rpm.

The 21 tooth left hand side jackshaft sprocket is the answer, though if a belt drive option became available i would jump at the opportunity.
Having said that, you would need to definitely need a 40 tooth or maybe a 42 tooth final drive front sprocket to enable original engine rpms for the previous road speed with a 36 tooth sprocket and a 17 tooth jackshaft sprocket, whilst keeping the 9T-48T right side jackshaft sprockets.

It would give benefits all round: higher engine rpms with noticeably slower pedal rpms.
Improved hill climbing power over and above the original jackshaft gearing, whilst still using the 24 tooth front sprocket to enable good crawling speed to negotiate your way around tricky obstacles, and at the same time giving even greater pulling power.
The ability to really slam serious pedal power into the system when the engine is operating in it's maximum torque range of around 3,800 rpm.

Overgearing (pedalling too slowly, putting too much energy into each turn of the crank) is what weakened my knees in the first place! Damn single speed jump bikes!
Belt drive would eliminate chain stretch (actually wear) issue, right?
Um, yes I think I agree but I just think of it the other way around since I'm coming from cycling, so pedal cranking speed is taken at 100RPM as a set figure. But now I'm confused.. You'll have to excuse me I've just been reading the piston ring thread and my head feels like it's been in the microwave.

Higher engine RPMs at the SAME pedal RPMs since my legs can only produce useful pedal power at a set RPM around 70-100RPM.
Yes, the 24T (or whatever final output) sprocket should still rotate at the same RPM, we're raising engine RPM here.. so it would lower your stall speed even further (mine as planned would be about 3mph with a 30T low range final drive sprocket, I think)
That's definitely what I was thinking, match both power sources to work together.

 

[FurryOnTheInside]

In some ways there are advantages to having a smaller diameter jackshaft sprocket, and a larger final drive sprocket, because you don't have to run the front derailleur right up against the inner face of the 48 tooth sprocket.

Either way, it's not a problem, because there is enough room to play with sprocket spacing if running a (front derailleur actuated) dual range ratio on the freewheel.

That is really good to hear.. So far I can only imagine how it all goes together. If that works well with the outer (input) chainring being smaller than the largest final/combined output chainring then that's wonderful news! Just don't want a tooth of the input chainring to catch the rear edge of the front derailleur.. these are the sorts of issues I expect (I always expect bad things lol!)