SBP Shift Kit - Ratios, Speeds and Pedal-starting

[Fabian]

I've looked at that, but it doesn't seem to be a problem as i've got half an inch of space between the 9 speed jackshaft chain and the 9 speed chain running to the rear cassette when it's sitting on the smallest cassette sprocket.

I'll just have to see how it works out when i move the chainwheel sprockets closer together.
One of two things will happen: it will either be a success or a total failure.

Fabian

 

[AussieSteve]

I doubt if it'll hurt anything even if they do touch now and then. They probably wouldn't snag.

 

[jon roemer]

Yeah, this bloody rain. Nothing else to do.
I'd much rather be riding.
At least I think I know what gearing I want.
I like that 11T/36T/24T combo.

... Steve

how is your bike set up is the chain from the motor going to the crank shaft so you can keep the original chain
or do you have a separate sprocket system

 

[funnygasser]

i have a question has anyone changed up the 17t sprocket from the engine output side of the shift kit to a smaller tooth to see if that increases power output?

 

[GreasyChris]

I'm pretty sure you need the 17tooth in order to get enough torque to turn the crank pedals. Sbp makes a 21tooth for the left side for even more torque. They also make a 9 tooth for the right side for 10% more torque and a 11 tooth for more speed. You can change your gear ratios more with different chain rings for the crank and different freewheels for the rear. I'm using a 44 and 40 tooth on the cranks and a 28-14 freewheel on the back and have no problem doing 75+kmh

 

[The_Aleman]

i have a question has anyone changed up the 17t sprocket from the engine output side of the shift kit to a smaller tooth to see if that increases power output?

You'd lower your gearing doing that, but the nasty side-effect is increased pedal cadence relative to engine rpm. IE: you'd run out of pedal assist before the engine got in it's powerband. This is why larger jackshaft input sprockets are offered by SBP - so that you can pedal-assist up to higher engine RPM than with the 17T. IMHO, it's best to go with the largest jackshaft input you can fit, this will give you the broadest combined powerband.

Another way to achieve that is by increasing the outer crank sprocket toothcount, but there are limited options beyond 52T for that unless you go custom. I have a 76T on mine, for example, which along with the 18T jackshaft input allows me to pedal-assist to 8K engine RPM. With a 17T input and 44T outer crank, I was only able to pedal assist to ~4500 RPM.

The jackshaft input, output, and outer crank sprocket all contribute to cadence:engine RPM ratio. Numbers lower than 25:1000 are ideal if you want your legs to actually contribute with a shift kit.

 

[extremeodd]

That is some awesome information Aleman!

I'm starting to think my next upgrade is going to be getting a SBP HD shift kit, it'll be nice to have the equivalent of a 29t-60t on tap would make riding the bike a fair bit more pleasant. Not to mention being able to actually pedal-assist the bike past 10 mph.

 

[The_Aleman]

If you use jpilot's gear calculator, you can play around with the numbers pretty easily to figure out your shift kit's ratios:

This is a "stock" shift kit, with 17T input and 44T outer crank:

Top section is internal chinagirl reduction, 2nd section is engine output and jackshaft input, 3rd section is jackshaft output and outer crank. In this case, the engine reduction to pedal crank is 30.67:1. Then we divide 1000 / 30.67 = 32.6 pedal cadence per 1000 engine RPM. Most people can't spin over 100 pedal cadence for very long, so one might find it tough to assist beyond 4K engine RPM. Pedal crank arm length affects this, longer cranks (such as 175MM) typically reduce your max cadence, while short crank arms (like 125MM) will increase it, at the cost of torque.

By entering that drive ratio and the rest of your gears into a second instance of the gear calculator, you can get your overall ratio:

Your ratio goes in first section, inner crank and rear wheel sprocket in the 2nd section. I used 36T inner crank and 28T rear sprocket (1st or 2nd gear on most cassettes) as an example. If you're using an internally geared hub, those values would go in 3rd section.

Jpilot's gear calculator thread: https://motoredbikes.com/threads/jpilots-gear-ratio-calculator.18543/

 

[extremeodd]

If you use jpilot's gear calculator, you can play around with the numbers pretty easily to figure out your shift kit's ratios:

This is a "stock" shift kit, with 17T input and 44T outer crank:
View attachment 88866

Top section is internal chinagirl reduction, 2nd section is engine output and jackshaft input, 3rd section is jackshaft output and outer crank. In this case, the engine reduction to pedal crank is 30.67:1. Then we divide 1000 / 30.67 = 32.6 pedal cadence per 1000 engine RPM. Most people can't spin over 100 pedal cadence for very long, so one might find it tough to assist beyond 4K engine RPM. Pedal crank arm length affects this, longer cranks (such as 175MM) typically reduce your max cadence, while short crank arms (like 125MM) will increase it, at the cost of torque.

By entering that drive ratio and the rest of your gears into a second instance of the gear calculator, you can get your overall ratio:
View attachment 88867

Your ratio goes in first section, inner crank and rear wheel sprocket in the 2nd section. I used 36T inner crank and 28T rear sprocket (1st or 2nd gear on most cassettes) as an example. If you're using an internally geared hub, those values would go in 3rd section.

Jpilot's gear calculator thread: https://motoredbikes.com/threads/jpilots-gear-ratio-calculator.18543/

That was what actually inspired me, before I made my post earlier I figured out the following:


When I calculated the gear ratios on SBP's kit I get the following:

Equivalent sprocket	Cog tooth count	Actual gear number	Speed at 8,000 rpm
60t	28t	1st	25.9 mph
50t	24t	2nd	30 mph
46t	22t	3rd	33 mph
42t	20t	4th	36.3 mph
38t	18t	5th	40.4 mph
34t	16t	6th	45.4 mph
29t	14t	7th	51.9 mph

 

[xspade17x]

You know they sell a pull start dont you,i have one and my engine starts with the first pull.

Mine starts with first pull as well
Although the line does break a lot. I literally just ordered a 5 pack of handles with ropes cuz it does break too often. I've gone thru 2 pull start mechanisms and 7 or so ropes between the two