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Schwinn Stingray convert to electric bike help?

My son is converting a Schwinn Stingray (chopper) bicycle into an electric bike. Today I am in search of a replacement #25 chain sprocket. The current sprocket (original - DNP Long Yih brand) is a freewheel style sprocket much like the freewheel one on your web site. Mine measures 3 1/4", I believe a 1-1/2 center hole with 20 teeth. You can see by the attached photos that there is no clear way to attach a lager sprocket.Two dark areas are not screw holes. Thus the need for help: 1. which #25, 60-80 tooth sprocket do I purchase and 2: how do I attach it? A provided solution would be most appreciated.

 

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Having the proper gear ratio between the motor and rear wheel is essential to a successful electric bicycle conversion. If the gear ratio is too high the bike will be slow and not be able to go as fast as it should, and if the gear ratio is too low the motor will not be able to operate at its optimal RPM which will significantly lower the power output and overheat the motor causing it to burn out.

In order to calculate the gear ratio and recommend the correct size rear sprocket I need to know some more information about the parts you are installing. Here is the information that I need:

1- How many Watts is the motor?
2- What is the motor RPM?
3- How many teeth does the motor sprocket have?
4- What is the size of the wheel on the bike measured from the floor to top of tire?

Once I have this data I can calculate the proper gear ratio for the bike and recommend a rear sprocket size. If the motor you are planning on using is not a gear motor, then there is a chance that a wheel sprocket large enough to obtain the proper gear ratio will not be available, in which case a gear motor may need to be used for the project.

First, thank you. I am not able to determine RPM's, however, it is a Currie Motor, 24V, 650W, #25 chain , 11 tooth sprocket, rear tire size is 20 1/4 (you have a replacement tire on your web site). Do you still need me to measure the tire?

You do not need to measure the tire because the size tells me that it is a 20" diameter tire.
Most Currie motors with #25 chain sprockets are for electric scooters and are rated between 2500-3000 RPM. I did the calculations and here are the results using an 80 tooth rear sprocket which is the largest size we have that will fit onto a freewheel.

With a 2750 RPM motor the results show a top speed of 22.51 MPH with an 80 tooth rear sprocket. This speed is higher than Currie electric scooters with 650 Watt motors, they are geared to go around 15 MPH. If the bike is used exclusively on flat ground with a lightweight rider then I think using an 80 tooth rear sprocket would be OK, however if the bike will be used to go up hills or inclines that may overheat the motor and cause damage. In that case I recommend switching over to a gear motor to provide the proper gear ratio.


If you want to try building the bike with an 80 tooth rear sprocket then our item # SPR-2580 and FWM-125 freewheel are the right combination. I can not say for certain that the FWM-125 freewheel will thread onto the Schwinn Stingrays rear hub. Item # FWM-125 has M34X1.00 threads and will thread onto wheel hubs with the same threads. If the rear wheel hub has a different thread than M34X1.00 then any 16 tooth bicycle chain freewheel that will thread onto the rear wheel hub will attach to the item # SPR-2580 sprocket using our item # HDW-100 sprocket to freewheel nut and bolt set. The existing 20 tooth bicycle chain sprocket is too big for the 80 tooth sprocket though.


I will post all of the parts that I discussed below.

80 Tooth Freewheel Sprocket for #25 Chain Item # SPR-2580 $19.95
Rear Wheel Freewheel Clutch (Right Side) Item # FWM-125 $14.95

Sprocket to Freewheel Nut and Bolt Set Item # HDW-100 $2.95


Please let me know if you have any questions.

Regarding this conversion, all was well received/ Thank you to your team. However, we have an issue with the chain in that it will not stay on. To be clear, we installed a MOT-36450-G Gear Reducer Motor. The motor is firmly attached and inline. We used the original wheel, sprocket, and chain (although shorter). We have set the chain with ample clearance with and without a chain tensioner - made it tight and a bit loose. When we give it power and the motor "kicks in" the chain comes off the motor sprocket.  Attached is a photo of its placement.

- do we need a larger sprocket on the motor?

- do we need a larger sprocket on the wheel?

- a different chain?

- is the motor location ok?

- .... 

I sincerely appreciate the assistance.

The motor location looks great to me.


The size of the sprockets do not matter at all in regards to the chain staying on. However if the speed of the bike is too slow then you could install a smaller wheel sprocket to gain more speed, and if the bike is too fast a larger wheel sprocket could be installed to slow it down and give it more hill climbing power. We do not have and can not get different size gear motor sprockets.

From the size of your sprockets and rear wheel it looks to me like the bike should go around 12-14 miles per hours with the current gearing. This is a good speed for a 450 Watt motor to go because it will provide a decent top end speed and also will provide some decent uphill torque. If the electric bike will be driven exclusively on flat ground then you could install a smaller wheel sprocket to gain more top end speed.

The used chain and used rear sprocket could definitely be causing the chain falling off problem. As sprockets and chains wear out during use the chain stretches and that causes the sprocket teeth to wear out along with it. Because the motor sprocket is new I recommend installing a new rear sprocket and a new chain. That way the drive train will be 100% new and will wear out together so everything meshes properly during its life cycle.

The chain falling off is most likely from the combination of using new and used sprockets and used chain, but it could also be caused in part by an alignment problem between the two sprockets. Here is an illustration that shows the most common sprocket alignment problems and what perfect sprocket alignment should look like.

I like to check sprocket alignment by eyeballing one of the sprockets against the other one like this.

Please let me know if you have any questions.
I just updated the above reply after reading your reply again and finding a few questions which I did not answer.

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