Sprocket mounting techniques?

Rocky_Motor

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Nov 14, 2011
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Fort Collins & Boulder
I'm making an electric bike (circuits and all) right now and have been thinking about how I'm going to drive the rear wheel. I have a 1kw electric motor that will be mounted above the rear wheel. It spins to ~3000 RPM at 48 volts. It's rated for 30 amps at that voltage, which is 1.4kw.

I definitely don't want to use one of those rag clamps that come in the 2 stroke kits. So I was wondering what you guys think I could do? I've seen that maybe I could buy a disc brake wheel and mount a sprocket onto the disc brake bolt holes.

I have access to a machine shop at the school here. They have lathes, mills, welding, etc.. I've been using the mill and TIG machine a lot so I'm semi comfortable with those. Any suggestions are appreciated! I wasn't so sure about the disc brake hub method since the bolts are only like 6mm and electric motors deliver 100% torque from the start.
 
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Ludwig II

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Jul 17, 2012
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I've got a disc mounted sprocket, and it occurs to me that if you had an adaptor bolted and stuck with a high strength adhesive to the hub, you could mount whatever sprocket to the adaptor.

Most people only think in terms of epoxies as a glue, but cyanoacrylates offer faster cure times and high strength these days.

If the machining is really fancy, you could even make up a female adaptor to fit the shoulder of the hub and do away with adhesives, and let the forces bypass the bolts entirely, and go directly to the metal of the hub.
 

Chainreaction

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If I had access to a machine shop I would make a split hub to fit inside your wheel that had extensions outside the spokes that fit split style sprockets. Of course offset the splits when assembling. #35 has a lot of choices in split sprocket tooth count. Of course making such a hub that you could get inside spokes onto axle housing would be a challenge.
 

Rocky_Motor

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That sounds like the mad mechanics sprocket axle clamp. My current wheel has 32 spokes which would render it unusable sadly. I could make one, but I don't want to put that much work into this part of the project. After I bought the motor and got my motor controller circuit working, I realized how to build a motor controller for brushless motors.

The brushless motors are the kind that are built into the hub and require no power transfer setups. The brushed motor I have, has an 11 tooth sprocket at the end of it. Sooo I was hoping to find a solution to drive the wheel that is better than a rag clamp and without extensive machining.

I'm guessing the disc mounted sprocket is my best bet. Epoxy is definitely very strong, I have used it on a number of projects. I'm not sure I understand the purpose of using it in this scenario though, perhaps I'm not visualizing it as you are.
 

Chainreaction

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It seems like the older I have gotten the more I seek the difficult solution. For example instead of just buying a jack shaft bracket from Sick Bike Parts I have made my own with a cheap flux core welder and a lot of filing to get the bearing holes right.
 

Ludwig II

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Jul 17, 2012
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The adhesive as well as the bolts is to ensure the little bolts and the thin metal they go in don't break.

A thought. A disc brake hub isn't circular, it has 6 sticky out lumps. Why not have pegs fitting in the gaps between them from the adaptor to a pair of semicircular plates behind the lumps, much like a rag joint, but with proper metal?
 

2door

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Maybe I'm missing something here, not being an electric biker and not knowing what's really available, but wouldn't you want to use some type of variable speed control for the electric motor. Yes, you are correct that an electric motor will produce most of its power almost immediately, which could be hard on sprockets/adapters/chains etc. but with a variable speed control you wouldn't be applying full power right from a dead stop.

If I'm way off base, just tell me.

Tom
 

FFV8

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Oct 29, 2013
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Don't over think this one.

The 6 disc bolts are more than enough to hold the torque.

A bike with a good disc brake is capable of locking the wheel & skidding the tire - which is the torque limit of the system. I have never seen a disc bike brake fail at the bolts or disc.

My sidehack is fairly heavy, and I run a 72 tooth 40 chain sprocket bolted to the disc brake hub:



Never have had an issue with the bolts taking that beating. I have plenty of mass, great traction & gobs of torque.

The front brake is a hydraulic caliper from a gy6 scooter. The 6 bolts will take anything that caliper can deliver.

I have seen several hub adapters slip. Put enough torque to them and away they go. I twisted one with a 50cc pitbike engine - and it was torqued as tight as possible.

The 6 bolts are enough.
 

fatdaddy

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May 4, 2011
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I've been running my Howard Adapter for two years or better with zero problems. Problem is, He quit making them and they only fit certain bikes anyway. I put it on and haven't had to touch it since. But I still recommend any good sprocket adapter over that stupid rag joint.
fatdaddy.usflg
 

Rocky_Motor

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Nov 14, 2011
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Sounds like the disc brake hub will be the best way to go! I'll look around for an adapter. I've heard mixed reviews on many vendors though. This site has some good stuff that I'd be looking for it seems:
http://custommotoredbicycles.com/sprockets_for_disc_brake_hubs_unviversal_disc_sprocket_adapter
If needed I can machine some changes. The grubee wheel looks pretty convincing too!


Maybe I'm missing something here, not being an electric biker and not knowing what's really available, but wouldn't you want to use some type of variable speed control for the electric motor. Yes, you are correct that an electric motor will produce most of its power almost immediately, which could be hard on sprockets/adapters/chains etc. but with a variable speed control you wouldn't be applying full power right from a dead stop.

If I'm way off base, just tell me.

Tom
You're absolutely right! My circuit gives me 256 steps of speed control. I figure that there may be situations in which I'll crank it to full throttle at a stop (for whatever reason). It'd be unfortunate if the drive-train explodes dnut I'm not sure if this motor will pop wheelies or casually accelerate in such a circumstance. I'm doubtful it'll do a wheelie, but I guess that depends on the gearing.

I was thinking I'd try a 72 tooth sprocket. The motor has an 11 tooth on it and a 26" wheel makes for ~30 miles per hour I think.
 
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FFV8

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Oct 29, 2013
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You indicated that you have a machine shop available to you, so making a sprocket adapter is simple.

With a 1.4 kw input, wheel stands are unlikely.

What pitch is the sprocket on your electric motor?

Your speed computation is a little off, with 3,000 rpm & 72-11 gearing (6.54:1) on a 26" wheel you are looking at 35mph.

Do you plan on a rear disc brake as well as a drive sprocket?

.
 

Rocky_Motor

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Nov 14, 2011
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I was just going to use a front disc brake and a rear rim brake. You're right, 1.4kw isn't a 'muscley' amount of power. Especially so as the motor is labeled as a 1kw motor.

Dangit, I was hoping the 72 tooth would bring it down far enough. I'm hoping to avoid having to go through more gearing. I can make the circuits for the BMS and controller, but I'm a little challenged when it comes to the mechanical side of things! :)
 

FFV8

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Oct 29, 2013
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Spring Valley NV
it all depends on the pitch of the sprocket on the motor

If it is #35 chain, 80 tooth sprockets are easy to come by.

Or you can change the motor sprocket.

.
 

Rocky_Motor

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Nov 14, 2011
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I'm unfamiliar with chain ratings, but it says to have a 8mm (T8F) chain compatibility. So I can get a 72 tooth like this for only $20. I love it! Still searching for an 80 tooth in this chain size.




I could get a different drive sprocket that would match the 80 tooth sprocket too. Perhaps that's my best route.
 
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