Chains Useablilty of chainrings/dual chain setup

budd0413

New Member
Joined
May 3, 2008
Messages
5
So I have a question about a different kind of setup for a multi-speed bike. Suppose you were to buy a standard run of the mill kit from any one of the manufactures. Instead of mounting a separate sprocket on the rear wheel to run the motor to why not keep the standard bike chain on say the biggest or outermost chain ring and run a short chain from the motor to the middle chain ring. This would allow you to shift the rear gear cog through all seven different gears. Of course you would no longer use the chain ring shifter. Is there a problem with this setup that I am not seeing or would this be a semi-simple way to have a 7speed motorized bike?
 


sparky

Active Member
Joined
Mar 19, 2008
Messages
1,429
If you had at least two front sprockets, you could do it. But there's no telling what your gear ratios are gonna be like.

Ideally, you'd want Ghost0's jackshaft idea that Large just linked too... but you'd prolly want two sprockets of a similar size on the front.

If you don't use Ghost0's idea... you'll always be pedaling, even when you're going 25-30mph.... not good.
 

budd0413

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Joined
May 3, 2008
Messages
5
picture a regular 21 speed bike. There's a 3 gear sprocket set that the pedals are attached to and a 7 gear cog attached to the rear wheel. Keep the pedals and the regualr bike chain set up complete but set the chain on the outermost sprocket of the three sprocket set. Then use the middle sprocket of that set for the chain coming from the motor. So the motor turns the pedal crank assembly the same way the pedals would. Then the normal bike chain can still use the real derailer to shift through the rear seven gears. Kind of hard to understand. I'll check that other thread out might be what im saying. Thanks
 

budd0413

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Joined
May 3, 2008
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yep thats what I'm looking for thanks alot. Thats what I was thinking always having to pedal, and I dont like the sound of that.
 
L

Large Filipino

Guest
It seems any time you get engine power to share the cog wheel/gear sprockets/whatever your gonna need a freewheel crank. Even if you can set up a rack mount to gear on your largest sprocket,your STILL gonna need a freewheel crank and then the shifting is for your feet and not the engine.
 

budd0413

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Joined
May 3, 2008
Messages
5
Yep, I wonder if the motor had a pull start if you could shave the pedals all together and still have enough power for the initial get up and go without burning up your clutch. then you could bypass the freewheel crank
 
P

Pete

Guest
I assume you are talking about engine power rather than electric ?
Have a serious look at how the ratio's work out. Bike gearing is designed to take low pedal RPM and convert it to high wheel RPM.

All the engine kits take high engine RPM and convert it to wheel RPM..

So consider the effect of turning your front cranks at approximatley wheel RPM, and how fast that would end up making you go.

Check here for a handy ratio calculator tool to help with crunching the numbers
http://www.motoredbikes.com/showthread.php?t=7711

You also have the problem that your pedals must rotate whenever the engine is applying power. I know there are a few threads here about incidents of stuck throttles and non freewheeling cranks, trying to get around roundabouts without comming off.

If it is electric you are talking about, check out http://www.cyclone-tw.com/ for one implementation. The down side to this setup is you usually need to replace your front crank and chain rings with their freewheeling crank, which is a custom unit and not particularly strong. You also get stuck with a 44? tooth chainwheel.

Alternatives for a stronger freewheeling crank ;
http://www.hostelshoppe.com/cgi-bin/readitem.pl?Accessory=1098307215
http://www.easystreetrecumbents.com/stuff/drivetrain.html
http://translate.google.com/transla...p://www.bike-elektro-antrieb.ch/zubehoer1.htm
 
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duivendyk

Guest
The problem with this arrangement is that you need a large gear reduction between the motor and the chainwheel something like 60 to 90 depending on the engine size,a chain drive can't do much more than 4/5 to 1,so you need another reduction of say 12 to1 between the motor and the chainwheel,built into the motor or with a jack shaft.Most geared motors with a built-in reduction also have the output on the left,which implies a jack shaft to get it to the right side and the reduction is much less than 12 to 1,so you need another reduction!.It might be possible to kluge up something with a tandem bike,dedicate the second crank to be motor driven,and have freewheel somewhere in this drive train, you still have to come up with the necessary reduction,but you have all sorts of room to do your thing! and another chainwheel available to drive.Pedaling speed is typically in the 70/90 rpm range,so you have to reduce from say 5000/7000 rpm to say 80 rpm (hence 70/90 reduction) .I think you can prob. make something like that work.But there is also the question if the chain&derailleur can take the torque&speed if you are careful about changing gears (throttle back), and your engine is not too large say below 3HP,you can get away with a lot,you also need a pull start &centrifugal clutch,JJ
 
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