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No Title -> Motors/controlers/batteries -> Bigger Sprockets better?
Post Info TOPIC: Bigger Sprockets better?
Nitoragro


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Bigger Sprockets better?
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I heard somewhere, i forget where, that larger sprockets are more efficient even if they have the same gear ratio, such as like a 10 tooth motor sprocket and 20 tooth drive sprocket is less efficient then 20 tooth motor sprocket and 40 tooth drive sprocket. Has anyone ever tested this, or even heard of it? I've done a little research about this on bikes this is a link for what I found. http://cyclingtips.com.au/2010/04/the-big-ring-mechanical-or-psychological-advantage/

I think it would be because it will put less stress on the chain because the chain doesn't have to turn as hard around a larger sprocket then this reduces friction on the pins connecting the chains. But with this it also adds weight, but not a lot. 



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Brendan_Smith


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Hi Ryan,
Yes this is certainly the case. Tension in the chain is what transfers the drive from the motor gear to the sprocket. If a 10-tooth gear is used, then when you look at the worst-case point in the chain link cycle, the tension in the chain is applied at a wide angle from perpendicular to the chain line (as the chain articulates towards straight). The chain also will deform more as there is a higher torque applied to the sprockets / higher tensile force in the chain, further reducing efficiency.
Reducing the pitch of the chain (number of links-per-distance) means you can make the same reductions with smaller chain wheels, saving weight, but likely will mean that the chain will deform more.
However, if you go too large of a sprocket set, or too short of a pitch of the chain, then the mass will get too high for the system (mass of the chain increases with the link count getting larger, and larger chain wheels generally are heavier), and you induce excess friction losses to the system as the chain in totality has to articulate too much.
There's a large amount of research on chain drive design; first or second-year mechanical engineering students go in-depth to it for industrial systems, and Bicycling Science by David Gordon Wilson has some good studies that were conducted on human powered vehicles (slightly less power than we have available, but very similar design philosophies and desired outcomes).
The vast majority of Bicycling Science was taken from the International Human Powered Vehicle archive of journals. I bought Bicycling Science to have it all at my fingertips and in a better-arranged format, but if you've not seen this archive, it might just whet your appetite somewhat! I would suggest you might even find enough information here to design the majority of your drive system...
www.ihpva.org/hparchive.htm


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Brendan_Smith


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Pages 14-16 of Number 51 (2001) and 3-9 of Number 50 (2000) have a wealth of information.

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Zaine Stapleton


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You also have a larger ratio between different gear combinations if running a bigger drive sprocket on your wheel (more options to choose from) versus going with a smaller drive sprocket (less options to choose from). 

For example - running a 60:20 combo (wheel sprocket:motor sprocket) comes out to 3:1

                     running a 61:20 combo comes out to 3.05:1

                     running a 62:20 combo comes out to 3.1:1

                    running a 60:19 combo comes out to 3.16:1

                     running a 60:21 combo comes out to 2.85:1

                     running a 60:22 combo comes out to 2.72:1

As you go bigger with your wheel sprocket, that ratio between the wheel and the motor becomes bigger yielding a more fine tuned adjustment.  

But going bigger with your motor sprocket on the other hand, that ratio gets smaller yielding a less fine tuned adjustment.  

I was able to figure this out when switching the max voltage on my car from 24 to 36 volts.  

I was also able to run the exact same gearing when I had it set up for 24 volts originally, but now on the updated 36 volt conversion using a bigger wheel sprocket.  

More volts in the system usually will need either a bigger wheel sprocket or a smaller motor sprocket.

Personally, I'd change the size of the wheel sprocket over downsizing the motor sprocket if doing a system voltage change.

For racing, then  I'd change the motor sprocket out depending on the track I'm at.   

But yes, it does come down to how many rpm's your motor is doing based on the system voltage of your car, last I remember Etek brand motors (Briggs and Stratton variety or Manta motors) typically run at 72 rpm per volt.  

Hope that helps a little!

Zaine



-- Edited by Zaine Stapleton on Saturday 19th of March 2016 06:41:46 AM



-- Edited by Zaine Stapleton on Saturday 19th of March 2016 06:42:22 AM



-- Edited by Zaine Stapleton on Saturday 19th of March 2016 06:44:23 AM

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