Calculating Mid-Drive drive train ratio + nuvinci. How?

[omnivore]

So, as I'm sure most know, I am completely new to all this. Sorry for yet another set of dumb questions.

My problem is I don't understand how the rest of the mid drive's drive train effects the main chainwheel to rear cassettes ratio. To further complicate things, I've purchased a Nuvinci IGH and I have no clue what type of ratio I need to keep the top speed I have now.

So speaking of the mid gear crankset my inside chainwheel that connects to my rear cassettes is a 44 tooth. the outside chainwheel that chains to the motor is a 48. I get 30mph top speed with this. I'll just be honest and admit I don't understand the relationship between those two side by side chainwheels and how they affect one another. Do I calculate them just as I would if they were tooth to tooth or do you calculate it differently when they are parallel of each other? would I divide 48/44 then 44/(we'll just say I'm in the highest gear I have on my cassettes)11 then mutliply them together? like 48/44=1.09 x 44/11=4 = 4.36?

Further, how does the Nuvinci play into this ratio mess?

I know, I'm pretty pathetic. please help.

it looks like the smallest cog I can use on the Nuvinci is a 16T so I will be going from a 11T/44T to a 16T/44T which is a drastic difference. Then on top of that the Nuvinci can go from 0.5 to 1.8 internally and I haven't the slightest idea how that affects the drive chain ratio.

So, anyone got a formula that will help figure this mess out for me?

 

[omnivore]

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[Avitt]

Sheldon Brown's Gear Calculator will allow you to compare the high end and low end metrics (including gain ratio, gear inches, or speed at specific pedal rpm) for the Nuvinci, with various sprocket and crank combinations:

http://sheldonbrown.com/gears/

 

[teslanv]

Here's a quick excel spreadsheet to calculate your gearing.

PM me if you want the excel file.

 

[omnivore]

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[spinningmagnets]

Here's a wheel diameter to RPM chart that lists the speeds you will attain. It may be useful at some point.

http://www.endless-sphere.com/forums/viewtopic.php?f=28&t=16114

You mention a GNG kit, and the stock motor has a Kv of 67 (that means 67 RPMs per volt) and your other post mentioned that you are using 48V, so (48 X 67 =) 3216-RPMs at the motor shaft.

I notice you have the chain-primary kit (http://www.endless-sphere.com/forums/viewtopic.php?f=6&t=60513&p=904023#p904023) what are the tooth-counts on the primary small drive sprocket and the larger driven sprocket? (the left side connecting the motor to the jackshaft)

The secondary reduction from the jackshaft to the crankset should be standard bicycle chain, what are the tooth-counts on those two?

Then what chainrings do you have avaiable to run from the crankset to the rear wheel?

The NuVinci is at a one-to-one ratio when it is in the middle position, below that is under-drive, and over it is over drive. There is a selection of tooth-counts on the sprockets available to mount to the right side of the NuVinci hub.

 

[omnivore]

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[omnivore]

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[omnivore]

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[teslanv]

Here's the calculation: This is "unloaded Speed" at 100% throttle

 

[spinningmagnets]

That graph looks good, loaded speed (wheel on the ground driving a load) might be roughly 20% slower than unloaded, so at the middle setting of the NuVinci range it "might" be around 29-MPH.

If this is true, I would consider using a larger sprocket than 16T on the rear hub, that will lower the top speed and improve the lower-RPM hill-climbing.

 

[teslanv]

What max speed are you currently getting on your 11T high gear? Both "Unloaded" (on a stand) and loaded...

 

[omnivore]

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[omnivore]

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[teslanv]

Hmmm.
Double check the tooth counts on the various sprockets. The crank RPM calc seems high at 150. My Bafang mid drive maxes out at 120 RPM.

 

[omnivore]

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[JennyB]

Gear inches may be an old-fashioned way to describe gearing, but it works neatly with ebikes.

The important number is 336: if a hub motor has enough power to run at 336 rpm the bike will go one mph faster for each inch increase in driven wheel diameter. A 20" wheel will get you 20 mph, and a 26" one, 26.

Now suppose you have the same motor and you want to go 26 mph, but you only have a 20" wheel. The wheel needs to turn 1.3 times for every turn of the motor. You might use a 26 tooth input and a 20 tooth output or 52:40 or 13:10 - it doesn't matter so long as the ratio is the same. When you use two or more stages of reduction or step-up just multiply the ratios together, as your spreadsheet is doing. The result is equivalent to a 26" directly-driven wheel, so is said to be a 26 inch gear.

A bicycle may have gears from around 100" to 40 or 30 or even less. Pedalling comfortably at 70 (where conditions permit in a 100" gear gives a top speed of 100*70/336=20.8 mph.

 

[omnivore]

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[omnivore]

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[omnivore]

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[spinningmagnets]

Chain skipping is why I recommended using a larger rear sprocket to lower speed and improve climbing, The other method would be to use a smaller front chainring, but that engages fewer teeth.

 

[omnivore]

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[omnivore]

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[omnivore]

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[omnivore]

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