Dual reduction, single reduction, chains and sprockets

[agniusm]

So I have couple of these motors that are spun @ 2900rpm. I am aiming at 30kmh loaded with 20" wheel. My calculations are that I need about 7.5 to 1 reduction. I would sure go with single stage reduction but that puts me in 83T sprocket market and it comes out at 345mm D bicycle sprocket when 20" wheel is 500mm.
I have few options. Do double stage reduction or go with smaller pitch sprockets/chains. Motor is about 350w nominal. Can I use #35 sprockets or even #25? #35 are 800kg tensile when bicycle chains are 1400kg.
Is there efficiency difference between sprocket sizes when compared?

 

[Chalo]

My first e-bike had a 400W nominal motor with #25 chain in a 1:12 reduction (12/144). The motor peaked at 24V 80A measured, though probably with low efficiency at that current level. My experience suggests the chain drive could have tolerated even more power.

 

[agniusm]

Its a big motor. It is also dc but unlike other wheelchair motors, this one is large. I bought simple cheap 500w and 800w controllers. What sort of material you have used for it? Would stainless suffice? I will test out sprocketeer app and watejet sprockets I need out of 3mm ss. For #25 it must be 2.5mm?

 

[Chalo]

I used 2.4mm thick 2024-T3 aluminum (Duralumin) for my big sprocket. I wrote a very small program for it, using just the profile of one tooth and a decremental loop with angular offset.

It's the small sprocket you have to worry about, because it has fewer teeth, less load sharing, and much deeper articulation. In my case, the small sprocket came with the motor and was made from hardened steel.

 

[nutnspecial]

I never heard that bike chain has a higher tensile strength to #35ansi?

For a 350w used that way, if #25 is appropriate you might also want to consider 8mm depending on it's relative availability and price for you, and how it compares with pitch.

You don't want to seek a slightly different balance of volts and amps to better accomodate speed preference?

 

[agniusm]

The chart is on wiki: https://en.wikipedia.org/wiki/Roller_chain
I have all the stuff so dont want to change anything so I would need to get new battery or controller. Its for 11 year old so it should be fine. I probably will go with #25 cause its smaller. I just cut out plywood samples to see how they fit.
The smaller is #25 with about 205mm in diameter:

 

[nutnspecial]

Looks like you have enough teeth on the small #25. Mcmaster lists #25 with a working load of 80lbs, and #35 as having 200lbs. Their tensile ratings are of course much higher.

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There's no way a bike chain is gonna hold up to what I put my heavier #35 through. I think wiki is mistaken and writer made an assumption based on the pitch of bike chain, because it's the same as ansi 40. Roller width and diameter, and sideplate thickness also certainly have bearing.

 

[nutnspecial]

Useful list of ansi chain criteria http://www.ocm.co.jp/en/han_info/05_01.pdf

However, some bmx chain is touted to have tensile strengths of 3300lb (between standard #35 and #40 tensiles), so color me confused. I guess 'working load' is more 'real world', using tensile strength, chain speed, and other criteria.

I found this useful when selecting chain http://www.gizmology.net/sprockets.htm, but it doesn't list working load of bike chains.
http://cozybeehive.blogspot.com/2011/05/bicycle-chain-stretch-test-results.html Wipperman tested 10 bike chains for stretch with up to 300lb loads. Who knows how long the last @ that loading though?

 

[Chalo]

Wipperman tested 10 bike chains for stretch with up to 300lb loads. Who knows how long the last @ that loading though?

Loading of chains is cyclic, but the force peaks are much higher than that in many cases. Let's look at an example.

A 175mm crank with 32t ring, which is the most common setup for today's single ring MTBs, has a crank length to chainring radius ratio of 2.7. So however much force is applied to the pedal is multiplied by 2.7 at the chain. Thus a 111 pound pedal force becomes the 300 pounds used in Wipperman's test.

A 111 pound person can apply that much pedal force just by standing on it, as if climbing up a step. I can apply more than three times as much force in the same way, meaning more than 900 pounds of chain tension.

The longest crank I use is 225mm. The smallest chainring I use is 28t. If I combine those things and apply my body weight to the forward pedal, the chain sees over 1300 pounds of tension. I try to use rings 38t and larger whenever possible, but my chains hold up just fine.