almost full sprocket utilization and engine efficiency (possible dumb question) =D

bakaneko

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so, i have been thinking about the rear sprocket and engine power utilization. cant work it out in my head but aren't we losing some energy/torque/speed due to not a 100% sprocket utilization. on most bikes ive seen the chain only engages about 60-75% of the rear sprocket due to the placement of the chain tensioner. does this mean we are losing some power from the engine while probably not 25-40% but perhaps some non-marginal amount. maybe it isnt power loss but more like torque to full power utilization.

if true, wouldn't it make sense to create a tensioner that tries to place the chain on the sprocket such that more teeth are in contact. if we think about the engine and sprocket as a closed system then i cant imagine energy being lost but more feasibly perhaps the energy/speed curve will be steeper to the plateau (more torque) if more teeth are engaged. and, does this doesn't even touch upon the drive sprocket.

so more torque and same top speed???

see professional attached illustration :D
 

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no.

power transfer has nothing to do with number of teeth in contact, other than the limit to how much can be transferred. look at gears, they only contact one at a time...

you get MORE loss with more contact...

the extra bearings needed for an idler sprocket simply adds to the losses. chains are close to 98% efficient when lubed and maintained. there are limits to the minimum numbers of teeth and the ratio between two shafts.
 
no.

power transfer has nothing to do with number of teeth in contact, other than the limit to how much can be transferred. look at gears, they only contact one at a time...

you get MORE loss with more contact...

the extra bearings needed for an idler sprocket simply adds to the losses. chains are close to 98% efficient when lubed and maintained. there are limits to the minimum numbers of teeth and the ratio between two shafts.

hmmm ok. so it doesnt even effect torque with more teeth engaged ie more power transferred but not total amount of power? i didnt really think energy was loss. i guess with more torque it will also decrease the top speed? this thought experiment is with a 36t sprocket.
 
With chains it is the bend that robs the power so that in theory for efficiency below 19 teeth should be avoided, 9 is a minimum.
The ideal number of teeth for 1:1 ratio is 25 teeth, with 23 considered a minimum for low backlash drives.
If I can remember right there is a rule about ideal tooth count for both sprockets is 50 combined for low ratio drives.
The normal maximum ratio is 6:1 or 19:114 which are ideal min/max counts.

The losses come from bearings, bending and sliding up the tooth (minimal).
120 degree contact is considered minimum but above 180 degrees less is considered better.
This all means the ideal placement of an idler is inside the chain spreading outwards on the slack side.
Slack is important, with any tension on the slack side robbing efficiency big time.

Steve (a machinist and millwright)
 
That's interesting. What do you think about sprung vs fixed idlers? I would instinctively think a light/soft spring with a minimal limit of travel is ideal.
 
On the idler question. I am thinking bout putting a spring tensioner on the top chain line mtn bike. i can hear it hit something over big bumps. i say yes to spring tensioner above and solid one below. that way if u go over big bumps on trails...then u have some give take. always use solid state tensioner bottom. it just works. can even use solid top but that something for yeah to mess with.
 
That's interesting. What do you think about sprung vs fixed idlers? I would instinctively think a light/soft spring with a minimal limit of travel is ideal.

I've used both in industry.
Spring idlers are for single direction steady loads. No reversing, power in one direction only, no shock loading.
The best were rubber cored tensioners like pictured below.
The rubber adds some damping to the arm. Spring tensioners are typically quieter.
66929-8694619.jpg

On a reversing, shock loaded, variable load system (like most bikes are) a fixed tensioner would be a better choice "by the book".

A light/soft spring tensioner with travel limited to minimum would be an ideal solution, I agree.
Have not seen one yet. Maybe something like this could be made into ideal?
roller_tensioner01.jpg


Steve
 
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