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

Discussion in 'Transmission / Drivetrain' started by bakaneko, Apr 25, 2016.

  1. bakaneko

    bakaneko Active Member

    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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    Last edited: Apr 25, 2016

  2. HeadSmess

    HeadSmess Well-Known Member

    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.
     
  3. bakaneko

    bakaneko Active Member

    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.
     
  4. butre

    butre Well-Known Member

    to a certain point, the less contact the better. look up parasitic load
     
  5. Steve Best

    Steve Best Active Member

    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)
     
    FurryOnTheInside likes this.
  6. FurryOnTheInside

    FurryOnTheInside Active Member

    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.
     
  7. Ytyukon

    Ytyukon Member

    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.
     
  8. Steve Best

    Steve Best Active Member

    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.
    [​IMG]
    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?
    [​IMG]

    Steve
     
    FurryOnTheInside likes this.
  9. butre

    butre Well-Known Member

    bicycles don't reverse under power though
     
  10. FurryOnTheInside

    FurryOnTheInside Active Member

    I think Steve means bump starting and engine braking.. Tension switching to the return (under) side, reversing the function, reversing the load.
     
    Last edited: May 1, 2016
  11. FurryOnTheInside

    FurryOnTheInside Active Member

    My thoughts are that a spring being constantly compressed wastes (some) power, but a fixed idler makes no allowance for the eccentric HT drive sprocket and/or wheel sprocket. That is why I thought a light spring is best. When the chain is at it's tightest spot in the cycle (once every 220 revolutions of the rear wheel with 44t rear sprocket) and when it's function is reversed, the soft spring would be compressed as far as it's (adjustable screw?) limiter, but when the chain is slackest it should be only compressed by the weight of the returning chain, so applying almost no load to the chain.
    The picture appears to show an adjustable preload, great for switching from bumpy roads to smooth racetrack; but no limit to the spring's movement other than it's own. The part it would like to see when I look at the picture you posted is a bolt poking down next to or inside the spring.
    I thought maybe there is something for "fixies" and single speed cycles built on bike frames with dropouts (lacking the proper rear facing forked wheel mount that you generally see on BMX bikes) but the idlers that I have seen so far appear to favour quick wheel installation over maximum efficiency so they are just sprung with lots of preload and really no limit until the chain pulls tight (so it does allow the top to go slack during pedal braking, eek!) just like an ordinary derailleur. Which does lead me to question the actual amount of power loss and whether it really is worth worrying about, whether it is fixed by the spring strength and preload or varies with the power put in.. and I really dont know. :confused:
     
  12. HeadSmess

    HeadSmess Well-Known Member

    no worry about getting the sprocket concentric...i am yet to find a chain that wears evenly anyways. after 10,000km or so...kachunk ka chunk kachunk.


    nope, the whole thing isnt worth worrying about. you can run an amazing amount of slack on a chain before it starts spitting. just set the fixed tensioner so you get 20mm or so slack in the tightest spot, then forget about it until it starts flinging off, breaks, or has worn enough to mandate checking...

    never have a tensioner on the tight side. a guide, maybe, but no tensioner. it has to take the pressure of engine power driving the load. they never hold up, add to the wear, blah blah.
     
  13. FurryOnTheInside

    FurryOnTheInside Active Member

    10,000km on one chain?!:eek:
    That'll make your sprocket concentric.
    ;):D
     
  14. HeadSmess

    HeadSmess Well-Known Member

    just replaced my moto chain after 20,000... thats about the standard life of a motochain ;) bit less, bit more, depends on the bike and the maintenance.

    and no... the sprockets get WORSE with the amount ive done on them. might be time to change em, as youre meant to when replacing the chain.... cost more than the freaking chain does though!
     
  15. FurryOnTheInside

    FurryOnTheInside Active Member

    Crikey, I've never had a bicycle chain last that far. I just replace when ten links measure more than 25.55cm (new is 25.4cm).. I couldn't say how far that actually is. Sprockets AFAIK only need replacing when they seem to be accelerating chain wear, or if chain replacement was delayed too long (25.7cm) then run the worn out chain and sprockets as long as possible then replace both together.
     
    Last edited: May 2, 2016
  16. skyash

    skyash Active Member

    I got that tensioner in the pic at work i have takein one home to try but will the rubber in sides hold up? And I like how thay have a kg on them. Mine is 0 to 15 kg and have a hard and mid hole.
     
  17. skyash

    skyash Active Member

    At the moment. Hard to see but it's a stock tensioner with a kick stand spring to the top back wheel fork adjustable.
     

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  18. Steve Best

    Steve Best Active Member

    I like your ideas Furry, but as usual, I think Headsmess has it pegged.
    Chains can run efficiently with huge slack. A light spring tensioner with limits might let it run quieter however. Is it worth the bother?

    The picture shown was a stock one from off the internet as an example and Headsmess is right, you'd never want to run it on the drive side.

    Steve
     
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