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Thread: automatic vs standard shift. final drive ratios.. someone care to help?

  1. #1
    Join Date
    Apr 2013
    Location
    3whinerworld.com
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    85

    automatic vs standard shift. final drive ratios.. someone care to help?

    Ok, so ankle deep into a franken quad project for my girlfriend (yamaha blaster + yamaha bravo 250 sno machine) = yamaha trail blazer lol.. anyways, trying to figure out the final drive ratio, I'm using the polaris trailblazer as a benchmark. it has (in stock form) a 2.68 reduction in the transmission, and 3.45 sprocket ratio. now those do add together right? as im getting a final drive ratio of 6.13 : 1 which is double what most quads have of 3.45 or so. I can only assume that the cvt transmission has an overdrive nature to it of somewhere 1:3. and needs that wholey crap gearing to make the final drive 3:1ish. that about right? and secondly the bravo has a 1.69:1 reduction. so assuming the clutches are the same size in the yammi vs a polaris, I would need to have a 4.44:1 sprocket ratio. to come up with the extra gearing from the low reduction of the yammi gear box. which is doable 12/52 will net that. but is all that correct? just seems weird to me they would have that much reduction..

  2. #2
    Join Date
    Jan 2014
    Location
    Massachusetts USA
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    783
    When dealing with ratios, the math is always driven teeth ÷ drive teeth (or driven diameter ÷ drive diameter if using pulleys). Work out the individual ratios then multiply them (2.68x3.45=9.246) so you may need a steeper sprocket ratio than you think. Also, CVT's usually give very little overdrive, something to the tune of 0.8 : 1. My guess is Polaris used the steep reduction to allow full primary clutch engagement at low traveling speeds, to prevent slippage from prematurely wearing out the belt.
    1985 Tri-Z 250
    1985 ATC250R

  3. #3
    Join Date
    Apr 2013
    Location
    3whinerworld.com
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    85
    yup.. sure looks like i'll need a deeper rear drive that way. on that math then, Polaris final drive 9.246 benchmark (a quad 100lbs heavier) divided by the yammie 1.69 = 5.49 which is a bit much for a rear sprocket, as the largest I can go that im seeing is a 11/52 combo netting 4.73. now big if here, If I could find a 12/23 sprocket set on the yammie gear box that would give me a 1.92 primary reduction. leaving 4.82 for the final drive sockets, which 11/52 will be close with a 4.72 ratio... anyone know if I can fid a set of yamaha gears like that?

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