Smaller Rear Sprocket

Discussion in 'Tech Area' started by toasterknight, Jun 3, 2014.

  1. toasterknight

    toasterknight New Member

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    Does anyone have any idea as to how much top end a 31 tooth rear sprocket would add to a K1? Also has anyone rode one at 55mph for any length of time?
     
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  3. racerx

    racerx Administrator
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    I think you may have this backwards...

    Mathematically, 15/31 would require engine rpm in the low 9000s, well within the limits of what these motors can spin. However, that's only on paper. Miles per hour and horsepower have a direct, mathematical link. Think about it, if a simple ratio change was all that were needed to increase speed, why would anyone bother with engine mods and all of their complexities? The answer is simple, hp determines speed potential; gearing is setup to allow the engine to reach its peak hp rpm at the max mph it can pull. Go too tall and the engine won't be able to reach peak power, it'll fall on its face. Go too short, it'll run out of revs before top speed potential can be realized.

    The short summary is that 55mph requires approximately double the stock output, under normal (i.e. on the flat, minimal headwind) conditions. Just going from 40mph to 50mph takes 60% more power.

    As for sustained cruising at low freeway speeds (50-60mph), the basic design of this bike has enough capability. I've been doing just that for well over 20,000 miles & nearly a decade-and-a-half now. You might check out the video thread. It's not the same as saddle time but, it's that POV. This is, however, an involved topic of its own. Unless you have the extreme good fortune to live in an area with perfectly maintained, smooth, pavement and little traffic, suspension and braking upgrades are part of the equation...for maintaining an OEM-like balance.
     
  4. toasterknight

    toasterknight New Member

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    Dang I live on the Oregon coast the roads aren't that great and I wanted to be able to do the half hour run to town. Fortunately no one would look twice at a bike going thirty on the highway.
     
  5. OLD CT

    OLD CT Well-Known Member

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    Careful you don't wind up street pizza for the vultures doing 30 on the highway.
     
  6. Adam-NLV

    Adam-NLV Well-Known Member

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    I have a 15T front and 35T rear but it's in my 1970 K0 with a Lifan 110cc. I purchased a 31 Tooth for the rear wheel but haven't tried it yet, the reason I want to swap it for the 35T is the front wheel lifts way to easy w/t 35T and a L110.----> I don't think that smaller 31T sprocket would be so great with a 72cc as racerx has inferred.

    I'm in the process of taking my 70 K0 back to stock with the exception of the front forks which I'm currently swapping to K1 forks and just gonna leave in the 110cc and will try out the 31T.
     
    #5 Adam-NLV, Jun 6, 2014
    Last edited: Jun 6, 2014
  7. motoman287

    motoman287 Member

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    Yep, as racerx alluded to, extra horsepower is required to take advantage of the smaller sprocket. I have a 117cc motor with a cam and exhaust, currently running a 17/31 combo, and I'm revved out around 60 mph. I'm going to drop another tooth or two in the rear, as the motor will pull it, seeing how it revs out as it does.
     
  8. racerx

    racerx Administrator
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    The key to getting optimal road gearing is figuring out what the engine can handle, as expressed in mph/1000rpm in top gear. .3mph/1000rpm can make the difference between minimal cruising rpm and an engine falling on its face. For the typical 110-125cc tune, capable of pulling 6.5-7.5mph/1000rpm, that's a difference of ~4-5%. Being more than about 3% off is where the difference can be easily detected on the old seat-of-the-pants dynamometer. It's usually possible to dial-in a combo closer to within .15mph/1000rpm of "ideal".

    The bottom line here is that doing a little bit of high-school-level math can save one a lot of trial & error, by eliminating all the variables of trans & primary ratios + differences in tire circumference. Even if you're flying blind, with a hitherto untested engine/tire combo, having the ability to know what changes you're making, with precision, will allow you to get where you need to be in the fewest number of steps. Should you decide to change tire size, at some later point, you'll know what sprocket changes to make, from the get-go.

    These engines don't make a ton of torque, thus small changes can have (seemingly) disproportionately large effects. Rolling along at 45-60mph, you might be surprised how much difference 300-500rpm less engine speed makes at (literally) the end of the day, in terms of rider fatigue alone. Consider the cumulative wear index, as the years & mile roll by. 400rpm x 60 minutes = 24000 crankshaft revolutions per operating hour. That works out to roughly 50,000 crankshaft revs per 100 miles. Consider the piston travel, valvetrain & transmission motion and the cumulative wear on those parts, the numbers can really add-up. Building a roadworthy mini-cruiser takes a fair investment. IMHO, maximizing usable mph and service life (a.k.a. optimal efficiency) doesn't cost, it pays...
     

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