Sprocket Size vs. Engine Output
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  1. #1
    Senior Member YBW's Avatar
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    Jul 2016
    Buena Vista, VA

    Sprocket Size vs. Engine Output

    I love modifying things, but I also love efficiency (which is often my justification for modifying things). While thinking through what I want my TW to do, I have seriously considered rebuilding an extra motor I have into a 250cc 6 speed, but the "snowball" costs of doing it are making it a hard sell.

    Sure, I suppose I could just tear apart my motor with 2000 miles on it and put in the 6 speed/larger crank/74mm jug on it and call it a day, but then it feels like the cam, carb, exhaust, etc. are hindering performance!

    My question is this; is there a way to calculate how a given sprocket change correlates to a given torque (I'm assuming) change? For instance, if I don't care about exceeding 55mph, I can obviously haul a lot more gear with a 50 tooth rear sprocket then I can with a 45 tooth.

    Of course increasing engine output will allow you to carry more weight at a higher speed, but I really don't need or want to go over 55. I am interested to know what size sprocket correlates to something like Sebastians 22hp build

    Disclaimer: Your engine will be less stressed carrying those loads even if you don't plan on exceeding a certain speed, though this doesn't take into account the potential stress whatever mods were performed to increase your output cause on the engine i.e. higher compressing causing higher operating temp.

  2. #2
    Senior Member kj7687's Avatar
    Join Date
    Jul 2010
    Southern California - Inland Empire
    Disclaimer: this might be more of me just rambling and reflecting on my own experiences (granted, I owned and rode many bikes over the years, and had several different gearing setups and lots of seat time on a TW) than legitimately helpful, but hopefully it can at least help a little.

    I'm not sure I could answer your question directly, but what I would say is that gearing changes basically affect two things: how fast you can go in any given gear, and how much torque you can put to the wheel within in each gear (I guess that's what you really want to know, specifically...). You may know this already. Anyway, I think that with stock gearing, 55 mph in 5th gear is already maxing out the engine in terms of comfortable sustainability for long periods (i.e. cruising no faster than that with stock gearing...). To get a TW to feel like a stock-geared TW with 22 horsepower... just by changing gearing...? Hmm. I don't think you would need that much, to be honest, as 22 horsepower still really isn't all that much. Probably 14/55 would do it, or at least certainly 14/58, assuming we are talking about 22 hp at the crank? For 22 rear wheel horsepower? -- 14/58 or 14/60...

    I would say that your final drive gearing choice should really come down to exactly what you want/need out of the bike. E.g. if you regularly need to do longer stretches of highway at 55, I personally wouldn't go lower than stock gearing, unless you place a VERY high value on off road performance versus street manners/cruising capability. IMO, 14/55 gearing is as low as I would ever gear a TW even if you do heavily prioritize off road capability/torquey feel, IF you need to travel at 55 mph a lot. If you really don't need to go faster than 55 mph regularly, then stock gearing is just fine as far as the motor's health/longevity is concerned (you can put MANY thousands of miles on a TW at 55 mph with stock gearing without hurting a thing). In other words, unless you want/need to frequently cruise at 60 mph or more, I would not use a smaller-than-stock rear sprocket.

    One approach is to gear the bike based on how high you want or need your maximum sustained cruising speed to be (gear it as low as you can while retaining the ability to cruise at your chosen max cruise speed without exceeding about 7,000 RPM). For a 55 mph maximum sustained cruising speed, stock gearing is perfect; for 60 mph, 14/45 or 14/44. Faster than that is possible, but it will start to make the bike feel VERY gutless and relatively useless off road, and even 14/45 is already going to struggle to pull 5th gear up some hills. For infrequent/limited cruising at over 50, maybe go to 14/55 for good torque/off road capability. If you hardly ever need to do any highway/over 50 mph, then 14/58 (light rider) or 14/60 (heavier rider) is fantastic for back roads and off road!

    If you haven't seen this handy tool yet, check it out: TW200 Speed Calculator
    Last edited by kj7687; 08-18-2018 at 10:59 PM.
    KJ, just KJ, ok.

    Current rides:
    2004 GMC Sierra 1500, 1999 Toyota 4Runner

    Past rides: 2015 Yamaha XT 250, 1997 Suzuki DR 200, 2007 Honda Ruckus, 2007 Yamaha TW 200, 2007 Kawasaki Ninja 500, 2009 Kawasaki KLX331S; 1994 GMC Sierra 1500, 1987 Nissan Pathfinder, 1992 Acura Integra, 1986 Honda CRX, 1989 Jeep Cherokee, 1994 Chevrolet Astro Van, 1979 Volkswagen Rabbit, 1984 Jeep Cherokee

  3. #3
    Senior Member YBW's Avatar
    Join Date
    Jul 2016
    Buena Vista, VA
    I was playing around with that calculator just earlier today! With a 50 tooth rear (and stock tire) 55mph is reached at right about 7000rpm. A 55 tooth causes it to be reached about 750rpm higher. Though I wouldn't want to ride at 7750 rpm for hours on end, I don't think the occasional couple hour jog would be severely detrimental to my bike.

    But I digress. You are correct, the "torque into wheel" aspect is what I'm trying to figure out.

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