Do you own (or plan to own) a classic 1962-71 BSA Thunderbolt, Lightning Rocket, Spitfire, Hornet or Cyclone 500, 650 or 750cc twin, or early 750 triple? The original 4-speed transmissions on these models are excellent, but did you know they're not the only ones available? The unit 4-speed (used on “A” Range models with VIN A50, A65, A70 and A75) is only one of the factory transmissions originally available for different purposes, such as side-car, road racing, scrambles etc., and they can be easily interchanged using the instructions in your service manual. In addition, even more choices can easily be made from the existing parts.
    Transmission internal ratios (what’s different between your transmission and the factory options) happens inside the transmission - the relationship between 1st & 2nd, etc.) have nothing to do with overall drive ratios (a function of sprocket sizes, which determines top speed). “Close” ratios, while a main-stay of bench race conversation, are absolutely not what you want for street use, or even for some racing venues.
    Even if you already knew all the available tooth counts and internal ratios (some are not common), actually comparing them includes:
    »   RPM % lost on a shift
    »   RPM % recovered on a shift
    »   actual RPM in the new gear after upshift
    »   lowest RPM reached in the entire range
    »   miles per hour at a specific RPM for corner changes
    »   top speed in high gear
    »   overall gear ratio including final drive sprockets
    This is a huge amount of work, but it’s essential if you race AHRMA or similar venues, since it permits you to re-think your internal choices after a hot lap to get the best possible selection for that track condition. Exactly where in the course your shift points should occur will change with any engine mods, such as exhaust length, cam choice, rider weight, sprocket sizes, etc. and there is no “right” ratio choice for every race - or even the same race under different conditions. Ever wonder why your opponent places ahead of you with a weaker engine? He may simply have made a more favorable ratio choice - now you can, too.
    Want to know more about what factors are involved in choosing a ratio? Click here: .

    I have written, after many, many hours of research, a spreadsheet that already contains the number of gear teeth (not sprocket sizes) as data, and accepts engine RPM, primary drive components, sprocket choice, and wheel diameter as variable inputs, and does all of these calculations and comparisons on all possible ratio combinations instantly & automatically. The

results can be printed out, and kept with your tools and race spares, and then used to analyze your race results and plan for the next race. Shown, right, as a sample, is part of the spreadsheet, click on it to see a larger version.

    I have also written an equivalent spreadsheet for the 1990-2003 Sportster and Buell 5 speed, including all early & late factory & Andrews ratios, and factory early, HCR, and Johnson Engineering “SportGear” high-gear sets (total of 21 different internal ratio combinations), click here to see more details: .
    Again, this is not yet another table of sprocket size vs. miles per hour, and does not require you to know anything about your transmission other than what parts you already have, and your purpose.
    The choices compared are all factory options including standard, close and wide factory ratios plus 19 other potentially useful different internal ratio combinations.     Please note: this is not a “coffee table book” or repair manual, it does not explain how to service your transmission or repair worn parts - use it with your service manual. It is a research tool for those who seriously want to improve the efficiency of their engines.     This product is a Microsoft Excel® “.xls” spreadsheet. If you have a PC-type computer (or Microsoft Excel software in your Unix or Mac-based system) this will open automatically with your existing program. Simply double-click the file name in your e-mail program (under “attachments”), and it will launch the program.
    If you do not have Microsoft Excel®, you can download “Open Office” for free, and use it to open the file. Get it here: .     Upon receipt of your PayPal

payment the file will be promptly sent to your e-mail address within 24 hours. Before purchase, you must read the copyright notice and use agreement; click here: .

Price: $8.95

. This item is subject to sales tax for New York residents at your local rate, or simply add $.78 to the price = $9.73 total.
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