Transmission internal ratios (what happens inside the transmission - the relationship between 1st & 2nd, etc.) have nothing to do with overall drive ratios (which is a function of sprocket sizes, and 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. Read more about selecting transmission ratios here: .
Now, how to apply this advice to your project?
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.
I have written, after many, many hours of research, several spreadsheets that already contain the number of gear teeth (not sprocket sizes) as data, and accept engine RPM, primary drive components, sprocket choice, and wheel diameter as variable inputs, and do all of these calculations and comparisons for all the possible ratios instantly & automatically. The results can be printed and kept with your tools and race spares, and then used to analyze your race results and plan for the next race.
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 what your purpose is). Other transmissions are under analysis, but the currently available products are:
Harley-Davidson 1956-90 Sportster 4-speed, including the 1956 KH (and any 1952-55 K & KH
using the later parts), 1957-69 Sportster & KH (with dry clutch), and any 1970-90 4-speed using
the dry clutch parts. The choices compared are the factory Std. Ratio, Std. KR Ratio, Std. “Close”
Ratio, Std. KR “Ratio C” Ratio, Special Close Ratios “D”, “E”, “F”, “G”, “H”, “J”, “K”, “M”, “N”,
Std. “XLRTT” Ratio, Std. XLRTT “Close” Ratio, Special XLRTT Ratio “C”, and Special XLRTT
Ratio “D”, and 52 other potentially useful different internal ratio combinations.
Harley-Davidson 1991-2003 Sportster & Buell 5-speed,
including all early & late factory and
Andrews ratios, and factory early, HCR, and Johnson Engineering “SportGear” high-gear sets,
and the Baker XL6 6-speed
(a total of 23 different internal ratio combinations)
Harley-Davidson 1936-* big twin 4 & 5-speeds, including all Andrews ratios (a total of 11 different
internal ratio combinations)
Triumph unit & late pre-unit 650 & 750, and Triumph & BSA 750 triple 4 & 5-speeds, including factory, Quaife & Triple Cycles ratios (a total of 39 different factory and potentially useful
alternate internal ratio combinations)
BSA 1954-63 A7, A10 & Gold Star 4-speed, (7 original factory ratios + 11 potentially useful
alternates,
total of 18 different internal ratio combinations)
BSA 1962-72 A50, A65 & A70 & 4-speed, (4 original factory ratios + 9 potentially useful
alternates requiring some fabrication, total of 13 different internal ratio combinations)
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 work automatically with your existing program. Simply double-click the file name in your e-mail program (under “attachments”), and it will launch the program.
Upon receipt of your PayPal
payment the file will be promptly sent to your e-mail address within 24 hours. Before puchase, please read the copyright notice and use agreement; click here: .
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H-D Sportster 1956-90 4-speed
Sportster, Buell 1990-2003 5-speed
H-D Big Twin 1936-* 4 & 5-speed
Triumph 650 & 750 4 & 5-speed
BSA 1954-63 A10 4-speed
BSA A50, A65 & A70 4-speed
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For most motorcycles, including the Harley-Davidson big twin, Sportster, Triumph 500, 650 & 750, BSA 500, 650 & 750, Norton 500, 600, 650, 750 & 828, the original standard 4-speed or 5-speed transmission ratios are good compromises for mixed street and moderate performance use, and are“staged” or“progressive” in that the engine speed loss on shifting from 1st to 2nd is higher than the loss on shifting from 2nd to 3rd, etc.
The purpose is to keep the engine in its torque range at higher vehicle speed, where wind resistance requires more power for acceleration. Wider gaps between ratios will allow a “stronger” (higher numerically, e.g. 2.90:1 instead of 2.50:1) 1st gear for better manners in traffic, but increase the RPM lost on shifting. Narrowing the gaps will increase acceleration at speed, and potentially improve top speed under certain conditions, but acceleration from stopped and operation in traffic will suffer.
The 1st gear ratio for most 4-speed transmissions is about 2.50:1, and 4th is almost always 1.00:1. The ratios of 2nd and 3rd are placed in between these two, and are discretionary to best serve the weight, intended use, speed, state of engine tune etc. of the motorcycle.
“Range” is the torque multiplication difference between 1st and 4th gears; wider-ratio gear-sets have more, typically between 2.8 and 3.2. This is the single most important determinant of low-speed acceleration.
“Progression” is the next factor. This is the
reduction or decay in the percentage drop in engine speed in the next gear (e.g. after shifting from 1st to 2nd). Most transmissions have some degree of progression in that the RPM drop
on the 1-2 shift is larger than the RPM drop on the 2-3 shift, which is in turn larger than the RPM drop on the 3-4 shift. The progression may not be linear (continuously reduced) or done in proportionate stages for various reasons, including a special need for a gear to reach a
specific speed or RPM for passing, racing, etc. or simply economic necessity: the parts were available.
The two factors are not mutually exclusive, but each limits the number of options for the other. A wide range, which gives a strong torque multiplication in 1st gear for excellent manners in low-speed traffic (especially with a smaller motor, heavy chassis or side-car) mean that the progression percentages must all be high. The
amount of engine speed (and therefore power) that must be lost on each up-shift is higher than would be the case in a transmission with less range (but less power in 1st gear). A numerically low 1st gear (2.02, &c.) reduces available torque in 1st gear, but allows more choices
of progression.
There is no choice of ratios that gives the “best” performance at all speeds, nor is there a choice of primary drive ratio or rear pulley ratio that gives the “best”; performance at all speeds. It simply doesn’t exist, all ratios are compromises, and not necessarily better than the original ratios for most use.
The advantage to a close ratio gear-set lies in the fact that the RPM loss at very high speed is reduced, allowing extra power to accelerate above 100 mph. However, of necessity the torque multiplier in the lower gears is reduced by the same proportion, and performance at low speeds is much worse. Even for road racing, the closest possible ratio is not always the best choice since some races begin with a flying start (favoring close ratios, where 1st gear acceleration is less important) and some with a grid start (favoring slightly wider ratios with high progession, where 1st gear acceleration is very important).
To give a larger spread between the gears, a wider ratio high-gear set can be installed in the other gear sets to give non-factory combination. It multiplies the 1st 3 gears a bit for better acceleration, then drops the rpm on the 3-4 shift. This allows a“taller” (numerically smaller) primary, pulley and/or overall drive ratio for higher top speed, without reducing acceleration in the lower three gears. Notice that the RPM loss on the shift is exactly the same, except the 3-4 shift.
In general, engines with smaller displacement (500), very long duration cams, ported heads, large carburetors &c. don’t pull well from low rpm., and when the 3-4 shift will benefit more from close ratios in the upper gears, and even more so as the maximum speed at a specific course increases.
If the shift takes place at a speed where air resistance is high (70+ MPH), closer ratios are better and the factory Group 1 “Std.” gear-set is at a big disadvantage with its 27.6% RPM drop (6,000 RPM drops to 4,344).
The Harley-Davidson factory racing Group 3 Special Close Ratio “E” gear-set loses only 12.6% on the 3-4 shift (6,000 RPM drops to 5,244), and the motor (rather than“falling off the earth”) is right in its torque curve and accelerates much better.
Group 4 close ratio boxes, such as Special Close Ratio “N”, have an even greater advantage but have the added disadvantage of less torque
multiplication in 1st.
If your engine has been specifically designed for a tuned RPM torque peak (or if that’s how the engine behaves), the transmission ratios must be chosen to insure that after each shift during a lap the engine speed recovers to a point above this peak at that specific track. From the negative viewpoint, tha ratios must be arranged to avoid dropping the engine into a &ldquohole” on an upshift, where power falls off disproportionately.
If the widest ratio change gives a 25% loss, the shift RPM is 7,000 RPM, and there&rsquos a torque increase at 5,000 RPM you’re safe: 7,000 - 25% = 5,250, the engine will be in this desirable range on acceleration.
If the widest ratio change is 30%, shift at 7,000, and torque at 5,500: 7,000 - 30% = 4,900, far below the power range and the acceleration (and perhaps the jetting) will be weak until you reach 5,500. You will definitely benefit from a closer gear set, or at least re-arranging the progression to reduce the 30% drop to a better number. Depnding on the bike and the track, adding to the drop in the previous gear pair (i.e., problem with the 2-3 shift: add some drop to the 1-2 not the 3-4) is the 1st choice but results will vary.
Individual tracks with combinations of maximum speed and corner speed will require different intermediate (2nd & 3rd) gears to allow downshifting for a specific gear to enter a turn, or to use only one gear during a turn to avoid traction loss. The key to analysis here is whether your favorite track has a spot where the engine is “flat” after shifting at an awkward moment in a turn, but better as it speeds up. Closing the ratio between these 2 gears will help, but of course it moves the greater RPM drop somewhere else. Click here to return to the product terms: .
