[Originally published in Motorcyclist Magazine: June 2003 Issue]
What if you could increase your bike's real-world acceleration by more than 10 percent for less than $150? This is not a smoke-and-mirrors bit; we're talking about gearing here. Your bike's stock gearing is aimed at a balance of power and maximum speed.
So what do you do? Pick up some new sprockets and gear down -- that is, shorten the overall gearing -- to achieve better acceleration. Sprockets are widely available from manufacturers such as Driven, Vortex, JT, DID, and AFAM.
Now that you've convinced yourself that you SHOULD regear, how far might you want to go? It comes down to two decisions; One, how much top end speed will you sacrifice for better acceleration? And two, are you willing to buy a replacement chain? Generally speaking, most sportbike chains and adjusters will permit a change of one tooth at the countershaft or three teeth at the rear wheel and still have the adjustment range use the stock chain.
We'll discuss the loss of maximum speed later, so the immediate questions are; How much should you shorten the gearing, and what will you gain?
To calculate your bike's increase in torque and RPM, use this equation; new rear/ new front multiplied by old front/ old rear, then subtracted by one, multiplied by 100, will equal the percentage of change in torque at the rear wheel.
So if you have, say, a GSX-R750 with stock 17/42 gearing -- a 17-tooth countershaft sprocket and a 42-tooth rear sprocket -- and are considering going to a 16/44 (i.e. minus one in front and plus two in the rear), the equation would look like this; 44/16 multiplied by 17/42, then subtracted by one, and multiplied by 100, which equals an 11.3 percent acceleration increase. You read that right: At any rpm in any given gear, you would have an acceleration increase of 11.3 percent in this example. This causes a change in engine speed at any given vehicle speed. If your bike was previously turning 5000rpm in sixth gear at 65mph, you would see 5565rpm at the same speed.
Here's how we figure the top speed sacrifice -- with gear ratios and rolling radius plugged into another simple formula. Pull out the owner's manual for your bike and check the part in the back where gear ratios are listed. (This example is from a Honda CBR929RR.) Primary reduction; 1.521; first, 2.692; second, 1.933; third, 1.600; fourth, 1.400; fifth, 1.286; sixth, 1.190; and final reduction, 2.687. The final reduction is also known as the final gearing -- the combination of countershaft and rear wheel sprockets. In this case, a 16-tooth countershaft and a 43-tooth rear.
What do these numbers tell us about our top speed with the new gearing choices? Let's do some math. Assuming your bike would normally reach top speed in top gear, start with the sixth-gear ratio (1.190) and multiply it by the primary reduction (1.521) and the final gearing (2.687). For this bike on stock gearing, the overall ratio in sixth gear is 4.863.
To determine the bike's speed relative to engine speed, use this equation;
MPH = (RPM x wheel radius) / (overall ratio x 168).
MPH = (RPM x wheel radius) / (overall ratio x 168).
How big is that rear wheel? You can do some quick math using the size stated on the sidewall. Let's use a 180/55-17 tire. We know it's 180mm wide and the 55 indicates an aspect ratio of 0.55:1, which makes the tire 99mm tall. Add half the diameter of the 17-inch rim, 216mm, and you get 315mm, or 12.4 inches, as the wheel and tire's radius. (You knew that 1" = 25.4mm, right?).
Consider checking the website for your bike's tire manufacturer...many list overall diameters for given sizes, and divide by two.
Back to figuring top speed. As a rule of thumb, most bikes reach terminal velocity at or near their power peaks (there are notable exceptions, so spare us the letters -- we know). For the CBR929RR, that's 11,000rpm. Plugging the figures into the equation above gives us 11,000 (RPM) times 12.4 (radius) equals 136,400. Factor the other side, 4.863 (ratio) x 168 = 816.98. So divide 136,400 by 816.98, and you get 166.96 mph theoretical top speed.
Now, let's change the final gearing, substituting the stock 16/43 combo for a shorter, better accelerating 15/45 pairing. Do the math and you'll see the theoretical top speed drops 17mph to 150mph.
Are you upset at being able to go ONLY 150mph on the street? Or are you happier with an 11 percent boost in useful torque the rest of the time?
- Todd Robinson
Shortly after writing this article, we became the USA importer for Healtech Electronics...maker of the now-legendary Speedohealer speedometer/odometer correction device, the industry benchmark. That was a lot of years, and tens of thousands of Speedohealers ago.
This fixes not just factory speedometer error, but the issues created when changed sprocket gearing, which is speedometer and odometer error. Remember that the 11.31% torque increase discussed in the article above would create that same speedometer error (reading high) on top of existing factory error, and would also introduce an 11.31% odometer error (reading high, e.g. more miles than you're actually traveling).
Anyhow...enough of the sales pitch...if you're interested...read more about the Speedohealer...otherwise...I hope you enjoyed my article about putting more power to the ground with sprocket gearing...