A Bigger Rear Sprocket Makes More Horsepower
4.8 hp loss with -1/+5 gearing
The first thing most of us do after making a performance modification is throw our gear on and give the bike a good “seat-of-the-pants (SOP)” dyno run. The SOP dyno test, for those not familiar with the term, means simply to get on your bike, twist the throttle and feel what effect a modification makes. If you made a good choice and installed the parts correctly you come back with a smile on your face. Certain mods, however, are not so easily evaluated. Changing your gear ratio with new sprockets for example can have a noticeable effect on how the bike feels, but is it safe to assume that what feels faster has actually increased your horsepower?
The gears in your transmission are torque multipliers, and the final-drive ratio of your countershaft and rear sprockets also have an effect on torque. Changing the final-drive ratio is one of the easiest ways to modify the feel, acceleration and top speed of your bike. Lowering your gear ratio with the popular down one tooth in the front and up two teeth in the rear (-1/+2) sprocket set makes a huge difference on how the bike accelerates off the light. Because of this positive effect, we’ve heard many riders claim that a bigger (more teeth over stock) rear sprocket and/or a smaller front countershaft sprocket actually makes more horsepower. How could this possibly be a myth when you can hop on the bike and feel the difference?
Does a bigger rear sprocket make more power? Before we answer that, it’s important to know what your sprocket ratio means in terms of power. The chain and sprockets transfer the engine power to the rear wheel. The sprocket sizes determine how many times the countershaft must turn in order to spin the rear wheel. This ratio is your final drive, which is a numerical indication of how many turns the front sprocket makes in relation to the rear. Equal size front and rear sprockets would be a 1:1 ratio of power transferred from countershaft to rear wheel. To make better use of available power, a lower final-drive ratio is used to multiply torque from the engine and transmission to the wheel.
Knowing what sprockets you have is the first step to making ratio mods. If the bike is stock, you can refer to the manual or, if you look, most sprockets are clearly stamped with a number that tells how many teeth it has. If not, you can count the number of teeth on the countershaft (front) and rear sprockets and do the math. Changing the size of either sprocket is what modifies the final-drive ratio, and determining what ratio is best depends on what type of riding you’ll be doing. A lower gear ratio will bolster your bottom end for better light-to-light performance, while higher gear ratios will help out on the top end. Making a sprocket change on your motorcycle would be comparable to changing the ring-and-pinion gears on a car. If you’re confused by gear-ratio terminology, just remember that lower or shorter gears are numerically higher. Higher or taller gears (highway gears) are numerically lower. For example, a 3:1 ratio is lower than a 2:1 ratio.
So if torque is multiplied through gear ratio changes, does this equate to an increase in power? For this month’s Myth test we grabbed some sprockets, chain and wrenches for back-to-back dyno testing on a 2008 GSX-R1000. The tests would be comprised of three stops on the dyno with three different sprocket setups: baseline stock (17/43, 2.5:1 ratio), Vortex -1/+2 (16/45, 2.8:1 ratio), and Vortex -1/+5 (16/48, 3:1 ratio). Lower gears might feel like power has been boosted, but the dyno won’t be so easily deceived.
The first dyno session with OEM gearing netted our baseline data: 153.9 HP @ 73.1 LB-FT of torque. For the second session, we replaced the sprockets with the Vortex 16/48 (-1/+5) set connected by an RK 530-pitch GXW chain. In theory, since no engine mods were made, there should be no change in the power numbers. So it’s interesting to observe not only a change, but an actual drop to 149.1 HP @ 69.8 LB-FT of torque. Next, we knocked off the extra links from the RK chain (we started with the bigger sprocket so that we could use the same chain) and switched to the Vortex 16/45 (-1/+2) set and the results fell in between at 150.6 HP @ 71 LB-FT of torque to verify that power readings were directly affected by sprocket ratios.
So did lowering the final-drive ratio actually reduce power? Theoretically, the three runs should not have produced a measurable power difference since no engine mods were made. The curves are nearly identical all the way up to peak RPM; however, the lower numbers meant that something had indeed changed the rear-wheel power. Since nothing from engine to output shaft had changed, the indicated loss in RWHP is a direct result of the lower final-drive ratio’s increased rate of acceleration. The ratio changes matched by corresponding drops in RWHP readings verify this. The lower gearing therefore multiplies torque, increases the rate of acceleration, but does not measurably increase or decrease horsepower at the crankshaft. We’ve proven that a sprocket ratio change does affect usable power measured at the wheel, but unlike popular belief, a bigger sprocket actually makes slightly less power on the dyno.