Recently, while running a 2003 Yamaha R6 through the gears, I noted that the bike seemed to lunge forward when shifting from fourth to fifth gear and again from fifth to sixth. It was almost as though the engine was relieved to be in the taller gear and was therefore rewarding me with more forward thrust. I was shifting the bike at redline, so how could this be? Redline is the correct shift point, right? So I pulled some dyno charts on the R6 and noted that peak horsepower is at 12,750 rpm, yet the factory tach shows redline at 15,500 rpm. With this much overrev, it made me think about what the ideal shift points should be.
After discussing the topic with several street racers, their consensus was that the bike should be shifted at redline for maximum acceleration. I kept digging into the topic and found others who suggested a bike should be shifted when the horsepower in the new gear and rpm exceeded the horsepower in the previous gear. This theory is aimed at keeping the bike at peak horsepower at all times. Others advised me that torque was the key, and that the bike should be kept at peak torque in order to maximize acceleration.
Luckily, two of my riding buddies clubbed me over the head with their logic and informed me that even though these theories provide at least some truth, they ignore the gearbox and are therefore flawed. Even when a bike is run beyond its peak horsepower and torque, it will still typically deliver more driving force to the ground in a lower gear than after shifting to a taller gear. This is because the lower torque value translates to more driving force at the rear wheel when it is multiplied through the lower gear. On the other hand, if power falls off too much, more driving force will be available in a taller gear and lower rpm meaning you should shift before redline.
We can calculate driving force at the wheel for a given gear and rpm by using the equation:
Driving force (lbs.)=T x G x P x S x R
P=primary reduction ratio
S=final drive ratio
R=radius of rear tire (ft.)
For example, our Yamaha R6 shows a peak of 44.7 foot-pounds at 11,750 rpm and 35.5 foot-pounds at the 14,750 rpm rev limiter. The torque multiplication provided by first gear yields 771 pounds of driving force at 11,750 rpm and 612 pounds at the rev limiter. In sixth gear, driving forces have dropped back to 293 pounds and 233 pounds because of the lower torque multiplication of the taller gear. Figure 1 shows the driving force in each gear for the R6.
Let's take a look at the right way to determine when to shift a bike to garner the maximum forward thrust. Note from Figure 1 that the driving-force lines cross each other at a certain road speed. To maximize acceleration, one should shift to the next gear at that point when the driving force applied to the road in the next gear is equal to the force applied to the road in the present gear thus maximizing driving force at all times. While this sounds pretty simple and is easy to see on the graph, it is not that easy to calculate.
As you can see on the R6's graph, the gear ratios get closer and closer as they go higher, which means the rpm drop from the fifth/sixth gear change is not the same as it was from first to second. If you look at the actual gear ratios for the R6 shown in Figure 2, you will see that the percent rpm drop at each shift decreases as the gear increases. The first/second gear change results in a 31.6 percent drop in revs, while fifth to sixth nets only a 9.0 percent drop.
Because gear spacing is closer in higher gears, on many bikes particularly bikes with a lot of overrev like the R6 the ideal shift point may not be at redline in the last few gears. For example, the R6 needs to be held to redline before the first shift; the lines for first and second gear on the driving-force graph never touch. But from then on, the ideal points start to change due to the lower rpm drop in these final gears. The fourth/fifth shift should be done at 126 mph, or 13,352 rpm instead of the 14,750 rpm redline. Then the fifth/sixth shift needs to be pulled all the way back to 13,244 rpm (140 mph on the graph).
Another complication is that factory tachometers are notoriously inaccurate. In almost every case they read a higher-than-actual rpm. In the case of the Yamaha R6, the factory redline shows 15,500 rpm, and the rev limiter kicks in at an even higher number on the tach while the dyno shows just 14,750 rpm; many bikes are even more optimistic. When making these calculations, you need actual dyno data for your bike for the most accurate results. The rpms we list are actual and not indicated per the tach-we'd tack on the percentage error to find the indicated shift rpm. Figure 3 shows the optimum speed and rpm for each shift on the R6.
We enlisted fellow gearhead and mechanical engineer Ron Grigsby to build a spreadsheet for almost any bike that can be downloaded from our Web site at sportrider.com/tech/146-0402-art-shift-2.xls. In order to determine these optimum shift points, a few pieces of data must be known. First, you will need a dyno chart. The more detailed your torque data for the bike data points at least every 500 rpm and preferably every 250 rpm the more precisely the shift points can be calculated. You will also need to know the primary and final reduction as well as the individual gear ratios (first through sixth). They are usually published in your owner's manual or on a specification sheet. And finally, find or measure the radius of your bike's rear tire. With that data, you can determine the actual force being applied at the rear tire's contact patch throughout the rev range for each gear.
Using the spreadsheet, you can simply plug in your gear ratios and the torque for your bike and it will create a chart that displays the actual driving forces in each gear. Simply pick the exact point where the lines cross for a given gear change, and that is the ideal shift point.
In summary, if you shift your bike at redline in every gear in hopes of getting the best acceleration, you may not be getting what you think you are, particularly in the higher gears as our calculations show. On bikes with peak power very close to redline, this effect may not be as large as it is with the high-revving 600s. To get the exact data on your bike, arm yourself with a good dyno run and your bike's gear ratios and take a trip to our Web site to get your optimum shift-point calculations and road-force plots.
|gear||ratio||overall ratio||percent rpm drop at shift |
Figure 2: The individual gear ratios and resulting overall ratios for the R6 show that, in the higher gears, the drop in rpm decreases with each shift. This is why the ideal shift point is different for each gear.
|Optimum shift points for the R6|
Figure 3: The optimum speed to shift is where the lines cross, maximizing driving force.
Figure 1: This graph shows...
Figure 1: This graph shows driving force in each gear for a 2003 Yamaha YZF-R6. Note that in the higher gears there is more driving force available by using the next gear than by winding the engine out in the lower gear.
Figure 4:Driving force for...
Figure 4:Driving force for a Suzuki GSX-R1000. While the effect is not as pronounced as on the R6, you can see that in the higher gears, a lower-than-redline shift point is necessary to maximize driving force.
Figure 5: The Honda RC51's...
Figure 5: The Honda RC51's driving-force curves. Because the curves fall off so quickly, gear selection and rpm are crucial to maintain drive.