DYNO CHART SMOOTHING
Name SAE/5 STD/5/4/3 STD/2 STD/1 STD/0
Steve 76.3 77.8 78.0 78.4 78.6
Joe 74.8 76.2 76.3 76.4 76.6
FOURTH OR FIFTH GEAR?
HORSEPOWER VS. MPH
Using Joe’s Road King as an example, note how the rear tire’s rolling resistance affects the horsepower curve (black line) above 120 mph. By the time the engine hits the rev limiter at 140 mph, the curve is in a steep dive, especially when compared to the blue line, which is the horsepower in fourth between 100 and 115 mph. Also note that I exceeded the tire’s 130 speed rating for less than two seconds.
Some operators test five-speed Harleys in fourth gear, which I’ve heard is “the industry standard,” while I usually use fifth. So, which gear is right? What about on a six-speed sportbike? Which gear will read the highest horsepower and torque? These are all questions I’ve answered dozens of times while working at the dyno. The answers lie in gear ratios and design of transmissions, as well as tire speed ratings and pressures. Confusing, isn’t it? Well, let’s apply a little brainpower to some facts, and we’ll have all the answers before the next dyno run.
Let’s start with the basics, okay? We are trying to measure the engine’s horsepower and torque output. Where are we measuring it? At the rear tire, where it counts, right? Did you know that power losses occur between the engine crankshaft and the contact patch of the tire? These are called drivetrain power losses. The primary drive (whether it’s via chain, belt, or gears), transmission, final drive (be it chain, belt, or shaft), and the tire all introduce losses. All have mass, and power is used to get them moving. In this argument, we’ll ignore the primary and final drives, since their losses are most likely pretty constant and independent of the transmission gearing and rear-wheel speed.
The transmission in a sportbike is similar to the manual tranny of a frontwheel- drive car: It’s a simple two-shaft system. The input and output shafts are mounted side by side. Each has five or six gears, which mesh with their counterparts on the other shaft. The gear ratios are determined simply by the number of teeth on the drive gear divided by the number on its driven mate. With no reason for a 1:1 ratio set, there usually isn’t one, and at least two ratios are overdrive. Power is always transmitted through one gear mesh. For our purposes, this means a nearly constant power loss, regardless of which gear we’re in on the dyno.
A Harley-Davidson Big Twin transmission, on the other hand, is patterned after the four-speed (or three-speed — remember them?) gearbox of a rear-wheel-drive car. The output shaft is concentric with the input shaft. The only difference is that power goes in the front and comes out the back in the automotive application, whereas Harley mounts one shaft inside the other so both shafts come out of the left side of the transmission. In high gear, the input and output shafts are simply locked together, giving a 1:1 ratio and minimal power loss. (If the neurons are flowing, the lightbulb above your head should be starting to come on about now.) The input shaft also meshes with a gear on a countershaft mounted beside or below it. This countershaft is often referred to as the cluster gear or shaft because it also has a gear for each of the transmission’s lower gears. To complete power transmission in the lower gears, the output shaft has mates for the gears on the cluster. The ratio of each lower gear is a combination of the reduction ratios of the input-to-cluster and cluster-to-output gears. So in each of the lower gears power goes through two gear meshes, producing — you guessed it — double the power loss.
The other major inconsistent factor we have to deal with is the rolling resistance of the tire that we’re measuring the engine’s horsepower and torque through. Harley tires typically carry an H speed rating for sustained maximum speeds of 130 mph. Sportbikes run W (186 mph) or Z (149+ mph) rubber. A tire running above its speed rating, or an underinflated tire, starts to heat up and will eventually fail. Heat indicates friction in the sidewall, and that translates to rolling resistance and increased power losses. With a Harley on the dyno, it shows up as a power loss that increases with speed. In contrast, sportbike tires show zero loss on the dyno, even when approaching 200 mph.
Okay, so where does this all lead? With a constant one-mesh loss, regardless of gear selection, and a tire that adds no loss, a sportbike’s horsepower and torque graphs are virtually identical, whether displayed in third, fourth, fifth, or sixth gear. There’s not much point in printing one here because all of the lines run together. But it’s a different story on a five-speed Harley. With the double-mesh loss in fourth and no loss in fifth, horsepower and torque will always read higher in fifth, right? Not so fast, or, rather, too fast for the H-rated tire, that is.