Hey gang, For those knowledge junkies out there wondering how our beasts shift thru the gears here is another dissertation: Where’s it all stack up at It all happens on two shaft assemblies. In between the two case halves right behind the crankshaft. The first shaft Yamaha calls the “main axle” we’ll call it the clutch shaft, as it is the same shaft that the clutch hub stacks up on drives power into from the crankshaft. The second shaft Yamaha calls the “drive axle” but we’ll call it the output shaft as it has the sprocket attached to it and this is where the power exits or “output” to the chain. How the gears stack-up The gears stack up on each shaft in a kind of reverse order. One shaft starts with the biggest gear and works it’s way to a small gear. The mating shaft does it in opposite order. When a big gear drives a little gear it can spin the little gears shaft several times for every one time it rotates. The opposite is true for a small gear driving a big gear, it has to rotate several times for the big gear to go around once. We call these differences in rotation “ratios” The first I mentioned would be called a “fifth gear ratio” where as the last I mentioned would be called a “first gear ratio”. A ratio is the rate at which one shaft turns relative to another. Some basics Before we got into detail let’s understand a few basic things about our transmissions. We have shafts that are “splined” and on these “splines” we have “sliding gears” that slide left to right but cannot rotate because they have teeth on their inside diameter (ID) that locks them to the shaft. Each of these sliding gears is controlled in it’s action by what is called a shift fork. Shifting forks are shaped like the letter “C” they wrap around grooves or collars on the sliding gears so that as the gears rotate these forks can still hold the gear and slide left to right or side to side. These forks are controlled by a shift drum which we will discuss later. We also have gears mixed in with these slide gears that do not slide along the shaft they are fixed from going side to side with snap rings and washers, but they can do something different than the sliders, they can rotate freely around the shafts diameter. They do NOT have splines on their ID. All the gears that rotate or spin freely on a shaft have what we call drive lugs on the sides of them. Drive lugs are kind of like protrusions sticking out from the side of the gear so that when a nearby gear “slides” right up tight they engage with each other through their sides and lock both of them to the shaft. The spinning gear can’t spin anymore because it is “Lugged” to the sliding gear which remember what we said about all sliding gears – they can slide because they are splined to the shaft. The stack-up This is where it gets confusing, remember what we’ve discussed so far and follow closely. It helped me to even draw a picture as I went along. Got get a picture of the transmission page at Yamahas website for a 98 YZ400F. Print it out and follow along as I show the flow (via the numbers on the picture) of power from the clutch shaft thru to the output shaft when the transmission is shifted into each of the various gears. It might help to write down next to the gear what its name is from the part description as well (IE first gear second gear etc…). Power Flow In neutral all sliding gears are turning on their perspective shafts and mating with their perspective free spinning gears on the mating shaft. For any gear to be selected a slide gear must move over and “lug” a free spinning gear to it’s shaft thus making power connection from the clutch shaft thru a gear set to the output shaft. First = 5th gear (#15) slides to the right and lugs 1st gear (#14)(free spinning gear), to output shaft (#13). Power comes in the clutch shaft (#1), goes thru the 5th gear ratio (#14), into the lugs of the 5th gear slide gear (#15), which is splined to the output shaft (#13), thus transmitting power out. (Power flows from item number1-14-15-13) Second = 4th gear (#21) slides to the left and lugs 2nd gear (#23)(free spinning gear) to output shaft. Power comes in the clutch shaft (#1), goes thru the 2nd gear (#9 & #23) ratio, into the lugs of the 4th gear slide gear (#21), which is splined to the output shaft (#13), thus transmitting power out. Power flows from item number 1-9-23-21-13). Third = 5th gear (#15) slides left and lugs 3rd gear (#19) (free spinning gear) to the output shaft (#13). Power comes in thru the clutch shaft (#1), up into the 3rd gear slider (#6), thru its splines, thru the 3rd gear ratio (#6 & #19), into the lugs of the 5th gear slider (#15), which is splined to the output shaft (#13), thus transmitting power out. (Power flows from item number 1-6-19-15-13). Fourth = 3rd gear (#6) slides left and lugs 4th gear (#7) (free spinning gear) to clutch shaft (#1). Power comes thru clutch shaft (#1), up thru splines in 3rd gear slide gear (#6), thru its lugs into 4th gear (#7), thru the 4th gear ratio (#7 & #21), which the 4th mating gear (#21) is splined to the output shaft thus transmitting power out. (Power flows from item number 1-6-7-21-13). Fifth = 3rd gear (#6) slides to the right and lugs 5th gear (#3) (free spinning gear) to the clutch shaft (#1). Power comes thru clutch shaft (#1), up thru splines in 3rd gear slide gear (#6), thru its lugs into 5th gear (#3), thru the 5th gear ratio (#3 & #15), which the 5th mating gear (#15) is splined to the output shaft (#13) thus transmitting power out. (Power flows from item number 1-6-3-15-13). Shifting Three shifting forks that are slid side to side in the transmission by what is called a “shift drum” control all gear changing. The shift drum is rotated a small amount every time you depress the shifter. Fro example; as you depress the shifter down into first the drum rotates about 30 counterclockwise, where as shifting into second rotates the shift drum clockwise the 30 degrees coming back from first gear and an additional 30 degrees to get into second gear. Along about now your saying OK but how does this rotating shift drum make the shift forks slide side to side as you noted above to change gears? Simple: Imagine cutting grooves around the OD of the drum. Now place pins (that are part of the shift forks into these said “grooves”. As I rotate the drum around and around nothing would happen to the forks because the are just setting there with their respective pins resting in the grooves. But what would happen if I took and put an “S” curve in one of the grooves, meaning that it wasn’t a straight line all the way around the shaft, it wiggled side to side as it went around the shaft. Now as I rotate the shift drum the respective fork would be forced to slide side to side as it’s pin passes thru the “S” curve. The shift drum has three grooves in it, one for each of the three shift forks. Each groove has a precise path that it related fork has to follow as the drum goes around. Each groove has it’s “S” curves precisely placed so that the three forks are sliding left or right at the right time. Bored yet!