March 12 2013 by Jason Fogelson
Do you ever wonder what is really happening inside your engine when you pull in that clutch lever and stomp your heel down on the shifter? I'm going to try to shed a little bit of the light of knowledge into that dark space, with the hope that a little bit of understanding will help you ride better.
I'm no mechanic -- I've proved that with several projects on my poor old Sportster, Manny. But I have learned that if I don't force anything, and if I take the time and read all of the directions, I can figure out a few things. I still leave the big projects to the professionals, but I got tired of seeing mechanics rubbing their hands together with glee every time I pulled in to the shop for a minor repair. I decided to gain a basic understanding of how my motorcycle works -- so now when a mechanic says "Seems like we might need to tear this down and get a look at the dogs," I know that he isn't referring to fence removal.
Which brings us back to the transmission. I'm going to assume that you ride a Harley-Davidson, but even if you ride a BMW, Honda, Yamaha, Suzuki, Kawasaki, Triumph or other motorcycle, your bike probably has a manual transmission that works in a very similar fashion. Mopeds, scooters and a few new Honda and Yamaha models feature automatic transmissions, but they are by far the exception in the two-wheeled world.
The name "transmission" is descriptive and accurate. The transmission is in between the engine and the final drive -- it "transmits" power from the engine to the wheels. Transmissions are necessary in motorcycles in order to extend the bike's operating range. An engine has a maximum speed, called "redline." By connecting the engine to progressively larger gears, the transmission helps the engine to cause the wheels to turn faster without exceeding the engine's maximum speed.
The simplest kind of transmission to observe in operation is a bicycle's gear set. A series of concentric gears are connected to the rear wheel hub, and derailleur that's connected to a shift lever. Move the shift lever, and the derailleur moves the bike's chain from one gear to the next. If you want to start off quickly, you choose a low gear, and the chain gets moved to a smaller gear wheel. Once you get moving, you select a higher gear, the chain moves to a bigger gear wheel, and you get more speed out of each rotation of your pedal. Think of yourself as the engine. By changing gears to match conditions, you can maintain a constant pedal velocity and effort, and still achieve a much greater range of bike speeds.
Your motorcycle transmission operates on the same principles, though with a different operation. Instead of concentric gears on the rear hub, the transmission is housed in a transmission case -- on new Big Twins, it's a six-speed transmission in a case that's shared with the engine; on new Sportsters, it's a five-speed transmission in a separate case. Here's what you need to know about what's inside those cases:
The transmission gears are constantly rotating, constantly meshing in contact when the engine is running. A series of plates with pegs, or "dogs," sit on an output shaft between the gears. When you push down or pull up on the shift lever to engage the gears, a shift fork moves the shaft and plate toward the gears, and the dogs slot into corresponding holes in the gears. Now, the gear is driving the output shaft, which is connected to the final belt drive, which drives the rear wheel, and you're moving.
So, how can knowing this help you operate your transmission better?
First, it's important to select the right gear for conditions. Use your transmission to keep your bike in its sweet spot. If your engine speed is too slow, you lug the engine and expose it to potential damage. If your engine speed is too fast, you're wasting energy, stressing your engine beyond its intended limits without getting more action out of it.
Second, visualize those dogs, and you'll realize that smooth, quick shifting and clutch operation are essential to their long term health. Shift too slowly, and you may be causing the dogs to skim the surface of the gear rather than engaging, and you'll hear a grinding sound. That's actual grinding that you're hearing, as you wear away your shift dogs and damage your gears. It will get worse the more you do it, until eventually you'll notice difficult shifting and slipping gears.
Third, don't force things. If there's resistance when you try to shift, back off. You may need to adjust your clutch. You may need to check your fluid and oil levels. You may need to visit the mechanic for some clutch or transmission work. Don't force things -- you'll only make it worse.
Trust me, I know. Unfortunately.
Today's transmissions are better, stronger and smoother shifting than ever before. I've greatly simplified (maybe oversimplified) the explanation of how motorcycle transmissions work -- if you want to know more, do a little research, and you'll discover that there are many modifications and adjustments you can make to your transmission to get even more performance out of your motorcycle.
At the very least, learn enough so that when the mechanic tells you that you've got a problem with your dogs, you know enough not to get shafted.