Motorcycle Technology and Parts of a Motorcycle

[JohnBM01]

For the novice users or for those just wanting a technical chat on motorcycles, I've offered this thread to all of you. There are even some parts of a motorcycle I'm not sure about in my education and development of motorcycles. Part of my bringing of motorcycles to GTPlanet has to do with technologies as opposed to the bikes themselves. I'll start this thread off with two items of interest.


"All-Wheel Drive" Systems
All-Wheel Drive is new to me for motorcycles. Why called drive when you can't drive a motorcycle? My understanding of AWD for motorcycles is that you're able to equally deliver power to both wheels on a dirt bike, much like how AWD delivers equal power to all four wheels on a traditional 4WD or AWD car. How the system works and the benefits of AWD for motorcycles is still very foreign to me.

The Sprocket
I'm not sure about how sprockets work. My guess is that the sprocket (as well as the number of teeth on a sprocket) has to do with the transmission for a motorcycle. I'm probably wrong, but I've seen certain aftermarket sprockets in ads and stuff.



This thread is about certain technologies and parts of a motorcycle. I even mentioned what I learned longer swingarms do. Feel free to contribute with your questions and answers about parts of a motorcycle.

 

[wfooshee]

On your sprocket question, changing the sprocket sizes is like changing the final drive ratio on a car. A larger front sprocket or smaller rear will result in a taller ratio, less revs per distance travelled. A smaller front or larger rear sprocket gives a lower ratio, more revs per distance. The former might be used for fuel efficiency or to reduce harshness, the latter would be used to improve acceleration and off-the-line performance.

To me, the coolest bike tech out there is the front end. Nearly every bike there is carries the front wheel in a pair of telescopic forks. Forks allow the wheel to move up and down, and the assembly can be turned to allow steering. Simple, and elegant.

A couple of problems with forks are that they flex under load, and they don't work well under braking.

By their nature the forks are two separate spring and shock systems carrying a single front wheel. If they behave differently from each other then strange things can happen. One fork softer than the other will cause the axle to want to go out of horizontal relative to the bike. Side-to-side flex can take the wheel out of the bike's centerline. Fore-and aft flex that's different in each side will actually steer the wheel slightly. Anybody who's seen the bars wiggle if they ride hands-off has seen the effect of at least one of these conditions.

As for braking, look at what happens when you grab the front lever: The wheel applies a force against the bike's travel at the axle, but the axle is not connected to the bike itself. The only connection is at the steering head, maybe 2 or 3 feet above the axle, a pretty serious offset. As the axle tries to slow down, the motorcycle tries to overtake it, and the offset of steering head above axle causes a huge compression action at the front end, soaking up a lot of suspension travel that could be put to better use against bumps or whatever else the pavement presents. BMW has a couple of alternative front ends on their production bikes, and another system is in use on the Bimota TESI, and was also seen on the Yamaha GTS 1000.

The BMW telelever system puts a ball joint between the fork legs just above the wheel, and a swingarm from the frame is attached there. The swingarm carries the spring and shock, the same way the rear swingarm does, so the forks are simple tubes, not the actual damping and spring system. They just slide. The top has a nearly conventional steering head. What's gained by this system is a reduced offset in the braking force. The ball joint is much closer to the axle than the steering head, so the force applied by the motorcycle's mass over the front axle under braking is reduced: the swingarm carries the force into the frame.

BMW's Duolever system is similar, replacing the steering head with an upper arm, and the forks don't telescope at all. The strange-looking piece in front is how the steering is carried from the bars to the forks.

The most exotic front end, though, has no forks at all. The Yamaha GTS (a nice thread on the pashnit board here) and the Bimota TESI use a front swing arm which carries the front wheel on a steerable hub. The result is zero offset on the braking force; the swingarm is a direct route to the frame. Dive under braking is greatly reduced, leaving the full suspension free to do its real job: control the front wheel. The downside is a rather complex steering system. The handlebars come up from what would be a conventional steering head, and there is a system of rods and cranks to carry the steering inputs down to the hub. Each of those links will have some kind of bearing or bushing at each end, so the cumulative wear as the system ages might introduce a significant sloppiness in the steering feel. Also, unless the swing arm is unreasably wide, maximum steering angle is less than a conventional fork suspension. But show up somewhere on one of these, you will be the center of attention for a while!

 

[vr6gti72]

Nice write up:tup:

 

[wfooshee]

I wanted to do a post about shaft drive and rear suspension problems caused by shafts.

Your most basic shaft drive suspension is a simple swingarm with the shaft running inside one side of the swingarm. Why, it so happens that I ride such a bike, my FJR1300. Here's a bare chassis pic.

Let's revisit that BMW R1200 from my previous post in this thread, also a simple swingarm shaft drive:

Now, because the shaft is u-jointed at the front end only, it carries the torque reaction from the the rear drive gearset forward to the bike. Hard throttle tries to raise the front of the shaft as the shaft tries to pivot the opposite direction from the wheel. Here's an ultra-crude drawing:

The result of that reaction force is the rear of the bike tries to get jacked up by the drive shaft, causing an artificially (and worse, temporarily) stiff suspension. Of course, such a shaft layout is simple, easy to build, easy to maintain, and certainly cheaper than the designs I'm about to show.

To redirect that torque reaction you put a u-joint at the rear of the shaft as well, and carry the rear drive in a separate section from the shaft housing/swingarm. BMW calls their rear suspension the Paralever, and Kawasaki uses something similar on the new Concours 14.

Bare chassis Concours:

What all those links do at the rear end is separate the shaft from the torque load. Time for another crude drawing:

What happens now is the torque reaction is carried along the upper and lower links rather than by the drive shaft, and the direction of the force does not load or unload the suspension.

And by the way, there is a rumor going around, spread by the ill-informed, that because of all this torque reaction nonsense, shafties can't do wheelies. This is at best, not correct:

Not me (my bike is silver), and I've never taken mine that high. Just a couple of little 4- or 5-inch Superbike-type power wheelies when I get a good twist in second gear.

 

[Wolfe]

Well done, wfooshee. 👍

 

[wfooshee]

Transmissions. I saw in the beginner bike thread where someone referred to his worry about starting out and being able to get the thing to shift, and that got me thinking.

Crowd response: Oh, crap! Not that again!

Someone might be afraid of a motorcycle because they can't, or never bothered to, drive a manual shift car. The assumption is that because they don't know how to shift gears, they can't operate a motorcycle.

Nothing could be more incorrect. Motorcycles are different. Not just in number of wheels, lack of weather protection, no place for that 1800-watt stereo, and so on. The transmissions are completely different (sort of) from automotive transmissions.

Both kinds are called constant mesh. That means that all the gear pairs for all ratio are always engaged with each other. The 1st gear drive gear is meshed with the1st gear driven gear, the 2nd gear drive gear is meshed with the 2nd gear driven gear, etc. So how do you have all the gears meshed all the time without something coming out in pieces? Simply put, one of the gears just spins freely on its shaft. The drive gear is splned to the transmission's input shaft, while the driven gear spins freely. When all the gears are spinning freely, the transmission is in neutral.

The engage a ratio (shift into a gear) a wheel splined to the output shaft, but free to move back and forth on the shaft guided by a sliding fork, is pressed up against the driven gear of the set you're selecting. This is the point where motorcycle boxes differ from car boxes.

In a car box, the spline wheel is actually a ring large enough to go over some teeth on the hub of the gear. These teeth have the same spacing and shape as the splines, so the inside of the ring fits them the same way it fits the splines on the output shaft. When it slides of onto the gear hub splines, it locks that driven gear to the output shaft, and that gear is selected. If the ring and the gear are going different speeds, you get the grinding, crunching sound you all know from hearing bad shifts. The mechanism to prevent that is called a synchronizer. It's basically a cone-shaped friction surface on the ring that engages a similarly shaped surface on the gear hub, bringing the gear to speed before the splines engage. The driver may feel that as a momentary pause in the shift lever. Sometimes you have to be patient with the shifter, and engage the gear gently to avoid the crunchies.

In a motorcycle gearbox, the splined wheel that moves to engage a driven gear has large pins sticking out the side of it, called dogs. The dogs mate up with holes in the drive gear's hub. There are 4, maybe 5 dogs on the wheel, and a fairly large number holes, spaced correctly so the dogs don't have to go very far around the gear to engage. There are no splines, no synchronizer cones, no friction surfaces. The dog wheel moves over and engages with a snick, and you're in gear. Very smooth, very easy, very quick.

Of course the other main difference is that a car is usually shifted with an H-pattern lever which everyone is very familiar with, and the bike has a sequential shifter which scares people as being strange or different. The car lever's position in the H determines the gear that's selected. On the bike how many times the shift pedal has been clicked up or down determines the gear it's is in. Click it down as many times as it will go and you're in first gear, and it shifts up a gear for every click up from there,down a gear for every click down. There is a half-click space between 1st and 2nd for neutral, and on a bike, a light indicates that neutral is selected. While riding, engage the shifter firmly and positively. You can't be "gentle" and tentative, or you get excessive dog wear. You actually want it to engage sharply and quickly.

At this point I need to express very strongly that you NEVER EVER shift a motorcycle that is not moving, except to get 1st from neutral, and even that is only with the engine running. The reason is that when nothing in the box is turning, the dogs will not engage, yet the shift fork is moved fully into its position. If that is the case, something has to give, and you end up with bent shift forks, subsequently resulting in poor shifts, or actual failure to shift into certain gears. If you're test-riding a bike and you find one gear that engages harder than the others, or slips into a "false neutral" between gears, this might be the problem, and you want to avoid that bike. Tearing into a bike tranny is expensive because the transmission case and engine crankcase are usually the same casting, which means the engine has to come out of the bike for the repair.

Another difference is the clutch. Most cars have a single-plate dry clutch. Excessive slippage generates heat and wear, and leads to clutch failure. Motorcycles have a multi-plate wet clutch, meaning there are several clutch discs in a bath of engine oil. Multiple discs spreads the friction load, and the oil flow carries heat away. Part of riding a bike is slipping the clutch intentionally, which goes against everything you were ever taught when learning to drive a stick-shift car. Slow-speed maneuvers like turning around in a parking lot are performed at a steady engine speed using the clutch for bike speed, more slip to slow down, less slip to speed up. It's called the friction zone, and it's the first thing they teach you in a motorcycle course.

One other difference I want to mention concerns maintenance. You might notice that motorcycles have a rather short oil change interval. This might be partly because the engine is tougher on oil, higher RPMs, tighter tolerances, etc., but it's mostly because the oil goes more places on a bike. As mentioned already, it's used in the clutch, and the same oil circulated through the engine is also used in the transmission. Remember, the crankcase and the transmission are the same casting, same pan. Gear surfaces put stresses on oil that engine bearings can't even fantasize about, so change the oil often.

So back to the beginner. Shifting a bike is easy. Easier than a car. The clutch is easier, too. You might stall the bike, but you're not ever gonna see the bucking, stalling, jumping kind of thing so many folks do when learning in a car.

Take a motorcycle course. Why poeple spend thousands on a bike and cringe at 2 or 3 hundred bucks which will increase their skill and confidence in a single weekend, and maybe save their life, is beyond me.

 

[homeforsummer]

So back to the beginner. Shifting a bike is easy. Easier than a car.

Depends what you're used to. Having driven cars for five years, when I hopped on a motorcycle recently I found it difficult to get to grips with, despite me knowing what everything did already. That said, it didn't help the clutch and gearbox were past their best...