oldfrt

It's a good thing there are people "racer" and others who are willing to give something back to the sport. It makes it better for the rest of us. Actually, anyone participating in these forum threads is giving something back by sharing their experience and knowledge.

I had a R1200ST and traded on the ZX. The ZX is basically more exciting than the R12ST, handles almost as easily, tracks like on rails, has good tractable power at lower rpm and gut wrenching power above about 5000. Its really fun and not hard to ride. I'm faster in the corners on the 675 but the ZX is still a lot of fun. Its reasonably comfortable and smooth just riding the slab. It's loafing along at 80 mph at 4000 rpm in 6th. Redline is 11,000. It's not a boring bike. I do also use it like a SUV for daily chores and commuting. Joe Rocket soft saddlebags will carry a bunch of groceries.

Other than some dirt bike riding in the early 70's I just started back on the street with a SV650 in 99. Now I have a Daytona 675, KLR650, and just recently a ZX14. I find that the more I ride, the more I get out of it and also realize how unskilled and ignorant I am. My riding goal is to improve my skill and knowledge and so far I am really enjoying doing that. I love riding the back roads in the mountains and the local twisties. If I lived in an urban area I probably wouldn't ride. Well maybe, but the fun factor would be far less.

It's hard to describe in words what is so simple if a sketch could be drawn.

I didn't say a line from the contact patch to the c.g. , I said that the resultant of the additional forces at the contact patch would pass under the swingarm pivot. The additional forces at the contact patch are the balancing moment couple force acting vertical, and the horizontal friction force driving the bike and overcoming the inertia of the bike. The balancing moment couple force is required to balance the overturning (think wheelie) moment required to keep the bike stable. The overturning moment is the accelleration force acting at the c.g. times the height of the c.g. to the ground. The initial weight component force at the rear contact patch doesn't need to be considered because it is already balanced by the suspension, so we only need to consider the additional forces. The driving force for the bike is the horizontal force at the contact patch produced by the motor and drive chain. Otherwise the bike would only spin the rear wheel. All of the forces making the bike move forward are transmitted to the ground through the contact patch.

I shouldn't have referred to gravity, accelleration, and wheel reactions as external forces. I was only trying to simplify the problem. Here are the forces. The bike is accellerating forward and is resisted/balanced by the horzontal traction force at the rear wheel. The bike has weight which is resisted by the upward pressure on the wheel and these forces are vertical and pass through the axles. The forward accelleration force is through the c.g. of the bike and creates an overturning moment which is balanced by an additional load vertically downard at the rear wheel and upward at the front wheel. The rear wheel traction force has to get from the rear wheel/swingarm/suspension to the c.g. of the bike allowing that it is also part of the mass of the bike. All I am saying is that the resultant of the additional upward component due to balancing the overturning moment and the horizontal traction force balancing the accelleration at the rear wheel passes below the swingarm pivot and causes the rear to rise. I am discounting the rotational inertia of the rear wheel which I say is not significant to the discussion in my opinion. Visualize the bike rolling backward and applying the rear brake. This creates the same forces and may make it easier to visualize what I am trying to convey.

I was trying to clarify and simplify the problem by only looking at the external forces on the bike and those initiated by the bike. Getting into the sproket, swing arm, and chain are "internal" , or component, forces and can be ignored if one is only trying to balance the external forces. The external forces are the weight of the bike acting vertically and momentum or inertia acting horizontally, and the forces resisting the inertia and weight which is the upward vertical force at each wheel passing through the axles and the tractive force at the rear wheel which is horizontal. My point is that the horizontal traction at the rear wheel is a force that passes under the swing arm pivot, and is therefore trying to rotate the swing arm downward. I don't think torsional inertia of the wheels and rotating mass is a large consideration in our discussion. I've given some thought to the quick turn and countersteering and as I've said before, it's my belief that countersteering causes a turn to be initiated because it makes the wheel LEAN not turn in the desired direction. I think the small degree of turn in the handlebar actually follows the lean. If you think about a turn where the bike is leaned over hard, there is only a small degree of actual turn in the handlebar, the bike is actually tracking as if it were inside of a conical structure. The people that ride in the spherical cages are actually not "turning" at all when they get to the point of doing vertical loops or going almost horizontal around the circumference. As for the quick turn, there is no limit to how fast the turn can be initiated except how quickly the bike can be leaned to a stable turning lean where the resultant of the gravity and the centrifugal accelleration passes through the c.g. of the bike and thru a line between the front and rear contact patches. Lucky for us, countersteering actually is an aid in leaning the bike. I've been away from the forum for a couple of days myself and probably will be off for another couple. I hope we are in agreement on these issues. If not, I'm sure you will tell me. It's an interesting and complex subject. I went to the Level 1 course back in May and learned a lot. I try to practice those lessons each time I ride now, and have become noticeably faster (to myself), but more importantly, more comfortable and competant in my riding. That translates also to safer, I hope.

My static test was not a good example because there is no difference from it to the engine force pushing against the wall. Instead of giving up I'm going for one more shot. Frequently in engineering we isolate the forces we want to analyse and ignore the others. Lets just look at the forces we add if we push back on the bike from the wall. The horizontal wall force is resisted by an equal and opposite force on the rear wheel acting at the surface of the pavement. Just taking this force acting parallel to the surface of pavement at the contact patch, it appears obvious that this force is tending to rotate the swing arm downard. This will happen regardless of angle of the chain to the swingarm in my opinion. OK, now I give up. PS I left out another force required to balance the system, that is an additional force upward on the rear wheel and downward on the front wheel to balance the overturning moments caused by the horizontal forces. I don't think this affects the gist of the discussion and only adds more confusing complications. Therefore ignore it.

I give up

I don't disagree with the chain force tending to rotate the swingarm. It may be more of a factor in moving the swingarm than the external forces I referred to. A detailed analysis could be made if the geometry, weights, and forces were known. If you say that the horizontal force at the rear wheel contact patch is irrelevent to the discussion, then I don't agree with that. In fact a simple static test could be done quite easily. Put the bike in gear (motor off) and push the front wheel toward the rear allowing the rear wheel to resist the movement. The pushing should be by a flat vertical surface pushing horizontally rearward. No front brake, obviously.

Racer, I agree. As I stated earlier, I am an engineer (structural) and analysing force systems is something we do routinely. It really isn't necessary to bring out all the details of the forces in the chain, sprocket, swing arm, etc. If you just look at the forces acting externally on the bike (wheels) then you can visualize how the bike will respond. If the bike is stationary and pushing on a wall the front wheel has the bike weight component acting upward vertically thru the center of the axle and the wall is pushing backward horizontall also through the center of the front axle. The extra horzontal force compresses the forks and the front squats. The rear wheel also has the weight component pushing vertically up thru the center of the rear axle, but now has an additional horizontal force at the contact patch which is equal to the force pushing on the wall at the front. The horizontal force at the rear contact patch tends to rotate the swing arm downward causing the rear of the bike to rise. If the bike is free to accellerate forward (not against a wall), the only difference will be that the front will rise, instead of squat, because there is no horizontal resisting force on the front wheel, and there is the overturning moment caused by the inertia of the bike thru the center of gravity of the bike acting rearward. This overturning moment unloads the front wheel and the front rises as the forks expand. The rear rises also because the forces acting on the rear of the bike are similar to the situation when the front is pushing the wall.

Some more of my thoughts on the above issues. The rear rising during accelleration is due to frame (swingarm) geometry. Although there is weight transfer to the rear, it is more than offset by the torsional bending tending to rotate the swingarm downward. Force causing the bending is the horizontal traction force at the rear wheel. The quick turn is (in my humble and often screwed up) opinion, initiated by gyroscopic precession. When force is applied to turn the wheel left the precession force causes it to respond by immediately leaning to the right and this is what initiates the turn to the right. Also note that there is physically no problem with going from a straight line to a turn instantaneously. The limitation is in the ability to quickly lean the bike. Hard countersteering accomplishes this. (I think that's pretty much what racer said above)

And if you push the front tire against the wall with the engine running and start to let out the clutch, does the rear end squat? That's the easiest way to start to understand the effect of engine power on the rear during acceleration. (And NO, it does not squat.) There are multiple forces that you have at your disposal to use to your advantage. You can argue the physics until you are possibly convinced on how to do it completely wrong, or you can start with some basic theories and ideas and see how they work when you are actually riding a motorcycle. Theory without application is worthless, but a lot of fun in the off season. Dang. I was wrong in that the front doesn't rise because the wall is compressing the forks due to the trail angle. This may give the impression that the rear is rising and the front squatting, but the weight should transfer toward the rear wheel. I now also see that the rear wheel traction has the effect of unloading the rear shock and spring which may totally counteract the loading of the rear shock from weight shift to the rear. I was overlooking the suspension geometry. So I concede, you're right.

I'm an engineer, not a racer, but the rear squatting thing under accelleration is not so difficult to understand and really not complicated. Acceleration merely transfers weight from the front to the rear. A lot of acceleration produces a wheelie and then you have 100% transfer. The weight transfer compresses the rear suspension resulting in squat at the rear and rise at the front. The opposite happens when you decellerate using the front brake. The front dives and the rear rises. 100% transfer results in a stopie. If you push the front tire against a wall the rear should squat and the front rise also.