| Author | Message | ||
     Dynarider |
If you are interested, the equation for "I" for a disk is simply 1/2mr2 where m is the mass and r the radius. For an annular disk we just subtract the missing inner portion so we get 1/2m(ro2-ri2) Where "o" and "i" subscripts indicate outer and inner radii, respectively. ================================================== I hated algebra & other math related courses in school over 23 yrs ago. I hate them on the net even more. You are making my head hurt Blake  | ||
| Blake |
Rick "Finally someone puts that info up there short, simply, and succinctly. Thanks Steve." Hey, the man is a pro! But not a single formula or diagram. Hmmph!  Jprovo, The XB9 does not have what would be considered a low CG. I think you are confusing "mass centralization" with "low CG." One means that most of the mass of the bike is closely bundled; the other means most of the mass is close to the ground. Why bundle the mass closely? Because angular inertia is the enemy of quick/precise handling and inertia, as posted above is proportional to mass times radius squared; the radius in this case being the distance from the mass center of the bike/rider to each component. Sum them all up about each of three primary coordinate axes (X, Y, and Z) then add the individual "I" of each component alone ( as if rotating about its own centroid) and you have the total inertia of the entire system, bike and rider. Dyna, You might try increasing tire pressure, especially if it is at the low end of the specified range. Higher tire pressure can help mitigate a bike's tendency to weave. Basically you are in effect stiffening the suspension. | ||
| Peter |
Don't stop. It's doing my head in trying to follow it, but I'm enjoying learning. Pity about the ego push and shove content in there though..... be better if that was gone. PPiA | ||
| Blake |
Another moto-gyro illustration just popped into my head. Wanna hear it? Here it is.  Ever see the arena cross guys cross it up as they are 30 feet in the air flying past the checkered flag? They turn the front wheel full to right side lock and the bike, in mid air flops over onto its left side. That is a perfect example of gyroscopic precession of the front wheel acting to roll the bike over onto its side. So, does this mean that precession can significantly affect the aggressive turning/leaning of a sport bike? You might think so, but alas no. Why? 1. Mass and inertia of the 250cc motocross bike are much much less than a sport bike. In other words a motocross bike has little resistance to rolling/leaning. 2. The rider being off the bike in mid air further reduces resistance to rolling/leaning. 3. The significantly larger diameter motocross front wheel has more gyroscopic strength than a smaller diameter sportbike wheel. 4. To amplify the effect, the rider pushes himself away up and to the right away from the bike, thus helping it to rotate. 5. Even with all the above noted advantages, the motocross rider must still turn the handlebars an extreme amount, more than 45 degrees, to achieve a noticeable roll of the bike in mid air. If he were to steer just a few degrees or even five degrees, as in steering a sportbike aggressively at high speed, the result would be virtually imperceptible. Greg, do you agree? Steve? ![:]](http://www.badweatherbikers.com/buell/clipart/proud.gif) | ||
| Court |
>>>>Don't stop. It's doing my head in trying to follow it, but I'm enjoying learning. Pity about the ego push and shove content in there though..... be better if that was gone. Make that two of us....I've got an entire library of math and MENSA stuff and learn more here from a reasonable discussion including the likes of Steve A and Blake than I could in most books. This discussion has also sent me to Amazon.com at least 4 times. I find myself growing more in awe of the "human component". The science, the hard facts and numbers, are fascinating but what is incredible is how many of us have assimilated the ability to feel/recognize/act intuitively on these things. In a way, I hope with the talent assembled here that it evolves into an "anhedreal & dihedral" converstaion that explains the dynamic stability of the Harrier..but, hey...that's just me. Thanks for the intereesting discussion. Court | ||
| Reepicheep |
No kidding! Isn't there some sort of "affiliate program" with Amazon where the linking site gets a little kickback for any sales resulting from a sale? Blake... you oughta set that up. SteveA's chapters (along with the rest of the book) should show up at my door monday Yay! Don't know when my first roadracing world magazine will show up though... better make sure my wife did not intercept the bill | ||
| Rick_A |
Well Blake...I read Steve's and I'm like...yeah, sure. I read yours and it's like bla bla bla...I just don't wanna get down into the math part, see...it's too simple without it!  Blake, I think Jprovo was just comparing the XB with the tubers. In that case it's true...and certainly with a full tank of gas, no? Sitting atop a tube frame Buell over the fuel frame it's readily apparent that the fuel framers carry their weight lower. Even the steering heads and upper frame sections are lower. You're "in" an older Buell and atop the new jobbers. Just put the two side by side. | ||
| Blake |
Rick, Oh yeah? Well, bla, bla-bla, bla-bla-bla-bla! Or in the famous words of Charlie Brown's teacher, "Wah, wah-wah, wah-wah-wah-wah." | ||
| Aesquire |
Court, The Harrier anhedral is easily explained, swept wings increase apparent dihedral so to avoid excessive roll stabilty and dutch roll effects they built the wing with "droop" (1 piece carbon fiber on the av8b) to compensate. They do the same thing on Hang Gliders to keep near neutral stability. Roll control on a flex wing uses weight shift to create wing warp (aileron effect). Uh... no gyro's here, so I suspect I'm off topic. lol | ||
| Aesquire |
Had a wicked tank slapper on a Suzuki GT380 on Rt 490. I had a cooler full of ice & beer on tail rack & the bike "walked" across 5 lanes of rush hour traffic, it quit by itself (whee!!) just before I slapped a retaining wall. I was accelerating through the big "s" turn just south of downtown when the bars nearly ripped out of my hands. Sounds like "weave", crappy forks/shocks & rear cg, f & r tires moving with aprox 2 hz freq. (didn't measure, was busy) (and very lucky) It left an interesting scuff pattern on the road. | ||
| Jima4media |
Steve, I heard a rumor that Honda is going to drop the V-5 MotoGP motor this year in favor of a V-6. Have you heard anything like this? And also what do you think of Tony Foale's latest book on Motorcycle Design? Is Cycle World going to do a review of it? Jim X-2.5 | ||
| Jima4media |
Blake, You said... "The XB9 does not have what would be considered a low CG." Compared to what? A V-Rod? A sidehack? Jim | ||
| Pilot |
"It left an interesting scuff pattern on the road" and I would bet interesting paterns between your butt cheeks and seat surface. | ||
| S320002 |
Blake, Blake, Blake you really disappoint me. Quote: "Please explain to me, however, as you asserted earlier, the interaction that exists between linear and angular momentum. Never mind, I'll save you the trouble. There is none. The two are completely in inexorably separate, 100% independent of one another." ------------ Remember Sir Isaac Newton? You obviously don't agree with him. Suppose you are standing in the path of a motorcycle traveling at 60 mph (lots of linear momentum). If the rider doesn't change the angular momentum, and thus the linear momentum, of the rolling tires by applying the brakes, it might change your mind. By the way. Did you read all of Stave_A's post? Sure this subject is simple, but only if you ignore enough of the facts. Still no equations. Just logical explanations. Greg;-) | ||
| Jprovo |
Blake, I wasn't confused at all. To quote the Buell 2003 sales brochure (not the greatedt souece of engineering, I know): "The principle of mass centralization dictates that the bike's weight be as low and as close to the center as possible" If the XB's are ideed built to Buell's principle of mass centralization, the XB series should have both a low cg and a relatively (compared to other motorcycles) small mass moment of inertia (I). "The XB9 does not have what would be considered a low CG." What published data are you getting that from? James | ||
| Blake |
Jim, Yeah, the XB9's center of mass is definitely higher compared to a typical cruiser. Greg, I see your point. You are confusing the interaction of torque, traction, thrust, and speed with momentum. For example, clamp the brakes and initiate a skid on an icy slippery road. Angular momentum is instantly zero, while linear momentum remains virtually unaffected. Or run the bike on a dynamometer. Angular momentum goes from zero to maximum for the rear wheel and engine spinning at top speed then back down again; linear momentum remains zero. I do see your point, though. | ||
| Blake |
James, I'd guess that the "close to the center" part is accurate, with "center" meaning center of mass. Keep in mind that the total "system" includes a rider of significant mass who is perched atop the seat. A too low center of mass on a sport bike with the rider perched up high makes for a bigger total inertia about the roll axis, which is not a good thing. Lower than the tube frame models? I wouldn't argue against that. | ||
| S320002 |
Blake, ---------- Quote: "Please explain to me, however, as you asserted earlier, the interaction that exists between linear and angular momentum. Never mind, I'll save you the trouble. There is none. The two are completely in inexorably separate, 100% independent of oneanother." Next Quote (partial): "I see your point. You are confusing..." I am not in the least confused. I will try try to make this as simple as I can so that YOU do not become any more confused than you are. Also to avoid confusing the issue we will define rolling as; a tire (mass) rotating around its axle (axis) along a flat surface in a straight line without sliding. 1. If a tire (mass) only spins about an axis, it only has angular momentum. 2. If a tire (mass) only moves in a straight line without spinning, it only has linear momentum. 3. If a tire (mass) rolls in a straight line. Because it is spinning AND moving in a straight line, it has both linear momentum (traveling in a straight line) and angular momentum (rotating about its axis). Any questions? Still no equations.;-) Greg | ||
| Jprovo |
Blake, What about the “low” part, is Buell trying to mislead me as to what their principle of mass centralization dictates? (I doubt it) I am well aware that the rider is part of the system. If you compare the following seat heights for motorcycles that I consider being in the same class as the XB9R (sporty streetbikes and sportbikes), you will see that the seat height of the XB9r is advertised as being the lowest. Therefore, rider position does not support your global statement: “The XB9 does not have what would be considered a low CG." Seat Heights in inches, 2003 Model year motorcycles, from the manufacturers websites/brochures BMW R1100S, 31.5 Buell XB9R, 30.5 Ducati 749, 30.7 Ducati 749S, 30.7 Honda CBR600F4i, 31.7 Honda CBR600RR, 32.3 Kawasaki ZZR600, 30.7 Kawasaki ZX-6R, 32.5 Kawasaki ZX-6RR, 32.5 Suzuki GSX-R600, 32.7 Suzuki Katana 600, 30.9 Suzuki Bandit 600, 31.1 Suzuki SV650S, 31.7 Suzuki SV1000S, 31.9 Triumph Daytona 955i, 32.1 Triumph TT600, 31.9 Triumph Sprint RS, 31.7 Average of 17 Bikes = 31.59, Buell XB9R = 30.5 I would contend that the Buell XB9R does have a low CG compared to bikes it it’s class. This is primarily due to the application of Buell’s Principle of Mass Centralization: “Keeping as much weight as LOW and centered as possible” (my emphasis) James | ||
| Dynarider |
You can have a low seat height yet still have a higher cog if more weight is carried higher in the frame. I would think a BMW with its engine arrangement could possibly have a lower cog simply because the heads, pistons, etc are placed much lower in the frame than is possible on a Buell. They may not have optimal mass centralization, but they could have a lower cog even tho the rider sits up a tad bit higher. | ||
| Blake |
Greg, Yes, I have some questions. I agree that in specific instances there exists a "relationship" between the linear and angular momentum of a motorcycle. Is that different than saying in general that there exists an "interaction between linear and angular momentum"? Because that is what I understood you to say. I'll leave that for you to decide. To aid your decision here is a definition of "interaction" as it applies to physics... in·ter·ac·tion noun Physics. Any of four fundamental ways in which elementary particles and bodies can influence each other, classified as strong, weak, electromagnetic, and gravitational. re·la·tion·ship noun The condition or fact of being related; connection or association. You say that a moving and rotating wheel has both linear and angular momentum. No argument here. They can certainly coexist within the same body. Isn't the concept of "interaction", which is what you initially asserted, quite different from the concept of "coexistence"? You say "this subject is simple, but only if you ignore enough of the facts." I would say... Neglecting unrelated, inconsequential, and insignificant factors, the primary mechanisms governing the steering of a motorcycle are simple. Here's a question you keep dodging... Please explain how the complexity of any system could render moot the laws of physics, specifically... Though stubbor as I am, it is becoming clear to me that in the realm of confrontational rhetoric, where you obviously wish to sequester this debate, the continuation of this discussion may lack merit. If you don't care to contribute any valid evidence or science to supoprt your point, then I'd rather just leave you to disagree with me, the laws of physics, the experiences of roadracers everywhere, documented results of empirical scientific experimentation/testing, the results of computer generated engineering simulations, and of course acclaimed credible experts such as Kevin Cameron. I give up.  James, You may be right. I did not know that the XB9's seat height was significantly lower than other sportbikes. Cool! Dyna has a point with the boxer motor thing, even the Duc with it's low front cylinder might fit that same theory. | ||
| Dynarider |
Stolen from Ducati.com.... But how can handling and stability co-exist? The modifications described above were obviously "calibrated" in order to enhance the benefits without encountering the natural contraindications. For handling purposes, the potentially negative effects due to the increase in the wheelbase are widely compensated for, and exceeded, by lowering the center of gravity. In addition, getting the rider closer to the center of gravity also reduces the moment of total inertia of the "rider-bike" assembly with respect to the roll axis, making the variations of the bike's tilt faster and making it possible to take curves in rapid succession at high speeds and with minimum effort. | ||
| Dynarider |
This was written with some help from Aprilias engineers... The process of making a countersteered right turn can be broken into five somewhat arbitrarily divided steps: 1. You initiate the turn by applying a torque to the handlebars, steering the front wheel to the left. 2. The wheel steers to the left. The rate at which the steer-ing angle increases is set primarily by the moment of inertia I, of the wheel, fork, and handlebars around the steering axis, and by the "trail" (described later.) 3. As the bike is now turning to the left, a centrifugal torque leans both you and the bike frame to the right. Gyroscopic action also leans the bike to the right, but, as I will show later, its effect is negligible. 4. Transmitted by the fork, the increasing lean attempts to lean the front wheel over as well. For the first time, gyroscopic action becomes important, as the wheel responds to this "leaning" torque by attempting to steer to the right, thus counteracting the steering torque. The steering angle stops increasing. 5. The leaning torque overcomes the steering torque and the wheel steering angle decreases. Note that the lean continues to increase because the bike is still turning left. 6. As the bike has now acquired substantial leaning velocity, the lean increase cannot end instantly. Driven by the still increasing lean, the wheel steering angle passes smoothly through zero and then points right. The centrifugal torques reverse direction, eventually halting the lean increase and balancing the gravitational torques. As no more leaning torque is applied to the wheel, the steering angle stabilizes, and the bike executes the desired right turn. Interestingly, gyroscopic action's effect on bike steering is generally indirect. A bike is two gyros liked through the steering axis. Applying torque to the front gyroscope produces a pitch (lean) at right angles. The bike continues to lean until a new equilibrium position is found. That position is determined by centrifugal force acting in one direction, and the yaw of the bike (induced by torque input to both the front and rear gyros by the pitch (lean) torque input) around the radius of curvature of the turn. The engine in racing (where RPMs are high) provides most of the direct gyroscopic effect, which is the reason that Yamaha and Suzuki have produced their GP bikes with two counterrotating crankshafts. | ||
| Blake |
Where did you find that. I owe you a beer. | ||
| Dynarider |
Found it here. It was written in collaboration with aprilia engineers & I would definately think they know what the hell they are talking about.http://ihome.ust.hk/~westland/new_page_3.htm | ||
| Dynarider |
Truly is amazing what info you can find on the web. I love it. | ||
| Dynarider |
This is on there as well. Decribes what shifting does to the cog, etc. Interesting.  Taken from http://www.motorbikeracing.com/ and shrunk to maintain page readability. | ||
| Jima4media |
Dyna, Thanks for that reference page. Other than the fact that it should be centripetal force instead of centrifugal force, they don't cover camber thrust at all, they only talk about trail and not rake angle, and there is a spelling error - the wheels are LINKED and not Liked, it covers almost all the bases. I'm sure there is more content in the actual book. Jim | ||
| S320002 |
Blake, "Here's a question you keep dodging... Please explain how the complexity of any system could render moot the laws of physics..." I have yet to say anything in an effort to "render moot" the laws of physics. You however, have on occasion made statements that, if true, would indeed render moot the laws of physics. Your attempt to deny the coexistence of angular and linear momentum in a rolling wheel is only the latest. "I would say... Neglecting unrelated, inconsequential, and insignificant factors, the primary mechanisms governing the steering of a motorcycle are simple." You asked for Steve_A's help. Then you ignore it when he says a german PHD could not adequately describe this subject with 50 non-linear equations. You also seem to have ignored his statement that some long distance wheelie riders use electric motors and gyroscopic precession to steer their bikes. "I agree that in specific instances there exists a "relationship" between the linear and angular momentum of a motorcycle. Is that different than saying in general that there exists an "interaction between linear and angular momentum"? Because that is what I understood you to say. I'll leave that for you to decide." C'mon Blake, you're not really that stupid. This discussion has always been as it applies to motorcycles. "...I'd rather just leave you to disagree with me, the laws of physics..." I have never disagreed with the laws of physics and I don't always disagree with you, only when I think you're wrong. If you keep to the point and leave out the insults this discussion will more useful. If you want to give up that's up to you. Greg | ||
| Blake |
Jim, Actually the diagram is correct in labeling the apparent outward force the "centrifugal" force. The "centripetal" force acts opposite of the centrifugal force and is the real force that causes a vehicle to turn. It would be drawn pointing to the right and acting on the contact patch of the tires. Using the imaginary centrifugal force allows us to explain what in reality is a dynamic scenario as though it were a simple static equilibrium problem. If something is in static equilibrium, it isn't going to turn. But since we are just trying to explain how a bike keeps from falling over, the static free body diagram, as a snapshot in time, is valuable. What really happens is that there is no "real" centrifugal force acting on the bike/rider, so to maintain equilibrium the bike, in the case of the above diagrams, steers/accelerates laterally to the left. |
