| Author | Message | ||
     Blake |
Brought from the general discussion board. To good to lose.  | ||
| Road_Thing |
If all the forces sum to zero, why doesn't it fall over? | ||
| S320002 |
Road_Thing, When the forces sum zero, call it neutral if you like, the bike doesn't "fall" up or down. Think of it as a helium balloon floating at a fixed point between floor and ceiling. The force pulling it down (gravity) is equalized by the forces holding it up. (The helium doesn't really hold it up, but that's a different story.) Greg | ||
| S320002 |
Blake, Before you chastise me. Your vector diagram is correct. It is the free body diagram that should have a resultant of zero. That is also corect. Please excuse my rustiness. Must be my old timers desease. Road_Thing The post above is still correct when applied to a free body diagram. Greg | ||
| Steveb |
Hey all, This has been going on so long I forgot where all it has went. But *precession* operates 90% ahead of the input force. With a one finger test on a bar end while traveling at speed, or with a spinning bicycle wheel held on the fore fingers, very little if any motion is necessary to cause precession. It is a small force vector of little motion that, through the rotating mass causes a powerful effect. At 60 with your hands off the bars and only one finger pressure straight forward, you can achieve quite a turn, but the displacement of the wheel left or right is very small. If you haven't done this, you really can't appreciate the power of just one finger. Paul,(snail) had so much fun with this on SacBorg, I'm really lost on this thread, however, if you haven't given it an honest test, I don't know how anyone could give an opinion. Blake, am I even warm here? I repaired gyrocompasses in the Navy even if they didn't teach me how to spell it. Quite a feat that a Gyro remains on North and flat to the earth's surface while it navigates the globe. Steve B | ||
| Blake |
Goofs in the free body diagram (FBD)... Dang! JB found another one. Good call JB! The two I noticed as I posted the FBD are... 1. The cg, (red dot), is shown located along the bike's centerline. Since the rider is hanging off, it should be shifted to the bike's right (down and left, towards 7 o'clock on the page). 2. The front wheel is shown aimed straight ahead; it should be turned in with the curve a little bit. But JB, It's not my goof that the rider is too stupid to look through the curve!  Good call JB! You are too observant. Greg: That's an interesting point about track grip being more than a typical coefficient of friction times weight. I guess you'd call it an effective rolling coefficient of friction? I'm sure, like most mathematical models of the real world, the scenario could fill multiple disertations. Track temperature, track roughness, track material, humidity, tire temperature, tire pressure, tire profile, tire wear, speed, lean angle, total weight... all variables that would influence the actual grip. Still, there will be a coefficient that when multiplied by the weight of the bike will yield maximum grip for a specific condition, right? That would be the applicable coefficient of friction. The mechanisms leading to that coefficient are imaterial to the validity of the calculation of grip. Oh, and since it took you soooo long to recognize that my vector addition illustration was correct... NYAH, NYAH, NYAH-NYAH NYAH! Consider yourself chastised. MikeJ: As you say, the tire does indeed exert forces on the track surface. Likewise, the track exerts opposing forces on the tire. Try pushing your right index finger into your left palm (hold your palm stationaty, just resisting the force exerted by your finger). Which one is pushing on which? They both both push on each other, right? The definition of a free body diagram (FBD) is to show a discreet body (in our case the bike and rider together) and all the forces acting on it. Since we are not concerned with the behavior of the track, we can ignore the forces acting on it. Mechanical/Structural Engineers would be utterly useless without their FBD's. Road Thing: Think of it this way... If there is zero net force and zero net moment acting on a discreet system (bike and rider), what is there to make it change its orientation? Like Newton used to say... Absent the influence of any net external forces acting on them a body at rest will remain at rest, and a body in motion will remain in motion. Steve: Precession of a motorcycle's front wheel makes the handlebars progressively more difficult to turn as speed increases. The amount of moment (torque) translated by precession into leaning the bike is equal to the moment (torque) that the rider applies through the handlebars about the steering head. At high speeds (70 mph+) you are in effect trying to roll the bike by pushing on the handlebars. The gyroscopic action (angular momentum) of the front wheel simply translates your applied torque to the handlebars into a moment that tries to roll the bike and rider. To get an idea of how much effect this has, imagine your motorcycle with a full weight/size dummy effigy of you on it skewered horizontally lengthwise through it's cg (along the centerline}. Now imagine the skewer is a big axle with roller bearings supporting it on each end, one in front and one in the rear and a laterally (left/right) oriented 25" long bar welded to the rear end of the skewer/axle. Imagine how difficult it would be by cranking on the bar to get the bike/rider rotated 60 degrees from vertical on that axle at a rate that would equal the rate of lean when entering a hard curve. That's close to 700 LBs of bike/rider you are trying to rotate within a fraction of a second. It can give you a hernia, trust me. Gravity and centrifugal force are a LOT stronger than we are. Imagine how difficult it would be to turn the same skewered bike/rider with little superbike clipons. The smaller moment arm (width) of the clipon reduces the amount of torque you can apply since you can only push so hard. If you are real strong and traveling at high speeds you can take advantage of the front wheel's gyroscopic action to help get the bike turned. Otherwise, it's mcuh more of a hinderance to being able to turn-in quickly. Zat make ANY sense? The rest of y'all, sheesh, everyone's an art critic here eh? I happen to SUCK as an artist, so please forgive the minimalist stick figure. Blake (maybeanothersketchisinorder?) | ||
| José_Quiñones |
All this is interesting, but the only thing I know is that when I push the right grip, I go right, but when I PULL on the left grip, I go right even faster! It's pretty fast, so be careful. | ||
| Werewulf |
i know this will hit a nerve with a lot of long term riders, sort of like like the earth is flat theory. i took the advanced msf course etc. i fully realize that countersteering is a controversial subject and ive read about it for hours. sort of like the bike school that has the demo on a bike that you cant steer by leaning. i used to dread going thru deals gap on my fatboy, because of the fireworks display and the chuckles from the squids on the rice bikes as i ground off my floorboards. i experimented with my daily trek thru the turns. i found that if i didnt lean in the curves and just countersteered the bike, i could go thru the gap with no dragging. my body stayed vertical and the seat rotated under my butt. my eyes were always level with the horizon. later when i bought the buell i used the same technique and it worked like a charm. it also lets me brake in the curves without losing control. i can go thu the course with the rest of them and have recieved some interesting remarks about my unusual riding style. all i can say is that it works for me and i chalked up 30,000 mountain miles last year with it this year.! im not saying that i would work for anyone else. | ||
| Bullitt |
Hmmm - I thought it worked just the opposite way -that you hang off the bike (thus moving the cg outward) so the bike itself doesn't have to lean over as far. Sitting bolt upright and countersteering only would require the bike itself to lean more, and start dragging the hard parts earlier. Or am I in left field on this? | ||
| José_Quiñones |
It sounds like you're dirt tracking it through the turns.  | ||
| Werewulf |
i know that for someone who has been riding the conventional way for a long time, this is blasphemy! i originally got the idea from a book that i read on the web. i wish i could recall where i found it, but the author was deemed the "antichrist" of the motorcycle world. i thought i would try it and it worked. the proof was when i went thru the turns at the gap, same speed, same curve and the fatboy wouldnt drag. the guys that i ride with have argued that it cant work, contradicts physics and is just plain unamerican, but when we ride, they admit that my bike has no problem staying with theirs. im not trying to change anyones ideas on proper riding technique, but theres more than one way to skin a cat. and as they say" it worked for me". | ||
| Buellistic |
WEREWOLF: Kenny Roberts went to Europe with this ability to ride this way and showed them how to ROAD RACE!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!! In buelling BUELLISTIC and/or Hardley-Harley | ||
| Blake |
WWulf: I usually don't hang off much myself. But you can rest assured that hanging off, REALLY hanging off, will significantly increase your corner speed and prevent scrapage. If you are seeing the opposite, something else is entering the equation. It is important to commence hanging off BEFORE entering the corner. Me, I usually leave my butt planted but lean forward and in when approaching a curve, just before pushing on the bars to get the bike leaned over. Leaning also seems to direct my visual focus more towards the inside of the curve, so I also gain more benefit from the natural tendency to go in the direction I am looking. It just makes agressive cornering a LOT easier for me. | ||
| S320002 |
Blake ??? Without accounting for all the mechanisms that effect traction, partial calculations would yeild invalid results. One very important variable you did not mention is speed. Because rubber is a "viscouis solid" it does not imeadiatley conform to the irregularies of pavement. Thus a high speeds it tends to "float" more on the higher spots. In addition you must to account for the dynamic coefficient of friction which is always less than the static coeffcient of friction. If traction calculations were this simple a tire engineers raceday might go something like this: 1. Throw down patchs of tire compound on the race suface. 2. Place the appropriate weight on them. 3. Measure the force required to get the patches to slide (coefficiant of friction). 4. Declare "This'ns got the highest CF, put 'm on and race 'm." Perhaps we should use the term coefficiant of traction? "There is a time in the life of some projects when it becomes neccseary to shoot the engineer and get on with it." Fotunately none of my bosses ever felt it was neccesary. Greg | ||
| Steveb |
Werewulf, I find your experience very interesting. Peg weighting and tire angle to the ground can produce some unexpected results. Pros use peg weighting on tracks, but dirt riders must use it for increased corner traction or while on a side hill. On a slippery side hill, weight the outside peg, and perhaps shift your weight as far back as possible for rear wheel traction. I believe Keith Code recommends weighting the outside peg and pushing through that point through the body to the opposite shoulder, then the arm to the inside bar to initiate a turn. Blake, A motorcycle is the closest thing to magic that I can imagine. The shorter bars on a road racer also (I'm so sure it's an accident ) put your body forward so that you have your upper body weight transfered to the bars and you can selectively put more weight (push fwd) on one bar or the other, especially if you use the outside peg as pressure point. All, One thing I think we can agree on is that the motorcycle is an incredible device that gives us great joy in life no matter how we get around curves and corners. Steve B |
