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Difference between revisions of "BMW Paralever"
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Let's be clear on some terms I'll use here. The "rear drive" is the bevel-gear containing casting that is at the end of the swingarm; it contains the flange that the wheel bolts up to. Whenever I refer to "rotation", I am specifically referring to the rotation, or angular change in position, of the rear drive unit as the suspension moves up and down. The wheel rotates while you ride but that's not important, and the Paralever links rotate around their end points also, but don't let that distract you. We are only concerned with the change in the angle of the rear drive unit (shown as a circle in the wireframe diagrams). | Let's be clear on some terms I'll use here. The "rear drive" is the bevel-gear containing casting that is at the end of the swingarm; it contains the flange that the wheel bolts up to. Whenever I refer to "rotation", I am specifically referring to the rotation, or angular change in position, of the rear drive unit as the suspension moves up and down. The wheel rotates while you ride but that's not important, and the Paralever links rotate around their end points also, but don't let that distract you. We are only concerned with the change in the angle of the rear drive unit (shown as a circle in the wireframe diagrams). | ||
Also, I will mention the forces acting on the rear drive: there is a road force (pushes forward on the tire's contact patch during acceleration) and this is balanced by the inertial force of the bike which pushes backwards at the swingarm pivot. These two forces, since they are not directly opposing each other, create a twisting force or torque in the swingarm/drive/wheel assembly that we're looking at. | Also, I will mention the forces acting on the rear drive: there is a road force (pushes forward on the tire's [[contact patch]] during acceleration) and this is balanced by the inertial force of the bike which pushes backwards at the swingarm pivot. These two forces, since they are not directly opposing each other, create a twisting force or torque in the swingarm/drive/wheel assembly that we're looking at. | ||
[[Image:Torque.gif]]{{clear}} | [[Image:Torque.gif]]{{clear}} | ||
Lastly, it's worth keeping in mind that most of my diagrams portray the two Paralever links as being parallel and of equal length. This isn't exactly correct, but making this assumption clarifies the difference. At the end I'll show how the actual geometry differs. | Lastly, it's worth keeping in mind that most of my diagrams portray the two Paralever links as being parallel and of equal length. This isn't exactly correct, but making this assumption clarifies the difference. At the end I'll show how the actual geometry differs. | ||
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[[Image:Diagrams.gif]]{{clear}} | [[Image:Diagrams.gif]]{{clear}} | ||
This is a simple result of the fact that the swingarm and rear drive are one unit, hinged at one end: vertical movement will cause rotation, and rotation will cause vertical movement. Think harder about the latter part of that relationship: the torque of the wheel and drive will move the suspension vertically. This torque-induced suspension movement, sometimes called "the shaft effect" or "shaft jacking" is considered to be a drawback to traditional shaft-drive designs. The Paralever drive unit, on the other hand, does not rotate as the suspension moves up and down - it stays at the same angle as the chassis because of the parallel links. Since there is no direct relationship between vertical suspension movement and drive unit rotation, the torque of the wheel does NOT result in suspension movement. This, in a nutshell, is what the Paralever is about. Understand this paragraph and you're almost finished. | This is a simple result of the fact that the swingarm and rear drive are one unit, hinged at one end: vertical movement will cause rotation, and rotation will cause vertical movement. Think harder about the latter part of that relationship: the torque of the wheel and drive will move the suspension vertically. This torque-induced suspension movement, sometimes called "the shaft effect" or "[[shaft jacking]]" is considered to be a drawback to traditional shaft-drive designs. The Paralever drive unit, on the other hand, does not rotate as the suspension moves up and down - it stays at the same angle as the chassis because of the parallel links. Since there is no direct relationship between vertical suspension movement and drive unit rotation, the torque of the wheel does NOT result in suspension movement. This, in a nutshell, is what the Paralever is about. Understand this paragraph and you're almost finished. | ||
If that didn't work, try this: Think about the motion of the contact patch with respect to the bike. Since the Monolever is one solid piece (in our example), every part of that system rotates around the swingarm pivot. As the contact patch pushes forward, it can actually move forward with respect to the rest of the motorcycle by pushing downwards. | If that didn't work, try this: Think about the motion of the contact patch with respect to the bike. Since the Monolever is one solid piece (in our example), every part of that system rotates around the swingarm pivot. As the contact patch pushes forward, it can actually move forward with respect to the rest of the motorcycle by pushing downwards. | ||
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==Real World functionality== | ==Real World functionality== | ||
Now to be truly accurate, and get to the real nitty-gritty of the design: the geometry of the Paralever isn't an exact parallelogram. As shown below and in the original picture, the forward pivots are closer together, somewhat negating the theoretical benefit. Consider the two extremes: if the two forward pivots were in the same place, forming a triangle out of the rear drive, swingarm and torque arm, all three of those components would rotate around that point exactly like a Monolever. If the linkages form a true paralellogram, like some of my illustrations show, the rear drive won't rotate at all, which would be like having a swingarm of infinite length. By moving the forward pivots closer together (going partway back to being a Monolever) they create the effect of having a much longer swingarm than they really have. The "virtual pivot point" of this system is somewhere around the steering head - a very long swingarm indeed, which resists the shaft effect as mentioned. Additionally, moving the pivot points closer together changes a few things. For one, they gain ground clearance: with the true parallelogram arrangement I described earlier, the forward pivot would just be too low when the suspension compressed over a bump. Primarily, though, they reintroduce enough "shaft effect" to counteract the squat that would otherwise happen under acceleration. | Now to be truly accurate, and get to the real nitty-gritty of the design: the geometry of the Paralever isn't an exact parallelogram. As shown below and in the original picture, the forward pivots are closer together, somewhat negating the theoretical benefit. Consider the two extremes: if the two forward pivots were in the same place, forming a triangle out of the rear drive, swingarm and torque arm, all three of those components would rotate around that point exactly like a Monolever. If the linkages form a true paralellogram, like some of my illustrations show, the rear drive won't rotate at all, which would be like having a swingarm of infinite length. By moving the forward pivots closer together (going partway back to being a Monolever) they create the effect of having a much longer swingarm than they really have. The "virtual pivot point" of this system is somewhere around the [[steering head]] - a very long swingarm indeed, which resists the shaft effect as mentioned. Additionally, moving the pivot points closer together changes a few things. For one, they gain ground clearance: with the true parallelogram arrangement I described earlier, the forward pivot would just be too low when the suspension compressed over a bump. Primarily, though, they reintroduce enough "shaft effect" to counteract the [[squat]] that would otherwise happen under acceleration. | ||
[[Image:Wireframes.gif]]{{clear}} | [[Image:Wireframes.gif]]{{clear}} | ||
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This is a separate issue from the ends not being in alignment... as in, not being phased correctly. | This is a separate issue from the ends not being in alignment... as in, not being phased correctly. | ||
[[Category:Suspension]] | |||
[[Category:Suspension | |||
[[Category:BMW suspension]] | [[Category:BMW suspension]] | ||