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The '''DW link''' is a patented bicycle suspension system designed by mechanical engineer Dave Weagle. This particular suspension design has been successful in the mountain biking community due to its position sensitive [[Suspension_(vehicle)#Anti-dive_and_anti-squat|anti-squat]] response. |
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==Development== |
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The development of the DW-link was envisioned by Dave Weagle as a recreational mountain biker. Using his background in four-wheeled vehicle suspension dynamics, Weagle started to investigate the linkage axle path of bicycle suspension systems. The analysis method that Weagle developed stands as the first published text on the analysis of linkage suspension systems for chain driven wheels. Weagle's research was directed towards what is known among mountain bikers as "suspension bob". Analysis revealed that the "bob" was a result of the combined effects of mass transfer during acceleration and the unbalanced forces of the rider's legs moving up and down. It was realised that the use of anti-squat could counteract the mass transfer forces that partially produced "suspension bob". |
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The DW link was developed in conjunction with [[Iron Horse Bicycles]], and the first DW-link bicycles were released in 2005, most notably the Iron Horse Sunday. Weagle's design currently holds US Patent 7128329 [http://patft.uspto.gov/netacgi/nph-Parser?Sect2=PTO1&Sect2=HITOFF&p=1&u=%2Fnetahtml%2FPTO%2Fsearch-bool.html&r=1&f=G&l=50&d=PALL&RefSrch=yes&Query=PN%2F7128329]and is pending worldwide. |
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== Overview == |
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Bicycles have a relativley short [[wheelbase]] to [[Center of mass]] elevation ratio, when compared to other vehicles such as cars. Because bicycles are pedalled, their forward acceleration tends to be in surges while each pedal is under power. In unison of [[Newton's Third Law]], the bicycle experiences a force in the opposite direction to travel. This force is partially responsible for the compression of suspension under power known as "suspension bob". This reaction force opposing the direction of travel has a significant effect on the suspension because the centre of gravity of the rider and bicycle is a relativley large distance from the centre of the suspension system. Therefore, more torque is produced about the centre of mass than would be present if the centre of mass were closer to the centre of the suspension system. |
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Two techniques to combat suspension bob are: |
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* Increase the [[suspension (motorcycle)#Damping adjustment|compression damping]] of the suspension |
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* Increase the [[spring rate]] of the suspension |
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However, neither of these solutions are ideal as it hinders the suspension's ability to absorb small bumps or low-speed impacts while the bicycle is coasting (Note: "low-speed" does not refer to the velocity at which the vehicle is traveling, but the speed at which the suspension is compressed). In the case of excessive compression damping, this problem is known as [[overdamping]]. |
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The DW-link uses an anti-squat suspension design to counteract forces responsible for "suspension bob" and consequently removes the need for excessive compression damping. This allows the suspension to be much more active over low-speed impacts, allowing more traction. |
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== Mechanics of the DW-link == |
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The DW-link uses anti-squat to eliminate "suspension bob". Anti-squat is defined as the tendency of rear suspension to compress under acceleration. The anti-squat used in the DW-link system is achieved by a minimisation of torque about the centre of mass. |
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To understand how this is achieved, one must first understand why suspension is prone to squatting without anti-squat. |
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===Squat & anti-squat=== |
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All forces produced from the rear wheel which act on the rear suspension act along a line which can be drawn from the contact patch of the tyre to the centre of mass. This is because the centre of mass is defined as the point about which forces appear to act. There is no official term for this vector, however, a popular term is virtual pivot axis. |
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As stated before, torque is produced about the centre of gravity under acceleration. The result of this torque on the suspension system is dependant on the position of the pivot point relative to the virtual pivot axis. If the suspension pivot lies above or below the virtual pivot axis, torque can be produced about the suspension pivot. This toque is the product of the vertical distance of the suspension pivot from the virtual pivot axis and the reward inertia force. |
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There are three possible situations: |
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* The suspension pivot is below the virtual pivot axis. |
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In This case, the suspension will experience "squat" under acceleration. In other words, the suspension will compress, and "bob" under acceleration. This is because there is a vertical distance between the virtual pivot axis and the suspension pivot. Because the suspension pivot is below the virtual pivot axis, and the inertia force is towards the rear of the bicycle, the torque produced acts to compress the suspension. |
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* The suspension pivot above the virtual pivot axis. |
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In this case, the suspension will experience a jacking force, or the opposite of the previous situation (The suspension will extend under acceleration). Because the suspension pivot is above the virtual pivot axis,and the inertia force is towards the rear of the bicycle, th e torque produced acts to extend the suspension. |
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* The suspension pivot lies on the virtual pivot axis. |
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In this case, the suspension will not compress or extend under acceleration, because no torque can be produced about the suspension pivot. This is because there is only the rearward inertia of acceleration, and there is no vertical distance between the virtual pivot axis and the suspension pivot. |
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This last situation is the technique employed in the DW-link to eliminate suspension bob. However, it is more complex as the virtual pivot axis will not remain in the same location as the suspension is cycled, so consequently, the suspension pivot needs to change location in a similar manner in order to consistently eliminate "suspension bob" throughout the whole range of suspension travel. |
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===Consistent anti-squat and virtual pivot points=== |
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[[Image:IHsunday.jpg|thumb|Virtual pivot point on an Ironhorse Sunday bicycle]] |
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In order to maintain a consistent amount of anti-squat throughout suspension travel, the location of the suspension pivot point must change as the suspension, and consequently the virtual pivot axis moves. This can be achieved by the use of multiple linkages, as seen on all DW-link bikes. All DW-link bicycles use two linkages to achieve the correct placement of the pivot point. The instantaneous point at which the two bisections of the linkages intersect is known as the instantaneous pivot point, or virtual pivot point. By carefully designing the linkages of the suspension system, it is possible to design the desired amount of anti-squat into the system at various points. |
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===Braking=== |
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Another major advantage of the DW-link is the lack of need of a floating brake system to achieve optimum suspension performance under braking. Typically, on [[Bicycle_suspension#Single_Pivot|single pivot]] suspension systems, a large amount of squat is experienced under braking without a floating brake design. This is a result of the rotational inertia of the spinning rear wheel being converted by the brake into a torque on the axle which compresses the suspension. Subsequently, the suspension is less active under braking, resulting in a diminished amount of traction. |
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The DW-link eliminates the need for a floating brake design because, as explained previously, no external torque can be produced about the rear suspension system. |
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===Minimisation of chain growth=== |
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The DW-link system has been designed to minimise chain growth through suspension travel. "Pedal feedback" is defined as a sudden increase is pedalling resistance when the rear suspension encounters a bump. By minimising the chain growth, "Pedal feedback" is also minimised. This enables the suspension system to pedal much more efficiently over rough terrain. |
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==Licensing Of The DW-link== |
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Currently the DW-link has been licensed by Dave Weagle to the following bicycle companies: Ibis, Independent Fab, Iron Horse Bicycles (Now Dissolved), Pivot Cycles and Turner Bikes. |
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==References== |
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*[http://www.tonyfoale.com/Articles/Dive/DIVE.htm Virtual pivot points and Anti-squat explained] |
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*[http://www.dw-link.com/physics.html DW-link physics explained] |
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*[http://www.ironhorsebikes.com/tech/dw-linkTech.html Ironhorse bicycle company technical information] |
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[[Category:Cycling equipment]] |
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[[Category:Cycle types]] |
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