macribs wrote: ↑
Nov 28, 2017 6:05 am
So as the swing arm compresses the linkage works the rocker arm. The rocker arm compresses the shock while also working the opposite side of rocker arm to move the "wishbone". The "wishbone" got a pivot point on the frame too.
So here is where the linkage outsmarted me. What is the job of the wishbone? How does the movement of the wishbone influence the shock movement or the forces working on the swing arm? Looking at the pics below we can see the shock got a straight line from the frame mount to the swing arms "hi point".
I can briefly remember to seen a build or build sketch that used a similar swing arm. Will a swing arm alone shaped like that have any positive effect on shock and damping forces? Or does that curved swing arm not make any difference without the linkage too?
The linkage setup here makes it so the ratio between the movement of the rear axle and the movement of the shocks 'rear' axle (the 'shock attachment' point in FlightService's image above) changes throughout its range of suspension compression. In this case, the ratio decrees - thereby increasing the force required to compress the suspension further. That way it feels 'soft' for the first say, 1/3rd of the compression, stiff for the middle 1/3rd, and very hard for the last 1/3rd, making it very difficult to bottom out. The same thing would happen if it was simply attached to the swingarm as in the 2nd pic (no linkages) but the difference in how 'stiff' it gets would be much less. A linkage like the 1st pic can have a change in ratio from 1:4.5 that decreases to 1:2, where the 2nd pic will change from perhaps 1:3.4 to 1:3.0. Note these are not the actual numbers, just guesstimates for comparison.
Another way of thinking about it is a fulcrum/leaver setup:
the 1st bike with its linkages effectively means that the fulcrum moves to the right as you compress the suspension - so your 'effort arm' gets smaller, and the effort required to compress the suspension increases. For the 2nd bike pic, the fulcrum doesnt move, so the effort required remains approximately the same, no matter how much the suspension is compressed (in truth the fulcrum does move in both cases, but far less for the 2nd bike). The wishbone/dogbone setup simply allows the designer ( ie sur ron) to tune where the fulcrum is positioned at any given point in the suspensions compression. In particular, it allows for a 'progressive' rate of compression, so the effort required to compress the suspension increases exponentially, giving good compliance through most of the range of travel, while still preventing bottoming out.
hope that helps... haven't had my morning coffee so that might actually be gibberish.