ReX
TY 4 Stroke God
zoomzoom said:
It's the front pivot arm that goes between the tunnel just behind the drive axle and the front of the slide rails.
It was historically called a w-arm because it looked like a w. The mono-shock suspension "officially" has a "pivot arm 1" or "front pivot arm" at the front, but most people have been calling the front arm a w-arm still.
Of the two suspension arms, the pivot arm has to handle much higher loads. The rear arm and scissor only provide vertical support and lateral control of the skid (and lateral control only if the rear arm and bushings are relatively tight). The front arm (w-arm) has to provide not only vertical and lateral, but it also controls the position of the skid longitudinally (along the direction of travel). This means that whatever tension is in the track around the rear axle, twice that force is pushing the skid forwards and the w-arm has to handle that load as well. In addition to this if the bushings in the rear arm or the scissor pivots are worn, most of the lateral support is handled by the w-arm.
If you're accelerating over stutter bumps, each time the track is pushed hard into the snow there's a sudden large forward push that the front pivot arm has to handle. If you're cornering over stutter bumps under power the pivot arm is really worked hard with a combination of the above as well as high lateral loads. All of these loads are highly cyclic and are all in phase (peak loads occur at the same time). If the trails are bumpy (and most are) the cyclic nature of the loads make the front pivot arm highly susceptible to fatigue failures if it's not effectively overdesigned (maximum stress is much lower than allowable yield stress – at least a factor of 2 with a decent margin remaining).
