"Adult rider with full rear rack" must be considered a stable condition, as it meets Honda guidelines for proper usage. In this case – a 200 pound adult loaded with 120 pounds of gear laying low in the center of the rear rack – 37% of total weight remains on the front wheels, about 339 pounds weight above the front axle.
The interesting part is where clear instability begins. "Kid with Fat Guy on the end of rear rack, uphill on 20 degree slope" would probably meet anyone’s definition of unstable, and indeed only 18% of the weight is on the front, with 166 pounds. This is clearly not stable. I didn’t calculate hitting a small rock at high speed under this scenario, but an additional upward lift on the front would be provided (!), which is a real problem for poorly distributed rear weight. Also, downward forces for uphill travel are related to a cosine function (largely because rider Cg is above seat height), so after 10 degrees of slope, and certainly 15 degrees, the tilt has progressively MORE affect to move Cg rearward. As the quad tilts up, the riders lean back, in a feed-back loop. Uphill travel includes not only "59 mph up a 30 degree slope", but also crossing that 5 foot deep gully with a steep bank, or maybe riding on a sand pile. They are the same thing for instability scenarios.
I can see why "2-up" riding could be a very bad idea. If the load is distributed as a normal packed rack would allow, i.e., 120 pounds packed low in the center of the rack, then stability is not particularly an issue, regardless of driver size or load type. However, once a passenger sits on a rack, what if he or she sits at the back, and not the center? If a seat is mounted on the rack, where is it mounted? In the center, or at the back? What if a kid is hauling a fat guy? And the seat is 4" higher than the rack? You see the point – weight distribution is beyond the control of the manufacturer and too complicated for lawmakers controlled by word processors. Just say no!
However, these calculations do show that certain conditions are stable, as defined by the calculations for "Adult rider with full rear rack". Several scenarios fall within these parameters (see chart). I can not say what is the "absolute stability" limit, but I think the comparisons speak for themselves.
I would be interested to do a "2-up" set of calculation, but I would need the location of Cg (put two scales under front and rear wheels, respectively), as well as seat positions and rack positions relative to the front axle..
Comments welcome…)
Menard LaPue
