A portion of the passenger's overall weight is transferred to the fully sprung part of the bike thru the rear footrests. This amount is more than I guessed it would be-44 lbs for me as a passenger or about 28 pounds with my wife as passenger. This loading makes the rear suspension more active which will improve comfort for the rider on a rough road. Unfortunately, it makes things worse for the passenger, at least for the passenger's tailbone. The more active the rear suspension is , the bigger the jolt received by the passenger as their part of the seat hammers solidly into their backside. In the same way it is quite likely that stiffer rear springs, while giving less comfort to the rider, will quiet things down for the passenger. I can see now that this effect (less give in the rear suspension with heavy springs) is actually an improvement for the passenger.
The difference in ride between the front part of the seat and the rear is night and day.
The difference in ride between the front part of the seat and the rear is night and day.