OK, when in doubt go to the source. I found this online in the Segway HT Reference Guide, reprinted without permission from:
http://www.segway.com/support/docs/Reference_Manual.pdf.
Weight Limits for Riders and Cargo
Segway sets weight limits for two reasons: rider safety and to reduce risk of damage to the Segway HT.
Dynamic Weight Limits
To keep the Segway HT and rider upright, the Segway HT must always have enough power to be able to turn its wheels forward and backward. Exceeding the weight limits, especially when combined with other variables that require more power (see list below), will increase your risk of falling or damaging the
Segway HT. Variables that require more power include:
> Higher payloads (weight of rider and all cargo)
> Steeper slopes
> Bumpier surface conditions
> Higher speeds
> Abrupt maneuvers
Since the Segway HT can monitor the amount of power being used, the Segway HT will engage the Speed Limiter (and potentially perform the Stick Shake Warning) in response to these variables. It is important that all riders learn to anticipate and recognize when the Segway HT is reaching the limits of performance.
Minimum Weight Limit
The riders weight must not be less than 100 lbs. (45 kg). If the rider is below the minimum rider weight limit, she may not be able to ride safely because she
cannot shift her weight far enough back (behind the centerline of the wheels) to safely slow down and stop, especially when riding downhill. Also, riders below the minimum weight limit might fail to properly activate the Segway HTs balancing system.
Handlebar Cargo
The total weight of any Handlebar payload plus any other attachments hanging from the Handlebar should not exceed 10 lbs (4.5 kg). Exceeding this limit interferes with the Segway HTs balancing ability and could cause the Segway HT to accelerate on its own, risking injury and damage.
Structural Weight Limits
Increased risk of damage to the Segway HT is another consequence of exceeding the maximum weight limits. Heavier payloads place greater stresses on the Segway HT. Segways testing has shown that the stresses created by a payload within the total payload weight limit (with any cargo not exceeding the applicable cargo weight limits), ridden on a wide variety of terrain in accordance with the instructions in the Segway HT Riders Guide, the Segway HT Reference Manual, and the Segway HT Safety Video, will not damage the
Segway HT.
Just like the dynamic weight limits, several factors affect the loads transmitted to the Segway HT:
> Skill level of the rider
> Payload (weight of the rider and all cargo)
> Surface condition (obstacle height, etc.)
The stresses created by exceeding the rider or cargo weight limits, especially when riding on uneven terrain, could damage the Segway HT.