It's important to note that we designed the Stage One slider system in response to customers asking for a lightweight and collapsible slider system. A rail system that could be broken down and transported in a smaller transport bag (even a carry on bag) that had a good weight to load capacity ratio. In general we've found with medium wind conditions the Stage One can handle 15 pounds of additional load not counting the 2 pound weight of the cart and a 1 pound slider motor (18 pounds total), but this 15 pound load specification assumes a maximum span of 40" from support to support.. Since the Stage One system is theoretically infinitely expandable (only being limited by the amount of rail and length of belt) users tend to want to go beyond the 40" span.. As the supports start to spread out to 60" to 80" this will have a very significant effect on the stability of the rig. Basic structure dictates that the farther the span the less load we can place on that rail span before it bends, this is particularly the case at the center of the span (farthest from the two supports).
Stage R Weight
A two axis Stage R system weights about 7 pounds. Your average DSLR with a normal or wide angle lens is in the 4-5 pound range. Additional accessories like clamp plates etc. can add a pound or so which can bring your total load into the 12-14 pound range.. This basically maxes out the capacity of the Stage One rails which means unsupported spans beyond 40" between supports is NOT ADVISED. You certainly won't break the system by increasing the span but you will start to see some instability at the center of the span in the form of deflection (bowing) or 'bounciness'
Height is another very significant factor! As you raise the height of your load (typically what happens when you introduce pan and particularly tilt axes) this introduces strong 'twisting' (technical term: moment) forces. In essence the taller a rig gets the stronger these twisting forces become and exponentially so! Therefor whenever possible try to keep the center of gravity low as possible in your setup to help prevent these powerful twisting forces caused by tall rigs.
Single Axis Stage R is a Great Compromise
One of the best ways to increase stability and keep the rig low when using the Stage R rotary in combination with the Stage R is to drop one of the axis of motion and use a single . If you can achieve the shot with single Stage R allowing for slide/pan or slide/tilt this cuts the weight that the Stage One has to carry as well as the height of the rig. We've found that with a single R unit the load usually falls below the 10 pound mark so spans up-to 60" are quite stable due to the reduced height and weight of the system.
Stage One Plus Advantage
Early 2015 we introduced the Stage One Plus design which helps with the rail structure in two ways: the first is the way the mid-span supports clamp onto the rails and provide solid mount points, this makes it possible to position the mid-span supports 10-20" in from the ends of the rails and reduce the overall span in the center. A 60" length rail can be cut to a 40" span easily by spacing the mid-spans from the ends 10". These clamping supports with the extended clamp arms help with twisting forces. Together, using the Stage One Plus to reduce the span and increase the twisting resistance helps a lot with the extra weight of the Stage R system.