No other design in the history of wheelchairs has proved as successful as the folding vertical X-frame.  First patented in the U.S. in 1937, X-frames have served countless users around the globe for the past 69 years, on manual and powerchairs alike.  And, it's no wonder that the folding X-frame is the global mobility standard - it allows a thin, compact structure for transport and stowage, while easily unfolding into an entirely functional, full-size wheelchair.  With such a blend of functionality and portability, it's easy to see why the folding X-frame has held its reign for decades as a mobility mainstay. Still, the folding X-frame isn't without its flaws.  The folding X-frame, as a mechanism, has stability issues, where the X-frame doesn't completely rigidize a wheelchair - after all, an X-frame is designed to fold side-to-side, and, as such, is susceptible to energy-robbing frame movement, as with the side frames moving somewhat independently of the X-frame.  Over the years, several manufacturers - Etac, Kushall, and Quickie, to name three - produced "folding" wheelchairs that did not use an X-frame, striving to create alternative folding yet rigid structures.  While the unconventional folding mechanisms provided rigidity and stability over a conventional X-frame, the nontraditional folders lacked practicality toward folding, where the user had to remove wheels or strong-arm mechanisms to collapse the wheelchair (whereas with an X-frame, simply pulling up in the center of the seat folds the wheelchair, complete).  As a result, users had to make trade-offs, to hassle with a nontraditional folder for the sake of performance, or give up some performance for the sake of easy folding.  Was there a way, though, to maintain the practicality of an X-frame while optimizing rigidity? The answer was, yes.  Rigidity comes from forming a solid link between two points.  With the seat tubes floating atop the side frames, as with a conventional X-frame, some play was always involved, where the seat could sway atop the side frames (an affect that was more prevalent on some models than others based on quality of design) - that is, the two sides of the wheelchair were never entirely locked out because there was no true rigidizing point between the top points of the side frames.  Rigidizing the tops of the side frames is especially important to address side loads, as with wheel camber and turning, where the top of the side frames want to pitch inward.  If the X-frame could be entirely seated internally within the side frames rather than on top of them, where the X-frame's seat tubes parallel the top of the side frames, preventing them from pitching inward, an inherently more rigid, stable structure could be formed.  Put simply, an internal X-frame, where the seat tubes sat beside the side frames on the same plane, would better prevent side loads from buckling the side frames, better rigidizing the wheelchair.

In the 1980s and 1990s, European and Asian wheelchair manufacturers moved to internal X-frames, where it has become a commonplace in their ultralight folding wheelchair markets.  Oddly, the North American market has been slow to adopt the internal X-frame, but with companies like TiLite using the concept with success for several years, Quickie's bringing the Neon model from Europe to North America (AKA, the GTX), and Pride introducing its version, consumers, providers, and therapists are catching on to what most of the rest of the world has known for years - that is, an internal X-frame can dramatically enhance performance. While the performance benefits of an internal X-frame are clear, the functional impact is less obvious but profound, fostering easier user transfers.  On a conventional X-frame, the seat tube sits atop the side frame, forming a blunt edge - a blunt edge that's inherently prone to snagging clothing and body protuberances when transferring.  Additionally, a conventional X-frame's seat tube is a tricky hand-hold point, as the minute one pulls on it, as with a slide transfer, the wheelchair is prone to folding.  By contrast, an internal X-frame resides inside the side frame, eliminating a blunt seat tube, and, for transfers, allows one to pull on the side frame, not the X-frame, dramatically reducing the likelihood of the wheelchair unexpectedly folding. With such meaningful benefits, it's easy to not find downsides to internal X-frames; however, there are points to consider.  When folded, internal X-frames are typically 1" wider than conventional X-frames - whereas a conventional X-frame may fold to 4" wide, an internal X-frame may equal 5" (this is specifically due to a conventional X-frame keeping the side frames in its vertical path when folded, and an internal X-frame dictates that the side frames remain slightly outside its vertical path when folded).  Additionally, drop seating (very rare on active, high-end ultralight wheelchairs), may have compatibility issues due to mounting hooks and inside widths.  As such, when considering an internal X-frame, relate all of your needs, just as when selecting any wheelchair, determining the pros and cons toward your mobility needs. Now, conventional X-frames remain a wonder of the mobility world - they're simple, sturdy, and functional - and they perform well under almost all conditions.  However, if you're looking for the ultimate in folding performance, think outside of the box - or, "inside," as it may be - and take an internal X-frame for a spin.