At the time that I designed the Birdcage research vehicle in 1991 with a biomedical engineer, I was aware that a Net Ramp Angle (boot board + binding ramp) of more 3 degrees had a significant, perceivable, negative effect on skier stance and balance. Although I didn’t know what the range of the optimal Net Ramp Angle was (and still don’t), I knew that Net Ramp Angle is affected by the length of the Achilles tendon and that this aspect affects the synchronizing of peak arch tension with peak Achilles tendon tension that occurs just before the heel separates from the ground to initiate propulsion. I refer to this as the Reference Shank Angle. It is the foundation on which to build a strong stance from the bottom up.

Since my experience prior to 1991 had demonstrated that more than 3 degrees of Net Ramp Angle was too much, a decision was made to fix the Net Ramp Angle of the base of the small Birdcage, shown below, at 2.5 degrees and the base of the large Birdcage at 2.35 degrees.

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The small Birdcage fit US men’s size 4 to 8 feet. The large Birdcage fit US men’s size 8 to 12 feet. Thus, all skiers with feet in a Birdcage size range had the same Net Ramp Angle.

Since the base of the Birdcage acted as the boot board, there was no removable boot board as in most ski boots. The base shown above was made from high grade aluminum and was in the order of many times stiffer and more torsionally rigid than was necessary to withstand the expected loads of skiing without deforming. This is an important factor that will be discussed in a future post.

Since 1978, I had suspected that the plastic shells of most, if not all, conventional ski boots were undergoing significant deformation under loads typically of racing. So I began stiffening the bottoms of ski boot shells with a torsion box structure similar to those used to stiffen skis. Recent studies have not only confirmed my suspicions, but shown that the deformation that occurs can be far worse than I suspected. Consistent with good practices of science-based research the entire Birdcage was engineered with excess structural capacity so as to ensure that it could easily withstand the maximum loads imposed on it without significant deformation as this could disrupt the processes of skier balance and control.

An important aspect of the Birdcage was continuity of the surface structure that the foot rests on. A one piece top sheet comprised of 5 mm thick high grade aluminum formed into a tub was secured to the base with screw fixations. The 3 black strain gauges shown mounted on the base (2) and side plate (1) in the photo below of a left foot Birdcage show the continuity of the surface under the balls of the feet. Continuity of the surface under the ball of the large toe is especially critical.


In order to ensure each test skier was as close as possible to the Reference Shank Angle, the start and end points of shaft rotation and the forward end point at which resistance was introduced were adjusted to peak Achilles tension – Shank Reference Angle so as to make the effect of ramp angle as consistent and neutral as possible without fine tuning it to each test skier.

The photo below shows the rotation resistance control mechanism on the back of the Birdcage.


This important aspect is discussed in detail in US Patent No. 5,265,350. FIG 56 below from the patent, shows the means to adjust the rearward travel limiter of the shaft to set the shaft forward lean angle, forward travel limiter of the shaft to set the limits of forward shank movement and journal resistance means for 10 to 12 degrees of low resistance shaft rotation.

Fig 56

A reasonable starting point for a boot board standard would include the following:

  • A ramp angle of 2.5 degrees with a  shim kit with 0.1 degree shims for the heel and forefoot to facilitate fine tuning based on ski testing.
  • A top plate surface that the foot rests on that is monoplanar (flat in the long and transverse planes) with the transverse plane parallel to the transverse aspect of the base of the ski.
  • Torsional qualities that when integrated with base of the boot shell maintain deformation of the boot board and boot shell base within agreed upon acceptable limits.
  • A top plate surface that the foot rests on that is contiguous under the heel and the balls of the feet, especially under the ball of the big toe under the the 5th metatarsal.