Making a low DIN tech binding

By Christian
We recently acquired a pair of Atomic Backland boots on the cheap (broken/missing hardware - which I fixed) that fit our oldest daughter, who's 7. The only problem is that she's a DIN 1.75 (or maybe 2, soaking wet). The tech bindings with the lowest release values I could find were the Dynafit Rotation 7 (2.5-7) and the Hagan Pure 8 (4 - 8). But neither of those quiet cuts it. Rather than spend another season hauling two separate pairs of skis around for her (one for up, one for down) I decided to try and swap the springs on a set of tech bindings to get a lower release value - and I feel like I succeeded.
Before we get started comes the obvious warning - I don't know anything about ski bindings. If you modify safety equipment based on a description somebody posted on the internet of what they did (for example, this blog post) and things go wrong you have nobody to blame but yourself. 
The first step was to pick the binding to start with. After looking at as many photos online as I could find, and making some guesses as to the internal mechanisms, I decided on the Hagan Pure 8. Also, it's a lighter binding and it's always nice to save a few grams.

I thought the easiest way was to just make a little slideshow of some images I took disassembling the binding. Assembly is just the reverse.

Discovering the depth gauge fits inside the spring opens up some possibilities...

Once you've got them out you need to put different ones in, if you want to change the release properties. I was pretty pleased to discover that I could poke the depth gauge on my callipers down the middle of the springs and then use the callipers to press them onto my scale - thereby measuring force and displacement at the same time to determine the spring rate! After taking some measurements and making some guesses I bought and tried out about $100 worth of candidate springs (shipping was a significant fraction of the cost, so I ordered a bunch). None of them seemed like the "perfect" ones - I couldn't get the same conversion for horizontal and vertical - but in the end I think the best option is to use part number ​pc054-312-12000-sst-1130-cg-n-in from "the spring store" for the horizontal release and take the old horizontal release spring and use it for the vertical release. Note that the new horizontal release spring is way softer than the original, but a lot of the horizontal release actually comes from the toe. In fact, at the lowest setting, the spring is basically doing none of the retention - but you still need a spring of some kind, otherwise the tower won't stay pointed the right way.

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What are the new release values and how did I determine that? First a question - What's the difference between the Vermont Release Calibrator and a funny-looking metal foot and a torque wrench? Answer - Several thousand dollars. Since I didn't have several thousand dollars for this project I just made my own funny-looking metal foot and used the torque wrench I already own.

Horizontal (using PC054-312-12000-SST-1130-CG-N-IN, all values "ish"):

• 3 -> Now 2 (limited by toe, I think)
• 4 -> Now 2
• 5 -> Now 2.25
• 6 -> Now 2.5
• 7 -> Now 2.75
• 8 -> Now 3 

 Vertical (using old spring from horizontal release, all values "ish"):

• under 3 -> Now 1.75 (there is some adjustment range below 3 where everything seems to still work)
• 3 -> Now 2
• 4 -> Now 2.5
• 5 -> Now 3
• 6 -> Now 3.5
• 7 -> Now 4
• 8 -> Now 4.5 

And there you have it - a 1.75 (ish) or at least a 2 release value on a tech binding. But can kids even use a tech binding or are they too fiddly? F seemed to be doing ok - here's her downhill transition on her first day testing out the new system: