Inserts: Weak link of 2-pin Tech System

Crispi Telemark Tech Toe Inserts
Staying connected when you want with a 2-pin tech system requires tight tolerances – especially with the boot insert.

What started out as a simple test of the Meidjo, a telemark binding with a 2-pin toe and NTN clamp, turned into an investigation of tech toe inserts and what makes them good or bad. Tech systems are an interaction of components on a petite scale, so small variations can have big consequences. While my initial interest was spurred through examination of a telemark adaption to Dynafit’s 2-pin tech toe, the general conclusions are the same, although the application may be different.

Everyone that needs to know figures out pretty quickly that tech bindings are not as solid as alpine bindings; they’re more prone to pre-release. That said, they work remarkably well and, contrary to first impressions, are quite solid and amazingly bombproof. The recent flurry of innovation with tech bindings has even eliminated a lot of the tendency for releasing before an alpine binding would, but not completely. There remains a level of uncertainty inherent in the tech system which can be attributed primarily to the boot inserts.

Dimensional variation responsible for pre-release

NTN TTS Telemark Tech Toe
Jeff Campbell et al measured the forces applied with various models of Tech boots and bindings. Clearly some combinations are more prone to release than others.

This issue has plagued AT skiers for years, but they’ve come to accept and deal with it. One area that is not intuitive to a resort skier is the potential for unwanted release when touring. To prevent this occurring, especially when traversing steep, hard snow, tech toes can lock out the release. In reality, it still remains possible, though much less likely.

Some bindings have a stronger “lock out” retention force than others. Fritschi’s Vipec merely increases retention force by approximately 2 on the DIN scale, and others try to approximate infinity with a ridiculously high, non-linear value.

Here’s the rub. Even when release is locked out AT skiers know you can still release if there’s dirt or gunk in your inserts preventing the jaws from closing completely, making it easier to trigger a release. In effect, the pins let go too easily if the boot inserts are too wide or too shallow, either by design, poor manufacturing, or from being gunked up. You can see in the graph above, as the pin gap increases, on most bindings the clamping force reduces, which is when the jaws open up and let go of the boot.

This was the inescapable conclusion presented by Jeff Campbell, Irving Scher, Bruce Jahnke, and David Carpenter to the 21st International Congress on Ski Trauma and Skiing Safety held in Cortina, Italy in March 2015. After measuring and comparing the inserts from five different boot makers in 10 models of bindings from five manufacturers they hypothesized that:

“…constraint forces imposed by tech bindings on AT boots are highly sensitive to variations in tech insert geometry.”

In particular, they noticed that the height of the front wall of the insert (Dim B) relative to the depth of the cone (Dim A) strongly influenced the results. In their words:

“…the difference between Dim. B and Dim. A had the largest influence on release torque (40% +/- 12.8%) while the clamping force had the least influence (14.9% +/- 1.8%).”

Field Results

Let me tell you straight up, their hypothesis looks correct. It isn’t just about one particular dimension, it is the full 3D shape, from the width of the inserts at the depth of the cone but also, especially, the height of the sidewalls of the hole the pin mates into.

I came to the same conclusion, albiet less precisely, after analyzing the results of a simple field test. My research led me to their research. Here’s how I found out.

Tech Toe Telemark Bindings Tech Inserts NTN
Dimensional tolerances and the ability for them to add up create the potential for inconsistent results.

The first generation Meidjo had a problem with the jaws staying closed when locked out. The stainless steel pins that the toe springs wind around were soft enough they would bend, allowing the pins to disconnect. Stiffer hardened pins was the proposed solution, but on their maiden tour, we found that a pair of Crispi Evo’s was still popping out way too easily. Of course we doubled checked that the inserts were clean, and the binding lock was fully engaged. They were. Meanwhile, my Scarpa TX’s were indicating that Meidjo’s harder steel pins solved the problem.

A quick comparison at the trailhead with a Dynafit Vertical toe suggested the problem was with the inserts of Crispi’s Evo, not Meidjo. Back at the office, I used a pair of Crispi Shivers (same lower boot shell) and compared how easily they could be pried loose when in tour mode with every pair of tech toes at my disposal: Dynafit Radical, Verticals and the TLT, G3 Ions, Marker Kinpins, The M Equipment’s Meidjo, and OMG’s TTS toe. Only the Radical and Ion held fast no matter what boot was used. In descending order of perceived retention were the Kingpin, Vertical, Meidjo, Dynafit Low-Tech, and the TTS toe. The Vipec wasn’t tested since there is little chance those would ever be used with a compatible NTN boot.

NTN Tech Toe Telemark System
Ideally, all these bindings might lock out when you ask them to, depending on the circumstances. But this makes clear, it takes two to tango and the weaker insert is obvious except with a few bindings. Bindings not tested might do as well, or worse than the examples listed here.

I freely admit this was not a calibrated, precisely repeatable test. Nonetheless I checked each brand of boot, in each brand of binding more than once to attempt to create repeatable results. While the actual force may have varied, the force I applied with my arm versus release make me confident the relative holding power of the various bindings with each insert would be confirmed with a more scientific test setup.

Of course, I measured the width of the inserts with a micrometer and even though my numbers are in the same ballpark as those I obtained, I do not believe my measurements were accurate enough to claim an undeniable difference in the Scarpa inserts versus those in the Crispi boots.

Crispi was queried about the dimensions of their inserts. Their response was to state their inserts were “certified;” to what was not clear. There may have been translation issues at their end, but Crispi has a track record of not responding to my questions. Their loss.

Exactly what are the dimensions supposed to be?

NTN Telemark Tech System
A close up of the critical dimensions as defined in a Fritschi dealer manual for adjusting the pin gap. Dim. A and B indications added for clarity with this article.

Next step was to try to confirm what the dimensions were supposed to be. In 2014 Dynafit readily said the pin gap, when the binding was closed, was 2.31”, which converts to 58.674mm. Being that Dynafit is an Austrian company I expected a metric value; say, 58.50mm plus or minus some ridiculously small number. Since then, repeated attempts to get the exact number on their inserts have been fruitless. So I pestered other manufacturers. Same results, a generic, “we won’t give you that.”

I finally found someone to share the magic numbers, but without confirmation I really don’t know if it’s true or not, but it certainly is in the ballpark; 57.5 mm ± 0.15mm. I’m guessing that is the minimum distance, and the value is probably more like 58.5mm since Jeff Campbells graph above suggests the number is somewhere between 58 and 59 mm. The tolerance seems accurate, based on the documentation in Fritschi’s service manual to retail shops about how and why the pin gap on their Vipec may need to be adjusted.

Intolerable Tolerances?

Crispi Scarpa Tech Inserts Telemark Boots TX Pro Evo
An inexact comparison of how deep the pins seat in the boot inserts of a 2010 Scarpa TX-Pro w/genuine Dynafit inserts VS a 2014 Crispi Shiver with their own inserts. It isn’t dramatic, but the pins appear to seat deeper in the Scarpa boot.

Now if that’s the actual spec and everyone adhered to the tolerances it then there wouldn’t be any issues. However, in the real world there’s a whole lot of variation going on, but few know how much, and fewer still proclaiming it.

Fritschi’s tech binding, the Vipec, needs to mate precisely with a specific pair of boots for the lateral release at the toe to work reliably. After sampling the field of available boots and their inserts they decided the Vipec needed to allow for up to 1.2mm of adjustment. That’s one piece of public info indicating there’s a whole lot more variation to inserts than ± 0.15mm.

The other is the aforementioned report by Jeff Campbell et al who determined it isn’t just the basic spacing of the inserts, what his team labeled the A dimension, or the B dimension, the distance between the tips of the cones of the inserts, but the difference of the two, B-A. In effect, assuming the cone shape is precise, the height of the front sidewall above the cone.

In their tests they found that Dim. A varied among the samples from five manufacturers by 0.25mm while Dim. B varied as much as 0.5mm. A tolerance of ± 0.15mm allows for a variation of up to 0.3mm, so the 0.25mm measurement for variation among inserts is “in-spec.” However, as Campbell’s group noted, the variation in dimension B exceeded that and when tolerances stack in the wrong direction, the variation could be up to 0.75mm, which is clearly too much.

This replicated my experience exactly. Although my measurements may not be precise to a hundredth of a millimeter, nonetheless my measurements indicated Crispi boots had a shallower front wall to the insert which allowed the pins to pop off way too easily in tour mode.

Scope of the problem

I’ll admit that this has been an ongoing issue with the tech system for years that, by and large, has been ignored because it is a relatively small problem. Or is it?

We know from field evidence that a minority of users experience pre-release, either when skiing downhill or even when locked out and skinning up. Due to the inconsistency of the problem, and from available manufacturing tolerances, it is clear that very small changes in dimensions can have a strong influence on how the tech system behaves, either toward reliable retention when desired, or its counterpoint, unreliable release.

There is no actual industry standard defined for insert dimensions, only the proprietary dimensions developed by Dynafit. Based on results these dimensions appear to work well when sufficient quality control is exercised. Despite the fact that Dynafit has more experience with this design, nonetheless it is but one companies input and an alternate view might be beneficial in resolving these issues.

For instance, consider G3′s Ion. They changed the position the pin arms swivel about and dramatically changed the retention force of the binding. This is one potential solution, and in my limited sampling of boots I can confirm that Ion holds on where other tech toes cannot, even with a suspected bad insert.

It is also possible that alternate geometry in the insert can, not only be more likely to release, but might also be less likely to release. Such was the historical case of the inserts used in the first production models of Garmont’s Cosmos and Celeste. The entire AT community was in an uproar when Lou Dawson noticed that the Cosmos inserts exhibited more slop yet required more force to release.

Garmont made an issue of how Lou’s hand test wasn’t a real test. In his defense Lou was reporting what he perceived to be a difference from the norm. Never mind that everyone had been complaining for years that AT boots tended to let go too easily and this might be a way to overcome that. Lou’s demonstration of the change in releaseability is compellingly damning, but not a legitimate, by the book test.

I spoke with someone inside Garmont who prefers to remain anonymous, but back then he admitted the inserts were a bit deeper to combat the tendency of tech systems to release too easily in tour mode. Except that design decision was poorly defended, perhaps because, regardless of intent it was a flawed design, but also because when the news broke, Achilles Morlin, Garmont’s owner, was in the midst of selling the ski boot assets to Scott Sports. Lou’s findings were a nuisance and Achille’s unwillingness to address the points Lou raised only fueled the view that this must’ve been a manufacturing mistake, not a design variation with potential, albiet unproven, benefits.

It is interesting to note that since that time, tech binding makers have worked to compensate for a limitation that might be solved easier and better with changes to the insert; or it might not, depending on the inserts in question.

The reality is this probably doesn’t affect the majority of AT boots, but enough to be aware it could be your next pair. In the fringe world of NTN, it potentially affects half the available models.

A Practical Solution?

As mentioned earlier, the easiest short term solution is to favor boots with genuine Dynafit inserts. However, I might caution you to against using this as the golden rule for tech inserts because Fritschi engineers are adamant the 1.2mm variation affects all brands of inserts. Furthermore, who’s to say that Dynafit’s dimensions are truly optimal? I’m not saying they aren’t the best currrently available, but there might be a slightly different geometry that can improve the reliability of the whole system. For such a quest, we consumers can hope that all the manufacturers will work together to determine what this might be. Experience suggests that manufacturers are reluctant to cooperate and would rather duke it out litigating patents.

As Jeff Campbell and his associates conclude,

“Further testing is needed to understand how the variability reported here, along with other variables such as material hardness and the surface profiles of tech inserts affects retention/release characteristics [of] tech boot/binding systems when heel pieces are engaged.”

Clearly there needs to be greater attention to detail with regards to the dimensions, tolerances, materials and processes when manufacturing tech inserts. Until such time as this is tightened up, if you have a problem with your tech boot/binding system, maybe the trouble is with your boots, not the binding. And until inserts are more universally consistent, binding makers will be asked to take up the slack. The Ion and Radical are examples of bindings that have.


Brand (2015) Insert Mfg.
Atomic Salomon
Black Diamond BD
Crispi Crispi
Dalbello Dalbello
Dynafit Dynafit
Fischer Dynafit
K2 K2
La Sportiva La Sportiva
Roxa/Moment Dynafit
Salomon Salomon
Scarpa Dynafit
Scott Dynafit
Tecnica Tecnica

It is also worth pointing out that I ran in to this problem with NTN telemark boots outfitted with tech fittings which are expressly not recommended for use with a tech binding due to the softness of the sole and thus, unreliable release. However, from experience I know that the system works well from a tele perspective where downhill safety release is “not required.” But there are other issues. Crispi inserts seem to be harder to hold on to in tour mode than Dynafit inserts, but in telemark mode, the forces are much different and the problem effectively disappears. Since the forces are different, perhaps the insert geometry should be modified too.

Let’s not get ahead of ourselves though because that’s a riddle no one is interested in solving; not for a long while yet. The bigger issue for telemarkers on the bleeding edge of technology is the already limited choices for boots with tech inserts, let alone whether those inserts are any good or not. The good news is next season (2016/17) Scott Sports will deliver the NTN Voodoo with genuine Dynafit inserts.

For AT skiers, if you find you have a problem with pre-release, getting a different pair of boots will probably solve the problem and there are a lot more choices available, so the solution is theoretically easier.

For boot makers, unless you’re willing to seriously reverse engineer what Dynafit has learned over decades of development, you’re probably better off using their inserts; after all, they’re still the number one brand of tech binding sold so your boot better work with their two pins too, or else…

Related Posts

Wild Snow AT Boot ISO standards

Tech Talk: What’s a MM between 2-pins?

Wild Snow: Garmont Cosmos Review Details (Including Achille’s response – fast fwd to 7 minute mark on Lou’s video demo)

Ed. Note: Lou Dawson has reported on this phenomenon before but not many people paid much attention since it was tech minutia on a tech binding which squares the degree your eyes glaze over in comprehension. I didn’t give a hoot either until it suddenly affected me.