A few posts ago we reported on the broken Silvretta Pure Freeride binding that occurred during our Trooper Traverse trip. During a trip to Jackson last winter I got to know Reiner Gerstner, the head guy who oversees Silvretta and Dynafit. Reiner saw my post, and we’ve sent quite a few emails back and forth. One of his communiques was particularly specific, so I edited for clarity and got Reiner’s permission to post here (see below).
When I met Reiner he struck me as an honest and sincere guy — I’m confident his take below reflects his experience with the Pure backcountry skiing binding line and any problems that have cropped up. I agree that the kind of breakage we experienced is probably quite rare, and since all ski bindings break given the right circumstances, perhaps what happened to us is not that big a concern. On the other hand, our problem was not minor as it rendered the binding mostly dysfunctional, and it happened not while cranking aggressive turns, but while walking near camp and sticking a ski tip into a pile of snow, then working the ski to free the tip as one does while trail breaking deep snow. Also, as far as I could tell the binding was not adjusted out of spec, though I couldn’t tell how the forward pressure adjustment was set. The release DIN was set at 7. One mitigating factor: let’s not forget that this type of binding break happens in touring mode, so at least it’s not going to cause a fall during extreme skiing.
All this considered, while I’d still recommend the Pure for less aggressive skiers of average weight, I can’t recommend any of the Pure models for larger aggressive skiers. More power to you if you’re in that category and happy with your Pures, but that’s my take. Also, what concerns me is that the Pure Freeride gives one the impression that it’s marketed for agro skiers using massive boots such as the Scarpa Tornado (as Mike who broke the binding was using). If that’s indeed the case, it’s possible Silvretta is attempting to sell to the wrong target group. At any rate, here is Reiner’s message (edited for clarity):
Like I explained to you at our last meeting most of our staff are active in ski mountaineering and know the consequence of equipment failure. Therefore we do everything possible to avoid problems — if you still had a problem with our equipment we apologize.
Lets talk about the case you mention on your website. In the first series of the Pure binding we had a few break and immediately reinforced the toe unit with ribs. For this generation of bindings, with thousands out on the snow we have only had one case of breakage such as yours and we were able to examine the binding, so we’ve got a pretty good idea of what may have happened to you.
Here is our take:
Because of the mechanics of the Pure binding there is a specific problem which is rare but still possible. This specific case occurs with the binding in touring mode (plate can move up and down for touring). And specifically it occurs if there is a very stiff boot with a very stiff sole (alpine or alpine-like boot) and the front piece is adjusted too low, so the safety release could not let the boot come out of the binding (too much pressure on the toe area). Moreover the user is big, binding release is set to maximum DIN, and the user falls to the front over the skis such that the knees land beside the skis (we call it knee fall), which means the binding plate stands at a high angle to the ski and presses the binding toe against the ski with extreme force.
Since the toe unit is hinged and can move for and aft, when this type of fall happens a lot of power transmitted through the toe unit as it hits the ski. This power pushes the boot back against the heel unit, then the heel unit moves back along the frame rails to the limit of its range and bottoms out. The resulting force can break the heel unit off the binding frame. Like I said, this is rare but it can happen.
In our minds even one case of a broken binding is one too much so we will search hard for a solution even for this very rare case. If you have any ideas on how to improve the binding please let us know, as we consider you as one of the most technical advanced ski mountaineering specialists. We will let you know as soon we have the idea how to improve. And we will inform you about everything regarding this case.
Best greetings from Munich,
Reiner’s explanation sounds correct to me. Examine the Pure Freeride backcountry skiing binding on a workbench. In touring mode the Pure only allows about 60 degrees of lift before the toe unit hits the ski. Other randonnee bindings have much more range of motion (e.g., Fritschi 90 degrees).
Continue your exam, and you’ll see that when you’re in touring mode and take a forward fall, the hinging toe unit allows your boot to be forced backward with what could be extreme force. To compensate for this the forward pressure spring allows the heel unit to slide on the binding frame, but once that movement bottoms out, something has to give. One would hope the vertical heel release would then actuate, thus saving the binding, but forces in this situation are complex. It appears the rearward (damaging) force on the binding is high, while the input of force required to activate a release is low in comparison. In our case during the Trooper Traverse, I believe it didn’t even take a fall to cause this type of damage, just aggressive movements of the ski by a big strong guy with big boots.
|Silvretta Pure ski binding on the bench, showing how the binding frame can only rise about 60 degrees before toe unit impacts ski, then drives boot rearward, thus stressing the heel unit.|
To answer Reiner’s query for suggestions: the obvious idea is that the binding heel unit simply needs to be stronger, so a vertical release will occur BEFORE the rearward force breaks the binding.
Another suggestion is that the binding toe be constructed in such as way as to allow more pivot range while in touring mode before the binding toe hits the ski. And when the binding toe does hit the ski, perhaps it would be good to have a soft bumper in that area rather than hard plastic.
Note that the reason for this problem with the Pure is that the binding touring pivot is located farther to the rear than most other backcountry skiing randonnee bindings. This helps with ergonomics, but introduces a problem with toe clearance because the toe unit drops farther during a stride. Also, let’s not forget that virtually all AT bindings exhibit rather self-destructive behavior when subjected to the “knee fall” in touring mode. Something has to give in this situation. If a vertical (forward) release does not occur, then the binding will either break or rip screws out of the ski. The more range of motion the binding allows the less common this situation is. Dynafit and Fritschi, for example, allow quite a bit of range so problems with knee falls are super rare. Pure has less range — thus our concern after seeing this problem in the field.
Perhaps some of you engineers out there have ideas? Comments are enabled!