Originally Posted by royal
great explanantion, also it was my understanding that there was/is a related issue with the marker M4-MRR toe's. if the forward pressure was too high the boot could move forward as the toe wing opened. the toe then did not have enough force to push the boot rearwards and back to center. thus leaving the boot toe stuck partially out of center and forward. I was taught to always be very careful with marker forward pressure settings for this reason. maybe a result of the pivot locations and toe wing design(no rollers)?
I have always liked the single pivot toes because of the long elastic travel and any external recentering forces help the binding get back to center as the boot pushesd back on the opposite toe wing thus "pulling" the toe back to center not just relying on the internal spring force of the binding. although, I must say I've never felt less safe in non single pivot bindings. another reason I like the single pivot toes is that I can easily dismantle them clean,deburr, and regrease them. I've yet to find an alternating pivot toe that was serviceable. (I do recognize that in todays litigious society that this is a disadvantage from the manufactures point of view, but I like that I can get inside anyway)
@royal Vector analysis does not have to be imagined. Forces, torques and moments can be measured. Functional requirements can be measured. Forces can be measured between boot contact surfaces of toe components and the boot. McMaster-Carr sells special, thin, 'foil' that registers compression forces by color. You can place this foil between the contact surfaces of the binding and the boot to see where and how much force is exerted at any point on these surfaces at which you chose to locate the paper — during any phase of rotation and translation of the boot relative to the ski. From this exercise, you will see that the location and magnitude of the forces are Very Different from what you are expecting.
There are so many ways to analyze this issue that you raise. While performing your analysis, pls keep in mind the following:
(1) From a 'release' perspective, lateral toe release only deals with tibia fracture mitigation: in one recent key study, tibia fractures have a prevalence of less than 1.7% of all skiing injuries — and an incidence of over 40,000 mean-days-between-injuries (MBDI) (ref: Shealy & Johnson), though within certain sub-groups (children and racers) the prevalence is higher and the incidence is not favorable. Children have a tibia fracture prevalence of 5.1% of all skiing injuries and an incidence of 36,400 MBDI (the smaller the incidence-values, the worse the condition). The racing sub-group is similar. Never-the-less, in the early 1970's, leg fractures had a prevalence of ~25% of all skiing injuries and an incidence of ~2,000 MBDI (again, small incidence-numbers are not good). Therefore, because toes have a proven biomechanical effect on tibia fractures ( other studies point to the correlation ) — toes are doing a good job at meeting the release objective, today.
(2) From a 'retention' perspective, there is no public epidemiological data anywhere in the world that provides prevalence or incidence data pertaining to upper body injuries that arise from pre-release. That also means there is no public data available that differentiates toe pre-release from heel pre-release. We (the industry) are working on ways to start gathering and reporting that epidemiological data, publicaly.
However, standard test methods quantify lateral toe retention. ISO 9465 expressly pertains solely to lateral toe retention — and provisions within ISO 9462 expressly and extensively quantify lateral toe retention. There are no public standards for measuring forward pre-release. Among ski binding engineers, we know that the ISO test methods for pre-release are not representative of what transpires on-snow that often cause toe pre-release (these standards are too liberal). Therefore, I have my own on-slope proprietary 'skiability' test methods that quantify pre-release — I have my own proprietary laboratory test methods that quantify lateral pre-release and forward pre-release. Each of the major European binding companies have proprietary test methods that quantify pre-release. None of us are divulging all of these proprietary on-snow and laboratory retention test methods to anyone. These are trade secrets.
Both functional requirements (FR's) involving release and retention correspond to associated design parameters (DP's). Utilizing Axiomatic Engineering, a matrix can be constructed to evaluate whether FR's and DP's are decoupled or cross-linked. We can imagine DP's for decades, but testing (testing, testing, testing) for compliance with FR's takes imaginary DP's out of the picture. A binding's function either meets FR's or not.
Single pivot toes score significantly worse than non-single pivot toes in all FR tests for retention, on-snow and in the lab — though all single pivot toes presently on the market meet all ISO standards for release and retention.
Circling-back to DP's — The critical force that interacts with toe retention function is the centroid of the compressive forces that act between the toe and the heel during boot deceleration relative to the ski (during fakie's the direction of the centroid is inverse) plus the compressive force that arises between the toe and heel when the ski flexes. When a skier is skiing forward, the centroid of the forward sheer forces acting on the toe is far greater than the centroid of the aftward forces acting on the heel — due to the vector addition of the forward deceleration component when skiing forward. When the centroid of the forward shear forces that act forward from the boot is centered between any off-center toe pivot, that centroid together with its perpendicular bisector that connects to the center-line of an off-center pivot generates a torque that acts to recenter the boot. When the same centroid has displaced beyond the center-line of an off-center pivot, the polarity of the torque inverts — driving the boot to release. With single pivot bindings, as soon as the centroid displaces laterally, the centroid of the forward shear force immediately forms a torque that acts to release the boot. This is why in ALL tests, single pivot toes MUST BE adjusted higher than properly designed off-center pivot toes (including alternating pivot toes) in order to produce equivalent retention during real-world on-snow forward deceleration and/or during ski flex. The centroid of the forward shear component is cross-linked with the interaction between release and retention in single pivot toes; the same centroid is functionally-decoupled from interaction between release and retention in off-center pivot toes. The interaction between the boot and these elements of the bindings are described as 'kinematics'.
Circling back to another DP — elasticity: Elasticity is essential, but 'more' or 'better' elasticity will not necessarily cause 'more' or 'better' retention if the above kinematic interaction is weak. For example, Spademan (and many other bindings) has elasticity far superior to those cited in a post above. However, Spademan had poor retention. Why? Poor kinematics. Retention (an FR) is a consequence of many DP's, including but not limited to the combined interaction of (a) kinematics, (b) elasticity, and especially (c) FRICTION REDUCTION.
Regarding service/maintenance: There is one alternating-pivot toe presently on the market that can be completely disassembled for cleaning — but you should ask those who are presently in control of that technology whether such disassembly voids the warranty BEFORE attempting cleaning (and you may consider asking them if their products meet ALL ISO ski binding standards). Proper ski binding FR's rely on conformance to many critical but subtle assembly specifications, such as for example — washer polarity in order to keep the burr-edge away from a resin housing. That's just one example out of many examples regarding critical assembly specifications that must be met in order to assure proper FR's of ski bindings.
Testing, testing, testing — provides facts that quantify these issues.
Howell Ski Bindings
Stowe, Vermont USA
Edited by Richard Howell - 1/17/14 at 1:14pm