Climbing skins are an essential part of the equipment carried by most modern backcountry skiers. They are removeable traction enhancers that allow the skier to walk uphill on snow. They are removed before downhill travel. Climbing skins are long-term reusable items with a lifetime expectancy from one to five years or so. The existence of climbing skins makes it possible to use a pair of skis that would otherwise be good for only downhill travel, as a general-purpose vehicle for ski touring any distance over snowy terrain -- even if it's far from level. The goal of climbing skins is to allow for both significant forward traction when travelling uphill, and significant forward glide under the skier's stride when the ground is relatively level. In actual practice much of the glide has been traded for increased uphill traction.
Construction
A climbing skin consists of a strip of material cut to approximately the length of a ski; it has a reuseable and very surface-specific adhesive on one side, and a one-way traction material on the other. It also has a means of attaching the forward end to the tip of the ski. There is, in some cases, a means of attachment to the tail of the ski as well.
Materials
Seal skin was the material of traditional climbing skins. This worked very well, with the water-resistant fur giving excellent traction over snow and ice, while also providing a nice slippery one-way glide -- exactly what worked so well for the original owners of the pelts. Seals being lovable and in short supply, modern manufacturers have switched from seal skin to mohair and nylon.
Mohair climbing skins give a good combination of traction and glide in drier snow conditions (see snow to water ratio). Mohair's relatively long, fine fibers spread and flex to acquire traction in even very soft, light, powdery snow, yet the same flexible fibers easily comb themselves straight and fold down flat when the skier is gliding forward. With inherently limited water resistance, mohair can ice up in wetter snows in case of significant temperature swings. In North America, mohair is more often used for climbing skins in the lighter, drier snows of the interior ranges and less often in the wetter maritime mountain zones.
Nylon has become the material of choice for the bulk of modern climbing skins because of its water resistance. To envision nylon's particular utility in climbing skins, imagine a pile fabric made of individual nylon bristles attached to some sort of woven backing material, each bristle being approximately 3mm to 5mm in length, and with the entire rank and file of these bristles attached to the backing material in such a way that every bristle is angled in one direction, toward the tail end of the skin. Think of a legion of Roman soldiers kneeling and holding spears thrust forward and upward at particular angle. Typical pile angles in nylon climbing skins are about 15 to 30 degrees away from lying purely flat. Although nylon does absorb some water, it is relatively waterproof for its low cost and is willing to act reasonably hydrophobic (for a few hours per application) when treated with a light wax. Another reason nylon has captured the widest market share in climbing skins is its durability; nylon skins can take many direct hits from rocks and roots and can handle a lot of walking across patches of snow-free ground before losing any noticeable degree of function or longevity.
Adhesives
The existence and popularity of climbing skins has necessitated a great deal (relative, at least, to a pretty small niche within the outdoor products industry) of adhesive research and development. The success or failure of any model of climbing skin rests as much in the reliability of the adhesive side as with the qualities of the traction/glide side. The adhesive side of a climbing skin has to stick to the base of a ski that can be anywhere from far below to well above freezing. It needs to stick even in the presence of water; it needs to displace water. It needs to be entirely removable, coming off of the ski entirely when the user pulls hard enough in the right direction. The skin must remove from the ski leaving no trace whatsoever of adhesive, or else the ski will not glide smoothly over snow in downhill use. This would ruin the entire backcountry skiing scenario. So this adhesive is both required to stick to something that is patently nonstick (a ski base which is often waxed to make it even more slick) and to fully desist from sticking when asked to. It's a wonder of physical chemistry that these things don't have to be semi-sentient to perform as needed.
The adhesive side of a climbing skin is usually the weakest link in the life expectancy of the product; fortunately skins can be reglued if there be sufficient perseverance (read: masochism) in their owner. See maintenance discussion below.
Attachment
In addition to the adhesive layer which temporarily/removably connects the skin to the base of the ski, there is always a mechanical attachment system. This is necessary to keep friction on snow and collisions with obstacles from forcibly removing the skins from the skis (replicating the motions that the user would make to peel the skin and its glue cleanly off of the ski). Primarily, there is always a system of tip attachment which connects the forward end of the skin strongly to the tip of the ski. Tip attachments usually consist of a wire or cable loop that hooks over and around the tip of the ski, either with or without a rubber section that provides for some variable stretching and tension in the skin-to-ski relationship. In addition to tip attachments, there is the potential for tail attachment for additional steadfastness.
Tip-only attachment systems consist of just a tip loop connected to the skin, with no other provision for fastening the skin on the ski besides the adhesive coating running the full length of the skin. In this system the skin is usually trimmed to length several centimeters shorter than the length of the ski; in particular the skin should terminate some distance ahead of where the ski's tail curves up away from the snow surface (if applicable, in modern "twin tip" skis). A tip-only system can be perfectly adequate if glue quality is well maintained, particularly if most usage consists of very long climbs uphill followed by nice long descents. Where the tip-only system falls short is in situations where no current adhesive can possibly be good enough -- in very wet conditions where the skins are liable to absorb a good deal of water, especially in case of ambient temperature changes that can freeze the water thus absorbed. In such cases the ice in the skins (or ice, if any, stuck to the bases of the skis) can make it nearly impossible for the adhesive to do its job adequately; thus additional mechanical attachment beyond just a tip clip is desirable.
Tip-and-tail attachment systems are more expensive and weigh slightly more; that is their only disadvantage. A tip-and-tail system has a fixed clip for one end of the ski and a clip with a rubber "stretcher" at the other end. The stretch built into the system puts a light end-to-end tension in the skins, which assists the user in applying them straight and true (see discussion of application, below) with a minimum of fuss. Formerly it was common worldwide to have the stretcher at the tip and a fixed clip riveted to the tail end of the skins. In recent years American manufacturers have switched to locating the stretcher in the tail assembly and keeping the tip clip static. The move from tip stretchers to tail stretchers removes some bulk and clutter from the tip ("shovel") end of the ski; as a result some skiers feel the new system is a good deal more wieldy.
Tip clips are available in several styles and many sizes to account for the many widths and shapes of ski tips, from skinny pointy traditional tips to modern wide skis with a more blunt (or even squarish) front end. It is advisable to carry a spare tip clip on any backcountry adventure (they don't weigh much nor do they take up much space) and possibly two spares on longer, multiday tours. The consequence of losing or breaking a tip attachment is that the user must either refabricate one somehow, or revert to the most primitive of means of attaching skins for the remainder of the trip: lashing them temporarily to the ski with duct tape or other forms of strapping. It has often been said that any good backcountry repair kit should contain duct tape and baling wire, and the importance of skins to the overall endeavor is one of the main reasons why.
Tail stretchers are available in several different styles, perhaps not worth describing in specifics since they may be expected to continue changing and developing in an active market. It is worth noting that before tail stretching tip-and-tail systems became commercially available, many skiers were fashioning their own tail stretchers as popular homemade add-ons to their tip-only attachment systems. The most common design of homemade tail stretchers involves a stretchy cord that is sewn to the tail of the skin and made long enough to wrap all the way past the tail of the ski, then extends forward along the upper surface of the tail of the ski and eventually attaches tightly to either the ski binding or some other attachment (like a snap or hook) fastened to the surface of the ski.
Sizing and initial setup
Proper sizing of climbing skins is a very common question, given the rapid changes in dimensions of skis throughout the past several decades. Once upon a time skins were sold at various lengths; this is in most cases no longer done since it is not efficient for packaging and display reasons. Skins now are available in one length which is long enough for the longest skis and easily cut down to match shorter ski lengths. This implies a lot more materials are being wasted, but it looks as though that concern is outweighed by the need for simplicity in packaging and display (the need to not confuse the consumer with a two-dimensional matrix of choices of what will fit their ski). Here and there you can still buy your climbing skin material, pre-glued, off of a large spool, cut to length at a retail shop, along with tip and tail attachments a la carte. Manufacturers, however, seem to be discouraging this even though some of them still make such spools available.
With the variable of length out of the way, the remaining size concern is width. Before the modern trend of significant sidecut in ski shapes, skis were more straight-sided and it was easy to select a width of skin that needed little if any additional trimming other than being cut to length. Currently, most desirable skis show major changes in width from tip to waist to tail, which makes a single-width strip of climbing skin material functionally inadequate on the snow. A simple strip is unable to offer enough traction near the tip and tail of modern ski shapes in all but the gentlest terrain and least demanding snow surface conditions. For this reason it has become popular to buy climbing skins to match the width of the tips and tails of a pair of skis, then trim the width of the skins to custom-fit them to mirror the profile of the ski from end to end. When done correctly, this results in a climbing skin that is congruent to the shape of the base of the ski, just slightly smaller -- small enough to ensure that the steel edges of the ski will always be exposed and able to grip the snow in sidehill (i.e. traversing) situations.
An additional setup concern in the case of tip-and-tail systems is the need to set the tension in the stretchers. This tension should be just barely more than enough to straighten the skin out when the tip and tail clips are fastened to the ski, but not enough to pull the skin tight like a bowstring. Insufficient tension would allow the skin to creep around on the base of the ski over the course of a longer tour; excessive tension can contribute to pulling the skin adhesive loose from the ski at unwanted times.
Application and Removal
Application: for tip-only sytems, simply fasten the tip clip to the ski tip, then stretch out the skin, align it carefully with the base of the ski, then smooth the skin onto the ski, rubbing from tip to tail, ensuring no wrinkles or bubbles are between the skin and the ski. For tip-and-tail systems, fasten whichever end is the fixed end, then stretch out and align the skin with the ski base, then fasten the clip at the stretcher end, then smooth the skin onto the ski and ensure no wrinkles or bubbles.
Removal: for tip-only systems, locate the tail end of the skin and peel it off of the ski base, continuing to pull on the skin (experiment to find an angle of pull that most easily breaks the adhesive bond with the ski) until only the tip clip is left attached. Finally, disengage the tip clip from the ski, then fold and store the skins. For tip-and-tail systems, remove the stretched end first, then pull at an angle to remove the skin from the ski base, then remove the static end, fold, and store.
Folding and Storage
Of primary importance to the longevity of climbing skins is that the user prevent any material from getting trapped in the adhesive. Skins like to get full of dirt, dust, pet hair, and plant parts. This decreases their useful life as they sink toward a state in which they will eventually have non-working adhesive at the edges, which leads to snow penetration between skin and ski. Any such snow penetration quickly ruins the straight tracking of the ski as the user steps forward and spells a quick end to the utility of the skins (at least until the glue can be cleaned and possibly renewed).
It is therefore necessary to fold climbing skins in such a way that the adhesive is exposed only to itself or some other item that is clean, free of any loose material, and able to let go when the skins are ready to be deployed again. The typical storage of skins has been to fold them in half, glue-to-glue, trying to avoid too much area at the edges where one skin does not overlap the other. There is now available a plastic non-stick mesh material that is meant to be trapped in the middle like the contents of a sandwich when skins are folded onto themselves, which lets the glue release from itself much more easily when the user is preparing to apply the skins to skis. This plastic mesh also contributes greatly to the longevity of skins by minimizing the difficulty of pulling the adhesive apart from itself when deploying/applying the skins, which consequently minimizes the tendency of the adhesive to pill up (agglomerate) and thereby move itself increasingly from areas of less adhesive to areas with more, until eventually there are sticky messes in some spots and non-adhesive areas in others. Most skiers claim to be too speed-minded to bother with these plastic "cheat sheets" on wilderness ski days; nonetheless, the plastic has been proven to help skins last much longer.
Storage temperature: climbing skins must be stored in a dry place at moderate temperatures, certainly not anything that feels warm to the touch. A very small amount of unwanted heat is sufficient to greatly increase the strength of the bond between the adhesive and itself in a folded-over skin, making it nearly impossible to unfold and redeploy without damaging the adhesive. Above all, do not hang your skins on your skis in direct sunlight for time enough to get them warm to the touch or even nearly so. This can result in a great deal of the adhesive becoming stuck to the ski instead of the skin. This is a Very Bad Thing.
Maintenance and Repair
Most of the time there is very little required maintenance for climbing skins aside from proper storage. The edges can be lightly singed with a lighter, hot knife, or other controlled heat source, in order to prevent the base fabric and/or the pile from fraying at the sides. If re-gluing is necessary, the preferred alternative is to soak the entire pair of climbing skins in kerosene and have a nice bonfire. There is a black art of re-gluing skins but most people avoid it to the point of not asking even a hated enemy to perform this unctuous task in one's stead. Other alternatives are exploited, such as selling the pair of skis to someone else, and the matching pair of skins at the same time.
If skins absolutely must be reglued, here are some tricks. First, if it appears that just a slight retouch in certain places would be sufficient, then buy a small can of skin glue from a mountaineering/skiing shop and dab on a very minimal amount with a brush, only on the driest areas. Too little is easily remedied after testing, but too much or too thick of a coat too quickly can lead you to botch the job and begin all over (or abandon ship). Second, if mere retouching won't be enough, then you'll need to remove most or all of the old glue and do a complete reapplication. Heat and solvents (hint: citrus-based solvents are effective enough, while less likely to ruin your health, compared to the #1 skin glue remover, xylene) work but require a ridiculous amount of scraping and a vast quantity of vapors inhaled. The modern trick is to lay brown paper strips (such as might be torn from grocery bags) over the skins after tacking the skins to a work surface, glue-side-up, then run a hot iron over the paper, which then absorbs the glue and associated dirt and contamination into its other side. 100 percent of the existing adhesive does not have to be removed, just enough to remove most contamination that could interfere with adhesion. After glue removal, new glue is then applied, either from a can, or from a product called Glue Renew Strips, which are essentially an iron-on replacement glue layer, gauged to the proper quantity and thickness, with a release paper strip to be thrown away after successful application.
Operating Technique
For the most part, you'll get used to the unusual sensation of walking around on skis with one-way carpet affixed to the underside. There are just a couple of important pointers. One: slide the ski forward, do not lift it up from the snow surface unnecessarily (this leads to icing in the skins). Two: learn a good lunge step; this helps to get the most forward gliding efficiency from the skins, and can be delicately moderated to discover a degree of lunge that gives a bit of glide but stops short of incurring more energy cost to the skier by requiring an unnatural motion. Above all, you're not just walking around, toy-soldier style, on planks; you're engaging in something similar to classic cross-country skiing, just with a heavier kit.
Acquire a bar of skin wax, also known as "glop stopper", and make somewhat frequent use of it (rubbing the wax against the skins as you deploy them on the skis, leaving a light coat of wax), at least when you can bring yourself to remember. This will be necessary at some (not necessarily foreseeable) point in order to keep your climbing skins more hydrophobic, to avoid excessive moisture absorption and therefore icing or snow-clumping. Total breakdown of the system of travel can occur if too much frozen material is able to attach itself to the skins and skis, leading to a rig that is too heavy to walk with.
Do dry your skins well between uses, just don't use too much heat to do it. Think air motion rather than heat. Some skiers carry their climbing skins inside of their clothing when they're not in use (such as during the descents in a multi-lap backcountry ski day) in order to avoid icing. Of course if they do ice up you're in better shape if you have a tip-and-tail attachment system or at least a good roll of duct tape.
Alternative Means of Traction
Some of the older traditional means of bringing removable traction to the base of a ski include grip waxes, rope, and other objects lashed to the ski. Grip waxes deserve their own writeup; they are temperature-specific compounds that adhere just well enough to a target range of snow types but allow for glide when static friction is broken. They are applied temporarily and wear off over some distance traveled. There is also a foul substance called klister which bridges the gap between wax and tree sap; this helps supply traction over extremely soft and wet snow. Rope, long ago, was twisted and tied onto wooden skis in order to make progress uphill, then simply untied and removed for downhill travel. Tree branches or other vegetation could also be lashed or strapped to ski bases if needed. These methods are worth knowing in case of emergency situations in modernity, but remember that without a means of strapping or lashing they are not much help.
Skis are also available with mechanical traction built into their bases; these bases are known as "no-wax pattern bases", a somewhat confusing term since glide wax is still useful on the non-grip parts of such skis. These are lightly touched on in cross country skiing and should also be covered in their own writeup.
"Kicker Skins" are also available. The name is derived from "kick and glide" technique in cross country skiing. These are skins cut to roughly a third of the length of the ski, in order to give a moderate amount of traction underfoot while detracting as little as possible from the glide which the tip and tail sections of the ski can provide. Compared to full-coverage climbing skins, kicker skins provide far more glide but will not grip on steep hills anywhere near the gradients that can be ascended with full coverage. The point of kicker skins is to replicate the function of grip wax or a no-wax grip pattern, but in a modular, removable fashion, on a ski that was otherwise made to be smooth overall and oriented toward gliding downhill only.
Anything Else?
Climbing skins have recently been available with whimsical patterns such as bovine spots and tire tread, this despite the fact that skins spend most of their time where they cannot be seen. Attaching your friends' skins backwards (swapping tail end for tip end) will result in an intense but short-lived comic episode. It is possible to ski downhill with skins affixed to your skis, though this takes some practice in order to do it with any grace. This is sometimes done when a short downhill section interrupts an otherwise long and continuous uphill journey. It is also possible to remove and even apply climbing skins to your skis without taking your boots out of the bindings; proof of this is left as an exercise for the reader.