Bootfitting for Backcountry Ski Touring

Spent more quality time backcountry skiing and lift skiing with my latest test boots. The stock liner is a robust inner boot that works for most people, but in my case, no joy. I’ve got narrow feet, and want room for my toes in a backcountry skiing and ski alpinism boot, so I went with a shell size larger than I’d have picked for alpine skiing. Even after careful molding of the G-Fit, with no sock and a thick custom footbed, I ended up with a hair too much room. Solution: grab a pair of thermal molded liners I had kicking around from another boot, and bingo, perfect fit.

My Thermoflex liners being baked at Sportfeet in Aspen.

As it nearly always does, a boot fitting problem yields to innovation and experimentation. Here are a few tips I’ve learned from this and dozens of other such incidents:

Pick the correct shell size for backcountry skiing by placing bare foot in shell, touching end with toes, then seeing how many stacked fingers you can fit behind your heel. Any more than a 2 finger stack and the backcountry skiing boot shell is too big. Less than one-and-one-half fingers and it’s probably too small. In some cases (as in mine) you’ll be between shell sizes. If that happens try fitting the smaller shell first, but remember it’ll probably be colder and you may have problems with getting enough length for your toes. The larger shell will be warmer and easier to get comfortable for touring, but you may have problems getting the fit tight enough for downhill skiing.

Be willing to experiment. Try different liners and play around with custom footbeds. Different brands have VERY different fits, so don’t get stuck on one brand/model.

When molding thermo liners for backcountry skiing (or alpine skiing) use a nylon stocking instead of a sock. When doing so, mold with the usual toe cap (under the stocking), but add a small wad of duct tap to any toe areas that tend to need more room (cover with tape so the spacer doesn’t stick to the inside of the toe cap). Liners molded this way may feel slightly tight for a few days, but after a few days use they’ll usually pack out to perfection. If not, re-mold with a thin sock.

You can mold thermo liners at home, but doing so can be a real freak show the first time you try it. Best is to do it at a shop with expert help, then mold at home after you’ve seen how the pros do it.

Here are thermo boot liner baking instructions gleaned from my own experience and various sources on web:

Put the liners in a convection oven at 210 – 250º F (depending on brand) for 12-13 minutes, or use regular oven with plank of wood on aluminum foil to keep radiant heat from scorching the liners. If you use a regular oven, pre-heat then turn off soon after placing the liner in the oven, to prevent scorching. With nearly all ovens, use an accurate oven thermometer to check temperature.

Most liners use similar temperatures, yet care is required. Sometimes you can find the correct temperature in the pamphlet sold with the boot. Other times, the information will be arcane and difficult to acquire. Examples:

Scarpa’s heat blower system is set at 246°F, this is an Intuition liner so their excellent home molding process applies. If using an oven I’d tend to go a little cooler, try 240°F.

Atomic Memory Fit oven operates at 117°C – 242.6°F. This would probably work for using an oven at home, provided your thermometer was accurate.

When the liners are “baked” in an oven they will look big and puffy. It’s better to heat them in the boot as you won’t have to struggle with placing the wormy puffed liner into the boot before you put your foot in.

If you have any sensitive areas on your foot that could result in rubbing/pressure problems, duct tape foam spacers on such areas to create extra room in the liner. Ditto for ends of toes that may not have enough room. If you use thick spacers, bevel the edges.

Put your footbeds on the bottom of your feet, and a toe cap over your toes. If you don’t have a toe cap, use tips from 2-pair of socks. Place a women’s stocking foot over everything to hold the parts together. Don’t wear any sort of sock, as the liners will pack out and compress quite a bit as you use them, and fitting them without a sock will make them tight enought fit perfectly with a sock after just a few days of use.

The toe cap should cover your toes and come down to the ball of your foot.

Prepare backcountry skiing boot shells by making sure buckles are easily worked and power strap is out of the way. Switch boots to walk mode. Sprinkle some talc powder in shells to ease entry of puffy baked liner — or better, heat liner while it’s in the shell (see Intuition method linked above). Make sure there is nothing inside the boot that will catch the liner and tear it, or keep it from going in smoothly. If such things exist, cover with some duct tape or something so the liner can go in easily. Many ski touring boots have a vertical tongue of plastic in inside the shell in the rear, take care this doesn’t get folded down while you’re molding.

Following must be done quickly:

(Assuming you’re using the oven method), take the liner out of the oven and wrap it around your foot, making sure the overlap or tongue is configured correctly. Have an assistant shift the liner around so that the seam on the bottom of the liner sole goes exactly down the center of your foot. Having help from an assistant is important.

Stand in the liner and pull up on the cuff to make sure your foot is all the way in. Then put your foot (with the liner) into the shell. This method prevents wrinkles from forming in the liner due to compression, and it also prevents the footbed from getting warped. When placing foot in shell, have your assistant hold the shell open and help the liner slide in.

To further make sure that your foot is all the way in the liner, have your assistant hold down the shell of the liner, and lift your heel and the liner up about 2 inches. Then while pulling up on the liner, push the liner back down into the shell with your foot. Do this procedure twice, but don’t over-stretch the liner by pulling it up too much towards your knee. Too much stretching and pulling will result in the liner being too thin and high. The foam inside the liner, when heated, is easily damaged.

Tighten the boot buckles half-tight, so that the liner just molds to your foot. Do not tighten the buckles any further while the liner cools.

Hit your heel on the floor several times, and flex a few times, to get your foot into the heel pocket of the boot. Do the process with the other foot. Then wait 10 minutes while standing with your toes up on a book or some kind of spacer that’s an inch or two high, so your heels are low. Try to maintain a static stance that’s similar to your downhill skiing stance. If you require cuff alignment (canting) be aware of which way to bias the mold. For example, if you are knock kneed use a slightly bowlegged stance while you wait for the liner to cool. Stay in the boots until the liner cools completely.

To fine tune, you can reheat parts of the liner with a heat gun to puff it out a bit, or compress on a boot press if it’s too tight in spot areas. Most liners also “ski in” a bit, so slight imperfections in the molding process can be remedied by just going out and using.

You can re-bake liners a number of times, but each time they will puff slightly less.