You might think that ski manufacturers would have some standard for determining where your feet will stand on your skis relative to their length. As far as I can tell, you would be wrong, because they vary dramatically…and manufacturers will even vary the recommended mounting point from year to year!
- Mounting bindings too far forward on a ski makes it feel “short” and unstable for its length, makes skating awkward, and possibly increases the risk of ACL injury.
- Mounting bindings too far back on a ski makes it feel “long” and sluggish for its length, decreases rebound, and when taken to an extreme, makes it difficult to hold a carved turn.
After years of experimentation with many different skis and with bindings that allow fore-aft adjustment, I’ve found that one method consistently produces good results for me with almost every ski:
Ball of Foot at Center of Running Surface (BOF/CORS)
Known as BOF/CORS, or just BOF, as in “I mounted these at BOF”. I did not invent this technique: I’m sharing it because it works so well for me.
For those who aren’t sure what I mean by “running surface”, here’s an illustration (marked as “running length”). It’s the part of the ski that contacts the snow when you’re standing on it.
Diagram is from skibuilders.com, a great resource for anyone interested in ski technology and construction. Click the picture to visit their site.
Our aim is to put the ball of your foot over the exact center of the running surface, because it’s where your weight should be while in a proper skiing stance. (If you feel like you’re falling forward when you try, or you spend a lot of time leaning on the back of your boots, you probably have too much forward lean and/or ramp angle in your boots and bindings, which is a common problem…but that’s another article for another time.)
I have never found going forward of BOF to be beneficial, and I have found that going behind BOF more than 15-20mm generally makes a ski feel sluggish and unwieldy. If a ski is for hardpack only, BOF is fine: if a ski is to be used offpiste, 15mm behind hardpack will usually give it a touch of extra float without sacrificing hardpack performance.
It is often instructive to find out where BOF/CORS hits relative to the manufacturer’s mark, even if you’ve already mounted your skis. Alpine skis today are generally marked with a midsole mark, which shows where the middle of your ski boot should rest. (Most boots will have this mark molded into the shell already.)
How To Find BOF/CORS For Your Own Skis and Boots
You will need: a pencil, crayon, marker, or something that will mark your boot and the ski’s sidewall (a grease pencil is great if you have one); a tape measure; a business card or credit card; a small hammer or large screwdriver; and a straightedge of some kind.
Step 1: Find the running surface.
- Hold skis firmly together, base to base.
- Slide the card down between the tips until it stops. Keep the bottom edge of the card horizontal or you will get inconsistent results.
- Mark the sidewall of the ski there.
- Slide the card up between the tails until it stops.
- Mark the sidewall there.
- I recommend repeating this for both edges of the ski, and measuring several times until you get a consistent, repeatable result.
Step 2: Find the midpoint between the two marks. This is easy: just use a tape measure and divide the length of the running surface by two. (If you’re bad at math you can fold the tape measure in half to find the midpoint.) Mark this midpoint on both sides of the ski’s sidewall.
Step 3: Find where the ball of your foot lives in your ski boots.
- Put one ski boot on.
- Find where the middle of the ball of your foot is by tapping the shell with the screwdriver handle or small hammer.
- Mark the side of your boot there.
- If your boot doesn’t already have a midsole mark, measure the length of the sole, divide by 2, and make one. Note: it is good to doublecheck the manufacturer’s mark, because they have been known to be wrong!
- For greater accuracy, put on the other ski boot and repeat these steps. The ball of your foot should be in the same place on both boots. If it isn’t, you either have weird feet or you need to remeasure.
Step 4: Make the new midsole line on the ski.
- Put the ski on a bench or floor.
- Align the “ball of foot” mark on your boot with the mark on the ski from Step 2.
- Find the midsole mark on your boot, and mark the sidewall of the ski where the middle of your boot is.
- Repeat this for the other side of the ski, and use your straightedge to draw the new midsole line across the topsheet of the ski.
Step 5: Make a midsole line on the other ski at the same place. I recommend measuring the first midsole line using a tape measure pulled straight from the tip, and repeating that measurement to mark the second ski.
Congratulations! You have just marked the midsole mounting line for BOF/CORS. Again, I’ve found BOF to be best for hardpack/groomer duty, and ~15mm behind BOF to help for skis used offpiste.
It is often interesting to compare this to the manufacturer’s recommended line—especially for skis which are already mounted and you’ve already skied.
Caveats!
- This method obviously does not work with reverse cambered skis.
- This method may produce nonsensical results with skis that have very gradual or subtly rockered tips (e.g. Moment, some K2) and no tail rocker.
- Measure twice, cut once. It’s always best to double-check your measurements.
Basically, this method only works when the running length of the ski is relatively constant and easy to determine, and when any tip or tail rocker is strong enough to shorten the ski’s effective edge. If you are in doubt or the results seem nonsensical, simply use the manufacturer’s recommendation.
I hope this information is useful! If anything isn’t clear, or if you have your own opinions on choosing a mounting point, please leave a comment.
Also, if you’ve measured BOF on your own skis, please leave a comment that tells us where your new mounting position ended up relative to the manufacturer’s recommendation. (Please include your BSL.)
Hi,
I’ve had a look at Sal Lord 177 and Sal Amber Origins 158.
From the ski specs the Lord mid sole mark is 791mm from the tail. The white line across the ski is only 770mm from the tail when measured with a straight tape, 772mm if the tail curvature is followed on the top sheet and 777mm when measured along the base. I have also attempted to measure the centre of the running surface (CoRS) along the ski. This comes out at 89mm ahead of the white line, or at 859mm from the tail. It is probably incorrect as the skis have rocker.
I have also checked the boots which have a nominal BSL of 297mm, actual measurement is 295mm. The position of the ball of my foot (BoF) is approx 40mm ahead of the mid sole mark at 147.5mm.
On the ski, the BoF is approx 810mm from the tail and 49mm behind the CoRS, possibly incorrect.
On the Amber origins, there is no mid sole mark. Using boot MSM, the mark is 635mm from the tails. The CoRS is 45mm ahead of the MSM, and just about ties in with the BoF. The BoF is then 680mm from the tails and Sal quote 686 for the MSM.
Perhaps Sal MSM quoted measurement is actually the BoF??
Thank you very much for clarifying the ball of foot and centere of running surface, this is how i have imagined the correct boot positioning to be, now all i need is a mechanic to put my boot where i want it!
ATB Barry
Barry:
It's always worked for me. Given that a correct stance should position your weight directly over the ball of the foot, BOF/CORS makes sense.
If you want your skis mounted forward or back, it's best to clearly mark, on the ski, where you want the midsole BEFORE giving your skis to the shop! In my experience, directions like “I want these skis 3cm forward” often get lost in the shuffle.
Of course, I now mount all my own skis in order to avoid such problems. If you're comfortable with a drill — and ready to assume liability for your own mistakes — go here for paper templates that will make the process much easier. (Hint: it's best to practice on a pair of dumpster skis before mis-drilling something brand new that you just spend $600 for.)
JS
I own two pair of skis: The Völkl Mantras and the Völkl Ones. The bindings on both skis are mounted on the Völkl reference line. The Ones ski great, but on the Mantras I often find myself leaning back in the boot to make the them go faster. Would moving the bindings a bit backwards help me get a more neutral to forward stance? Any help appreciated.
Peter:
I'm not sure about “going faster”: it mostly has to do with balance in a turn. Try sideslipping straight down a flat, steepish groomer: if your tips tend to hang up more, you might be too far back, and if your tails tend to hang up, you might be too far forward.
JS
This is a nice, thank you.
Question. Have you ever tried to sort this out for a ski that has rocker?
I am measuring a ski that has a combination camber and a good amount of tip rocker. I measured CORS and Center Of Side Cut (measured the same as CORS, not center of radius). There is 12cm between these two points on the ski with CORS closer to the trail and COSC
It turns out the factory mark is 1 cm behind center of the two (+5cm CORS -7cm COSC). Not sure if this was intentional or not, I haven’t measured rocker skis before.
I think that the ball of foot should be centered on the center of the apex formed by the side cut. Find the center of the side cut radius by measuring the width of the ski at the center. It increases toward the tip and toward the tail. Find the center of this apex and mount the ball of foot at this center.