The friction coefficient of hiking shoes varies by model, and by wear, i.e. miles spend on the trail. In this article I’ll show you my method for comparing the friction coefficient of my favorite hiking and shoes: the Salomon X Ultra GTX line of shoes, the Merrel Moab waterproof line, and my Keen Newport H2’s.
Perhaps at some point REI will let me bring my testing equipment (a polished granite chess board, and a measuring tape) into their shoe department, and I’ll do a larger comparative study on the latest shoes. At least I can grab a snapshot of their friction coefficients when they’re new. But obviously I’d like to see how they hold up over miles of use on the trail, which I’ll do below with my personal collection.
If you want to see the results, scroll down the the ‘Results’ section or click the corresponding tab in the Table of Contents.
Table of Contents
- Why is the friction coefficient of hiking shoes important?
- Introduction to understanding the friction coefficient for hiking shoes
- Methodology
- Results
Why is the friction coefficient of hiking shoes important?
The friction coefficient of hiking shoes is important because a higher number translates to less slippage on the trail. And less slippage means two things:
Safety
You’re less likely to slip, and either fall, or twist part of your body such that it creates a strain, sprain, or irritates a bulging disc.
Efficiency
The less effort your muscles and balance center are using to compensate for slippery shoes, the more energy efficiency your muscles retain. That means more muscle energy can be expended pushing you forward on the trail, or less energy is being used avoiding slippage. Either way, you’ll be less tired when dealing with uneven, or high grade terrain.
This phenomenon was actually noticeable when I first started using my new Salomon X Ultra 4 GTX hiking shoes, after years of using my old X Ultra GTX’s.
I’ve been going to the same trail, over and over, recently to train for an upcoming Yosemite hiking trip. The trail – the Grotto Trail from O’Melvany to Mission Point – mimics the elevation gain for the Yosemite Valley hikes I plan to do. This particular trail is hard in that it has very steep areas of loose dirt and gravel. You struggle up AND DOWN these areas when tackling the trail.
There was a large subjective difference in using my new Salomon’s, that of course have a lot higher friction coefficient than my older ones, as the outsole is newer, stickier, and has intact tread. I felt A LOT less tired climbing that 1000 feet over 1.25 miles.
Related: See my original review of my old, second edition Salomon X Ultra GTX hiking shoes by clicking this link. A review of my new X Ultra 4’s will be coming soon, after I’ve used them hiking some bucket list trails in Yosemite.
Introduction to understanding the friction coefficient for hiking shoes
The friction coefficient for any two contacting surfaces is unique to those surfaces. The friction coefficient between my hands, when I rub them together to create warmth, is different than that when I rub my hand against a glass table, or rub it against a granite rock on the trail.
Thus to compare friction coefficients between various hiking shoes, you need a test surface that is the same for every test you do. Further the coefficients you calculate for this test surface is unique to that surface. I’m using a polished granite chessboard for my testing, but of course the friction coefficients for this board and my hiking shoes are going to be different that the coefficients found out on the trail.
But by using a slick surface for my testing, I’m designing that my calculated coefficients are on the lower end of real world use on the trail. That is to say a friction coefficient for a given shoe might be 0.71 for my chess board, but on the trail, it very well could be an average of 0.80 for the trail’s various conditions.
Indirectly measuring hiking shoe’s friction coefficient using an incline
To test my shoes’ friction coefficients I place a particular shoe onto my polished granite chess board, then I slowly raise one side of the board, creating an incline. At some point, as I’m raising the board, the shoe begins to slip down the board at a constant, non-acceleratory, velocity. This is the point where the force of friction between my shoe and the chess board is equal to the force of gravity vector pointed in the direction of the inclining chess board, i.e. the prorated allotment of gravity that is proportional to the magnitude of the angle the board makes with the flat ground below it.
Force of friction for a hiking shoe on an incline
The amount of gravitational force needed to overcome the force of friction for a hiking shoe on an inclined chess board is based on the cosine of the angle of the incline, the friction coefficient of the shoe/board, and the weight of the shoe.
F = u * cos(a) * w
Gravitational force in the direction of the incline
The gravitational force in the direction of the incline is based on the sine of the angle, and the weight of the shoe.
F = sin(a) * w
The point at which the shoe starts to move, via gravity, down the incline
The point at which the shoe starts to move is the point in which the two forces, described above are equal to each other:
u * cos(a) * w = sin(a) * w
But what you’ll notice is that if we algebraically divide each side by w, that variable dissolves to a multiple of 1, i.e. it cancels itself out.
u * cos(a) = sin(a)
And if you do the same operation for the cosine of a, you’re able to isolate the friction coefficient to one side of the equation:
u = sin(a)/cos(a)
And it turns out, mathematically that sin(a)/cos(a) is equal to the tangent of a, or tan(a):
u = tan(a)
So all we really need to calculate a hiking shoe friction coefficient is the angle of the incline at which a shoe starts to slide down it. We don’t even have to know its weight.
Methodology
As previously stated, I would simply place a hiking shoe onto my polished granite chess board and lift one edge of the board to create an incline. I would then raise that edge higher and higher, until the first signs of constant slippage were noted. Next, I’d measure the height of the bottom edge of the board, at which that occurred.
The shoe was placed in a matter where it was facing the inclined edge of the board, to simulate a person walking up a hill. It was also placed in the middle of the board, for the sake of balance.
The length of the board along the incline was noted to be 13.875 inches. The heights of the incline for various shoes was generally between 5 and 9 inches.
Using these heights, and the length of the board, I would calculate the angles of the incline, for my shoes, in the following manner:
A = sin-1 (h/13.875)
Then once, I had the angle, I would simply calculate its tangent
tan(A) = u = coefficient of friction of the hiking shoe.
Results
| Hiking/Outdoor Shoe | Height of incline of 13.875″ long chessboard at which shoe begins to slide | Friction Coefficients |
|---|---|---|
| Salomon X Ultra 4 GTX (new condition) | 8 | 0.71* |
| Salomon X Ultra 2 GTX (used, 8+ years old) | 7 | 0.58 |
| Merrell Moab 2 waterproof (1.5 months old, daily use) | 8.375 | 0.76 |
| Merrell Moab 2 waterproof (6 months old, daily use) | 7.75 | 0.67 |
| Keen Newport H2 (5 years old, occasional use) | 6.25 | 0.50 |
* The Salomon X Ultra 4 GTX outsole, for whatever reason, doesn’t lie flat on the ground, when they’re not in use. Because only a portion of the available area of the outsole was touching the chessboard, it therefore has a practical friction coefficient higher than the measured one.
References
Determining the Coefficient of Friction (by Ron Kurtus)
Finding an Angle in a Right Angled Triangle
Further Reading
Check out my other hiking gear articles on my page dedicated to such! See ya out there….