Does Friction Depend on Contact Patch Width?
An Experiment - Part 1: Unloaded Static Friction
Abstract (aka tl;dr)Made a bunch of measurements and calculations of same durometer and material skateboard wheels with different width contact patches to compare how the contact patch width affects grip. Turns out it doesn’t as is to be expected.
IntroductionSome of you probably know me as the annoying fucking guy who constantly argues that wheel width has no effect on grip. Some have probably gotten tired of it. I have too. But instead of giving up on it I decided to go full in.
Materials & MethodsSo I got myself a set of 54 mm 99A Spitfire Formula Four Classics and a set of 58 mm 99A Spitfire Formula Four Conical Fulls (Pictures 1 & 2) and devised a bunch of experiments to actually get proof of how it actually is.
Picture 1 - The wheels
Picture 2 - The wheels again
According to the Spitfire website (
https://www.spitfirewheels.com/formulafour/) the contact patch width for the 54 mm Classics is 16.5 mm and for the 58 mm Conical Fulls it is 27.3 mm. I measured this myself too and it seem to hold true (Pictures 3 & 4). So the Conical Fulls I tested are 65% wider than the Classics.
Picture 3 - Conical Full 58 mm width
Picture 4 - Classic 54 mm width
According to many people here on Slap (& elsewhere) the Conical Fulls should therefore have 65% more grip.
More proper term for grip would be static friction. The force that is required horizontally to impact on the object to make it break into a slide. This can be measured in various ways. I got myself a 800x710 mm sheet of plywood from a local shop (Picture 5). I used birch plywood cuz it was cheap and it really doesn’t matter what it is for this test.
Picutre 5 - The plank
I placed the skateboard on the plywood sideways and lifted the plywood slowly until the board starts sliding. I measured the angle where the board starts sliding with my iPhone 8 and an app called Bubble Level which shows the angle to one decimal accuracy. To keep things repeatable I taped the phone to the plywood so it wouldn’t slide or budge. Lifting I did by hand slowly trying to avoid vibrations that would help the board breaking loose.
To make sure the board wouldn’t roll off from the side of the plank I tightened the axle nuts so tight that the bearings were essentially locked (Picture 6).
Picture 6 - Nuts cranked the fuck down
I made 10 replicate measurements with each set of wheels.
Both wheel sets used were brand new and in packaging. The plywood sheet was also new and fairly clean. I “broke in” both sets of wheels by tilting the plywood sheet with the skateboard until it slid to the bottom. I did this 10 times after which I rotated the skateboard 180° and repeated the procedure 10 more times. I did the same for both wheel sets.
For the setup I used a symmetrical Real Ishod 8.3 Twin Tail Slick that has been slightly skated, Thunder 149 Titanium Lights which have also been very lightly skated. (Picutre 7) Bearings were very lightly skated Spitfire cheapshots (Picture
, bolts Thunder Allen 1” and I also used Bones 1/8” risers just cuz they were on the setup to begin with. I didn’t use the axle washers to help me lock the wheels better by over-tightening the nuts.
Picutre 7 - Board and trucks and such
Picture 8 - The bearings
Coefficient of friction is what is used to describe friction between two surfaces. From the sliding angle the coefficient of friction can be obtained by taking the tangent of the angle. With the coefficient of friction you can determine how much force will be needed to get the object to slide. If we assume an object to weigh 50 kg and the coefficient of friction of being 0.5, on earth the normal force the surfaces pushes back up on the object is (50 kg * 9,81 m/s^2) = 490.5 N. With the coefficient of friction being 0.5 we would need a horizontal force of 0.5*490.5 N = 245,25 N to get the object to slide.
ResultsThe coefficient of friction for each measurement can be seen in figure 1.

Figure 1 - Measured static friction coefficients of both wheels
There are slight variations in the coefficient of frictions but overall they are fairly close to each other. This chart alone doesn’t really visualize the situation so I calculated values for both wheels based on the other wheels’ measured data using the assumption that contact patch width is directly proportional to friction. These values along with the original measured values can be seen in Figure 2.

Figure 2 - Measured static friction coefficients and calculated “assumptions” for both wheels
ConclusionsAs can be clearly seen from the bars in figure 2, increasing/reducing the contact patch width does not increase/reduce the friction of the wheel in proportion or at all.
A possible source of error on my experiments are vibrations on the sheet of plywood due to me lifting it by hand. However I did try to minimize bad vibez by taking it easy and listening to some good music while lifting. There were also no significant outliers in the data suggesting the presence of bad vibez.
Another possible source of error is contamination of the sliding surfaces. I made sure not to touch the sliding surfaces with my hands and all materials were clean and new. Nor were there any noticeable changes suggesting contamination would have at some point affected the slide.
EDIT: Another source for variability between wheels is manufacturing tolerances. I have no idea how uniform the batches coming from the Spitfire factory are but I would think there might be slight variations in various properties of the wheels between batches.
This is part 1 of my friction testing of skateboard wheels. This test only takes into account static friction on an unloaded skateboard. Possible deformation of the wheels under load and its effect on friction has not been taken into account in this experiment.
In conclusion it seems that the laws of physics also apply to skateboard wheels. I will continue my tests however just because I’m fed up with wrong information being spread here about this subject.