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A fun and friendly write-up on the basic scientific concepts which govern the urethane longboard wheel.
By Adam Ornelles
Rolling Resistance & Comfort
Rolling Resistance & Comfort
Rolling resistance is one of the two major factors in losses while longboarding*, below is some data from a field test of various wheels we had on hand in 2022 (not a tier list). *Aerodynamics begins to easily outweigh rolling resistance once you reach ~13 mph.
This data is a compilation of hundreds of roll-down tests with control tests in between, to verify accuracy over the same day over 10 hours. Watts in this dataset are relative to each other for this case, watts are the best metric to quantify energy loss, but in this test we don't have a good exact measurement of absolute resistance since there were other sources of loss such as bearing efficiency the absolute accuracy is likely low, while precision is likely high.
A takeaway you should have is that harder wheels may feel fast, but they aren't always fast on real pavement. The sound and feel of a fast wheel is the sound and feel of energy loss, although many prefer the placebo effect of a hard wheel. Just as how the professional cyclists stuck with rock hard 19mm tires for two decades before switching, despite scientific proof wider 25-28mm tires are as fast at more comfortable pressures, longboarders will continue to believe hard narrower wheels roll faster, due to their feel and sound.
*Rolling resistance value also includes bearing losses.
What causes rolling resistance?
What causes rolling resistance?
One process, hysteresis, is the loss of energy during the deformation of the urethane. In theory, the greater the deformation, the more potential for energy loss. This is the intuitive "less deformation, less losses" thought process with softer wheels.
Conceptually, urethane and it's ability to rebound are similar to the compression of air in a pneumatic tire, with the exception that urethane is incompressible and "deflects or deforms" rather than compressing. This means the sidewall shapes of a longboard wheel (shape factor), and urethane depth, makes more of an impact on ride quality than a typical air tire, where compressible air volume is more important.
However, we live in an imperfect world. Hysteresis is just one of two processes. Vibrational loss from slight imperfections in road surfaces can become a dominant factor, meaning we must have softness to comply, without becoming so soft we see losses from excessive deformation.
Energy Loss From Compression (Hysteresis)
Energy Loss From Compression (Hysteresis)
Urethanes, with perfect "low frequency resilience" a factor controlled by what we call rebound, would return 100% of the energy lost after deforming. 60-80% is more a realistic return with rubbers and urethanes.
For longboarding, the holy grail is a SOFT urethane formula, with perfect low frequency resilience to return the rolling energy, while still complying to road imperfections. An issue with this outside impossible physics is the lips over flexing, providing bad grip.
As total deflection of a urethane body (%) raises, it begins to lose efficiency due to hysteresis, which scales with compression stress. The goal is to find the balance between deformation for efficiency, with deformation for reducing vibrational losses.
Impactors on Compression and Efficiency
Impactors on Compression and Efficiency
Total urethane deflection, therefore energy loss, is a function of a few factors. Wheel diameter and width, as well as shape, control the max deflection depth. Urethane Properties, such as polymer structure and rebound impact return, even when durometers are the same.
The Shape Factor of a urethane body impacts the manner in which it deflects, since it is incompressible. With tires, the main factors are contact patch and inflation, since the air inside the car tire is compressible. It's different with urethane wheels. The durometer, contact patch size, and unsupported area of the wheel itself, will impact the wheel's grip and total efficiency. This is important to note, as core placement and lip shape impacts how the total body of urethane deforms, for example.
Effective Contact Patch
Effective Contact Patch
The effective deformed contact patch of a Balut, visible on my glass plate contact patch tester.
One key example of shape factor and incompressibility impacting a wheel is that the marketed contact patch size often differs from the effective contact patch when a wheel is loaded. As shown below, a wider wheel deforms less vertically under load, but the actual contact patch area remains nearly the same for wheels of similar durometer (or tire pressure in the example). By reducing contact patch, you increase the internal stresses in your tire or urethane, making a narrow wheel potentially more inefficient at a certain point. Narrower is not necessarily faster.
Intuitively, this could be described by the sensation of a narrow slide wheel feeling "in the pavement" as it slips, they experience greater internal stress than a wider wheel of similar durometer, allowing them to shred higher durometer thane rather than slip. These internal stresses can be viewed below in the next section on urethane properties.
Urethane Properties
Urethane Properties
Here is a table summarizing some modeled wheel deformation produced to benchmark testing data, while developing the Pantheon Karma. The higher the deflection, the higher the comfort. Deflection percentage is what impacts rolling resistance loss and describes total compressive strain on the urethane. Modulus is the average strength of the urethane, which is highly proportional to durometer but may vary between formulas, it's a good indicator of skin durability.
Impacts on Wheel Grip and Durability
Impacts on Wheel Grip and Durability
The deformation of urethane makes a large difference on grip. 5% deflection on a wheel's urethane seems to be the sweet spot for grip. Enough compression to strain the polymer, but not enough to tear too quickly. Sliding over the pavement, in the pavement, or icing out. For rolling resistance, ideally you would have LOW deflection %, with high total deflection to reduce vibrational losses. In reality, you can't have too deep urethane depth, as grip will degrade.
When designing a wheel, our objectives are to reduce urethane depth as much as possible, to prevent the wheel from being too flexible, resulting in unpredictable grip, while still maintaining a good level of internal stress for predictable grip and roll efficiency. Choosing urethane depth vs grip becomes a trade-off when designing an LDP wheel versus a downhill wheel.
Wheel Size
Wheel Size
Earlier, we discussed how wheel size can help reduce rolling resistance in terms of hysteresis due to vertical contact patch deformation. A larger wheel also decreases the attack angle on road imperfection, reducing vibrational losses. (A wider one can also help average out the imperfections in a road to some extent.)
At high speeds, smaller attack angles (from larger wheels) minimize vertical displacement, which in turn reduces the energy lost to “bouncing” or vertical deflection. As we mentioned earlier, much of the rolling resistance doesn’t come from hysteresis losses, but from these tiny vertical oscillations.
Fairly intuitively, as wheel size increases, attack angle on a 5mm bump decreases, this is proportional to our reduction in vibrational losses with all size imperfections.
Acceleration, Angular Momentum and Wheel Weight
Acceleration, Angular Momentum and Wheel Weight
Of course, larger wheels aren't all sunshine and rainbows... Linear energy, the direct acceleration of the weight of wheels and board, and rotational energy, the angular momentum to spin up our wheels, will be used to visualize the disadvantage of "big honking wheels" below.
In this acceleration example I took an extreme downhill start to visualize how an acceleration, due to work done by gravity and pushing, becomes an important thing to account for. Angular momentum and acceleration is only relevant in changes in velocity, which can be speeding up, or slowing down. Being relatively heavy, the angular momentum of your wheel can make acceleration more difficult even if it rolls more efficiently. The difference in rolling efficiency between the 80mm Kegel and 100mm Boa is easily outweighed by the forces of acceleration.
It would be much easier to get to speed on the Kegel, which would have knock on effects for the rest of your downhill, for example. Maintaining a higher speed thanks to a higher initial input results in drastic improvements in speed over the same elevation change (be it from a hill, in the classical rolling ball example, or a better initial acceleration).
Supplementary Materials
Supplementary Materials
To see how much your wheels and bearings matter, versus other factors, checkout the calculator.
Here are some unorganized notes on urethane properties relating to longboard wheels if you'd like to read more.
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