Why use polyurethane rollers (wheels)

• “Polyurethane is a non-compressible elastomer, meaning its volume will not change. Polyurethane formulations are able to store up and return energy; for example, if you squeeze a urethane product in one place, it will bulge the same amount in another place, and then return to its original shape once the load is removed. This property makes polyurethane an excellent material for load-bearing applications. In fact, when comparing urethane and rubber of equivalent hardness, urethanes have higher load bearing capacity than rubbers. For this reason, many industries prefer standard urethane over rubber.”

“With dynamic applications, the greater the cyclic deflection of the elastomer, the greater the heat buildup. Generally, an in-use deflection of 5-10% is acceptable, although a deflection of under 5% is desirable. If the deflection extends beyond 10%, then the part has a greater chance for blowout. Therefore, specifying a harder elastomer is usually recommended to minimize deflection. However, there are typically other performance attributes that also need to be considered, such as traction, where use of a softer formulation may be desired.”

Note: Diminishing returns if we keep deflection under 5%, although 5-8% may be more realistic for rough surfaces.

“Dynamics are a function of load and speed. If either is too great, then the part may not be able to dissipate the heat fast enough to avoid a blowout. This can happen even with a properly designed part. Sometimes it may be necessary to redesign the application. This can be accomplished by redesigning the roller to have a larger diameter, effectively reducing the speed. The part can also be made wider to distribute the load over a wider area.”

Note: Longboards heat and blowouts aren’t an issue, but those are the same forces that cause hysteresis, aka rolling resistance losses.

Urethane Hardness

Note: Translates to perceived hardness of a wheel.

90a skate wheel is 350% as ridged as a longboard wheel (PSI, not overall).

OR applicably for longboards a 72a wheel would feel 50% softer than 80a wheel.

Theoretically a 65a would ride a lot like a 70a wheel, which is interesting.

(base) Urethane Hardness

Note: Translates to perceived hardness of a wheel.

90a skate wheel is 350% as ridged as a longboard wheel (PSI, not overall).

OR applicably for longboards a 72a wheel would feel 50% softer than 80a wheel.

Theoretically a 65a would ride a lot like a 70a wheel, which is interesting.

(base) Urethane Hardness

Note:

Longboard wheel range will change +- 25% by quality of urethane.

Modulus (PSI) = 24.691x - 1389.5

From 68-80a

Calculating Optimal Depth

1. Bonded interfaces: Deflection depends on core support

• “…the condition of the parts loaded surface, has an effect on deflection.  When a part is compressed it wants to expand out laterally and that movement changes the shape factor.”
• “However, if the surfaces are bonded to a substrate such as metal, the shape factor remains the same.  Below is a graph showing the differences in deflection of a bonded part, a dry and clean part and a lubricated part.”

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Note: Supported urethane deflects, 60% LESS than unsupported urethane. Aka a lip of a wheel has a huge contact patch when put under strain relative to supported, if it isn’t too floppy. Which tracks.

1. Bonded interfaces : Determine core influence on urethane deflection (modulus of elasticity)

Note: true modulus of elasticity depends based on support

2. Shape Factors: Deflection vs wheel shape.

• “Since the blocks will not change in volume, the reduction in height is caused by the freedom of the sides to bulge. The rectangular block deflects more than the cylindrical one because the sides of the rectangular block provide a greater area free to bulge”
• “If the elastomeric part does not deflect enough to do its job, the designer can reduce the shape factor by increasing the height of the urethane pad. In reality, he does no more than increase the area free to expand under load.”
• “If the pad deflects too much, he may decrease the area free to expand or he may increase the hardness of the elastomer.”

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Note: Aka, narrower and taller wheels will naturally deform more.

2. Shape factor: Determine functional urethane strength�by shape

Note: “How to calculate strength based on shape.”

Calculating Urethane Deflection

L = Total loading or weight in pounds (not psi)

D = Deflection in inches (not a % deflection)

Y = Young's Modulus (see Modulus of Elasticity diagram above)

W = Width (length of contact surface) of the roller/wheel in inches

a = Inside radius of the polyurethane in inches

b = Outside radius of the polyurethane in inches

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This equation accounts for the shape factor for a wheel

Deflection = Shock Adsorption

Using % deflection we can now determine shock adsorption

• Deflection is directly proportional to shock absorption
• We can’t quantify comfort, but we do a relative calculation versus other knowns!

One can assume shock, AKA KE, is linearly reduced in relation to deflection %