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Technology - Wheels

Overview

Your choice of wheels is critical in completing your longboard setup. No one wheel will work for every rider in every situation. If fact no one wheel will work for you. Many factors need to be considered in picking your wheels. In addition to the type of riding (e.g. downhill, freeride, cruising) you plan on doing other factors have a significant impact on performance. Your weight, pavement type, and especially temperature are critical factors in choosing your wheels.

No one can tell you which wheels are best for you. To optimize your ride you need to learn the basics of wheel design and construction and how they affect performance. With this knowledge you can test different wheels in different conditions and find the setups that work best for you.

Anatomy of a Wheel

All wheels consist of a hard core over which urethane rubber is cast. All cores have recesses on both sides that accept the bearings with a spacer in between for mounting to the axle. This is where the similarities between different wheels end. Core size and set (location within the wheel), formulation of the urethane along with the diameter, contact patch (width of wheel contacting the pavement), lip design and overall shape of the wheel all vary greatly from wheel to wheel and have different effects on the performance of the wheel. Below is a brief discussion of these factors so that you can learn to test and select the right wheels for you.

Urethane Formula

Urethane comes in a variety of formulations that can be adjusted to select the specific properties desired. The key properties related to skateboard wheels are rebound, compression set, tear strength and hardness.

Rebound is the most important urethane property in a skateboard wheel. More rebound lowers the rolling resistance, making the wheel faster and allowing it to carry more energy. When weight is put on a wheel it flattens on the bottom. As the wheel rotates the urethane compresses when underneath and expands again as it rises. This compression requires energy and causes rolling resistance which slows the wheel. How much energy is used depends on the compression set and rebound of the urethane. The more rebound, the less energy required. Rebound is chiefly determined by the quality of the base material used.

Tear strength is a measure of the force that it takes to actually rip or tear the material. Urethanes generally have high tear strength and abrasion resistance which makes them ideal for skateboards wheels. As the urethane grips the pavement in a turn the g-forces try to tear the bottom layer of urethane off. High tear strength wheels resist this and will last longer.

A wheel that is gripped to the pavement but tearing off in a controlled way allowing the board to drift in a controlled way is ideal for freeriding. This is where urethane is being left on the pavement and we refer to it a “greasing.”

A wheel with low tear strength will drift (or grease) easily making it easier to ride for some riders. It is also tearing up the urethane faster wearing out the wheel more quickly and resulting in an expensive urethane habit. Metro Motion wheels are an example of this. (Our riders can use up a set of Metro’s in an afternoon.).

If the tear strength is too high, however, the urethane will not tear and the wheel will grip until it breaks loose and slides on the pavement. We refer to this as “glassing.” In a slide (as opposed to a drift) you have lost control like a car on ice. As speed reduces the wheel can “rehook” or re-grip suddenly. This loss of control and sudden rehook is often not fun, can be unnerving and maybe unsafe.

Durometer

Durometer is the measure of the hardness of rubbers. The shore A scale is used in the range of harnesses found in longboard wheels. Other shore scales, such as Shore D, are used for different ranges of hardness. Durometer measures the depth of an indentation in the rubber created by a standardized handheld instrument.

Durometer is an often underappreciated parameter considered in choosing your wheel. For a given wheel and road conditions, lower durometers will drift/grease and higher durometer will tend to slide/glass. Too low and you are wasting urethane, too high and you lose control. Maximum grip and control is found at the highest durometer before the wheel begins to slide/glass. The problem is that this point is influenced by several factors including the rider’s weight and most importantly temperature. Read more on this in the Drift vs. Sliding section below.

Diameter

Why are longboard wheels bigger when pro ramp and park skaters choose small ones? Small wheels are fast on smooth clean surfaces but are much slower on asphalt and concrete. In addition, small wheel are slowed by pebbles, cracks and road debris. It is no fun when your board comes to a sudden stop when it hits a crack or rock.

A larger wheel will ride up and over obstacles such as cracks. The same obstacle hit by a smaller wheel will hit more in the front of the wheel, pushing it back. Large wheels ride over road debris while smaller ones are slowed or stopped. For these reasons they are safer at high speeds.

 

Larger wheels do have a downside however. As the urethane between the ground and the core get thicker, the more it compresses under your weight. As discussed above in the formula section, this compression takes energy and increases the rolling resistance of the wheel. One way to reduce this is to use a larger diameter core. This will reduce the thickness of the urethane from the outside to the core while maintaining the outside size of the wheel.

eXtreme Sports Physics has some excellent information on the physics of skateboard wheels and wheel diameter. Click these links to read Part 1 and Part 2 of the article.

Contact Patch

Contact patch is a measure of the width of the wheel that is in contact with the pavement. This is different than the overall width which may be quite a bit wider depending on the overall shape of the wheel.

A wider contact patch will spread the weight of the rider over a bigger area. This will reduce the urethane compression and reduce the rolling resistance described above. It also makes the wheel heavier and more expensive to manufacture due to the additional expensive urethane itself.

A wider contact patch can also act to slow you in a turn where the out of the wheel is traveling a farther distance than the inside. Since both edges are travelling at the same speed friction is created in in the turn.

Lip Design

The area of the wheel at the edge of the contact patch is called the lip. The shape of this lip has a noticeable effect on the performance of the wheel. At one extreme is a square lip which is good for maximum grip. It is the last to break loose and slide but tends to gran on rehook. A sharp lip will often tear up and become ragged reduce this grip. Some wheels will add a bevel to the square edge help the edge last and reduce the sudden break loose and rehook.

On the other end of the spectrum is a lip with a full radius. Rounded lips will slide easier and in a more controlled fashion. They also offer more control on rehook. Many riders prefer the control gained over the grip lost with round lips. The grip lost can be offset by selecting other wheel characteristics such a urethane formula.

Square Lip Full Round Lip
High grip Less grip
Sudden break loose and rehook Smooth break loose
Drift improves with wear More control on rehook

Core Size

 

Almost all cores used in longboard wheels today were developed for in-line skate wheels. They tend to be small compared to the diameter of longboard wheels increasing the amount and thickness of urethane. A larger core size (outside durometer) reduces the urethane and therefore cost. It also reduces the rolling resistance described above. Our testing has found that there is an optimum thickness of urethane between the core and the outside of the wheel that requires larger cores. But you may prefer the suspension effect of thicker urethane with smaller cores.

Small Core Large Core
More urethane – more expensive Less wasted urethane
Usually made with cheaper urethane Higher quality formulations are affordable
Suspension effect Consistent performance
Side wall can flex Less side wall flex
Can have less control on turn or drift Less forgiving in turns
Flexes to hold pavement longer in turns Faster
Grabs late then can hop and rehook Drifts easier
Coning of side-set hub can fix but slows More control
(like BigZigs) Consistent performance

Core Set

 

The location of the core within the wheel has a very large impact the on how a wheel will perform initially and over its life. The set of the core refers to the location of the core within the wheel. A side set wheel has the core on one edge of the wheel while the core is in the middle in a center set wheel. Partial Side Set or Offset wheels have the core somewhere in between.

When riding a wheel, the urethane under the core carries the majority of the load, while the unsupported urethane away from the core flexes inwards. This reduces the effective contact patch and wears the area over the hub faster than the unsupported area causing the shape of the wheel to cone as the outside of the wheel flares. This effect is most pronounced in side set wheels.

A side set wheel will ride on the smaller contact patch and slide easier when new. As the wheels wears the coning will increase grip and slow the wheel. This also gives a “cushy” feel to the ride. Since urethane is wearing faster over the hub the life of the wheel is less than that of center set wheels.

Placing the hub in the middle a wheel better distributes the load and uses more of the contact patch. Center set wheels have more grip out of the box and may take a run or two to break-in. They will give consistent performance over the life of the wheel and last longer than side sets because there is less urethane compression and rolling resistance. Less wear gives them longer wheel life.

Side Set Partial Side Set / Offset Center Set
Quickest break-in Quick break-in Break-in needed
Large coning Offset coning Slight outside coning
Slowest Faster Fastest
Easy early slide Compromised A little harder to break-in
Grip increases with wear sliding and grip Highest grip
Shortest life Shorter life Longest Life
Slows with increased coning Coning still slows over time Best performing wheel

Drifting vs. Sliding

Drifting and sliding are not the same and are quite different as discussed above in the Urethane Formula and Durometer sections. In a drift the wheel is gripping the pavement and tearing off in a controlled way. This is where urethane is being left on the pavement and at Other Planet we often refer to it a “greasing.” When a wheel grips and will not tear until it breaks loose it is sliding on the pavement. We sometimes refer to this as “glassing.” In a slide (as opposed to a drift) you have lost control like a car on ice. In a drift, you maintain control as with drift auto racing.

When a wheel slides, it breaks loose with no control. Regaining grip and rehooking is more difficult and can be abrupt. The wheel can rehook, break loose, and rehook several times causing a hopping feel. In a drift with the wheel greasing, the urethane is still gripping the pavement and is tearing off in a controlled way. Wheel rehook is smooth and control is maintained.

As discussed above the quality and tear strength of the urethane has a significant role in sliding and drifting. But once you pick your wheel brand and model you still can adjust the drift/slide performance with the hardness of the wheel. A softer (lower durometer) will tear and drift (grease) easier than a hard one. As durometer increases the amount of tear reduces until you get to the point where the wheel will break loose and slide (glass). As the durometer increases urethane wear decreases and control improves. The point of maximum grip and control is a durometer just below that where the wheel slides.

This ideal drift/grease durometer depends on several factors. The weight of the rider and most importantly temperature will determine the best durometer for maximum performance. A heavier rider will tear up urethane faster and a higher durometer will give better performance and wheel life. On the other hand, a lighter rider may more easily slide rather than drift and may want to use a lower durometer.

Temperature, however, is the most important consideration. On cold pavement a wheel will more easily slide. For controlled drifting a lower durometer is needed. If the road is very hot the softer wheels will tear easily and grease too much making them feel slippery. Wheel life can be significantly shortened. Adding a couple of durometer points on a hot day will take advantage of the increased grip of the hot pavement and result in a better ride.

After thousands of hours of testing we have found that with southern California or spring/fall conditions and an average rider the durometer sweet spot to be around 80a. On a hot day (100F) we find that increasing the durometer a few of points (82a – 85a) works best. By reducing durometer to the 75a – 78a range we find that frozen pavement works great. In fact a $2.00 all day buss pass make for a great break from snowboarding on a clear winter day.

 

Our test riders often carry a Harbor Freight IR thermometer. [Picture] They will shoot the road temperature and pick wheel durometer accordingly. On a recent ride they recorded the air temperature as 40°, shaded pavement as 37° and pavement in the sun as 57°. The wheels started at room temperature but after a short run were at 98°. They keep extra hangers with different wheels pre-mounted and as conditions change make 30 second wheel changes by swapping hangers on their Revolution trucks. Pick up some extra hangers and experiment for yourself. We would love to hear about your results.

Wheel Life

As a wheel wears the thickness of the urethane between the core and the outside reduces. This thickness is important as our experience with Abec Flywheels demonstrates. After a break-in run or two a 76mm center set Flywheel (of the correct durometer conditions) drifts like a dream. Their good, but not best, quality urethane wears reasonably well over a several days or weeks and the diameter becomes smaller. When the diameter gets to around 70mm we notice a marked change in the grip. By the time we reach 68mm there is no longer any grip and the wheels are done. For us a 65mm NoSchoolz is done when it reaches 60mm. Pick up a cheap pair of calipers (at Harbor Freight for example) and track your own wheel life results.

To get best wheel life match your durometer to the temperature and your weight and watch how much urethane you are using. Keep a log and record your results to fine tune your setups.