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On 15 September 2011 at 13:52 | updated on 21 July 2018 at 17:23

Physics of the inline skate wheel

Physics of the inline skate wheel

Inline skate wheels are characteristic of our sport. If you consider speed skating as a rolling sport, wheels are the medium, the tool the skater has to master in order to gain speed. This article mainly focuses on speed skating wheels.

By  COMMUNIQUE DE PRESSE

Wheel dissection

Roues MPC Road War rougeThere is nothing theoretical about the characteristics of a skate wheel: They were designed bit by bit after attempts and mistakes. The list below examines the main characteristics, with a close-up on propulsion.

Resistance to stress and physical integrity

The wheel is an essential part of the skate that supports the body of the skater. It should keep its dimensions under all circumstances. It should support the weight of the skater, resist to shocks, to speeds around 15 mps in speed skating and 30 mps in downhill skating.

Elasticity and cushioning

The elasticity of the wheel contributes to its resistance to shocks, as well as to its flexion (see below). Shock cushioning works on two levels: High cushioning reduces the vibrations transmitted to the skater, as well as the propelling thrust. More specifically, it is preferable that the wheel serves as a low-pass filter, with good transmission of low frequencies and good absorption of high frequencies.

Dimensions

For most skates, wheel diameters go from 76 to 125 mm. Some slalom skaters, hockey players or juniors sometimes use 70 or 72 mm wheels. In street skating, there are also wheels with diameters around 50 mm.

The thickness of the polyurethane may be of 20, 22 or 24 mm — the latter being the most common by far. However the core remains of 24 mm. Let's note that for 608 bearings, the width of the frame is broken down as following: 7 + 10.2 + 7 = 24.2, which leaves a maximum tolerance of 0.2 mm near the axle.

Some frames enable to mix different sizes of wheels: Those kinds of set-up have widespread since the coming of 110 mm wheels. The most common hybrid set-up in 2011 was 110-110-100-110, but you could also find 110-110-100-100 and 110-100-100-110.

The maximum power developed by the skater depends on their ground clearance, which itself depends on the size and the set-up of the wheels.

Weight, moment of inertia and gyroscopic effect

The wheel should be as light as possible: Because of the movement of the skater, the wheel is the most extreme point of their body and influences their energy expenditure more than any other part of their equipment. Its moment of inertia works on two levels: High inertia for faster and constant speeds; Low inertia for more efficient starts, speed-ups and uphill speed.

The average weights of wheels used in 2011 are: 76 g for 80 mm / 80 g for 84 mm / 130 g for 100 mm / 160 g for 110 mm.

The gyroscopic effect is the force that impedes the pivoting of the wheel when rotating. Hardly detectable with 80 mm wheels, it is quite high with 110 mm diameters. To a certain extent, that effect contributes to a regular skating stride since it implies holding the frame in a constant axis. On the other hand, it is detrimental to quick changes of direction.

Rotation

Roues Gyro WalkyriesThe wheel should be able to turn under all circumstances. The rotation around the axle is taken care of by a pair of ball bearings, most of the time of 608 type (8-22-7 mm). The wheel should be balanced and shouldn't be deformed or squeezed permanently.

Friction coefficients

The grip of the wheel is very determining as for its propulsion capacities.

Coefficient of static friction: Essentially involved during the lateral push, this coefficient should be as high as possible on all possible surfaces. This condition being conflicting, there are special wheels for wet ground, i.e. for grounds with low static friction coefficient.

Coefficient of dynamic friction: It should be as low as possible to reduce the wearing of the wheels. This condition being conflicting with that of gripping on wet grounds, it explains why sliding hold for quite long once started.

Coefficient of rolling resistance: It should be as low as possible, on all the most varied surfaces. This condition being also conflicting, wheels designed for track (smooth surfaces) are usually different from those designed for road (rough surfaces).

Bi-density wheels

Roues bi matièreIn order for its weight to be reduced, a wheel is made of two elements:

  • The core (rim) of the wheel, that is only in contact with the polyurethane on-core and the bearings.
  • The on-core, that is only in contact with the core and the ground.

This bi-material design allows each part to be dedicated to a specific field, which avoids (or reduces) conflicting requirements. Only the circumference is concerned by the friction constraints; it is generally made of polyurethane. The core is in charge of shock resistance and dimension respect. A hollowed core obviously contributes to the weight reduction of the wheel: A 100 mm wheel with a solid core would weight 188 g instead of the average 135 g. The bigger the wheel, the bigger the effect.
This bi-material design has requirements, mentioned in the 'Flexion' paragraph below.
To be as rigorous as possible, bearings should also be taken into consideration. Yet, their stiffness is such that their impact on the considered effects is to be neglected.

Tri-material wheels

The higher average wheel size leads to an increase in the quantity of materials used in the circumference, which leads to several consequences. Since 2010, manufacturers have released tri-material wheels, just like car wheels, that will help illustrate our explanation.
A modern car wheel is mainly made of three elements:

  • A metal rim of great mechanical resistance, with very high elasticity properties,
  • An air gap enabling great volumetric adjustment to constraints, sometimes (but not always) contained in an air tube,
  • A tread made from rubber reinforced with textile fiber, that transmits energy, ensures friction, and holds the air gap in place.

That tri-material design allows to separate more specifically the roles of each part:

  • The rim, which doesn't need to bend sideways, can be made of a very stiff shock resistant material;
  • The tread, mostly dedicated to grip, can be made of a material with a high level of grip, but with lower mechanical characteristics;
  • The air gap deals with all deformations during operation, with almost no inner friction for better comfort, less noise and energy saving.

Motorbike wheels have more similitudes with skate wheels, since they are designed to be used on their flat edges and their sides. Creativity is already in action with multi-material wheels designed for circuits (soft material on one side only) or long routes (harder material on the flat edge and softer on the sides).

The same improvements for skate wheels are at aim.

  • The rim is still in charge of the geometrical integrity, the cushioning, and part of the sideways flexion;
  • The circumference 'core' is made of soft material to make the inner moves of the rubber easier and reduce inner friction; Some manufacturers also include a small volume of air there.
  • The hollowed circumference is still in charge of the grip and the lateral flexion, but its deformation to adapt to the ground is made easier by the presence of the soft core.

These wheels showing good results (2011 world champion in Yeosu), it is probable that, on the one hand, they become more popular, and on the other hand, their design keeps evolving towards harder outer materials, cores with bigger volumes, and other options...

Flexion: Modulus and mechanism

Test des roues Matter Juice 110 mmEach material has a modulus of elasticity in flexion (flexural modulus), describing the deformation of an object (usually long), one end of which is fixed while the other end is put to normal strain at the biggest axis of the object. Materials used in the manufacturing of a wheel have low flexural moduli ('stiffness'), which means that if you apply little strain, you get high flexion. During the lateral push movement, the wheel deforms laterally by lateral flexion, but only on the part of the wheel the circumference of which is in contact with the ground. When the wheel turns and moves a few centimeters forwards, that part is no longer in contact with the ground, and consequently no longer under stress. It then takes its initial shape back by mere elasticity. The next part is then flexing in turn. At each wheel rotation, each of its parts is put under strain, then released in turn.

At 10 mps (36 kph) a 100 mm wheel does 32 rotations: The wheel undergoes this flexion/release cycle 32 times per second. If the bond between the core and the on-core is insufficient, it is probably going to disbond. The risk of disbonding is all the more probable that the on-core and the core have different coefficients of flexion. The rim/core being particularly stiff, like for some wheels with aluminum rim, the latter won't bend at all under strain, which will create an angle between the rim and the on-core, leading to fast disbonding.
The match between the rim and the on-core has to be done on two levels: 1) The compatibility between the materials to get a good bond, and 2) around the same coefficient of flexion to avoid disbonding. The flexion modulus of a wheel determines its drift angle during propulsion.

Profile and footprint

The profile is the aspect of a straight section of the circumference. It ensures that the wheel and the ground are always in contact whatever the angle of push. The footprint is the area of contact with the ground. It depends on the hardness of the wheel and on the weight of the skater.
Speed skating wheels have an elliptical profile — slide wheels are generally more rounded. That profile leads to the following characteristics:
• When the skater is freewheeling, the wheel is vertical, the contact surface is minimal and friction is minimal and symmetrical.
• When the skater pushes sideways, the flat of the wheel is not in contact with the ground anymore, but a zone a little closer to the axis. The area of contact with the ground is bigger, there is more friction, more grip, and a couple of rotations tends to straighten the skate up to a certain angle ('capsize'): The push can be stronger.
• Elliptical profile enables higher flexion for the same hardness; Harder wheels, more wear resistant, but that remain efficient can be manufactured.

Shore A Hardness

Shore A hardness is a standardized measure of resistance to penetration. It is the function of a (leaning) strain according to the surface of pressed material. It is then similar to pressure. Wheel hardnesses in 2011 used to vary from 78 to 88 A for speed or leisure wheels, to 98 even 100 A for street wheels.

Shore A hardness has been a wheel identification measure for a long time. It may be dropped with the new tri-material wheel tendency. It remains essential to evaluate footprints.

Footprint evaluation

Roues RollX PulsRThe footprint of a wheel depends on the weight of the skater, on the number of wheels and on the hardness of the polyurethane; In some circumstances it may noticeably vary as a result of a violent push.

When the skater is in a normal push phase, they always have one skate touching the ground at all time. In the case of a 4-wheel frame, the stiffness of the latter is such that it is considered that the weight is equally divided between the 4 wheels (which is not always the case, though). With bi-material wheels, the sum of all 4 surfaces on the ground is a direct function of the Shore A hardness — which is no longer the case with hybrid wheels. Consequently, a manufacturer stopped announcing the Shore A hardness of their wheels, and replaced it with an evaluation of the surface in contact with the ground (which lacks clarity).
Flexion, profile and contact surface are essential components to the efficiency of a wheel; Strictly speaking, each skater should identify wheels adapted to their power, weight and the ground type they skate on.

Braking

Speed skating wheels may be used for braking. That use leads to rapid deterioration of the quality of the wheel and should be avoided. See paragraph on Resistance to wear.

Resistance to wear

Wear alters the wheel's footprint and its elliptical profile, changes the conditions of flexion of the urethane, causes bearing vibrations and possible loss of grip; All those elements reduce performance, especially on smooth surfaces (track). Most of the time, good resistance to wear is in contradiction with other qualities. Even if slight wear is a quality for the consumer, it is not the main preoccupation of the sports(wo)man who places this quality in last position.

Wheel mix

It is possible to mix your wheel, most of the time symmetrically on both skates. If you use two types of (new) wheels with different hardnesses and flexural moduli, you'll theoretically get intermediary areas of contact, so that you can increase the grip of the undercarriage without sacrificing other qualities. At that level, the stiffness of the frame is a parameter to be taken into account, which makes the subject particularly complex.

Wheel manufacturers

The manufacturing of modern wheels isn't a science but an art, that feeds with the knowledge and the experience of the manufacturers, but also with the feedbacks of the skaters and testers who translate the sensations they get on the field in terms of objectives.

OLS published an interview with Alain Myallonier in 2009. One of the special features of Roll'x, in addition to being French, is the mastering of the manufacturing of both the hub and the on-core.
Neal Piper may be the most famous skate wheel chemist in the world. He released a series of manufacturing videos, which are unfortunately no longer viewable today. All that is left is his presentation, which shows that chemists have a good sense of humor. His company, Aend Industries, works for Matter, amongst other brands. Neal is to skate wheels what Adrian Newey is to Formula 1: If you want to win, you need him. His videos let it be understood that his biggest volumes go to skateboard wheels.
Curst Labeda is an old acquaintance. He started his career on quad skates, he still skates today and still ranks in the Top-3s in veteran categories. The Labeda wheels had their moment of glory. Now he is the president of World Record Wheels, and makes wheels for great brands such as Hyper and Atom. He also started to make hybrid wheels, his last wheel is a perfect example: the Truth by Hyper. I had rather not comment on the commercial language linked to this wheel, but it is clear that the principle of the soft layer inserted in the center of the urethane has been tested by all.
MPC stands for Mearthane Products Corporation. Here is a true manufacturer that makes many more things made of polyurethane than just skate heels. MPC was the pioneer of hybrid wheels. The Black Track, that already contained the well-known T-ring, a ring inside the urethane, was already at the top of the world in 2006, with a first exhibition in 2004. MPC, after having known a dazzling ascent with the success of the Black Tracks and the efficiency of the Storm Surge, had to suffer devastating setbacks with the multiple breakages of their hubs. The corrections were not enough to ensure the efficiency of the next wheel, just as Yann explains in this post. This painful episode highlights that the design of the hub, and its consistency with the on-core, is almost as important as the urethane itself. With Aend and WRW offering hybrid wheels with comparable, even higher, performances, and with WRW offering competition wheels to the Storm Surge (H20), MPC has new efforts to make. It seems that the brand-new Typhoon by Bont looks a lot like an MPC wheel, with the faulty hub.

The price of speed skating wheels used to be around 8€ before the advent of hybrid wheels, it has now rocketed towards 20€/u. Their manufacturing hasn't changed much: The T-ring seems to be included by hand on the hub before injection. On the other hand, this increases the number of possibilities of wheels to test. It seems however that experimented manufacturers can come up with a correct hybrid wheel without much effort, now that the principle is set. Prices should then drop again.

Useful links

The recent technological innovations of skate wheels
The manufacturing of skate wheels
Roll'X Wheels: Which model for which use?
Wheels: Will the diameter race ever stop?
Wheel manufacturing: Visiting the Roll'X factory
Wheel maintenance

By XsFred
Thanks to lokonel and gui_gui
Translation: Chloe Seyres
Photos : rollerenligne.com
Released  on 15 September 2011 - Read 21983 times

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