The phenomenon of vibration in cycling: the Ortholite solution

Modern cycling encompasses a variety of disciplines (road, MTB, track, etc.) with an increasingly wide choice of bicycles and equipment. Taking the example of the wheel, the range of products on offer is vast, with multiple options. When replacing a rim, you have a choice of sizes, heights, widths, materials… Then there’s the spokes, hubs, tyres, etc. The same is true of the bicycle itself and its components. The same observations apply to modern disciplines and new trends: road cycling (sportives and endurance riding), MTB (XC, Enduro, Fatbike), triathlon, adventure cycling (allroad), and more.

However, there is a constant in all these forms of cycling: the bicycle serves as the interface between the rider and the ground.

Figure 1 – The three points in white are where vibrations enter the cyclist’s body

 

Whatever the terrain, cycling outdoors will always involve uneven surfaces (unlike track cycling, where the surface is very smooth). All these bumps in the ground produce vibrations, which are transmitted to cyclists through their bicycles via three entry points: the feet, hands and backside (Figure 1).

The intensity of these vibrations, in frequency and amplitude, depends on the bumps themselves (density, depth), wheel rotation speed over these bumps (and therefore on the number of times that the wheels hit them), the equipment used and the weight of the cyclist-bicycle system. These vibrations, however tiny, are a fact of life for cyclists.

Some bicycle races are actually renowned for their very rough surfaces: there are some sixty kilometres of cobbled sections in the legendary “Paris-Roubaix” race. The vibrations they cause the riders are severe, and every year the racers display great ingenuity in trying to limit them.

But what do we really know about the phenomenon of vibration? What types of vibration do we experience when we ride and, most importantly, do they have an effect on our body? That’s what we will find out in this paper.

A few reminders:

Figure 2 – Example of a sine wave

 

Oscillation can be defined as the back and forth movement of an object about a point of equilibrium (= reference). Vibration is therefore characterised by its period, frequency and amplitude.

The period T of a periodic phenomenon is the shortest time taken for the phenomenon to repeat in identical fashion. It is expressed in seconds.

The frequency f of a periodic phenomenon is the number of periods per unit of time, i.e. the number of times the phenomenon repeats per second. It is expressed in Hertz (Hz). The frequency is the inverse of the period.

The amplitude A of the signal (Figure 2) is the difference between the nil value (= 0) and the maximum value.

A little background:

The study of vibrations and their effects on health or on human motion is not new. Health authorities have long been interested in this field, including studying lumberjacks and the effects of the vibrations caused by chainsaws on the production of force. They observed a reduction in hand grip force after use of the chainsaw (Farkkila et al., 1980). Studies have also looked at helicopter seats and shown physiological and neurological changes linked to the vibration of the seat (Martin et al., 1984).

More broadly, we now know that vibration has an effect on health and can cause some disorders. Below is a graph of the effect of vibration on blood flow in the extremities (in this case the fingers). In the study, the author (Furuta et al., 1991) clearly demonstrates the effect of vibration on blood flow (red line) in the fingertips during periods of vibration (grey rectangles) at given frequencies. This reduction in blood flow can cause (short-term) changes in temperature regulation as a result of the constriction of blood vessels, and even the appearance or worsening of some potentially degenerative conditions, such as Raynaud’s disease or phenomenon (Olsen N., 1988).

Figure 3 – Localised reduction in blood flow (in the fingertips) when the hand is exposed to certain types of vibration (around 30Hz)

 

Taking a slightly earlier historical example, the Tacoma Narrows Bridge collapsed in 1940 after being subject to forced resonance. Resonance describes when an external force drives a material to vibrate at a frequency which is amplified such that it reaches a very large amplitude. On that day, the wind caused the deck of the bridge to oscillate up to several metres in amplitude (Figure 4) before collapsing. All bodies (including the human body and its organs) have their own resonance frequency.

   Figure 4 – Oscillation of the Tacoma Narrows Bridge

 

Vibrations can also cause kinetosis (motion sickness), headaches, numbness and pins and needles.
In the light of the results of these and other studies, standards were implemented to protect workers. The French Labour Code (articles R. 4441-1 to R. 4447-1, under ruling no. 2005-746 dated 4 July 2005) forces employers to reduce the risk of vibration.

 

What about vibration in sport?

The study of vibration later extended into the world of sport, including research into the vibrations transmitted by the racquet in racquet-and-ball games like tennis and squash. Apart from the usual conditions (e.g. epicondylitis, also known as tennis elbow), one of the common effects is that vibrations during exercise will cause a significant loss of strength in the recruited muscle.

 

Vibrations in cycling

From a performance perspective, the addition of vibrations will result in an excess of muscle activation for the same power output (and therefore an increase in the energy cost). This is especially true as there are multiple vibration entry points close to the main muscles involved in pedalling, like the gastrocnemius, vastus medialis and lateralis, rectus femoris, biceps femoris and gluteus maximus muscles (Duc S., 2005). Studies (Kavounoudias et al., 2001; Sonza et al., 2013) have also shown that the vibration of the feet interferes with the kinesthetic system, which informs us about the position of our limbs and the feeling of pressure, and is vital for maintaining our balance.

In addition, vibrations in the backside place a high demand on the intervertebral discs (which act as shock absorbers for the spine), leading to a risk of lumbago, sciatica and even slipped discs, according to the INRS (the French National Institute for Research and Safety; see: Pathologie lombaire, effet de la manutention manuelle, de la posture et l’exposition aux vibrations [Lower-back morbidity, the effect of manual handling, posture and exposure to vibrations]).

New trends in cycling, including very long races (Haute Route) and races on craggier terrain (allroad) are therefore leading us to focus increasingly on vibration. And to define it better, we need to know it better. For that reason we conducted studies on the ground to measure “in situ” the types of vibration to which cyclists are exposed, whether on or off road.

We recorded and analysed vibrations on the bicycle at the three points of interface with the body: the seat, handlebars and pedals. We also varied the conditions as much as possible to obtain the largest possible sample in terms of equipment and discipline:

We are therefore now in a position to state that the main range of vibration frequencies to which cyclists are exposed during testing is the 17-54Hz range on the road and 8-42Hz range on trails (allroad). Focusing more closely on resonance frequencies (cf. the Tacoma Narrows Bridge), we see that many of our limbs and organs have frequencies that may interact with the frequencies we encounter when we ride:

Figure 5 – Graph of road and allroad frequency ranges, compared with those of the human body

 

Armed with this new knowledge, we were able to run specific tests to assess different methods of reducing this level of vibration. Working on “soft” products such as garments and accessories (as opposed to “hard” wheel-related products), we could rule out mechanical filtering systems like the mass damper used in Time’s “Aktiv” fork. For technical and feasibility reasons, we also ruled out electronic systems (exposure to the elements, sweat, pressure, etc.). We therefore concentrated our work on the materials themselves. As the backside accounts on average for 60% of pressure across the three points of contact (compared to the hands and feet), we naturally started by working on the chamois, also known as the pad.

Using a (frequency-adjustable) vibrating platform, we tested more than thirty materials (foams and gels), and obtained a wide variety of results in terms of behaviour. This test allowed us to measure vibration quantity before and after the material was used. Below are graphs showing the behaviour of two very different materials, Ortholite foam (Figure 6) and gel (Figure 7), in response to the same impact.

Figure 6 – Behaviour of Ortholite foam in response to a one-off impact. The impact is well absorbed (orange arrow) and the residual vibration is very low (green arrow)

Figure 7 – Behaviour of gel in response to a one-off impact. The impact is not well absorbed (orange arrow) and the residual vibration is higher (it takes 6 times longer to be cushioned)

 

The Ortholite foam was therefore selected. Firstly, it absorbs impacts better than other foams. Secondly, it stands up very well to repeated compression. Finally, its open-cell structure gives it good moisture control characteristics.

After adding Ortholite foam to the pad, we tested the product as a whole on the vibrating platform, comparing it to existing products (rival pads). Here, too, the results are conclusive, especially when compared to pads with a gel construction. In the frequency range that can be very harmful for human beings (20 to 40 Hz), we reduced vibration by 20% across the whole frequency range, and by more than 30% at critical frequencies.

Figure 8 – Quantity of vibration at specific frequencies using different pads

 

The result is an optimised chamois which retains all its comfort and moisture control attributes while also better protecting the cyclist against vibration.

What about the other points of contact? Following our reasoning through to its logical conclusion, we also applied Ortholite technology to our shoes in the form of insoles (Figure 9) and in our gloves (Figure 10).

This means that Mavic’s Ksyrium Pro range offers not a partial but a complete solution, giving cyclists increased protection for long and comfortable rides.

Lastly, while vibrations are greatly reduced, cyclists will still feel them. The human body is well designed, so it has strategies to limit their effects! Unfortunately, this has a cost: an increased energy output, which is not used to move the cyclist forward. However, there is a way of limiting this “useless” energy cost, and we will discuss that in a future episode of Engineers Talk!

References:

  • Duc S. (2005). Analyse de l’activité musculaire du pédalage en relation avec la performance en cyclisme. Thèse de doctorat, Université de Franche-Comte.
  • Farkkila M., Pyykko I., Korhonen O., Starck J. (1980). Vibration-induced decrease in the muscle force in lumberjacks. Eur.J.Appl.Physiol., 43, 1-9.
  • Furuta M., Sakakibara H., Miyao M., Kondo T., Yamada S. (1991) Effect of vibration frequency on finger blood flow. Int Arch Occup Environ Health, 63:221-224.
  • Kavounoudias A., Roll R., Roll J.P. (2001) Foot sole and ankle muscle inputs contribute jointly to human erect posture regulation. Journal of Physiology, 532.3, pp.869–878.
  • Martin B.J., Roll J.P. & Gauthier G.M. (1984). Spinal reflex alterations as a function of intensity and frequency of vibration applied to the feet of seated subjects. Aviat Space Environ Med, 55, 8-12.
  • Olsen N., Petring O.U. (1988) Vibration elicited vasoconstrictor reflex in Raynaud’s phenomena. British Journal of Industrial Medicine, 45:415-419.
  • Sonza A., Maurer C., Achaval M., Zaro M.A., Nigg B.M. (2013) Human cutaneous sensors on the sole of the foot: Altered sensitivity and recovery time after whole body vibration. Neuroscience Letters, 533, 81– 85.

2 Comments

  • Why not try a wooden frame as AXALKO?
    The whole frame acts as shock absorber and transmits far less vibrations to the rider.

    • Mavic doesn’t produce any frame. Nevertheless, in spite of different properties depending on the type of wood, the geometry of the frame is also very important in the limit of the vibrations.