Bicycle Shock Absorption Systems and Energy Expended by the Cyclist

Sports Med 2004; 34 (2): 71-80 0112-1642/04/0002-0071/$31.00/0 LEADING ARTICLE  2004 Adis Data Information BV. All rights reserved Bicycle Shock A...
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Sports Med 2004; 34 (2): 71-80 0112-1642/04/0002-0071/$31.00/0

LEADING ARTICLE

 2004 Adis Data Information BV. All rights reserved

Bicycle Shock Absorption Systems and Energy Expended by the Cyclist Henri Nielens and Thierry Lejeune Saint-Luc University Hospital, Catholic University of Louvain, Brussels, Belgium

Abstract

Bicycle suspension systems have been designed to improve bicycle comfort and handling by dissipating terrain-induced energy. However, they may also dissipate the cyclist’s energy through small oscillatory movements, often termed ‘bobbing’, that are generated by the pedalling movements. This phenomenon is a major concern for competitive cyclists engaged in events where most of the time is spent climbing, e.g. off-road cross-country races. An acceptable method to assess the overall efficacy of suspension systems would be to evaluate energy consumed by cyclists using different types of suspension systems. It could be assumed that any system that reduces metabolic expenditure for the cyclist would automatically lead to performance improvement. Unfortunately, only a limited number of studies have been conducted on that subject. Moreover, the conclusions that can be drawn from most of them are limited due to unsatisfactory statistical power, experimental protocols, measuring techniques and equipment. This review presents and discusses the most relevant results of studies that focused on mechanical simulations as well as on energy expenditure in relation to off-road bicycle suspension systems. Evidence in the literature suggests that cyclist-generated power that is dissipated by suspensions is minimal and probably negligible on most terrains. However, the scarce studies on the topic as well as the limitations in the conclusions that can be drawn from most of them indicate that we should remain cautious before supporting the use of dual suspension bicycles on all course types and for all cyclists. For example, it should be kept in mind that most cross-country racers still use front suspension bicycles. This might be explained by excessive cyclistgenerated power dissipation at the high mechanical powers developed by elite cross-country cyclists that have not been studied in the literature. Finally, suspended bicycles are more comfortable. Moreover, the fact that suspension systems may significantly reduce physical stress should not be overlooked, especially in very long events and for recreational cyclists.

Since their introduction in the US in the early 1980s, mountain bikes have become increasingly popular. In the beginning, off-road riding capabilities and gear shifting levers located on the handlebar essentially contributed to their popularity. More re-

cently, shock absorption systems also known as ‘suspension systems’ or more simply ‘suspensions’ have been developed to improve comfort and performance. Nowadays, mountain bikes are the number-one sold bicycle in the US and in Europe. Most

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models are equipped with a suspended front wheel often called ‘front suspension’ (FS). Some come with suspension systems on both wheels generally known as dual suspension (DS) systems. When it seems obvious that suspensions improve comfort, only a limited number of studies have investigated the effects of such systems on the mechanics and the energetics of cycling. After a brief preliminary discussion of some relevant issues concerning bicycle suspensions, this paper will review and discuss available data on the effects of bicycle shock absorption systems on energy expenditure. As discussed by De Lorenzo et al.,[1] suspensions isolate the cyclist from vibrations[1] and terraininduced shocks[2,3] by allowing the wheels to move independently versus the rest of the bicycle. The cyclist and the bicycle equipped with suspensions are therefore able to travel a smoother path as only the wheels follow the contours of the terrain, improving comfort and bicycle handling.[2-7] Suspensions may also improve cornering,[2,3] braking capacity,[2,5] and more generally, bicycle control, handling[4,7] and traction[2,5] since they allow better contact between the tyres and the ground. These numerous advantages explain why all downhill mountain bike racers use front and rear large travel suspension systems that allow much higher speeds in downhill events. Alternatively, bicycle suspensions also have drawbacks. For example, DS bicycles are much more expensive and average between 1–2kg and even 3kg heavier than equivalent rigid models. Such a 10–15% increase in bicycle weight may be a concern during uphill racing and accelerations as it will demand an excess of energy expenditure for the competitor. Wang and Hull[8] calculated that a 1.8kg increase in bicycle weight would result in an extra 4.7 seconds for a 60kg cyclist to climb a 6% grade hill for 1000m at 6.5 m/sec. On the basis of such data, Wang and Hull calculated that such an apparently small weight increase would lead to a total time increase of 46 seconds on a championship race similar to the Women’s World Mountain Bike Championships held in Germany in 1995. More 2004 Adis Data Information BV. All rights reserved

Nielens & Lejeune

over, Wang and Hull[8,9] emphasised that suspensions may dissipate the power generated by the cyclist, which may become unacceptable for cyclists participating in events organised on hilly terrain. This issue will be discussed more extensively in section 1. Power dissipation may occur in the suspensions themselves or in the articulated bicycle frame that becomes more flexible,[10] especially in bicycles equipped with DS. 1. Bicycle Suspension Systems In general, most front and rear shock absorbing devices are composed of an elastic and a viscous element mounted in parallel (figure 1). Mechanical properties of both elements are generally separately adjustable on most bicycles. The elastic element is made of a steel spring that can be pre-constrained at different levels or an air chamber that can be preinflated at varied pressures according to the nature of the terrain and the cyclist’s preference. The viscous element is generally made of a piston and cylinder chamber filled with oil. The oil travels through orifices made in the piston. The total size of the orifices may be adjusted to modify the damper viscosity. Some simpler and cheaper systems include an elastomer part that has both viscous and To rider, bike frame

Viscous element (damper)

Elastic element (spring)

To wheel, ground Fig. 1. Components of a shock-absorbing device. Most shock-absorbing devices that equip modern suspended bicycles are composed of an elastic (spring) and a viscous (damper) element mounted in parallel between the wheel and the frame of the bicycle. Mechanical properties of each element can generally be tuned separately to adjust for terrain characteristics and cyclist preferences.

Sports Med 2004; 34 (2)

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Suspension element Rear suspension pivot

a

d

b

e

c

f

Fig. 2. Most common bicycle suspension designs: (a) front telescopic visco-elastic suspension system integrated in each arm of the fork; (b) traditional rear swing arm with one pivot point; (c) multi-bar linkage rear suspension systems with multiple pivot points; (d) unified rear triangle system that eliminates interactions between front chain-ring and rear suspension; (e) the Allsop’s Softride system; and (f) suspension seat posts.

elastic properties. On some bicycles, the suspension system may be turned off through command switches located on the handlebar allowing riding in rigid mode. While most FS systems are generally made from telescopic forks with visco-elastic elements in each arm of the fork, rear suspension systems are numerous. Figure 2 shows several systems that are commonly available on the market. Suspension systems are varied, from the simplest to the most sophisticated. Many mountain bikes are currently equipped only with a front telescopic visco-elastic suspension system integrated in each arm of the fork (figure 2a). More sophisticated DS systems are: traditional rear swing arm with one pivot point (figure 2b); multi 2004 Adis Data Information BV. All rights reserved

bar linkage rear suspension systems with multiple pivot points (figure 2c); and a unified rear triangle system that eliminates interactions between the front chain-ring and the rear suspension (figure 2d). Much simpler systems have also been developed that allow isolating the cyclist from vibrations and/or shocks generated by terrain irregularities. However, in such systems, almost the entire bicycle mass is unsprung. In the Allsop’s Softride system, the saddle is mounted on a flexible composite beam (figure 2e). Suspension seat posts are relatively cheap and simple systems that can replace traditional rigid seat posts on most bicycles (figure 2f). The location of the centre of rotation of the rear suspension swing-arm (i.e. the pivot point) is an important technical characteristic Sports Med 2004; 34 (2)

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Nielens & Lejeune

of rear suspensions as it may influence the magnitude of cyclist-generated power that is dissipated in the system.[8,11-14] Several authors have studied the optimal pivot point location of the rear suspension through mechanical modelling[8,9,13] and experimentation.[3] However, only very few scientific publications have addressed the issue of the optimal design for suspension systems. Accordingly, besides more precise knowledge about rear suspension pivot point location that will be presented and discussed more extensively in section 2, most arguments proposed by manufacturers favouring their particular system still need to be objectively assessed. 2. Mechanical Aspects: The Engineer’s Point of View By definition, the main function of suspension systems is to absorb energy. More specifically, the viscous element (damper) present in most shockabsorbing devices is precisely designed to dissipate energy transmitted to the suspension element. In the bicycle, cyclist and terrain model, energy can be generated and transmitted to the suspensions either by the terrain irregularities or by the cyclist themself[8,9] (figure 3). The aim of bicycle suspensions is to dissipate energy generated by the terrain allowing better comfort and bicycle handling for the cyclist. It is worthDeformable cyclist in movement Front shock-absorbing device

b

b

Rear shock-absorbing device

b

a

a

Fig. 3. Forces transmitted to the suspensions. In off-road cycling, forces transmitted to the suspensions may be classified in two categories: (a) forces generated by the terrain irregularities; and (b) forces generated by the movements of the cyclist which are applied on the handlebar, saddle and pedals.

 2004 Adis Data Information BV. All rights reserved

while noting that both the tyres[2,8,10,15] and the body parts of the cyclist[2,3] also dissipate energy generated by terrain irregularities and thus act as dampers. Downhill racers are confronted with important terrain irregularities on very steep courses travelled at high speeds. Hence, suspensions used in such events must be designed to absorb a large amount of terrain-induced energy. As a result, modern downhill bicycles are usually equipped with large travel suspensions with large energy absorption capacity. Unfortunately, suspensions may also dissipate power generated by the cyclist’s muscles through small oscillatory suspension displacements often referred to as ‘pogoing’[2] or, more commonly, ‘bobbing’.[3,8,9,12] Bobbing can essentially be generated by two mechanisms: (i) the displacement of the cyclist’s body parts; and (ii) the interaction between the forces applied on the pedals that are transmitted to the front chain-ring and the rear suspension. The first mechanism can be reduced either by tuning the suspension optimally and/or by the cyclist who can adapt his/her pedalling technique.[2,10] However, the magnitude of the energy dissipation that may occur through such a mechanism has never been studied in detail. It seems clear that such an issue will be difficult to address since it may be assumed that displacement of a cyclist’s body parts is probably highly variable among individuals.[14,16,17] The second mechanism (bobbing induced by interaction between front chain-ring and rear suspension) is graphically represented in figure 4. As recalled by Good and McPhee,[13] such a mechanism may become exaggerated in high-load situations such as climbing or sprinting. Several authors[3,8,12-14,17] have studied this phenomenon in more detail by modelling the bicycle and cyclist as a multi-body system with springs and dampers in order to evaluate the magnitude of energy dissipated in the rear suspension. More specifically, Wang and Hull[11,12] calculated that the power dissipated in the rear suspension was 6.9W when cycling uphill at 6.5 m/sec (23.4 km/h) a 6% grade smooth surface. This 6.9W value represents only 1.3% of the total power developed Sports Med 2004; 34 (2)

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Fig. 4. Rear suspension compression (a) and extension (b) forces in relation to pivot point location. In bicycles equipped with rear suspensions, the cyclist-generated pedalling forces are transmitted to the rear shock-absorbing devices. According to the location of the rear suspension pivot point, pedalling forces may cause the rear suspension to compress (a) or to extend (b), which generates cyclist-induced power dissipation (adapted from Wang and Hull,[12] 1997, with permission  Swets & Zeitlinger).

Chain force

a

Suspension compresses

Pivot

b

Suspension extends

Chain force

Pivot

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by the cyclist, which is in agreement with data presented by Kyle.[18] Noteworthy, power dissipated in the FS was found negligible. Wang and Hull[11] also validated their findings experimentally by measuring front and rear suspension displacements with linear transducers while a cyclist rode a commercially available DS bicycle on a treadmill at the same grade and speed. Later, Wang and Hull[12] studied the optimal rear suspension pivot point location in terms of energy loss minimisation. The vertical position of the pivot point was the most critical factor. Their model showed that power dissipated in the rear suspension could be reduced to 1.2W when the pivot point was positioned on the seat tube, 11cm above the bottom bracket for a 32-teeth front chain-ring. They also showed that optimal pivot point location was very insensitive to the fore-aft location of the pivot point, to pedalling mechanics, and to both spring and damping parameters. Due to the U-shape of the power dissipation versus vertical location of the pivot point curve, a sub-optimal pivot point location lead to only a small increase in energy loss. Wang and Hull’s results also indicated that the optimal pivot point location is directly dependent of the size of the front chain-ring, which is understandable because that parameter directly determines the position of the chain-line. Needle and Hull[3] conducted an experimental study with a DS bicycle with adjustable geometry and suspension parameters to verify the validity of both the previously proposed model and the location of the rear suspension pivot point obtained by simulation. In this experimental study of suspension displacements with only one cyclist, the optimal pivot point location was found to be 8.4cm above the bottom bracket, which was relatively close to the result obtained by simulation. Good and McPhee[17] developed a less complex four-body dynamic model of a rear suspended bicycle that was quite different than that of Wang and Hull.[11] The response of their four-body model was compared with simulation data previously obtained by Wang and Hull.[11] Although this four-body model was considerably simpler, it produced similar Sports Med 2004; 34 (2)

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results in terms of rear suspension displacements as a function of crank (bottom bracket) angle. Using a different optimisation method known as a ‘genetic algorithm’ to determine the optimal design of the rear suspension (pivot point location) along with their four-body model, Good and McPhee[13] showed that the optimal rear suspension pivot point was located 11.6cm above the bottom bracket and 2.7cm behind the seat tube. Such results are again very close to the first simulation data obtained by Wang and Hull. Finally, in a recent study by Karchin and Hull,[14] 11 experienced cyclists were asked to ride a custombuilt DS bicycle with adjustable geometry and suspension parameters at an approximate power of 300W (6% grade on a treadmill at 24.8 km/h) in a seated as well as in a standing position. By monitoring the suspension displacements with linear potentiometers, Karchin and Hull showed that: (i) the minimum power loss at the optimal pivot point height was quite variable among study participants (mean 0.89W, range 0.59–1.25) indicating large variability in pedalling mechanics; (ii) power dissipated in the rear suspension was considerably higher (mean 6.49W, range 0.7–13.48) in the standing position; (iii) no significant interaction between front and rear suspensions could be found; (iv) the optimal pivot point location is higher in the sitting position (9.77cm) than in the standing position (5.88cm); and (v) notwithstanding the wide variability in the minimum power loss among study participants, the optimal pivot point location remained consistent. In summary, all mechanical simulation and experimental studies specifically conducted to evaluate the power dissipated by suspensions in DS bicycles agree that the estimated power lost in the rear suspension ranges from 0.5–2W in the seated position when the pivot point is located optimally (±10cm above the bottom bracket on the seat tube). Such a magnitude of power loss remains inferior to 0.7% of the total power developed by the cyclist. It should therefore be largely compensated by the benefits provided by suspensions in terms of comfort and performance related to better bicycle handling  2004 Adis Data Information BV. All rights reserved

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(braking, cornering, traction etc.). However, in the standing position, power dissipated in the rear suspension may reach up to 5% of the total power developed by some cyclists. It must be kept in mind that many simplifying assumptions are made in simulation studies. Hence, numerous factors are neglected that may lead to a systematic underestimation of power losses due to suspension systems, e.g. multi-articulated DS bicycle frames become more flexible. It may be hypothesised that the amount of energy dissipated in the frame itself[2] may increase significantly. Such a source of energy dissipation is neglected in computer-simulation studies. However, it may be significant as witnessed by the efforts made by manufacturers of bicycle parts to reduce the flexibility of front telescopic-suspension forks by designing reinforced front wheel hubs and brake bridges.[1] Another method to evaluate the amount of power lost in suspensions is to measure the energy consumed by the cyclist riding FS or DS bicycles as compared with non-suspended ones. Section 3 of this manuscript will review such studies and focus more on physiological variables that can be observed in the laboratory or on the field. 3. Energetic Aspects: the Exercise Physiologist’s Point of View The energy consumed by the cyclist ultimately reflects the result of the interaction of the multiple variables from the terrain, bicycle and cyclist. On the same irregular terrain travelled at the same speed, a more efficient suspension system (i.e. a system that dissipates terrain-induced energy well without dissipating cyclist-generated energy) should allow the cyclist to expend less energy than with a less efficient suspension, since less cyclist-generated power will be dissipated in the suspension. At maximal energy expenditure rate, this more efficient suspension will ultimately allow the cyclist to attain higher speed. Berry et al.[4] were the first to evaluate metabolic expenditure by means of oxygen consumption ˙ 2) of cyclists riding different types of bicycles (VO (no suspension, FS, DS and rear suspension only) on Sports Med 2004; 34 (2)

Bicycle Suspensions and Energy Expenditure

a treadmill with a 4% grade and at a speed of 10.4 km/h with or without a 3.8cm high bump attached to its belt with duct tape. No significant differences in ˙ 2 were noted in relation to the suspension type in VO the no-bump condition. In other words, suspensions did not significantly increase energy expended by the cyclist riding on a smooth surface. They observed a very significant energy consumption increase in relation to the presence of the bump, ranging from a 63% increase compared with the no-bump condition for the non-suspended bicycle to 41% for DS bicycle. The DS allowed a very significant 11.5% de˙ 2 (p = 0.004) compared with the noncrease in VO suspended condition. Adding only a rear suspension to the bicycle already yielded a significant energy saving. Surprisingly, the FS alone did not succeed in lowering energy expenditure significantly, which suggests that FS does not succeed in reducing energy consumed by the cyclist when a DS system does. Such a finding contrasts with data of other authors that will be discussed further in the next paragraph. However, a type II statistical error may not be excluded due to the relatively small number of study participants (n = 6) included in this study. Seifert et al.[5] were the first to evaluate the energy cost of bicycle suspensions in a more natural outdoor environment. They designed a rather complex protocol during which cyclists were asked to undergo three consecutive experimental phases using different suspension types (no suspension, FS and DS). In one phase, 12 study participants rode at a constant speed (16.1 km/h) on a flat looped ~400m course built on hard level ground with 45 fabricated 5cm high wooden bumps. It must be emphasised that such experimental conditions are relatively close to those of Berry et al.[4] with 3.8cm high bumps encountered 42 times/min and a 4% grade compared with 5cm high bumps encountered 30 times/min and no grade, respectively, in Berry et al.[4] and Seifert et al.[5] protocols. Only a trend ˙ 2 (p = toward a significant reduction of mean VO 0.07) was observed with the suspended bicycles compared with the non-suspended one. Although experimental conditions were relatively similar, the results from Seifert et al.[5] contrast with the very  2004 Adis Data Information BV. All rights reserved

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significant 11.5% reduction in energy consumption observed in the DS mode by Berry et al.[4] Although up to 12 study participants took part in the first phase of the study where metabolic measurements were conducted, a type II error may still not be excluded. This hypothesis may be supported by the fact that mean heart rates recorded on the rigid bicycle were significantly higher than those on the suspended bicycles. In another phase of the protocol, seven study participants were asked to complete three different time trial courses as fast as possible including a downhill, a climb and a cross-country course. In the cross-country trial, the best performance was achieved on the FS bicycle in five of the seven cyclists with the mean finishing time being significantly smaller (p = 0.02) with the FS. This observation fits well with what is currently observed in competition. No significant differences in finishing times relative to suspension types were noted in the downhill and climbing trials. More recently, MacRae et al.[7] conducted a study on two different outdoor uphill courses aiming at comparing performances of six experienced cyclists riding bicycles equipped with FS and DS. The first course was a 1.62km asphalted road with a 14.2% mean grade. The second course was a ‘rocky and rutted’ fire access road that was 1.38km with a mean 11.3% grade. Traditional physiological variables ˙ 2) were monitored and (e.g. heart rate and VO mechanical power generated by the cyclist was measured and recorded during the runs by a Schoberer Rad Messtechnik (SRM) Training System (Weldorf, Germany). On both courses, performances in terms of finishing times were not significantly different whether the study participants used the FS or the ˙ 2 recorded during DS. Likewise, mean heart rate, VO the runs and blood lactate concentration measured in samples collected 2 minutes after the end of the trials were not different, suggesting that metabolic cost is independent of the suspension types. However, mean mechanical power generated by the study participants as recorded by the SRM system was almost 30% higher with the DS compared with the FS on both courses, which was very significant. In our opinion, the findings of this study must be Sports Med 2004; 34 (2)

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considered with caution. It is hard to explain how the study participants could have developed close to 30% more mechanical power on the DS bicycle with all physiologically recorded parameters remaining unchanged and when they were all asked to perform maximally on both bicycle types on both courses. The authors argue that the usual relationship between power output, cardiovascular and metabolic responses usually observed in the laboratory might be altered on the field. However, it must be noted that the SRM power-meter system has not been validated when used with DS bicycles. Another hypothesis could be that interactions between the front chain-ring (on which the SRM system is mounted on) and the rear suspension could have led to a systematic mechanical power overestimation due to so-called ‘kick-back’ effects.[12] Finally, Nielens and Lejeune[6] studied the metabolic cost of riding a bicycle equipped with different types of suspensions on a smooth surface. In this experiment conducted in a laboratory, one crosscountry DS bicycle was mounted on a bicycle trainer. Different suspension systems were obtained by successively replacing rear and both suspension elements by rigid links. The aim of the study was to specifically evaluate any possible cyclist-induced energy loss when riding a modern cross-country suspended bicycle after eliminating terrain-induced energy losses by riding on a smooth surface. Twelve study participants were asked to perform a 15-minute gradational cycling protocol starting at 50W with 50W increments every 3 minutes in the three ˙ 2, nor in suspension modes. No difference in VO heart rate in any of the stages of the tests was observed in relation to the suspension type, suggesting that suspensions do not generate any extra energy expenditure for the cyclist when riding on a smooth surface. However, in this experiment, the highest external power reached was 250W, which is probably significantly lower than powers attained by competitors in actual cross-country races. Moreover, in all suspension modes, the bicycle frame remained the same (a modern light cross-country frame with four-bar linkage front and rear suspensions) and energy losses may have occurred in all  2004 Adis Data Information BV. All rights reserved

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suspension modes in relation to the relatively high deformability of such articulated frames and forks. The results of this study should therefore not be extended to compare high-level cross-country competitors using traditional telescopic FS and DS without caution. In summary, Berry et al.[4] were the first to report laboratory data suggesting that the metabolic cost of riding a bicycle on a bumpy surface could be very significantly reduced by DS systems. Such observation illustrates the fact that terrain-induced energy can indeed be dissipated in the suspension system, allowing the cyclist to consume less energy to travel at the same speed over bumpy and shaky surfaces. The fact that suspensions may also dissipate the cyclist-generated power remains a major concern for bicycle manufacturers and competitors. The only three studies[4,6,7] that evaluated metabolic cost ˙ 2) of cyclists using suspended bicycles on (VO smooth surfaces failed to demonstrate any increase in metabolic cost in relation to suspensions. Unfortunately, the conclusions that can be drawn at this point are limited because of the paucity of studies that evaluated metabolic cost on the field, with the rather poor statistical power of some studies and, finally, with the relatively low value of the mechanical power outputs that have been investigated. 4. Bicycle Comfort and Physical Stress in Relation to Suspensions Seifert et al.[5] evaluated perceived riding comfort and rate of perceived exertion of 20 cyclists who successively rode non-suspended, FS and DS bicycles on a hard, level ground course with 45 fabricated bumps during approximately 1 hour. The DS bicycle was perceived as the most comfortable, and the FS as more comfortable versus the nonsuspended bicycle. Perceived exertion data favoured the suspended bicycles and no significant difference was observed between suspension types. In the same study, Seifert et al.[5] also reported lower creatine kinase levels in the venous blood samples of cyclists after riding the suspended bicycles (FS and DS) compared with rigid ones. Although no significant difference was observed between DS and FS for Sports Med 2004; 34 (2)

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creatine kinase change, these data strongly suggest that suspension systems are effective in reducing muscular stress. In summary, comfort is obviously an important issue for recreational cyclists who often favour softer and more comfortable suspension systems. Nevertheless, the fact that suspensions seem effective in reducing physical stress may become particularly relevant for competitors engaged in long distance events that may last up to 6–12 hours and for recreational cyclists who generally favour comfort. 5. Conclusion Bicycle suspensions have been designed to improve bicycle comfort and handling by dissipating terrain-induced energy variations. However, they may also dissipate the energy generated by the cyclist through small oscillatory movements often termed ‘bobbing’. Bobbing is generated by the cyclist’s body movements as well as by the forces exerted on the pedals that interact with the rear suspension. This phenomenon is a major concern for bicycle manufacturers and competitive cyclists engaged in events where most of the time is spent climbing, such as cross-country races. Any cyclistgenerated power dissipated in the suspensions will slow down the cyclist. Ideally, such a loss in performance that mainly takes place when riding uphill should always remain smaller than the gain in speed provided by suspensions through better bicycle handling, mainly in the downhills. Several mechanical simulation studies conducted by engineers suggest that if bicycle suspensions and geometry are optimised, cyclist-generated power that is dissipated when riding in a seated position is minimal if not negligible. However, several possible dissipation sources are neglected in such studies as they included numerous necessary simplifying assumptions in the models. Only a few studies have evaluated the effects of suspension systems on the energy expended by the cyclist in the laboratory and in the field. One study[4] showed that the energy expended by the cyclist could be very significantly reduced by a DS system when riding on an artificial bumpy course, sug 2004 Adis Data Information BV. All rights reserved

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gesting that suspensions dissipate terrain-induced energy variations well. Two studies[4,6] conducted in a laboratory reported no energy expenditure increase related to suspensions when riding on a smooth surface. Such studies indicate that if any energy was indeed dissipated in the suspensions, it was too small to be measured by traditional respiratory gas analysis methods. In other words, the magnitude of any energy dissipation by modern suspension systems must be very small, if any, and thus probably negligible compared with the advantages they provide. Such results are in agreement with data observed in mechanical simulation studies. In summary, evidence present in the literature suggests that cyclist-generated power that is dissipated by suspensions is minimal and probably negligible on most terrains. However, the scarce studies on the topic, as well as the limitations in the conclusions that can be drawn from most of them, indicate that we should remain careful before supporting the use of DS bicycles on all course-types and for all cyclists. For instance, the fact that most cross-country racers still use FS bicycles should be kept in mind. Finally, suspension systems clearly improve comfort and reduce physical stress. This issue should not be overlooked, as most recreational cyclists will ultimately favour comfort over performance. Acknowledgements The authors wish to thank Mr E.J. Conley for his valuable help in the writing of this manuscript. No sources of funding were used to assist in the preparation of this manuscript. The authors have no conflicts of interest that are directly relevant to the content of this manuscript.

References 1. De Lorenzo DS, Wang EL, Hull ML. Quantifying off-road suspension hub stiffness. Cycling Sci 1994; 3: 12-26 2. Olsen J. Bicycle suspension systems. In: Burke ER, editor. High-tech cycling. Champaign (IL): Human Kinetics, 1996: 45-64 3. Needle SA, Hull ML. An off-road bicycle with adjustable suspension kinematics. Cycling Sci 1997; 1: 4-29 4. Berry MJ, Woodard CM, Dunn CJ, et al. The effects of a mountain bike suspension system on metabolic energy expenditure. Cycling Sci 1993; 3: 8-14

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5. Seifert JG, Luetkemeier MJ, Spencer MK, et al. The effects of mountain bike suspension systems on energy expenditure, physical exertion, and time trial performance during mountain bicycling. Int J Sports Med 1997; 18: 197-200 6. Nielens H, Lejeune TM. Energy cost of riding bicycles with shock absorption systems on a flat surface. Int J Sports Med 2001; 22: 400-4 7. MacRae H-H, Hise KJ, Allen PJ. Effects of front and dual suspension mountain bike systems on uphill cycling performance. Med Sci Sports Exerc 2000; 32: 1276-80 8. Wang EL, Hull ML. A dynamic system model of an off-road cyclist. J Biomech Eng 1997; 119: 248-53 9. Wang EL, Hull ML. Power dissipated by off-road bicycle suspension systems. Cycling Sci 1994; 4: 10-26 10. Olsen J. Bicycle suspension meet Mr Simple Dynamics. Cycling Sci 1992; 3: 6-12 11. Wang EL, Hull ML. A model for determining rider induced energy losses in bicycle suspension systems. Vehicle Syst Dynam 1996; 25: 223-46 12. Wang EL, Hull ML. Minimization of pedaling induced energy losses in off-road bicycle rear suspension systems. Vehicle Syst Dynam 1997; 28: 291-306

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13. Good C, McPhee J. Dynamics of mountain bicycles with rear suspensions: design optimization. Sports Eng 2000; 3: 49-55 14. Karchin A, Hull ML. Experimental optimization of pivot point height for swing-arm type rear suspensions in off-road bicycles. J Biomech Eng 2002; 124: 101-6 15. Whitt F, Wilson D. The wheel: bicycling science. Cambridge (MA): The MIT Press, 1989 16. Daly DJ, Cavanagh PR. Asymmetry in bicycle ergometer pedalling. Med Sci Sports 1976; 8: 204-8 17. Good C, McPhee J. Dynamics of mountain bicycles with rear suspensions: modeling and simulations. Sports Eng 1999; 2: 129-43 18. Kyle C. Chain friction, windy hills, and other quick calculations. Cycling Sci 1990; 2: 23-6

Correspondence and offprints: Dr Henri Nielens, Sports Medicine, Cliniques universitaires Saint-Luc, Universit´e catholique de Louvain, Avenue Hippocrate, 10, 1200 Brussels, Belgium. E-mail: [email protected]

Sports Med 2004; 34 (2)

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