published in Human Power, Vol. 7 No. 2, Fall/Winter 1988, p. 4 - 6
In Oldenburg, a town of 140,000 inhabitants in the lowlands of North West Germany, bicycle traffic plays an important role in the traffic. In the inner city about 10% to 25% of the traffic is done by bicycles. All major streets have cycle tracks on the sidewalks. The situation of the bicyclists is not as good as in the Netherlands but better than in most German towns where only motorized traffic is promoted.
The traffic safety for the bicyclists in Oldenburg is not better than in towns without sidewalk cycle tracks. The accidents happen at the crossings instead of along the street as in towns without cycle tracks.
Most cycle tracks in Oldenburg are made of concrete bricks because they are cheap and easily removable for excavation. Riding on those surfaces is not very comfortable, but bicyclists are forced by law to use them and are not allowed to ride on the smoothly asphalted roadway pavement.
As a small group of physicists at the University of Oldenburg we are working on bicycles and HPVs. In order to initiate a public discussion about cycle-track quality we tried to quantify vibrational stress on the rider. The method we used is standardized by international ISO standard 2631 and German VDI (Verein Deutscher Ingenieure = Society of German Engineers) standard 2057. These standards are mainly used on tractors and other machines where the operational staff is exposured to vibrations. According to the standards the acceleration is measured at the interface with the rider. The acceleration is then filtered according to the frequency response of vibration of the human body. German VDI standard calls the resulting effective value K-Value. K-Values go linear with the acceleration but differ with frequency. In the VDI Standard this K-Value - there is no corresponding designation in the ISO Standard - is used to get exposure-time limits for health or capacity of reaction or comfort impaired. In ISO standard the limit of reduced comfort is identical to VDI, the German limit of reduced capacity of reaction is called exposure limit in ISO standard, and is the upper limit at all. There is no limit of health impaired in the ISO Standard; vibration should always be below the exposure limit (ISO) or limit of reduced capacity of reaction (VDI).
The frequency response of (wo)man and the exposure-time limits are empirical results. This method is very similar to the standardized measurement of loudness. But (wo)man has different frequency response concerning different directions and different parts of the human body. The hand-arm-system is, independent of direction, most sensitive between 8 and 16 Hz. The maximum sensitivity for vibration in direction of the spinal column of the body lies between 4 and 8 Hz.
We measured the acceleration at the handlebar of the bicycle and at the underside of the saddle in direction of the backbone. The signals of the sensors were amplified and encoded to a PCM-system on the bicycle and send to a data tape recorder in a nearby car by HF-telemetry. The data were evaluated with electrical filters corresponding to the frequency rating of ISO and VDI standard.
Because the standards allow the interpretation only of signals lasting longer than 1 minute time we cannot make statements about the effect of single bad spots on cycle tracks like potholes and very bad transitions between cycle track and driveway at crossings.
We carried out the measurement on eleven different surfaces:
We repeated the measurement with six different vehicles: a roadster bicycle, two touring bicycles, a Moulton bicycle, an OLF (Oldenburger Leichtfahrzeug = Oldenburg Lightweight Vehicle) and a car.
The roadster bike was similar to the typical bicycles in the Netherland and is the type most commonly used in Oldenburg. The tires were 37-622 mm (28 x 1 3/8 ") at 320 kPa (45 PSI). The saddle has large pressure and tension springs. The diamond-framed bicycle weighted 17 kg, the rider 80 kg.
Both touring bicycles had tires of the size 28-622 mm (28 x 1 1/8 ") at 600 kPa (85 PSI) front and 700 kPa (100 PSI) rear and an 'anatomic' saddle without springs. The bicycles differ in the position of the handlebar, wall thickness of CrMo steel tubes and weight. On no. 1 the rider's arm had an angle of approximately 70 degree to horizontal, no. 2 was with 45 degree more at the standard position on touring bikes. The wall thickness of the tubes of no. 2 were thinner (exact dimensions unknown). Bike no. 1 weighted 15 kg and its rider 80 kg, no. 2 14 kg and 72 kg.
The Moulton bicycle was an AM 7 with its 17" tires at 700 kPa (100 PSI). The suspension system is at front a weakly dampened steel spring and at rear a rubber-block spring with internal damping. Resonance frequency front and rear is approximatly 3 Hz. It had the same saddle as the touring bicycles. Weight of the bike was 14 kg and of the rider 80 kg.
The OLF is a three-wheeled recumbent prototype built by our bicycle research group (see HUMAN POWER Spring/Summer 1988). It has a soft spring suspension system with a resonance frequency of approximatly 1.5 Hz. The Moulton wheels were inflated to 600 kPa (85 PSI). The vehicle weighted 29 kg and the driver 82 kg.
The car we used was an old Volkswagen Rabbit. The whole payload weighted 280 kg and the driver himself 90 kg.
All vehicles had an additional weight of 11 kg for the measurement equipment. On the roadster and touring bicycles it was mounted in panniers on the rear carrier.
The valued intensities of vibration are shown in the graphs. The scale is logarithmic as is the human response. The bright column shows the intensity on the hand-arm-system, the dark that at the saddle in direction of the backbone.
For interpretation there are shown the exposure limits at different times of exposure per day as they are defined in the VDI standard. The labels at the lines mean:
If for example a column for a bike exceed the G 60 limit, then the health of the rider may be impaired if the total riding time of one day on that (or worse) surfaces exceeds 60 minutes. If the exposure time is below 60 minutes per day, "only" the capacity of reaction is impaired.
On many surfaces vibration on touring bicycle no. 1 exceeds the limit of health impaired. Only on asphalted surfaces and new cycle tracks is health not impaired. Especially the strain on the hand-arm-system is very critical because the static load is very high in the forward-leaning position. On touring bike no. 2 the intensity of vibration on hand-arm-system is only half as much, presumably because of the thinner wall thickness of the fork tubes. Up to good and medium quality asphalted surfaces, capacity of reaction is always impaired.
On rough cobble-stone pavement health is impaired on the roadster bicycle. On many surfaces the limit of impaired capacity of reaction is exceeded. Only on good quality asphalted surfaces comfort is not impaired.
With the Moulton AM 7 Bicycle the valued intensity of vibration is similar to the roadster bicycle despite of the low rolling resistance of the high pressure tires. The highly dampened rear suspension is especially troublesome. The vibrational strain is reduced by the front suspension much better than at the rear. But the only slightly dampened front suspension is unsatisfactory in driving behavior. On smooth surfaces swinging of the rear suspension in reaction to pedal forces is noticeable.
The much softer OLF shows distinct advantages over the other human-powered vehicles. Under practical riding conditions capacity of reaction is not impaired and in many cases not even comfort is impaired. It is possible to optimize the suspension system even further and hereby the valued intensity of vibration will be further slightly reduced.
The car was the most comfortable vehicle as to be expected. Comfort is impaired only on very bad roads. The suspension system of the car consists of three parts: the tire, the spring-suspension system and the seat. Our prototype OLF has only a spring suspension system, seat and tire springing can be neglected. The automobile industry of course has had much more time and money to reach the comfort of today's cars than our small group was able to spend on the development of our prototype HPV.
The intensity of vibration, at least on touring bike no. 2, increases approximately linearly with speed in the range between 10 and 40 km/h.
The intensity of vibration below a K-value of 1 is not measurable with the equipment we used.
To reduce the risk of impairing health and capacity of reaction (which means risk of accident) there are three possibilities:
The last point is the cheapest and is the most realistic. And it has additional benefits to inner traffic by lowering the speed of cars.
Maybe someone thinks vibration is a part of bicycle riding. It is one of the differences to the tin-box-encapsulated car driver. That may be applicable for sports riding and ego-trips on bicycles. But for the commuting bicyclists - and increasing the number of those is what we need to reduce the environmental disaster following abundant driving of cars in cities - vibrations are very burdensome.
Diagrams of K-Value:
a very old pavement of irregular field stones (Hochhauser Str. flank):
pavement of small cobble-stones (Werbachstr.):
pavement of normal cobble-stones (Elisabethstr.)
very old brick-stone pavement with stones on end (Hochhauser Str. middle):
brick-stone pavement with flat stones old (Marschweg):
brick-stone pavement with flat stones new (Damm):
concrete-stone pavement in the figure of a Y (Staugraben):
concrete-stone pavement rectangular shaped (Carl-von-Ossietzky-Str. cycle track):
asphalted surfaces in cycle track quality (Freibad cycle track):
country road quality (Kuepkersweg):
highway quality (Carl-von-Ossietzky-Str. drive way):
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