Surface Roughness - Introduction
Locally a number of riders have complained about the quality of the surface of bike paths (e.g. corrugations, ridges, etc.).
Unfortunately there doesn't seem to be any standard in the UK for the 'flatness' of such paths.
In Belgium they specify a maximum unevenness of 5 mm over a length of 3 metres - but they don't specify anything about frequency. If you had a sinusoidal surface with 5 mm peaks at 10 cm intervals - I don't think it would be very comfortable!
For cycling to be promoted successfully, cycle paths should not only be safe, but also comfortable to ride on.
That is why in 2010, the ministry bought 5 bicycles which measure cycle path flatness by recording vibration levels.
The goal of this project was twofold:
- Add vibrational comfort as a factor in assessment of cycle path quality.
- Give municipalities the means to manage cycle infrastructure in a systematic way.
During the audit, trained cyclists rode down all cycle paths of the 16 participating municipalities.
The results of the measurements were collected in an online database, which provides the participating municipalities with a complete inventory of (and statistics for) their cycle paths, allowing them to manage their cycle infrastructure in a structural manner for the first time.
Purchase of the measuring bicycles amounted to 16,335 Euro.
Extract from "Bicycling Science" by Whitt and Wilson MIT Press 1976
Rough roads affect bicyclists in several ways. The vibration may be uncomfortable and may require the bicycle to be heavier than if it were designed for smooth roads. And there will be an energy loss.
The energy loss depends on the "scale" of the roughness, the speed, and on the design of the bicycle. If the scale is very large so that the bicyclist has to ride up long hills and then to descend the other side, overall energy losses are small (and principally due to the increased air-resistance losses at the high downhill speeds). There are in this case virtually no momentum losses.
Now imagine a very small scale of roughness, with a supposedly rigid machine travelling over the surface. Each little roughness could give the machine an upward component of velocity sufficient for the wheel(s) to leave the surface (see Figure). The kinetic energy of this upward motion has to be taken from the forward motion, just as if the rider were going up a hill. But when the wheel and machine descend, under the influence of gravity as before, the wheel contacts the surface at an angle, the magnitude of which depends upon the speed and the scale of the roughness. All the kinetic energy perpendicular to the surface at the point of contact can be considered to be lost. Herein lies part of the reason for rough-road losses. Pneumatic tires greatly lower the losses for small-scale roughness because only the kinetic energy of part of the tread is affected, and the spring force of the internal pressure ensures that in general the tire does not come out of contact with the surface. The principal losses are due to the flexing of the tires and tubes (“hysteresis" losses).
At a larger scale of roughness, perhaps with a typical wavelength of 6 to 60 in. (0.152 to 1.52 m) and a height amplitude of 1 to 6 in. (25 to 152 mm), bicycle tires are too small to insulate the machine and rider from the vertical velocities induced, and the situation more nearly approaches the analogy to the rigid-machine case discussed above.
For this scale of roughness, typical of pot holes and ruts, some form of sprung wheel or sprung frame can greatly reduce the kinetic-energy or momentum losses by reducing the un-sprung mass and ensuring that the wheel more nearly maintains contact with the surface.
Use of Smartphones
I wondered whether smart phones would be sensitive enough to measure such roughness as they contain accelerometers, gyroscopes and orientation sensors as well as GPS.
A small app was written to capture such data. The graphs in teb linked pages show some preliminary results. A Smartphone was attached firmly to the rack of a touring bike which was ridden at about 10 mph over the section of interest. In each case the horizontal axis is time (in milliseconds) and the vertical axis is acceleration in m/s^2.
9.8 m/s^2 is g (the acceleration due to gravity).
Some of the graphs use data from an instrument (an accelerometer) which includes gravity
(i.e. the curves are centred on 9.8)
and some use a 'linear accelerometer' which removes gravity (i.e. the curves are centred around 0.0).