When viewing the results below it should be noted that, while experimental drag readings represent characteristics of the entire wheel, power reading only represent the rotating characteristics i.e. drag of the spokes (aero and wire) and skin friction of the disc and rim. This is because the motor is not working against the drag of the entire wheel. The wheel is being held in place by the balance, which is in no way connected to the motor.
8.1 Drag at zero yaw
Fig 8.1 shows the axial drag of the wheel at zero yaw angle for all the configurations tested. The full table of results from which the graph was plotted can be found in appendix C.
The results displayed in fig 8.1 follow a general trend to higher drags at higher speeds, although the drag of the spoked wheels does not increase at a consistent rate. Significantly the spoked wheels all have a similar shaped curve while the disc and tri-spoke have near straight lines. The results are consistent enough to postulate that spoked wheels have less drag at low speeds – below 25km/hr.
Although it was assumed that if the 36 spoke wheel has the highest drag at 40km/hr it will have the highest drag at all speeds, this is not the case in the results obtained since, contrary to expectation, the spoked wheel curves cross each other. This apparent inconsistency might be due to:
i) the high drag of the support jig and motor which makes the measurement similar to weighing a bag of sugar on the bathroom scales by weighing yourself and then you and the sugar! For tare values see appendix C.
ii) the spirit level style of the balance and the need to re-zero it regularly, which gave potential for human error in taking the readings.
iii) the very small variations in drag, so changes in the atmospheric conditions in the lab, errors in setting up the wheel in the test rig, and any inaccuracy of the balance may have significantly affected the results.
8.2 Power at zero yaw
Fig 8.2 shows the power required to drive the wheel at zero yaw angle for all the configurations tested. The full table of results from which the graph was plotted can be found in appendix C.
Fig 8.2 displays a set of results which are believed to be more reliable than those for drag. This higher degree of accuracy can be attributed to:
i) a more precise method of measurement - The voltmeter and ammeter used had digital displays, which involve no human error and gave readings to 2 decimal places.
ii) the high tare values (see appendix C), which were an advantage in this case, because they took out most of the inaccuracies due to the efficiency of the motor.
The below zero results are probably due to the difficulty of taking accurate tare values at low speeds. The bare hub and spindle have very little inertia and readings were erratic.
As far as the actual power results obtained are concerned, some interesting readings were taken. It was assumed before the testing was carried out that the disc would be the most efficient wheel, but the results indicate that this is far from true. It is mid way down the power scale along with 18 spokes. The tri-spoke was the easiest wheel to turn, being even more aerodynamic than the 6 spoke wheel. As expected, the power needed to turn the round spoked wheels increases with the number of spokes.
8.3 Theoretical results
The differences in drag between the various configurations are theoretically analysed in section 6. These results do not compare exactly with the experimental results, but show a strong correlation especially with the experimental results for power. They also show why there are differences between the data collected for each configuration. A more in depth theoretical analysis with fewer assumptions is needed to give precise results. This could not be done due to both time and software limitations.
The theoretically obtained results for all but the disc configuration are displayed graphically in fig 8.3 and 8.4. It should be noted that these are values for the spokes only and do not include the power to turn the rim or the drag it produced, the rim being the same for all tests this does not affect comparison of the results.
Fig 8.4 shows the tri-spoke as having a much higher power requirement than was found in the experiment. This is probably due to the assumption made to simplify the analysis that the spokes meet an uninterrupted free stream flow: in fact the flow is disturbed by the rim. This does not greatly affect the flow over the round spokes as they are angled out away from the rim towards the hub so most of the spoke can be seen by the free stream flow. The aero spokes however, go straight into the centre of the hub and so are completely hidden behind the rim. This will result in the aero spokes experiencing a lower pressure and probably more turbulent flow, which will mean less drag and so less power is needed to turn the wheel.
Fig 8.3 and 8.4 extend the power and drag curves to 60km/hr to highlight how aerodynamics becomes increasingly important at higher speeds.
8.4 Drag at different yaw angles
Fig 8.5 shows the axial drag of the wheel at varying yaw angles for the disc, 18 spoke and tri-spoke configurations. The full table of results from which the graph was plotted can be found in appendix C.
The results shown in fig 8.5 inevitably contain the same inaccuracies discussed earlier in this section, but these are not considered to be as great due to the greater differences in the drag measured and that these were the last tests to be carried out (practice makes perfect!).
All configurations show an increase in drag as the wheel is yawed. The 18 spoke and tri-spoke show very similar characteristics, with evenly spaced curves for each yaw angle, i.e. the rate of increase of drag with yaw angle is constant.
The disc wheel shows a greater increase in drag than the other configurations with a sudden increase in drag somewhere between 15 and 20 degrees. This same phenomenon was seen by Tew and Sayers and is contrary to the negative drag found by Greenwell et al. Greenwell’s finding seems improbable (despite being a common belief in time trialling circles) as a disc at an angle to the wind will generate lift which is associated with induced drag – not induced "push"! This forward force may be heavily dependent on the shape of the disc. The disc tested by Greenwell which gave the most negative drag had convex sides, another with near zero drag had flat parallel sides, Tew’s had straight sides which angled out to the hub while the researcher’s model has concave sides.
The increase in drag is probably due to the disc stalling which could clearly be seen in the smoke tests (see section 6). From this it can be deduced that the 18 spoke and tri-spoke do not stall, or at least not to the same extent, which leads to the conclusion that the flow passes between the spokes and through the wheel, increasing the flow on the adverse side and eliminating stall.
8.5 Power at different yaw angles
Because the wheel is not pulling itself along in the tunnel, the power readings represent only the drag of the spokes and the skin friction of the rim or sides of the disc. They will not be affected by changes in the profile drag of the wheel. Therefore a large variation in power needed to turn the wheel as the yaw angle is increased is not expected and little is seen in fig 8.6. On close examination an increase in power is seen when the disc is at 20 degrees yaw angle. This corresponds to the increase in drag in fig 8.5. It would be interesting to see a result for the power needed to turn Greenwell’s disc at this yaw angle – perhaps the decrease in drag would be at the expense of an increase in power?