Hello Anachronism and All, +1
(Your excellent executive summary)
Long version with excerpts from the reports here:
HPV #50 Spring of 2000
Spicer The data for the chain and the pulley tooth indicate that the component temperature rises with the input power.
For the pulley tooth, at 50 W input power, the maximum pixel value is approximately 23 units; at 100 W, 35 units and at 150 W, 65 units. These results are in rough agreement with the loss models presented previously where the frictional losses are directly proportional to the input power.
Unfortunately, the power loss in each of these cases is not proportional to the input power owing to the dependence of efficiency on chain tension. Using measured values for efficiency under the conditions for the data in Fig. 5 (97.2% for 150 W, 94.4% for 100 W and 85.5% for 50 W) indicates that 4.2 W of power were lost at 150 W input; 5.6 W at 100 W input and 7.3 W at 50 W input.
Obviously, for a lower power loss, the temperature rise should be lower if the lost power is converted entirely to heat by frictional loss. It would be expected that the temperature rise would be lowest for the 150 W input test since the measured power loss is lowest for this case.
DISCUSSION AND CONCLUSIONS Tests of efficiency for the derailleur type chain drive indicate that the overall efficiencies for the transfer of power from the front drive sprocket to the rear sprocket range from 80.9% to 98.6% depending on the conditions of drive operation.
Primary factors affecting the efficiency include the sizes of the sprockets in the drive and the tension in the chain. It was found that larger sprockets provide more efficient transfer of power while smaller sprockets proved to be less efficient. Simple, frictional loss models were developed that gave sprocket-size loss variations that agreed with those variations measured experimentally. Typically, a 2–5% loss difference was measured between the 52–11 and the 52–21 sprocket combinations depending on the drive operating conditions.
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HPV #52 Summer of 2001
Kyle (different apparatus than the Spicer tests) However, for determining the rank order between transmissions, since they were all tested under identical conditions, no correction is necessary. The efficiencies reported in this article include ergometer-wheel drive losses, so the actual transmission efficiencies would be higher by 2 to 2.5%.
27-speed: Shimano A Shimano Ultegra 27-speed mountain-bike transmission with three front chainrings (44/32/22 teeth) and a 9-speed rear cluster (12, 14, 16, 18, 20, 23, 26, 30, and 34 teeth). Because of time constraints, only 15 of the 27 gears were tested: (1) 22/34; (3) 22/26; (4) 32/34; (7) 22/20; (9) 32/26; (10) 44/34; (11) 22/16; (15) 32/20; (16) 44/26; (18) 22/12; (20) 32/16; (21) 44/20; (24) 32/12; (25) 44/16; and (27) 44/12.
CONCLUSIONS By viewing the curves, several general observations and conclusions can be made. 1.
Efficiency generally increases with the load—for all transmissions. Figures 1, 3, 4, 5, 7, 9, 12, or 14 all show this trend.
Although friction increases with chain load, rpm, and other factors [8], obviously the residual friction in a gear train becomes less important as the input power increases, while the friction factors that increase with load go up less rapidly than the load. 3. As the gear ratio increases, the efficiency tends to decrease for all transmission types. This is illustrated by the trend lines in figures 6, 8, 10, and 11. Even though the greatest efficiencies are sometimes near the highest gear ratios, the average efficiency decreases with higher ratios, (the high efficiencies were: Shimano 4 = gear 1, Rohloff = gear 9, Browning = gear 2, and Shimano 27 = gear 21).
An average 2% difference in efficiency is thus easily possible if the wrong gears are chosen. If racers, or even commuting or touring cyclists, could choose optimum gears they would be hundreds of meters ahead at the end of 60 km (37 mi). For example, if Lance Armstrong, in the Tour de France 58.5-km time trial (36.4 mi) were to choose the wrong gear, a drop of 2% in efficiency would cause him to be 410 meters behind (27 seconds) at the end of the time trial, easily enough to lose the stage [3]. Incidentally, Armstrong averaged about 54 kph (33.6 mph) for the time trial (58.5 km long = 36.4 mi). With commuting riders who travel 24 kph (15 mph), instead of 54 kph (33.6 mph), it only gets worse. A 2% drop in efficiency would lead to an 800-meter gap (about 2 minutes). The reason for the increasing gap is that the slower cyclist spends much more time on the course [3].
The point is, why waste energy when it is unnecessary. 5.
The tests show that some gears are inefficient. Derailleur gears On the other hand, factors affecting the efficiency of derailleur gears become clear by examining the curves in figures 10 and 11. For example, a 12-tooth sprocket seems to cause inefficiency. In the Shimano 27-speed, gears 4, 9, 15, 18, and 24 have the lowest efficiency. The two gears with the lowest efficiency of the 15 tested, both use a 12-tooth sprocket. The gears with 12-tooth sprockets (18, 24 and 27) have an average efficiency of 91.2%, while those involving 16-tooth sprockets (11, 20 and 25) have an average efficiency of 93.5%.
Apparently the sharp angle of chain link bend in the 12 causes increased friction compared to larger sprockets. So it appears that larger gears than 12 are necessary for efficient operation.
When there is a choice of gear ratios that are close, cyclists should choose the gearing combination with larger diameters [8].
Cheers, Neal
+1 mph Faster