Originally posted by octagon
throwing fresh fuel on the fire of cranks and speed: clearly, for any given amount of force, applying that force over a greater distance results in a greater total amount of work being done (very elementary so far) the result is that it is possible to apply more torque around the bottom bracket axle and therfore to run a higher gear. obviously the net result, at a given cadence, is to go faster.
newton being newton, and thermodynamics being thermodynamics, energy cannot be created, only transferred. this means that do do more work on the pedals, you have to do more work with your legs. this may take the form of travelling further (bigger circles, proportional to twice their radius, and hence twice the amount of increase in the crank length, hence twice the amount of work per leg) or pedalling faster. the only difference in the amount of total work expended over a given route at a given raod speed relates to the energy wasted as heat. this occurs either due to friction in the bearings of the bottom bracket (the loss in high cadence approaches) or deflections in the metals of the drive parts (the loss for high ratio approaches and long cranks) those using long cranks to push high gears at high cadences are probably introducing more potential for energy loss. not to metion potential weight gains assosciated with larger parts (ok all fairly small fish for the everyday rider!)
this is all fine if you're going to be working at near maximal energy throughout your ride, and your only concern is to get there as fast as possible. for riding in packs, at sub maximal effort, there is little value in doing more total work, and carrying more weight, and therefore pushing big cranks.
This is a really interesting thread, especially as I am about to go for a fitting for my new bike. Thanks everyone. I am 183 cm with an 89.5 cm inseam and am currently riding a 170mm crank. It has always felt too small for me and the bike shop doing the fitting said I should go at least 175. Perhaps I will test the 180 too.
Regarding power output, I looked for this on the web and found the following info:
http://www.ncbi.nlm.nih.gov/entrez/...ve&db=PubMed&list_uids=11417428&dopt=Abstract Eur J Appl Physiol. 2001 May;84(5):413-8. Martin JC, Spirduso WW.
The purpose of this investigation was to determine the effects of cycle crank length on maximum cycling power, optimal pedaling rate, and optimal pedal speed, and to determine the optimal crank length to leg length ratio for maximal power production. Trained cyclists (n = 16) performed maximal inertial load cycle ergometry using crank lengths of 120, 145, 170, 195, and 220 mm. Maximum power ranged from a low of 1149 (20) W for the 220-mm cranks to a high of 1194 (21) W for the 145-mm cranks. Power produced with the 145- and 170-mm cranks was significantly (P < 0.05) greater than that produced with the 120- and 220-mm cranks. The optimal pedaling rate decreased significantly with increasing crank length, from 136 rpm for the 120-mm cranks to 110 rpm for the 220-mm cranks. Conversely, optimal pedal speed increased significantly with increasing crank length, from 1.71 m/s for the 120-mm cranks to 2.53 m/s for the 220-mm cranks. The crank length to leg length and crank length to tibia length ratios accounted for 20.5% and 21.1% of the variability in maximum power, respectively. The optimal crank length was 20% of leg length or 41% of tibia length. These data suggest that pedal speed (which constrains muscle shortening velocity) and pedaling rate (which affects muscle excitation state) exert distinct effects that influence muscular power during cycling. Even though maximum cycling power was significantly affected by crank length, use of the standard 170-mm length cranks should not substantially compromise maximum power in most adults.
http://www.ncbi.nlm.nih.gov/entrez/...ve&db=PubMed&list_uids=12904940&dopt=Abstract Exp Brain Res. 2003 Oct;152(3):393-403. Epub 2003 Aug 01. Mileva K, Turner D
Neuromuscular and biomechanical coupling in human cycling: adaptations to changes in crank length.
This study exploited the alterations in pedal speed and joints kinematics elicited by changing crank length (CL) to test how altered task mechanics during cycling will modulate the muscle activation characteristics in human rectus femoris (RF), biceps femoris long head (BF), soleus (SOL) and tibialis anterior (TA). Kinetic (torque), kinematic (joint angle) and muscle activity (EMG) data were recorded simultaneously from both legs of 10 healthy adults (aged 20-38 years) during steady-state cycling at ~60 rpm and 90-100 W with three symmetrical CLs (155 mm, 175 mm and 195 mm). The CL elongation (DeltaCL) resulted in similar increases in the knee joint angles and angular velocities during extension and flexion, whilst the ankle joint kinematics was significantly influenced only during extension. DeltaCL resulted in significantly reduced amplitude and prolonged duration of BF EMG, increased mean SOL and TA EMG amplitudes, and shortened SOL activity time. RF activation parameters and TA activity duration were not significantly affected by DeltaCL. Thus total SOL and RF EMG activities were similar with different CLs, presumably enabling steady power output during extension. Higher pedal speeds demand an increased total TA EMG activity and decreased total BF activity to propel the leg through flexion into extension with a greater degree of control over joint stability. We concluded that the proprioceptive information about the changes in the cycling kinematics is used by central neural structures to adapt the activation parameters of the individual muscles to the kinetic demands of the ongoing movement, depending on their biomechanical function.
http://www.ncbi.nlm.nih.gov/entrez/...ve&db=PubMed&list_uids=12231284&dopt=Abstract J Biomech. 2002 Oct;35(10):1387-98. Zamparo P, Minetti A, di Prampero P.
Mechanical efficiency of cycling with a new developed pedal-crank.
The mechanical efficiency of cycling with a new pedal-crank prototype (PP) was investigated during an incremental test on a stationary cycloergometer. The efficiency values were compared with those obtained, in the same experimental conditions and with the same subjects, by using a standard pedal-crank system (SP). The main feature of this prototype is that its pedal-crank length changes as a function of the crank angle being maximal during the pushing phase and minimal during the recovery one. This variability was expected to lead to a decrease in the energy requirement of cycling since, for any given thrust, the torque exerted by the pushing leg is increased while the counter-torque exerted by the contra-lateral one is decreased. Whereas no significant differences were found between the two pedal-cranks at low exercise intensities (w*=50-200 W), at 250-300 W the oxygen uptake (V*O2, W) was found to be significantly lower and the efficiency (eta=w*/V*O2) about 2% larger (p<0.05, Wilcoxon test) in the case of PP. Even if the measured difference in efficiency was rather small, it can be calculated that an athlete riding a bicycle equipped with the patented pedal-crank could improve his 1h record by about 1 km.
Actually there are plenty of interesting academic articles on this site. Go to the first one above and click on the "Related articles" link to see the whole slew...
