There is a big misconception that shorter cranks provide superior acceleration compared to longer cranks, and this is mainly because short cranks are often associated with the explosive and strong track cyclist. In reality, the reason why track cyclists have to lift weights and work on explosive training is solely to compensate for the baggage short cranks come with- an increased force required to produce the torque needed to move a smaller lever arm.
WHAT HAPPENS WHEN THE CRANK IS TOO SHORT OR TOO LONG?
It's reasonable to think shorter is better, especially when you consider that since the gears are fixed, faster speeds can only be reached by hitting a higher cadence. But how do you know when the crank is too short?
When the cranks are too short, it will be too difficult to turn the cranks because your body simply can't generate the force needed to create torque. As a result, it will take longer to climb up the rpm's- your accelerations will be slower, especially from a low speed. Once you're at speed, you'll also find it impossible to get off the saddle because the cadence is too fast to maintain standing. You'll also experience more muscle burn as a result of the legs moving too fast for effective circulation to take place. If the blood doesn't have enough time to pick up the metabolic waste, accumulation will occur. When the legs move too fast, this also creates a high pressure area that restricts blood flow and contributes to more lactic accumulation.
Going too long is a big problem mainly because although you'll have the fastest jump off the line, your top speed will be too low to be competitive. Remember that as crank length increases, max cadence decreases. When the crank is too long, your top speed will be at a very slow and exhausting cadence. Accelerating the fastest is counterproductive if your top speed is so low that you'll always get passed right before the finish line!
THE OPTIMAL CRANK LENGTH FOR TRACK
The optimum crank length is the longest crank you can spin at the highest useable rpm. If your fitness limits you to 35mph, pick the longest crank that peaks out at a cadence that represents this speed. This gives the best of both worlds in a fixed gear situation, sacrificing the least amount of acceleration for the highest top speed.
Using a Pro as an example, Mark Cavendish uses a 170mm on his road bike and a 165mm on his track bike. If the correct testing was done, the 165mm is the longest crank Cav can spin at the highest cadence. Based on the gearing he prefers, going shorter will sacrifice too much acceleration and going longer will limit his top speed.
Interestingly, if you know your opponents' crank length, you'll be able to more accurately predict what strategy they'll gravitate towards.
Rider Strategy w/ Short Cranks: Riders on shorter cranks will gravitate more towards attacking down the slope to avoid the torque issues. After attacking, they'll try to spin as fast as possible and commit to whatever top speed they reach. This rider will also be more likey to sprint much further away from the finish line, allowing enough time to reach the fastest top speed and make it impossible to come around and pass.
Rider Strategy w/ Long Cranks: Riders on long cranks have the advantage if they cover most of the distance slowly to limit the sprint to a very small distance. In order to expose the weakness of a rider on a shorter crank, attacking from a slow speed on the flatter sections or the lower sections of the corner will cause the opponent on the shorter crank to struggle.
If you're interested in optimizing your crank length for track racing, please contact me at EatSleepTrainSmart@gmail.com.
Wednesday, February 10, 2016
Friday, January 29, 2016
|Everything you need to know about crank length in a flow chart!|
Power = Force (torque) x RPM (cadence).When changing the length of the crank, you're also changing the torque curve. Long cranks have a peak torque at lower RPMs and short cranks have more torque at higher RPMs. The flow chart above shows what to expect when the crank is too long or too short. What you'll take away from the information is that one crank can't do it all. There are specific advantages to going as long as possible and as short as possible. It depends on the type, duration, course conditions (grip) and course profiles.
***Short Cranks: The Major Flaw - Poor Accelerations
The reason why very short cranks produce terrible accelerations is due to the shifts. Everytime you shift, there's basically no power or zero watts. In a criterium, this problem makes it extremely difficult and impossible to be competitive for the final sprint. I learned this the hard way when I tried using a 165mm crank for the first time in a race. Normally, I have no problem matching attacks, and I'm usually one of the faster sprinters, but on the short cranks, I always lost 5-10 positions even during light attacks. It was the weirdest feeling to not be fatigued, but hopelessly watch everyone pass by, especially on the sprint.
In order to further understand why short cranks produce terrible accelerations, it makes sense when you consider this: If you had to shift four to five times over the course of a 10 second sprint, you've also lost momentum four to five times due to drag and friction while you waited for the chain to shift. Just the sole fact that so much time gets wasted on shifts makes short cranks the absolute worst choice for a fast acceleration or sprint. The problem isn't the short lever arm, it's the excessively high RPMS or cadence. There's no way to transition smoothly to standing unless you shift at least three to four times.
***Long Cranks: The Major Flaw - High Energy Demands
For super long duration efforts (2.5+hours), long cranks put the rider at a major disadvantage, especially if food isn't available or during solo efforts. This is mainly because it take more effort across the entire body to produce the torque needed to maintain speed. Since higher RPMS can't be achieved, the rider has to produce and maintain high power by putting more effort into each pedal stroke aka increase torque (equation above). This depletes muscle glycogen stores very quickly and once completely depleted, hitting the wall is inevitable. Of course, the solution around this problem is to learn how to conserve energy and practice eating during rides to determine how much, what and when to eat.
Time Trials, Team Time Trials & Triathlons (Long duration)
In general, shorter cranks reign supreme here because sustaining a higher overall power is the goal here. By increasing RPMs, torque doesn't need to be as high to maintain the same power, so the legs will be fresher over the course of the race. In terms of triathlons, the lower demand on the legs will help the transition to running. In all applications, the shorter crank will provide aero benefits, especially if inflexibility is a problem.
Time Trials, Team Time Trials & Triathlons (Short duration)
For short duration TTs, it's more beneficial to have a longer crank because the legs will be less likely to limit the performance despite the higher demand. You'll also have a faster start and acceleration towards the end which make a bigger impact on the shorter races. These benefits of course aren't worth it unless an aerodynamic position can be held with no interference from tightness.
Citerium (Flat courses, tight connecting corners and short hills)
You want to be on the longer side. The clearance to the ground is shorter, but I found this to be a non issue because accelerations occur after the apex where the bike is already returning to an upright position. Since crits require constant accelerations, the longer crank offers big advantages, especially on short hills where shifting would cause a massive drop in speed.
Very Long Climbs
Shorter tends to be better here because it's often a competition between power to weight ratios, so the climber who holds the highest overall power tends to win the race. While shorter cranks produce weaker accelerations, oftentimes a weak acceleration is all that's needed to mentally defeat rivals. In contrast, a longer crank will produce an intimidating attack, but it's extremely hard to stay below the redline afterwards. A longer crank can be effectively used in a long climb only IF the rider has excellent standing pedaling technique.
More info to come in future updates! Please post any questions in the comments below.