CYCLOCROSS TRANSMISSION/ CRANK SETS
Crank Length
One of the most important factors that will allow you to ride with the greatest amount of comfort, efficiency, power and duration is having the proper crank length on your bike.
Until recently, the conventional wisdom for determining the crank length was the assumption that most riders have very similar lengths of femurs. The assumed femur length was based on the average inseam length of 800mm, the old formula used was L x .219 = Crank length (L = inseam length in mm). A rider with a total leg length of 800mm would use a crank that is 172.5mm in length. In order to maximize profit, the manufacturers created cranks to fit only the majority of riders, thus cranks were only made in the lengths 165mm, 170mm, 172.5mm, and 175mm. The problem with this method of sizing (based on total leg length) is that if you lined up 20 people with 800mm inseams, you would find that none of them have the same length of femur. When pedaling, the femur is your body’s primary lever.
If you understand the use of levers, you would know that a longer lever makes the work easier, so you might conclude that longer is better. However, in the case of the human body, the weakness of joints, tendons, and muscles must also be considered when calculating the power the lever can produce.
In addition, crank length needs to have the proper relationship to your femur length in order to reduce the likelihood of injuries. Having cranks that are too long can cause painful knee problems because of the shearing forces behind the patella (knee cap). Studies have found that shearing forces are minimized if the angle between the thigh, and lower leg do not fall below 93 degrees. This is where crank length finds its moderation. Longer does make short-term work easier, but at the cost of injuring your knees. Injuries that are not treated and addressed by using the proper biomechanics while riding can shorten the years that any person can enjoy riding.
Here is the principle of short-term work. If you went to a gym to lift weights (leg press for example), and were determined to lift your maximum weight, you might find 5 reps at 400lb to be your maximum. In this example, you lifted 2000lb. On a different day go back, this time to increase the total amount of weight lifted…maximum reps at 50lb. You would find that you could lift many times the previous maximum of 2000lb, and that the speed at which you could accomplish each lift would be a lot faster. I will call this effect “long term work”. How does this apply to biking, and crank length…very directly, let me explain.
Most bike rides/ races last at least a few hours. If a rider were to start out riding in the biggest gear (big front chain ring and smallest back sprocket), and ride hard…that rider would go anaerobic in a few minutes. This would be the equivalent of riding a very slow cadence at high intensity (like lifting 5 slow reps. of 400 lb.). The point here is that maximum strength is limited to short term work. To be effective in long-term work, you must use a different strategy. Pro racers, and coaches have re-learned that working at the aerobic level, you can accomplish a lot more work, for a much longer duration of time. In fact, over time it is possible to increase your aerobic capacity tremendously. One of the finest examples of using the aerobic system is Lance Armstrong. If you watch his pedaling in the latest videos, you will notice that he maintained a cadence of about 100 rpm’s at a lesser watt level (the equivalent of 50 faster reps. Of 50 lbs.). It was only in his last 7 years of racing that he was coached to maximize his aerobic system. These were his most impressive racing years. Interestingly, in his younger years of racing, Lance relied more on strength, and a lower cadence of about 60-80 rpm’s. He was a good racer, but not a great racer then. Careful observation of many of the world’s greatest current racers will also prove the successful use of the high cadence method of training.
Other observations you will note watching the pro’s is that they ride without bouncing up and down on their saddle, they ride very smoothly, constant head level, not moving from side to side…at around 100 rpm’s.
Try a test the next time you go to the gym. Hop on a spin cycle with a cadence meter. Rev up to 100 rpm’s, and stay smooth for about 1 hour. If you have short legs, you will tire quickly, and bounce very rapidly with every leg’s up stroke. Conversely, if you have very long legs, you may feel extremely clumsy, like riding a child’s tricycle, and tire easily. Spin cycles are designed to fit the “majority” of riders, so if you are not in that narrow band of femur lengths, you will not do well.
This is where custom cranks come into the picture. With cranks that are proportional to your body’s femur length, you can ride with the same advantages the pro’s have, constant head level, high cadence, not moving side to side. This equates to less wasted energy, and a more efficient ride…you will ride further and faster, no matter your body size.
Here at Perfection Bicycles, we
specialize in totally custom bikes. We use the Accufit Designer system, invented
and developed by bike fitting guru Bill Boston.
Accufit is the best scientifically based fitting software available.
Many great riders have benefited from his work and research on the
design, and sizing of custom bikes. With
the Accufit method of sizing, your ideal crank length will be determined based
on a number of factors that are specific to your body.
We have access to custom cranks that range from 130mm to 220mm. If you want to crank smoothly, we can help you. Sometimes custom crank sizes also require a custom frame.
We are a Moots Cycles dealership. Moots is an American company, located in Colorado that makes the very best frames, with the best available titanium. Together, we can build a bike to fit your individual needs.