I think a lot about bike design and fit, this is probably an outgrowth of my inability to get a good fit from and off the peg bike combined with my analytical nature. There are lots of other factors to boot, but this is my main itch.
IMHO - most bike fitting methods are flawed - unless you've got an average build in which case an off-the-peg-bike will probably work for you.
IMHO most bike designs are flawed - unless you're a bike racer (or at least a wanna-be).
Some of my starting assumptions include:
- If the bike isn't comfortable for the rider, it will be hard to ride well or fast
- If weight isn't distributed appropriately, handling suffers - usually at bad times
- If you ride for any distance, stability is more important than quick handling. Moreover, a stable design can handle quickly on request
- Super light tires are less stable and more prone to slip on corners
- Light wheels and tires help with climbing and maybe sprints - but nothing else
- Most bicycle aerodynamics are bogus. The "aero" structures on a bike are too small & thick to manage air flows efficiently through the range of speed critical to a cyclist. Moreover, the effective direction of the wind is rarely directly head-on, negating the possible benefits from these "aero" structures.
- Body position is the most important and adaptable component of cycling aerodynamics
- Hard tires (high air pressure) works well on billiard tables and wooden tracks. In the real world, roads are rough and without the ability to absorb this roughness, the bike is bounced around - and energy is with each bounce
- A low bottom bracket may not improve handling. But, it does allow the top tube to be lower for a given frame size, or it allows for a larger frame size with the same top tube height off the ground.
- People often select or accept a fit based on what looks right. This is tantamount to what looks good - or in fashion. There is little evidence to demonstrate that fashionable design is related to either: a) going faster; b) enjoying a comfortable ride.
- More than a few people subscribe to the "no-pain, no-gain" theory in accepting uncomfortable fit. While pain is part of training, hitting ones foot with a five pound hammer will create pain, but only makes one slower and less fit. Be careful not to think that fit related pains are part of going fast.
If one accepts these thoughts, then a non-standard approach to frame design is the necessary outcome. We typically start by trying to establish the proper length of seat-tube and the height of the seat above the bottom bracket. As we get more sophisticated we also take into account the length of the top tube. Then folks worry about things like seat-tube angle, stem-length, and the relative height of the seat and handlebars above the ground.
I'm going to propose a different sequence of analysis. I won't give numbers or norms - because we aren't working with typical conditions. Some experience helps, but even more important is a sense of inquisitiveness and a willingness to experiment.
So, to begin, consider that if we could, we should lie on our bellies or backs to reduce air resistance. This isn't generally practical (recumbents notwithstanding). The first problem is that if we position ourselves as a pencil parallel to the ground, we can't see very well. So, that straight pencil approach becomes impractical quickly. Some athletes involved in HPV(human powered vehicles) tolerate the discomfort and complications of trying to achieve the pencil position, and they achieve good results in terms of speed. However, practically, this position isn't functional for day to day riding.
Secondly, there is the issue of tradition. Recumbents are real and well appreciated by a growing minority. But, for many of us, a traditional upright bike is necessary to enjoy cycling. Moreover, some of the sanctioned requirements of the various racing bodies specifically maintain this traditional approach - limiting the degree to which a bike can vary from its traditional form. In fact, a key difference between a Tri and a TT Bike relates to these limits, with the Tri rider allowed more latitude as to how far forward the tip of his/her saddle may be.
I confess to belonging to the traditional approach, and recognize that recumbents may be better bio-mechanical devices for self-powered transport. This however doesn't dissuade me from preferring upright bikes.
It is my contention, that the first point of fitting should be the handlebar/seat relationship. The importance of maximizing comfort and minimizing aerodynamic resistance is directly addressed by this relationship. The question is: How Low Should You Go? In reality, only the rider can say. At what point is there too much weight on the hands/wrists or is the effort to hold one's head up to see causing discomfort? Two fit factors play into this equation: a) reach from seat to handlebar; b) height of handlebar relative to height of seat. Finding the right combination of these factors is a matter of experimentation and experience.
One's upper body can maintain its position and posture relative to the seat while the handlebars move in an arc - the radius of which is the rider's arm and and the center of the arc is the riders shoulder. With arms extended straight ahead, less weight is on the hands and wrists than if the arms are pointed nearly straight down. However, with the bars farther ahead, more muscle strength is typically needed to support one's upper body.
Be aware that geometry is not the sole determinate of weight on the wrist and hand. The muscles in the lower back, as they apply pressure to the pedals, help to hold the upper body up. One can lean forward, take their hands off the handle bars, and keep from falling over if they push the cranks with enough vigor.
Also, the more flexibility one has in one's back, and the more freedom one has from various back ailments, and the stronger the back muscles - the more one can use the lower back muscles to hold one's self upright. And by extension, they can also accept a position that leans further forward because they will not be exerting as much pressure on the wrist and hand.
Finding the right back angle (relative to horizontal) and the right relative position of the handlebar to the seat establish a position of comfort that also maximizes the riders aerodynamic efficiency - and should be the first fitting goal.
The next step recognizes cycling as an aerobic activity. Oxygen uptake is critical to successful riding. Even if one isn't a racer, sooner or later there is a hill which requires big effort. So it is critical that ones position not impinge on the ability to breath. How far up one's thighs travel at the top of the pedal stroke determines whether breathing will be limited by their position on the bike. If the thigh doesn't rise high enough, some of the legs power may not be used. If the thigh rises too far, breathing will be limited.
It's relatively easy for a rider to recognize when their lungs are being constrained by their thighs. Factors that can affect thigh travel include the circumference of the belly, the length of the legs, the diameter of the thigh, the length of the crank, length of the leg, and position of the foot over the pedal. The factors of the rider's physiology are obviously independent variables. We aren't going to change these as part of the fitting process. So, once we know the highest appropriate position of the thigh, we can start to determine some other key fits.
Crank length is impacted by leg length and one's preferred cadence. Higher cadence is easier with shorter cranks - more power can be transferred each revolution with longer cranks. However, this is generally and minor consideration until all other aspects of fitting and performance are addressed.
Instead we should focus on obtaining a crank length within a range where: a) when the pedal is at the top of the stroke, the thigh is not limiting breathing; b) when the pedal is at the bottom of the stroke, the extended leg is still flexed and the pelvis doesn't rock from side to side. The sole of the foot above the center line of the pedal axle is a relatively minor adjustment that can usually be taken care after everything else by a simple adjustment of the pedal. But, the fore/aft position of the foot should be determined early in the process, as this is a key determinant of effective leg length.
As part of positioning the foot to pedal, we to ask two questions: a) is the rider able to pedal comfortable circles? b) does the rider suffer any Achilles problems? If the Achilles is comfortable, we can focus on circles - if not, we should move the foot forward on the pedal until the comfort problem is solved. Once comfortable, we can experiment to see where the rider can obtain his/her smoothest stroke. Again, this can take time - and is best determined by the rider for themselves.
Having got the foot position addressed, and knowing the preferred riding style and cadence, we can prescribe a crank length and try it out for comfort. Ideally, it should feel very natural for the rider - and typically it will allow the maximum extension of the leg (that is most riders will feel uncomfortable with a crank-limited stroke). From crank-length, and maximum thigh height, we can determine the distance from the seat to the center of the bottom bracket.
You will note that we haven't yet spoken of frame size or angles. What we have so far is a handlebar to seat distance, relative height of seat versus handlebar, a crank length, and distance from seat to bottom bracket. Interestingly, each of these factors can be adjusted within a minimal number of frame sizes if a shop is willing to swap seats, stems, crank arms, and handlebars. This doesn't necessarily mean that a good fit can be had this way, but many times it can.
To be finished later...
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