Aerodynamics, weight and rotational inertia, frame geometry, and handling are all extremely important considerations when evaluating a road, touring, or gravel bike, but it shouldn’t stop there. Energy absorption is an important and often ignored characteristic in a bicycle.
What causes fatigue?
Shock and vibration cause fatigue on a rider. Even relaxed rides can cause strain and fatigue when riding on rough surfaces, and this becomes even more important when riding longer distances.
In this respect, a bicycle is essentially a transmission device for forced inputs coming from the surface being ridden. We need stiffness in our bikes to maintain a certain level of handling characteristics and to promote a connected feeling between rider and machine. A stiff bike feels tight and responsive, and this is good. This stiffness also unfortunately puts a lot of stress on our bodies while riding. It’s essentially the compliance between the tire contact patch and our hands, feet, and butt. This includes the tires, axles, fork, bearings, frame, saddle, bar tape, and so on. For most of us there is not a single component there which we would be eager to soften up and reduce stiffness, and this presents a fatigue problem for our bodies.
Let’s start with the shock inputs we feel while on our bikes. Shock is a sudden single input like hitting a pothole, large rock, or dropping down off a curb onto the street. What defines how much of that shock force we feel in our bodies? Above else it is primarily based on the amount of deflection that occurs between the input source and our bodies. A stiff bike has as little deflection as possible. We compensate by getting out of the saddle and bending at the elbows and knees, limiting the shock forces to our wrists, knees, and ankles. What inevitably follows is a reverberation from the sudden spike in force that was just transmit through the bike. The shorter the shock impulse and the higher the impact force, the worse this reverberation or ringing is. It may not seem like a big deal once or twice, but over a longer ride this causes some real fatigue on the body.
There’s no way most people would sacrifice a stiff, connected ride for a little comfort through rough terrain, so let’s not even talk about springs and shock absorbers on a road bike.
What we can focus on instead is damping out some of that initial impact and the following reverberation through the application of structural damping. Think of a drummer hitting a cymbal and the ringing crash which follows. Imagine then hitting a cymbal that has a layer of rubber covering the underside of it. We’d hear just a dull thud. This is structural damping. What if that cymbal was made of steel? How about aluminum? Now let’s consider a carbon fiber cymbal – this wouldn’t resonate quite so much. With this idea we’re now on to something. The carbon fiber has resins in the epoxy which give it damping properties higher than steel and aluminum. Let’s finally consider wood. The damping coefficient in wood can be as much as ten times higher than that of carbon fiber! Unfortunately the stiffness of most hardwoods is much less than carbon fiber, which is an obvious trade-off. This means we’d need more wood to get a stiff bike, which means higher weight.
At Normal Bicycles we’ve combined carbon fiber and hardwood to achieve a frame with high stiffness and high damping, all while minimizing weight.
This is great, but as annoying as pothole shock reverberations can be they are in most cases likely not affecting our riding all that much. Let’s shift our focus now to vibration. This is a much more important road input to consider when it comes to fatigue. At the end of a long ride on rough pavement or gravel our bodies are most likely complaining, and it’s not just our leg muscles. Our hands, wrists, shoulders, knees, and butts are hugely affected by fatigue due to vibration exposure.
Vibration is transmitted from the road surface through our bikes and to our bodies based on input frequency, bike stiffness, and damping.
The input frequency will be somewhat random based on the road surface, but it’s also related to riding speed. Essentially, any time we’re riding faster than a crawl we are experiencing what I’m going to call high frequency vibration input.
Everyone talks about stiffness
Bike stiffness determines the excitation frequencies of the bike. Since we’re typically going for the stiffest bike possible, these excitation frequencies are generally high. These excitation frequencies are the points at which our bikes will ring like a tuning fork when struck. This is extremely important because when the road inputs approach the same frequencies that the bike will become excited, the result is an amplification of the vibration on our bodies. In this case our bodies would be feeling more vibration than the original road input. This is a disturbing thought!
The last consideration is damping. This is the material property that dissipates energy from shock and vibration. What happens mechanically is that the damped material is converting shock and vibration energy into a very small amount of heat energy. It’s not enough to make much of a temperature change, but this small dissipation of energy can have a significant affect on our bodies over a long ride. When a bike has a higher damping coefficient, those inputs that approach the excitation frequency of the bike are drastically reduced. We end up exposing our bodies to only a fraction of those vibrational forces.
The ultimate ride experience
The wood used on Normal Bicycles is engineered to enhance the damping of the frame while maintaining a high stiffness in a lightweight package.
Think about damping at the end of that next long fatiguing ride. A seemingly small thing like structural damping can have a big benefit for our bodies.