Wind and Rolling Resistance

Wind Resistance.  Wind resistance is a not-so-funny thing.  What makes it so devilish is that it actually gets greater the faster you travel.  It is almost like the wind is penalizing those who are the fastest the most.  Not only does it increase with speed, but it increases exponentially!  So if you double your speed, your wind resistance will increase four times (two squared).  If you triple your speed, your resistance will go up by nine-fold (three squared).  To overcome the wind resistance the cyclist must generate power equal to the wind resistance times the velocity.  Therefore, the power needed is the cube of the change in velocity.  So if you double your speed, you will need to increase your power by eight times (two cubed).

At around 20 mph, wind resistance makes up 90% of the total resistance.  At 25 mph, it makes up almost 100%.  As a result, streamlining is extremely important.

Rolling Resistance.  On the other hand, the resistance incurred from the tire rolling on the ground is constant at all speeds.  Thank god!  Otherwise, we would never get above 20 mph!  At around 20 mph it makes up only 10% of the total resistance and drops off to almost 0% by 25 mph.

Rolling resistance is made up of several factors, however.  These include tire pressure, tread, wheel diameter, and weight.  Higher body or bike weight, smaller wheel diameter, less air pressure, and a larger tread all increase rolling resistance and make it harder for you to move.

Weight.  While more weight will help you going down the hills, it hurts you everywhere else.  Plus, if the amount of uphill riding is equal to the downhill riding, the climbing will take much longer than the descending because of the large difference in speed, therefore, the heavier cyclist will not be able to catch up to the lighter cyclist.  So ride a light bike and keep off the fat!

Diameter.  The smaller the wheel diameter the slower you will go, but you must also account for decreased wind resistance.  A larger diameter may decrease your resistance but will increase you wind resistance.  Any wheel under 26 inches is probably going to hurt you more in rolling resistance than you will be benefiting from less wind resistance.

Air Pressure.  Less air pressure means more tire deformity when the tire meets the ground.  This equals to a larger surface area meeting the ground and an increase in rolling resistance.  It may be a smoother, more forgiving ride, but you will go slower.

Tread.   The larger tread is similar.  Just try a mountain bike tread vs. a road bike tread on the same bike and you will easily feel the difference.

Frontal Surface Area And Shape.  Although wind resistance is primarily affected by velocity, it is also dependent on the frontal surface area and the shape of the object moving through the wind.  The larger the frontal area, the greater the amount of oncoming air that strikes the athlete and the move power is needed to get through it.  How big you are and the position your body is in will affect your frontal area.  Since you can't do much about your size, you must concentrate on your position.  Sitting straight up may feel comfy, but you will go much slower and have to work much harder to go fast.  Riding in a tucked, aerodynamic position will make it easier for you to go fast, so get those aero bars!

Additionally, the shape your body is in will also have an affect on how easily you slide through the air.  Smoother surfaces create less turbulence and, as a result, less resistance.  Keeping extremities tucked in helps to keep turbulence to a minimum, which is another reason why aero bars are so useful.

The Keys.  The key points to remember are that:

  • Weight is always your enemy. Lower the weight whenever possible.
  • At any speed over 10 mph, wind resistance is your biggest enemy. As a result, use aerodynamic components and most importantly, position your body in the most aerodynamic way possible.

Remember these, and you'll be fine!