Bicycle Aerodynamics

Bicycle Aerodynamics Energy and Power use breakdown*

 

% of total power consumption:

Aerodynamics Total (combination of rider aerodynamics and bike aerodynamics) – 65%

Bicycle Aerodynamics ≈15% of total power use.  (≈25% of total aerodynamics)

              Wheels – 7-11% of total aerodynamics                       

Fork – 6-9% of total aerodynamics

Frame – 4%-9% of total aerodynamics

              Other – 2-4% of total aerodynamics

 

The bottom line:   In a solo event or triathlon, lowering total aerodynamic drag by 10% (from 7lbs of drag to 6.3lbs), without changing  power output, will cut 21 minutes of time (7%) from a rider who averages 20mph over 100 miles.   Time will drop from 5 hours to 4 hours and 39 minutes and average speed will go up to 21.4mph.  

 

How to go about it:  Get properly fit and comfortable first, then concentrate on equipment choices.  If you are a time trialist or triathlete, purchase aerobars and get them fit properly immediately.   Little details, like cable routing, are inexpensive and important with aerodynamics – get these taken care of.   Put in its most basic terms, all the other variables are pretty meaningless if you are so uncomfortable that you don’t want to, or can’t, hold an aero position.   

 

*Comprehensive studies have not been completed to show exact importance of all variables in relation to each other.  Results are estimates from a variety of research studies within the cycling industry.

Explanation and Tech talk:

 

Even though the rider is about 75% of the vehicle’s aerodynamic equation, the bike is still 25%.  25% is certainly still worth paying attention to.  Bicycle and parts manufacturers spend thousands on advertising how their products minimize drag and how that can help you go faster.  I will not argue that aerodynamics is very important.  If you are riding solo, you spend about 70% of your energy overcoming its resistance.  However, manufacturers are sometimes prone to exaggeration and generalization, assuming that consumers will just take their word at face value and not think about the details involved in aerodynamics.  Aerodynamics is all about details.  Understanding the principles of aerodynamics in cycling can help you make informed product decisions based more in fact than in claims, which in the end can keep you from just buying a “me too” aero look product and helping you search out the ones that can really help.   So, let’s see if we can make what is a rather complicated subject relatively understandable while simultaneously discrediting the design of manyHollywood spaceships…

What are the important principles and terms in bicycle aerodynamics?

Aerodynamic Drag consists primarily of three aspects:  Surface Character, Frontal Surface Area andShape. 

Surface Character:   This is the texture and pattern of the surface.  For example, the hair on a tennis ball or the dimples on a golf ball.   Companies are starting to experiment more with how to use surface to improve aerodynamics, especially at lower speed.  Zipp, for example, has started adding a dimple pattern to their disk wheels.  I will not discuss surface in this article much as it currently has limited application in the cycling industry and is not as big a factor as the other two aspects.

Frontal Surface Area:   As a vehicle is propelled forward, the front profile of that vehicle is what breaks through the wind first.  Therefore, the amount of mass or surface area that hits the wind first greatly shields and effects that which is located behind it.  For this reason, minimizing frontal surface area is an excellent step towards minimizing overall drag.

Shape:  The vehicle’s overall shape drastically effects its aerodynamic efficiency.  Shape is not the same as mass, not even close.  You can have a small spherical shape and it can be far less aerodynamic than a much larger elliptical shape. This is a big reason why a football can be thrown further and with more control than a volleyball.  The shape of an object effects the proportion of skin friction to pressure drag. Skin and pressure what?  Read on…

 

Quick Summary:    Aerodynamic Drag = Surface Character + Frontal Surface Area + Shape.   Because of the limited use of surface character in the cycling industry, we will focus on how shape and surface work to influence the flow of air.

 

Total Drag explained:

Total Drag is a combination of skin friction (“good” drag) and pressure drag (“bad” drag).  The proportion of skin friction to pressure drag are directly determined by the frontal surface area and shape of the object. 

 

Pressure Drag is most easily defined as turbulence.  The less of it the better.  Pressure drag is the disturbed air that spins off an object when air hits it.  Pressure drag slows a vehicle down more as turbulent air is the least controlled and most random form the air can be in and acts like an out of control barrier.  Blocky or round objects will have more pressure drag than oval or elliptical objects.  Air can flow around more elliptical objects smoother where as it is more likely to bounce off turbulently around blocky or round objects.   There are specific angles that we touch on below that have been found that minimize pressure drag.

Skin Friction is actually good drag.  Skin friction is a layer of deflected air that hovers right at the surface of an object.  Think of it as a coat that adds a little bulk, but that protects the layer underneath it and thus helps it go faster.  Skin friction flows smoothly around an object.  It is good because it can create an isolation layer around an object that can keep pressure drag (“bad” drag) from forming. 

Laminar Flow:   Undisturbed, smooth air.  Air is in laminar flow before it hits an object and eventually returns to laminar flow after an object passes through it.  Laminar flow is the most efficient form the air can be in, as it is undisturbed.  The quicker that air becomes laminar after going around an object, the less drag it will have.  Skin friction drag returns to laminar flow far before Pressure drag does.

 

Quick Summary:  Anytime an object passes through air there is going to be drag.  However, skin friction is smooth, and consistent drag whereas pressure drag is rough and chaotic.  In the total drag equation, proportionately, the more skin friction you have and the less pressure drag you have the smoother the air will pass around the object and return to laminar flow. 

 

So, what are the big goals when trying to minimize drag in a cycling position or product?

A)    Create a minimal frontal surface area that minimizes the initial turbulence and disturbances on the air.  Once the air is disturbed, it is much more difficult to calm it down again.  Do your best to  leave it undisturbed.

B)    Design a shape that encourages more skin friction and less pressure drag in order to minimize total drag and allow the air to return to laminar flow as quickly as possible.

How?

Sorry, more definitions…

Aspect Ratio:  Aspect ratio is not just a term used in aerodynamics.  Aspect ratio is a proportionate relationship between length and width of an object.  If we have a 4” long object that is 1” wide, its aspect ratio is 4:1.   If it is 1” long  and 4” wide the aspect ratio is a horrid for flying 1:4.  Aspect ratio helps to explain why a football flies so well when it is thrown length wise through the air, but acts like a wounded duck when thrown height wise.  You get the point…  Aerodynamically,  NACA (the aerodynamics research predecessor to NASA) studies showed that an aerodynamic aspect ratio of around   3:1 minimized drag.   

Shape Taper/Angle:  Directly related to the  3:1 aspect ratio is the taper and angle of the object’s surface.  Aerodynamically, non-round leading edge with a 14° taper leading back from to the widest point of the tube creates an object with a good aspect ratio and an aerodynamic profile.  Only a few of the tube shapes used in bicycles have a truly aerodynamic profile and taper to them.   Most, especially in difficult to work with materials like Titanium, look aero, but are not and often compromise the structural integrity of the design more than anything.

 

Quick Summary:  By using proven aspect ratios and taper angles of a shape effectively, drag off an individual object can be minimized. 

 

4.      Mitigating Factors.   Everything would be pretty simple if it were as easy as elliptical shapes always being best.  However, there are two things that throw a real monkey wrench into the principles above. 

 

1)  The parts on your bicycle and body are related to each other and effect each other.  The air flow around one will effect the airflow around the other.  These different layers create what is known as boundary layers. 

 

2)   The bicycle and rider are dynamic objects; there are many exposed and moving parts between the bicycle and the rider that create turbulence and lead to inconsistent and uncontrolled boundary layers between them.

 

Boundary Layers:   Boundary layers are layers of air created in the space between objects as the object passes through the air.  Boundary layers occur between a fork leg and the wheel, or between your legs and a seat tube or post,  for example.  

Boundary layers complicate everything discussed above because they can take all that nice flowing air that is going around objects, even objects with optimal aspect ratios, and can drive it into each other, thus causing pressure drag and turbulence.  Boundary layers and the fact that riders are dynamic are why all those wind tunnel tests on individual frames, forks, wheels and even built bikes have limited meaning and application.  A dynamic rider and other parts attached to them changes everything. 

 

Conclusions:  Do the mitigating factors mean everything we talked about above is meaningless?   No.  The concepts are all valid and valuable to understand because they allow you to look at the big picture and to take all the advertising about frame and wheel aerodynamics with a grain of salt.  Aerodynamics is not something that is simple or to be taken at face value – nor is it even something that even the most knowledgeable aerodynamicists claim to have full control over in regards to dynamic and low speed objects like bicycles.  There is just too much going on at one time and too many individually dependent variables for that to be allowed.   You can’t just build a bike of aero shaped tubes to go fast, the rest of the package needs to be aero in relation to it for it to help you out.   Some good rules to buy by:

 

1)     Cycling is a big picture sport.  Don’t buy a bike or a product just because it is aero on its own.  Buy it because it fits you well, rides well, is built well and meets all your needs as a cyclist. 

2)     Between you and your bike, you are by far the bigger air disturbance of the two.   The vast majority of the total aerodynamics equation is you, the rider.  Working on your riding position to make it aerodynamically efficient is the number one thing you can do to reduce aerodynamic drag and allows all the other technology to work better.

This entry was posted in Ask Fit Werx Article Archive. Bookmark the permalink.