Wednesday, September 3rd, 2008
By Sarah Shorett, co-owner of Fit Werx
Frame geometry differences can be a complicated subject. Hopefully this article can help make it a little simpler.
Think about bike geometry as you would shoes. There are many different types of shoes and all have a distinct purpose. If you are going for a hike on trails, you’d probably take your hiking boots, as they are very functional and adaptable for the many possible situations you may find on the trail. On the other hand, if you are dressing up for a night out, a hiking boot, while usable, would probably not be your first choice. Even if it means losing some of the positive characteristics of the hiking boot, you would probably choose a more specialized shoe (a high heel or dress shoe) that is designed to be more effective for what you are doing. Frame geometry and use is similar – the design of a frame follows its function, and you want to make sure that whatever you are riding matches up well with your needs.
Road frames, like adaptable hiking boots, are designed to work well in a variety of situations and positions, such as hill climbing, sharp cornering, descending, and riding in tighter groups or in town. A road geometry frame is designed to encourage a position that distributes the rider’s weight fairly equally (50%-50% ) between the handlebars and the saddle. This means a relatively upright riding position (compared to an aero position) that maintains an open hip angle (for smooth, powerful and efficient pedaling), while providing the rider with good visibility and control. The position and frame geometry are designed to encourage the rider’s hands to rest on top of the lever hoods where there is easy access to the shift and brake levers. This makes the position versatile, helping the rider maneuver the bike quickly and safely in groups or in traffic. The seat angle (the angle on the tube of the frame that holds the seat in relation to the ground) of a road frame is usually close to 73 degrees and is designed to allow the rider’s knee to be centered over their pedals for strong power and joint stability. The angles used in a standard road frame geometry have been around for well over 50 years and have proven to work well for a wide variety of riding.
A road geometry is versatile. However, depending upon your riding needs, “versatile” and “optimal” can be two different things. A road geometry and position is not optimal in regards to comfort and performance in some “specialty” riding positions, like the aero position many riders use for triathlon. For this, a triathlon geometry frame can be the ticket.
Think of a triathlon frame more like the high-heel or dress shoe. While high heels or dress shoes are not as adaptable as a hiking boot (you would not want to hike up a mountain trail in them), there are plenty of “specialty” situations where the high-heel or dress shoe is the more effective and appropriate footwear. This is where a more position specific frame geometry and design can allow for a specialized riding position to be more comfortable, while also enhancing stability and handling characteristics. Compared to a road frame, a triathlon geometry is designed around encouraging a more comfortable, efficient and aerodynamic position while the rider is in the aerobars. When a rider is in the aerobars, significantly more of their weight is distributed on the front of the bike (often up to 70% on the front and 30% on the saddle). This increased weight distribution towards the front of the bike dramatically alters the way the bike handles. For this reason, and because the rider’s hip angle still needs to be kept open for efficient pedaling as the front-end becomes lower, there are some design changes in triathlon frame geometry to encourage a stable, confident and comfortable ride in the aerobars.
One geometry aspect that changes is that the seat angle on a triathlon specific bike will usually be steeper (76 to 78 degrees) than the 73 degree angle often found on a road geometry. The steeper seat angle helps the rider’s hip remain open as they ride in the lower aerobar position, while also facilitating more hamstring activation to preserve the quadricep power for the run portion of the race. Triathlon frames also usually have a shorter top tube than a similarly sized road frame. The shorter top tube accommodates for the longer reach of the aerobars and helps keep the rider from being too stretched out when riding in the aerobars. From a handling and stability perspective, the head tube angle (the angle of the tube that attaches the front fork to the frame) is often more relaxed than a road design and the fork rake is also altered to extend the trail. In plain English, this basically means that geometry changes are made to increase the bike’s ability to track in a straight-line in a stable and predictable manner while the rider is in the aerobars. While there are other more subtle changes made to a triathlon frame geometry, these are the ones that are good to understand so that you can make an informed opinion on which will meet your needs best.
Optimally, every triathlete should have two bikes, a road and a triathlon bike, as each bike meets specific needs, helps develop different riding skills, and offers cross training benefits. However, this is not in every athlete’s plan. So, which style bike will work best for you? One place to start is by asking yourself some relevant questions: “ Do I ride more with tight groups or do I ride alone or in loose groups most of the time?”, “What kind of events do I ride in and what is the terrain like?”, “Is triathlon the focus of my riding or just a part of my riding?” and “Am I new to the sport and want to see if it will fit into my lifestyle or have I been doing this for awhile and am I pretty aware of my riding and exercise habits?”. Based upon your answers to these questions, you will gain insight into which direction makes more sense.
The most reliable way to know what is best for you is to work with a professional bike fitter to establish your riding position first and then use the information from the fitting to help find which style frame and what model frames fit you best. A good fitter will have the experience and knowledge to be able to make recommendations that will take the guess work out of the process and that you know will fit and work well for you – thus saving money and helping your bike work as it was designed. Also, remember that fit is dynamic and the position you were riding in two years ago when you first started riding may be different than what will work optimally now.
While road and tri frames are similar in their general use, they perform different functions. Each design came about as a result of a need and each one is most efficient and effective when used for the application it was designed for.
Posted in Ask Fit Werx Article Archive |
Wednesday, September 3rd, 2008
Mechanical, Compatibility & Friction - Energy and Power Use Breakdown*
% of Total Power Consumption:
Mechanics, Compatibility and Friction: 2%-100% of overall energy/power consumption
The bottom line: Friction from an adequately maintained drive train, etc. can use up to 2% of your energy. Component incompatibility and mechanical issues are deal breakers. From flat tires and loose and broken bolts to broken chains and incompatible brake shoes, if your bike breaks down or one part isn’t compatible with another you won’t finish the ride and could even injure yourself.
How to go about minimizing the negative effects: Keep your parts in good working order. Clean and lubricate your drive train and braking surfaces regularly. Inspect for worn parts and replace worn tires, chains and brake pads sooner rather than later. Finally, having a properly assembled bicycle from the start cuts back dramatically on future problems and increases the durability of the components while reducing friction. A good mechanic is important. Make sure that your bike frame and components are properly prepared (all frames should have their bearing surfaces and threads chased and faced with cutting tools before parts are put on) during assembly. Don’t balk at buying nice tires as they can save you from flatting and never use cheap tubular tires.
*Comprehensive studies have not been completed to show exact importance of all variables in relation to each other. Results are compilations from a variety of research studies within the cycling industry.
Posted in Ask Fit Werx Article Archive |
Wednesday, September 3rd, 2008
Weight of Bicycle - Energy and Power Use Breakdown*
% of Total Power Consumption:
Weight of Bicycle: ≈08% of overall energy/power consumption
The bottom line: First, never forget that you the rider are 85-95% of the total vehicle’s weight – the bike and components are only 5-15%.
Bicycle weight plays a comparatively small role in how a bike performs. Weight’s largest effect is on acceleration and the psychological benefits it can offer. Don’t compromise everything else to maximize a variable that is only 7-8% of your bike’s performance.
How to go about minimizing the negative effects: 1) Concentrate on your body’s weight first. 2) Before counting grams on parts, look at all your individual needs as a rider and never forget the big picture. If you are light and gentle on parts, go to town and feel free to go light! If you are not, don’t go too light or stability and durability will be compromised.
*Comprehensive studies have not been completed to show exact importance of all variables in relation to each other. Results are compilations from a variety of research studies within the cycling industry.
Explanation and Tech talk:
Psychology: Greg LeMond once said that he knew an extra couple ounces didn’t matter in the least. But he didn’t want to be thinking that he was at a disadvantage if Miguel Indurain passed him on a climb. The point is that weight’s importance is mostly psychological.
Weight does play a role in acceleration. However, not nearly the role the grade or slope of the road plays. It has been said that adding ½ a degree of slope is like adding 30 lbs. and that really isn’t too far off the mark. Rotating weight (wheels, for example) is more influential on performance than static weight. But the difference is not nearly the 6 times that is often claimed. I don’t have an exact number, and there are too many variables to determine it accurately, but it probably has two times the effect of static weight (frame, handlebars, etc…).
Basically, buy parts first on how well they address the major performance variables and then buy the lightest option that makes sense for your weight and riding style. If you weigh 190 lbs, don’t go gram counting with titanium pedal spindles that could break on you in the middle of a ride because they simply do not have the strength you need. This is especially important to consider when looking at frames, wheels and load bearing drive train parts like cranks and bottom brackets. We build up each customers bike as light as their budget and needs allow without sacrificing performance or safety.
Tags: Training
Posted in Ask Fit Werx Article Archive |
Wednesday, September 3rd, 2008
Rolling Resistance Energy and Power Use Breakdown*
% of Total Power Consumption:
Rolling Resistance: ≈10% of overall power/energy consumption
The bottom line: Rolling resistance is affected by friction caused by the weight of the vehicle (bike and rider) and how much of that weight has to be absorbed by the tires while riding. Rolling resistance is affected by vehicle weight, tire tread, casing, psi, the texture of the surface, and the vertical compliance of the components and frame. If you are currently riding equipment that has a high level of rolling resistance and you average 20 mph over 100 miles, you can cut 3-4 minutes off your time by minimizing your rolling resistance. If you average 15 mph, you can cut 4 to 5 minutes off. All the small pieces add up to the whole…
How to go about minimizing it: Get properly fit and comfortable first. Then, consider how vertically compliant the products you are using are. Use high quality tires with high thread counts and strong sidewalls and do not over-inflate tires – many tires should be run under their maximum recommended psi. When it comes to frames and wheels, look to maximize vertical compliance through more compliant designs or with suspension which will effectively reduce the amount of weight on the tires and lower the rolling resistance. The goal is to find the balance between torsional stiffness and vertical compliance, or find the few designs on the market that integrate well together and allow you to get the best of both.
*Comprehensive studies have not been completed to show exact importance of all variables in relation to each other. Results are compilations from a variety of research studies within the cycling industry.
Explanation and Tech talk:
Rolling resistance is the amount of energy required to overcome the friction between the road and tire. It sounds simple, but what effects it and how it works defies common thought. The key to understanding rolling resistance is to understand that it is determined less by size of tire contact patch than consistency of tire contact patch and that many variables from the vehicle’s tire pressure, tire width and tire construction, to its weight, to its frame design and how it effects sprung vs. unsprung weight all play a part.
We’ll discuss each variable individually, from less complicated to more. Just keep in mind that the real thing to understand from all this is that consistency, not current, tire contact patch is what really counts in minimizing rolling resistance. The methods of how to keep the tire contact patch consistent is where it can become difficult to understand.
Tire Pressure, Width and Construction: Narrower tires and higher tire pressures are not always better. If you are using the same tire pressure and have the same amount of vehicle weight above the tires, narrower tires will actually compress more than a wider tire because there is less initial surface contact on the road to absorb the shock. A narrower tire simply has less area to absorb the blow than a wider tire. Like most anything, spreading the impact across a greater area will reduce the effect of the overall impact. This is why 700c wheels with their longer contact patch will have lower rolling resistance than their 650c counterparts with their shorter contact patch. When it comes to rolling resistance, you should pick a tire based upon the quality of the casing and its ability to maintain its shape and choose other components based upon their ability to absorb shock so that the tire doesn’t have to.
Weight: Weight’s relationship to rolling resistance is indirect. On two completely rigid vehicles, the lighter vehicle (bicycle and rider) will have less rolling resistance because it will not put as much pressure on the tires as the heavier vehicle (bicycle and rider) and thus will be easier to lift up and over variances. However, vertical compliance in the wheels and frame changes this completely and the only way to explain how this works is by describing the somewhat complex difference between sprung and unsprung weight, which is found below.
Frame choices and sprung vs. unsprung weight: For those looking to minimize rolling resistance and understand exactly how a vehicle reacts to the ground beneath it, the vehicle’s weight needs to be broken down into sprung and unsprung weight.
I wrote a description of how sprung and unsprung weight works for Softride’s catalog once, so I hope you won’t mind me plagiarizing from myself in an attempt to explain what is not an easy concept. I worked hard to try to figure out a good way to explain this on paper, but didn’t succeed as well as I wanted. If you read it slowly and step-by-step, it might make sense.
“…resistance on a bicycle is determined by how much energy is required for it to move over the road. Even fresh pavement is riddled with surface imperfections that slow a bicycle down. Without suspension (vertical compliance), both the rider’s and the bicycle’s weight (an average of 175 pounds for both) is ‘unsprung’ and must be lifted up and over these imperfections for the vehicle to move forward. With suspension, the majority of the weight is ‘sprung’ and imperfections are absorbed by the suspension. On a ’sprung’ vehicle, only the unsuspended portion (wheel and lower frame) and a small amount of the rider’s weight needs to be lifted (about 35 pounds for both). It takes far less energy to lift 35 pounds than 175. Thus, to the road, a suspended vehicle feels significantly lighter than an unsuspended vehicle and will have less rolling resistance.
‘Sprung’ weight also directly reduces tire rolling resistance by keeping the tire contact patch more consistent. Tire rolling resistance is not as much about tire width or tire pressure as it is about consistency of tire contact patch. The more consistent the tire’s contact patch is with the road, the less rolling resistance the vehicle will have. Without suspension there is less vertical compliance and the majority of the vehicle’s weight is ‘unsprung’ and the road imperfections must be absorbed by the rider and tires. Therefore, the vehicle will be slowed as the tire deflects and deforms in an attempt to absorb the shock. Suspension, on the other hand, increases the portion of ‘sprung’ weight the vehicle has. By redirecting the load into the suspension system, the tires are kept from having to deflect as much. The more consistent the contact patch, the lower the rolling resistance and the less energy the rider will have to use to overcome the resistance.”
Conclusion: More vertically compliant frames (especially suspended frames), wheels and components have lower rolling resistance than less vertically compliant frames, wheels and components. The problem is that most highly vertically compliant frames lack much torsional stiffness to allow for climbing. One reason why we often recommend Softride beam suspension and Serotta’s ST systems to many riders is because suspension allows for torsional rigidity and vertical compliance in ways rigid frames cannot.
Posted in Ask Fit Werx Article Archive |
Wednesday, September 3rd, 2008
Stiffness & Compliance - Energy and Power Use Breakdown*
% of Total Power Consumption:
Stiffness and Vertical Compliance: ≈15% of overall energy/power use
The bottom line: Unless a design uses an effective suspension system, side-to-side stiffness and vertical compliance/comfort will be directly linked. In almost a 1:1 ratio and regardless of material, as a frame gets stiffer side-to-side, it becomes stiffer vertically as well and thus transmit more road shock. Stiffness of your frame and components relates to how efficiently the energy and power from your body gets to the road and powers you forward. While a small piece of total energy use, stiffer frames and components transmit power more efficiently. That being said, unless you are just doing uphill time trials, don’t compromise comfort or your aerodynamics for small gains in stiffness.
How to go about it: Get properly fit and comfortable first. Then, concentrate on matching equipment up with your position, weight, riding style, power output and individual needs. Larger and/or more powerful riders need stiffer components and frames than smaller and/or less powerful riders. Be aware that a production frame in a large size will be softer than the same model in a smaller size, thus what might be overly stiff for a small rider, might be too soft for a bigger rider. Make sure a design addresses your individual needs.
*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:
Much of the bicycle industry has done a good job of creating the impression that different materials offer different ride characteristics. Aluminum is supposed to be stiff and light, but is also known for diminished durability and harsh ride quality; Titanium is supposed to be light, durable, comfortable and compliant, but a little flexible; Carbon fiber is supposed to be light and comfortable while simultaneously enhancing drive train stiffness; Steel (Chromoly) is supposed to be “real” and provide a comfortable and snappy ride, but is known to be a bit heavier and more flexible than other options. Right?
Not necessarily.
All manufacturers are trying to build that perfect combination of ride characteristics where stiffness and responsiveness are maximized, while the ride is still kept silky smooth and comfortable. It is not too hard to find claims of a frame being stiff, yet compliant and comfortable, with fantastic vibration damping characteristics. However, the bicycle industry has never had a good baseline testing protocol to quantify how various materials and designs actually perform in regards to specifics like stiffness and comfort. Everything has pretty much been based on “feel”, which is not a very scientific or reliable way to test a piece of machinery. Automobiles provide a good model for how unreliable “feel” can be. A BMW 745i can cruise along the Interstate at 95 mph without feeling like it is going that fast, while a compact Ford Aspire will comparatively feel like it is going pretty fast at 95 mph. Likewise, a bicycle frame that is really stiff and transmits a lot of road shock, can feel fast while a frame that feels more comfortable and compliant can feel slower. However, as the car analogy demonstrates, such feelings can be misleading. I was involved in a test that was designed to find out a little more about what the reality behind the materials and designs is. We tested the stiffness of some common frame designs and material applications in both horizontal (power transfer) and vertical (comfort and compliance) plane. Some of the test results are below:
Torsional Stiffness of the Rear Triangle: This test applied pressure to the frame’s rear triangle side-to-side and measured how far the frame deflected in inches (moved) under a set pressure. The lower the number, the stiffer the bike is side to side, the less flex it will have, and the more direct the rider’s power will be transmitted to the drive train.
• Cannondale CAAD 3 Oversized Aluminum: .038”
• Softride Rocket R1 Aluminum: .039”
• Serotta Legend Ti OS: Oversized Butted Titanium down tube and chain stays: .045”
• Marinoni Lugged Butted Reynolds Chromoly: .045”
• Trek OCLV 110 Carbon: .052”
• Klein Quantum Pro Oversized Aluminum: .054”
• Seven Axiom Butted Titanium: .057”
• Kestrel KM40 Carbon: .060”
• Generic Welded Butted Chromoly Frame: .066”
• Litespeed Tuscany Production Titanium Frame: .074”
Vertical Frame Compliance: This test was conducted in a similar fashion to the torsional stiffness test, but it measured vertical deflection in inches. The numbers directly relate to a frame’s comfort and ability to absorb vibration. In this case, the higher the number, the more flexible, compliant and comfortable a frame’s rear triangle will be up and down.
• Softride Rocket R1 Aluminum: 1.4”
• Litespeed Tuscany Production Titanium Frame: .064”
• Generic Butted Chromoly Frame: .061”
• Kestrel KM40 Carbon: .060”
• Seven Axiom Butted Titanium: .057”
• Serotta Legend Ti OS – Oversized Butted Titanium down tube and chain stays: .054”
• Marinoni Lugged Butted Reynolds Chromoly: .052”
• Trek OCLV 110 Carbon: .052”
• Klein Quantum Pro Oversized Aluminum: .052”
• Cannondale CAAD 3 Oversized Aluminum: .049”
The results of the tests demonstrated a correlation between vertical compliance and torsional stiffness. With little variance, and the notable exception of the one suspension frame we tested (Softride Rocket R1), the frames that were stiffer torsionally were also stiffer vertically and the frames that were more compliant vertically were softer torsionally. There was also a good deal of range within materials depending upon their application in design. For example, both the Kestrel KM40 and the Trek OCLV 110 are made of carbon fiber, however the seat tubeless KM40 was softer in both the vertical and horizontal plane than the seat tube equipped Trek OCLV 110. Likewise, the Titanium Serotta Legend Ti OS, which was specifically engineered for bigger riders, was one of the stiffer frames in the test while the Titanium Litespeed Tuscany was one of the most flexible.
The Rinard Test was the most comprehensive tests in this regard. Visit this site for more information on theRinard Test and a detailed breakdown on the frames tested. What the Rinard test found corresponded with our testing. They found that the material itself matters little in regards to torsional stiffness and vertical compliance (responsiveness and comfort). What does matter is the size, shape and wall thickness of the tubing used and the manufacturing technique (carbon lay-up, lugged or welded…) and design of the frame.
There are no bad frame materials - there are only poor applications. Any material can be built to have characteristics that are on the other end of the spectrum of what is commonly thought. Aluminum can be soft and flexible (you may remember aluminum frames made by Vitus in the ‘80’s and early ‘90’s) and Titanium and carbon can be made so stiff and harsh that they would be unrideable. So, why do materials each have their own reputations in regards to ride characteristics? Certain materials lend themselves to certain production designs and it is these initial designs that deserve the credit, or the rap, for a material’s general ride reputation, not the material itself.
When choosing a frame or new bike, do not spend time making judgments about ride quality based upon the materials used to build a frame. Instead, approach your frame decision as an individual. Only consider frame options that fit you well, and then look at the design details and tubing to find the ride characteristics that will best match your needs, body and riding style. Finally, don’t forget that a bicycle is a sum of its parts. The other components (especially the wheels and the fork) that you use effect the way it will ride as much as the frame does and should be chosen based upon how they relate to the other parts around them. If you remove yourself from the advertising claims and choose your bike through a process that considers the big picture, I can promise that you will be happy with the long-term results of your new ride.
Important Considerations for Bigger or Smaller than Average Riders (under 150lbs and over 170lbs):
Keep in mind that most production frame tubing is designed for the “average” rider – usually a male who fits on a 55cm frame and weighs around 160 lbs. As production frames become bigger or smaller than this, or a rider heavier or lighter, the ride quality of the frame is going to change too. For better or for worse, when compared to the spec size (usually about a 55cm) a smaller than average production frame is going to be stiffer and less compliant while a larger than average production frame is going to be softer and more compliant.
If you are a larger than average rider, you need to be cautious of many of the more vertically compliant (more flexible) rigid frame designs on the market. While frames like Kestrel’s KM40 or a Litespeed Tuscany might be a good option for a lighter rider, as a larger more powerful rider, you could over-flex it. This can not only prematurely fatigue the frame but can also lead to shifting and stability issues while sacrificing your power because of too much flex. Lighter riders want to be wary of stiffer frame options as they become even more stiff in smaller frame sizes and a lighter rider simply does not have the mass to flex a stiff frame the way a heavier rider does. A Cannondale with its oversized tubing might not be the best decision. Without flex, a frame will transmit a lot of road vibration and will not be very comfortable. This is one reason we often recommend custom builders like Serotta, who not only build custom geometry frames, but also custom tune the ride by offering a variety of tubing size and shape to match your specific needs and frame size. It is also why, when a production frame will fit the rider well, we often recommend Softride. Beam suspension is the only design that currently eliminates material dependency and allows for a frame that is very compliant and simultaneously quite stiff side-to-side.
When looking at designs, keep in mind that the ride quality a frame is known for is usually based upon the experience of an “average sized” rider. What can ride great under a 160 lbs rider, might be too mushy for a heavier rider or might be too stiff and uncomfortable for a smaller rider. If you are bigger or smaller than average cyclist, it is even more important to approach frame and component decisions based upon your individual needs so that you don’t end up with a bicycle that is too stiff or too soft for your size and power.
Posted in Ask Fit Werx Article Archive |
Wednesday, September 3rd, 2008
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.
