Why go aero? Why spend so much time and effort positioning a rider? Why spend the time and money on the research and development of better systems, saddles and other equipment? Because as humans we have a limited amount of power and we have to explore and leverage everything physically possible to our advantage in order to reach our highest performance potential. At Cobb Cycling we strive to develop the industries most comfortable, innovative, rideable and reliable line of bike saddles and cycling products. Our products and positioning system allow riders to: Be comfortable, perform stronger and go fast!
WHERE DOES AIR GO?
You can feel air but you can’t see air, that is why it is much harder to understand where it goes. Air is like a “soft” liquid, it compresses a little and it moves away from resistance in objects.
A change in shape from a surface that is several inches away and downstream on a cyclist will effect the airflow at the earliest contact points of the rider and wind. Airflow is affected by the angle it attacks the rider, crosswinds make the air see the rider as a long cylinder shape; head winds show as air over an egg shape.
In the first photo (image 1.1), we see some basic airflow and how the rider moves the air creating a larger frontal area. Part of our goal is to lower this increase in frontal area and not allow it to ‘grow’ based on things we are able to control. Things that help with this are elbow width, knee positions and overall rider flatness.
In the second photo (image 1.2) we have the air flowing over the hands just at the outer edge of the forearm. You can see how the air reacts to the upper arm as a cylinder and flows down and around the thigh and hips. This riders loose jersey is contributing to this airflow direction, a tighter jersey would have pulled the air higher over the hips and allowed the air to flow smoothly off the rear.
The third image (image 1.3) shows airflow coming over the hands and making a sharp downturn because of the loose jersey. This causes a large increase in aero drag, meaning the rider has to pedal harder to force the air to make this turn. The final picture (image 1.4) shows airflow over the rider with the rider at a seven degree cross wind angle. The air is reacting to the long cylinder shape of the rider and you can see how the air”blows up” just past the upper arms. A tighter jersey and different helmet shape can really effect this. Hand height can also effect the ability of the air to sweep around the downwind side of the rider.
YAW & AIRFLOW
Let’s explore air movement over and around the front end of a bike and rider. We examine how air impacts these frontal areas which, in worst case scenarios, require you – the rider – to waste more power/energy to get down the road. A series of test for these specific purposes was conducted at the Texas A&M wind tunnel.
Air movement is both interesting and complicated. Getting THROUGH air represents the single biggest challenge to going faster for a cyclist. Air can actually be considered a fluid, which can help conceptualize its behavior. Air is compressible, difficult to see and unpredictable in its “reactions” in certain situations.
All of these things add up to make it a formidable challenge when one sets out to design a new bicycle frame or component. Simply flattened or “swoopy” shapes do not guarantee a low aerodynamic drag result!
In the first example, (image 1.5), photos were taken using “smoke” to help visualize airflow. The Texas A&M wind tunnel is used extensively by NASA, Harley Davidson, Indy Car teams, Navastar International and many other organizations because of the accuracy, high wind speeds and high rate of table/rider rotation it offers. In other words, speed and direction (relative to the object/rider) were controlled at all times while generating and recording precise data.
When testing objects for aerodynamics, it is essential to rotate the object in the simulated wind in order to measure the effects of crosswind airflow. Even for race cars traveling at almost 200 mph, crosswinds impact the performance of a vehicle. For cyclists, crosswinds are the ever present enemy, making the bike feel unstable, especially in gusty conditions. Savvy competitors soon learn to ride off to one side of the rider in front of them in “echelon” formation in order to benefit from the diagonal slipstream.
Conducting aerodynamic testing at a zero yaw angle, representing a perfect headwind, never tells enough of the story. It is important to test at multiple angles so that you have a better idea of how airflow will behave on most courses and in most scenarios – i.e. where all the air goes. Only then can you begin to refine the creation of wind-friendly equipment shapes and rider positions.
This series of photos illustrates how air changes direction over the rider’s arms as effective wind direction changes from zero to five to ten degrees of crosswind or, in overly simplified conceptual terms, “yaw”. During this test, the rider was pedaling because naturally leg movement affects air turbulence and flow.
The first picture depicts a direct headwind, or zero yaw, with air flowing neatly into the chest area and around the rider’s hips. Things begin to change quickly as we turn the rider to simulate a crosswind.
The second picture illustrates five degrees of yaw. The air has already begun to track more directly off of the low side or drafting hip area. You can see how the air becomes turbulent – read: disorganized – as it flows over the riders left hip. This makes the rider “appear larger” to the wind, requiring him to generate more power to maintain the same speed.
In the third picture on the far-right, the rider is turned to a ten-degree yaw, or crosswind angle. A ten-degree crosswind is very common, a situation that will feel familiar to those of you who have spent time on your TT/ aero bikes. At ten degrees, things get interesting: air behaves and flows quite differently than it did at small- er angles. It adds a significant amount of frontal area to the rider’s aerodynamic profile, requiring even more power to maintain a given speed. Air jumps from the right arm, across the open chest area and exits around the outside of the left arm. The air “exiting” the rider becomes much more disrupted, which drags additional air from the upper back into its vortex.
HELMET AIR FLOW
Why is one aero helmet faster than another? Why are most current helmets faster with the “Tail Up” than “Tail Down”? These are questions often asked and we will try to explain the mystery.
In the first set of pictures (images 2.1 & 2.2) we see the same rider, same smoke level, the only difference is the helmet position; “Tail Up” and “Tail Down.” We ran an actual aero drag test to compare the results. At a “0” yaw angle or head-on wind, the “Tail Down” was faster but if you average a run from “0” to “20” degrees, to simulate various crosswind conditions, the “Tail Up” is the better choice. In this particular case, the difference between the “0” and the “10” was 200 grams of drag or over .5 lbs of aerodynamic force per square inch of rider surface area. This rider had a total drag average of 6.3 lbs. So adding .5 lbs would be a big percentage of drag.
For the next example, the rider had a “B” style back, (for more info on back shapes see “Guidelines For Aero” ) meaning that most of his flexibility came in his shoulder blade area and he was not able to get low. (image 2.3 & 2.4) The rider also had very broad, square shoulders, and a narrow elbow position did not work for him. We ran several helmets but got down to a Giro Advantage and a L.G. Rocket. In the end, the Rocket was faster at 7.16 lbs vs 7.28 lbs for the Giro. We ran “Tail Up” and “Tail Down” over the yaw sweep but in this case, “Tail Down” was best. The outward flare at the base of the Rocket helmet seemed to help shield the wide shoulders from the oncoming air and make better lower back airflow. We did a lot of cockpit work and upper arm angle changes and ended up with a drag of 6.79 lbs. This will net this rider a large time gain.
This racer (image 2.5 & 2.6) produces a large amount of power for his age group. His biggest issue is that he has fused vertebrae in his neck and no flexibility at all. We tried several helmets and in his case, the Giro was the best. He could not hold a very tight “Tail Down” position but there was no real difference for him whether the tail was up or down. In the end, we moved him more forward and achieved a drag of 6.76 lbs. He has been racing competitively at an aerodynamic drag of 7.31lbs. In the next series of images (image 2.7 & 2.8) we have a simple look at “Tail Down” vs “Tail Up” airflow. In the actual drag measurements, the “Tail Up” was again better, but why was that? If you look at the smoke flow the air seems to go nicely over the “Tail Down” and flow off the back. There will be several things that contribute to the end result but a couple of things jump out in these pictures.
First, the air on the “Tail Down” position pulls all the way down to below the riders waist. The air on the “Tail Up” breaks free and flows off just past the shoulder blades. With the “Tail Down,” the air pulls down and tries to fill in the gap behind the neck and shoulders, on the “Tail Up” shot, the air jumps this gap and flows off the back, This rider has a much more aggressive position than most long distance triathletes, but the numbers stayed pretty consistent as we lowered his front end.
GUIDELINES FOR AERO
Helping a rider get more aero is always the goal in a good triathlon or TT positioning. There are some general rules to follow that will help you achieve 90% of all aero gains. It requires a trip to a wind tunnel or several weeks of very dedicated power meter testing to delve deeper into the last small percentage. At this time, I have not found any concrete relationship between back / shoulder shapes and helmet choice.
The position of the rider in the first image (image 3.1) in the following set of photos is quite horrible but typical of where we start quite often. This rider is too high in the front end and his forearms are pointing down, scooping in air. This is an Ironman distance racer so in the end, he was lowered about 12 cm and moved forward 6 cm. This built a livable trunk cylinder angle and took a lot of weight off his arms and shoulders.
The next rider (image 3.3) is a good example of a B style back combined with a muscular upper body. These riders are very difficult to get really aero. Elbow width is very important and generally you need to leave them just a little wide so air can pass through and over the hips.
In image 3.4 we have a girl who is an Ironman distance racer. Since she had broad shoulders, I brought her elbows in quite a bit to blend in with her narrow waist. Notice the angle of the red line, we were able to achieve a good “wing” to help her pull air off her chest and over her hips.
The following three photos are examples of different sizes and back shapes. Two riders have B style backs and one has an A style back shape. The rider in image 3.5 is an example of B style back and did have some flexibility problems but was able to achieve a good Lat wing angle to help pull air off his back. The rider in image 3.6 is about 6’3” and young, has excellent flexibility and is a good example of an A style back shape. This rider is a road racer and does a 40k max TT. The final rider in image 3.7 is an Ironman racer with a large hump by his shoulder blades and is another example of a B style back. Getting the Lat wing angle for riders like him will pay big dividends in long distance races.
CAN YOU GO TOO LOW?
The question, “what is the point of diminishing returns for a good aero position” comes up often. Several things can impact these answers. What type of racing is the rider competing in? Can the rider maintain this position for long periods? How much power do you lose in a real low position? All good questions with wind tunnel testing providing some of those answers. A Cat 1 road racer from California that had extensive TT experience was the test subject for this round of testing. He was set up on a bike with an adjustable stem to make changes easier. The stem was raised in 2 cm increments to map the aero changes, making a couple of runs in each height position to maximize the hand position in relation to the overall height changes. The absolute lowest setting was the fastest and the time gain from increased drag came on quickly. Using the time differences over a 40k course at a very reasonable 275 watts, the results were very clear as posted below.
When looking at the pictures above, even the highest position (image 4.4) is still a setup that could be considered a good aero position but giving up over two minutes in a 40k is never good.
For triathlons up to a half Ironman distance, the position variances needs to be considered. For a full Ironman racer, rider comfort in the neck and shoulder will have to be considered before the final height can be set.
To achieve the lowest positions, good pelvic rotation (see chapter 6) is very important. Choosing the right saddle to will allow for this will be a personal choice but very important.
Women seem to always have a slightly higher aero drag than men. I’ve studied this for years and consistently get the same results when testing fairly equal sized men and women, the women are always higher. I believe this is directly related to chest shape and breast size. I have also found that the rotation of a woman’s pelvis is very critical to lower drag. So, is going very low in the front end important for female racers? With this question in mind, we went to the wind tunnel for further research. For this test, we used a semi-pro road racer and experienced TT racer from California. She is 5’9” – tall and thin. I felt this would give a good comparison to my male test subject. For this test we used almost identical bikes except for the frame size difference, a 52 for her and a 54 for him.
Most women have a very difficult time getting low in the front end simply because the head tube lengths are too long, even on the shorter sized frames.
Another very critical part of the tests was teaching the rider how to rotate her pelvis forward, rolling over the edge of the pubic bone and resting more on the abdomen, rather than “soft tissue.” For women, learning this pelvis rotation is absolutely critical to achieve a rideable, low front end position.
Imagine that the front edge of the pubic bone is a pivot point, as you go over this point, the sit bones and other soft tissue parts are rotated up away from the saddle. You are then resting on the lower abdominal region and that area is perfectly happy to be ridden on. The results of the test were a little surprising but still show the importance of good positioning.
For a 40k distance at a very reasonable 200 watts average the results were:
Continuing the test for women and low positions, we used more smoke test to watch the air flow and I again believe the riders chest shape was a problem. This shows up when there is virtually no difference when making a 4 cm height change but as soon as the riders upper body comes up, where the shoulder pivot point and the lat point are close to level, the aero drag climbs very fast.
Most age group females triathletes can not get close to this low because of frame size limitations but there are huge time savings available with a little work. Using an adjustable stem which allows a much lower front end setting will often help racers regardless of the distance race they are training for.
ANALYZING BODY SHAPE
In order to position a rider on a bike, I have found that it is essential to have an understanding of the athletes structural form. This will help you consider more facts when setting him / her on the bike. I usually start this process by having the rider remove their shoes and walk way from me for about 25 feet. It is important that the rider be in bike shorts and have on a tight jersey or no shirt at all. I have them walk away and then return and stop with their feet in a natural position, two or three times as I make notes about different things I see. After a few cycles of walking, I begin a discussion about problems I noticed; the rider will typically confirm these things and are often surprised that I have noticed them. This is a great way to build trust with the rider and bring the rider more comfort. The following examples are real life case studies of problems and possible solutions that may correct them.
Our First example is a 30 year old racer who looked fairly normal at first glance. On his return walk, he stopped and his feet were offset from front to rear. (image 5.2) I had him walk again and he stopped the same a second time. This led me to look more closely at his pelvic structure. I discovered that he had a hip socket displacement that caused the left leg to be rotated back. He unconsciously had hidden or adapted this by standing with unequal footing. On the bike, I was able to offset this by simple cleat fore/aft alignment on the shoe
This next racer had been riding for several years but continued to have comfort problems on the bike. Persistent back and shoulder pain and hand numbness were among his primary complaints. I had him walk with no shirt and the spinal curvature was quickly evident. He also had a severely dropped right shoulder but did not seem to have any leg length difference. I worked with him on saddle rotation right / left, offset his elbow pads in width and set one aero extension bar slightly longer than the other.
This young lady pictured was challenging. She was a new athlete with new equipment. As she returned toward me on her walk, I noticed that she didn’t look right. It was hard to spot at first because she was not in racing shape yet but was very representative of a beginner athlete.
I had her place her fingers on her pelvic points and the hidden leg length difference became quickly apparent. She had adapted to this difference by having her knees offset underneath herself. The right knee was in toward her center line almost 3 cm more than her left. I fixed this with 1 set of cleat wedges used as risers and then offset her cleats to get her pelvic support more centered.
The female racer on the right would easily be one of the most difficult body shapes to get positioned correctly. She was preparing for a half Ironman and had been an athlete for several years. Persistent lower back problems, saddle issues, sore knees and hip problems were on her list. As I watched her walk, she was structurally straight. When she returned and stopped I spotted several of her problem areas. She had a tilted pelvis which caused a leg length difference. She had adapted to this by caving in her right knee an excessive amount, along with rotating her right knee outward. She wore orthodics for running but had tried and stopped using them in her cycling shoes. I addressed her issue by changing her saddle to model more narrow in the rear, and worked with her on saddle rotation right / left and changed her cleat alignments.
The next rider is a Cat 2 road racer that makes good power but had an apparent leg length issue. He had been involved in an unfortunate accident many years earlier that left him with a leg difference of almost two inches. Over time this has caused a pelvic rotation and spinal curve. He was using a spacer that was 3 cm thick and it actually had him balanced on the bike. I used two cleat wedges with the thick part to the inside to even out his foot pressure on the short side.
The next rider had an interesting problem also. His head was offset to his shoulders. We discussed injuries but found nothing signficant to his issue. He had very equal leg lengths and his pelvis and hip sockets were all in line. You could really see that his left trap muscle was over developed compared to the right side. I did not do anything about this because he was not having any issues. It is always good to observe these things however and have a discussion with the rider but not always necessary to address all issues.
One of the first things to look at when positioning a rider is their basic form. Teaching a rider to “rotate forward” on the seat makes many things come into place to achieve rider comfort. In the images you will notice the rider sitting back or non-rotated. This is usually caused by seat discomfort or bad technique. It pinches the diaphragm area which restricts breathing and causes neck and shoulder issues. You also see pictured a rider who has rotated his pelvis forward as indicated by the change from the green line to the red. With no other changes there is more open area in the diaphragm and less acute bend at the neck.
SHORT FEMALE POSITIONING
This female rider shows a very typical situation we face when working with riders, male or female under 5’4” using a 700c size frame. On many of the new frames, the head tube length is too long to allow for a more aggressive aero position. In this case I used a 40 degree rise stem and reversed it putting the pares below the top of the head tube. This particular rider is doing Ironman races, otherwise I would have tried to go even lower for better aerodynamic gains.
MUSCLE CROSS OVER
Finding the muscle “Cross Over” point takes practice but will soon become very obvious. It is important that the rider be under some work load to help you have a better feel of the muscle activity. The flex point is located above the midway point of the upper thigh, (indicated by the red X in the green circle). It is the IT Band (Iliotibial band) that you will be feeling. As the upper muscles, quads, rectus femoris etc. begin their contraction. You can feel the hamstring group relax or extend. The goal is to time these two reactions to the 12 o’clock position of the cranks.
Other indicators that you look for are the activity of the calf muscles (gastrocs, tibialis, peroneus) and their contraction in relation to crank position. These are indicated by the green and lime colored lines below.