A version of this article appeared in the January 2012 issue of Triathlete Magazine
Does Lower Always Equal Faster?
By Ian Buchanan and Dean Phillips
In the quest for more speed, many athletes assume that a lower aerobar height always equals greater aerodynamics and speed. While being able to maintain power in an aerodynamic position will help you ride as fast as possible, the thought that you always want to place your aerobars as low as possible to achieve this does not. Here is some of the “why”:
Biomechanics, Metabolic Cost and Power – Fundamentally, there is a point where closing off the hips too much (a byproduct of going too low) creates biomechanical and metabolic factors that compromise both power and aerodynamics.
When it comes to power development, too acute a hip angle creates a few biomechanical limitations, but the hip flexor muscles becoming too short to produce effective power is the primary one. The hip flexor muscles basically pull the femur toward the pelvis and, when the hip angle gets too tight, they can no longer pull effectively. To demonstrate this, while in a seated position, place your hand on your knee and push your knee up towards your chest. Keep repeating this but drop your chest closer to your knee each time. When your chest starts getting close to your knee – maybe 6-10 inches – you will find that you will almost be unable to lift your foot off the floor. Too much hip flexion makes two joint muscles, like the rectus femoris, too short for pulling and, to some degree pushing, effectively.
In addition to biomechanical limiters, there is also a point where the metabolic cost of maintaining a more acute hip angle can exceed the benefit, which will show up as diminished power or increased fatigue. In some cases power can be maintained with a really tight hip angle at lower power outputs, but diminishes with intensity. Regardless of whether it is biomechanics or metabolic cost, there is a point where lowering the bar further will simply cost too much power and energy to overcome any potential aero benefit.
Aerodynamics, Technique and Posture – Before readily available motion capture and wind tunnel access, we used a simple level and tape measure to find the high point of the back when the rider’s hips were in their most closed off position (top of the stroke). Even using this rudimentary method, a point where lowering the aerobars further actually made the rider’s back profile higher could often be seen. Today, motion capture and video analysis with the rider pedaling under load allows for quick back height comparisons between positions while helping the fitter also make sure the rider’s hip angle remains within their functional range of motion.
The point where the rider’s back elevates even though the aerobar is set lower often corresponds closely with the rider’s range of motion in a key muscle group. For example, if your hamstrings have a 110 degree range of motion off your sacrum and lowering the aerobars tries to make them stretch to 100 degrees your body has two choices:
- Strain the muscle group and experience discomfort while risking injury.
- Try to get the muscle back into its range of motion by rotating the hips/pelvis back and thus reduce how far the hamstrings have to stretch.
The body will naturally try to resort to option two and you will generally start to bend your back upwards (thus raising its profile) to try to open up your pelvis to gain power and comfort. In this case, you would be better off riding a higher aerobar position that allows you to maintain a lower overall back height while keeping your muscles in their effective and sustainable range.
Regardless of your riding position, you play a big role in your aerodynamics. Video motion capture can also help you gain a better understanding of how your technique and posture on the bike relates to your performance and comfort. By gaining awareness of where you need to sit on your bike in order to keep your posture aerodynamic, you can keep yourself in this position with greater regularity. For example, if your back is flat and aero when you are sitting centered on the saddle and with your hands at your shifters, note that if you choke up on the bars or start to sit forward you are likely compressing your spine and making your back higher. Even though “crunching up” may feel more powerful, the energy cost required to gain this minimal power is not worth the lost aerodynamics.
While a focused rider can tune their muscles and fitness to maximize their biomechanical range for a TT position, there is simply a point where the musculature of the legs, neck and lower spine become overly strained for the distance of the event if the bars are lowered and the hip is closed off further. Your riding position has to allow you to maintain good form if you hope to produce efficient power while allowing your back shape and height to remain as aero as possible comfortably.
The Extremes: There are exceptional athletes who somehow bridge aerodynamic positions that are much lower than average while maintaining power – Bjorn Andersson and Chris Boardman, for example. These riders seem to almost have developed a “hinge” in the bottom of their thoracic spine that allows them to rotate their upper body forward separate of their pelvis; this allows for an incredible combination of efficient power and exceptional aerodynamics. Bjorn Andersson, despite riding a lot more drop than average, appears to maintain an approximately 60 degree sacrum to femur angle that allows him to continue to engage the hip flexor muscles well. If he rotated the hips forward, so his back was flat, his hip angle would be too tight. Interestingly, it looks like even Bjorn is riding his aerobars higher than a few years ago, so there is a “too low” for everyone.
Aero Testing Data Support: Wind tunnel testing or using a field testing CdA protocol with a power meter can be very effective for precisely figuring out where a rider’s ideal aerobar height lies. Below are actual coefficient of drag (CdA) numbers gained using Dr. Jim Martin’s field testing CdA protocol that demonstrate where the “aerodynamic line” for aerobar height was for an accomplished athlete.
17cm drop: CdA = 0.286
19cm drop: CdA = 0.289
21cm drop: CdA = 0.295
This athlete is tall and capable of riding a fair amount lower than average comfortably, but the data shows that anything below 17cm of drop actually started increasing drag and costing him time. Not taken into account in the raw CdA numbers was the power differential between positions, which further benefited the 17cm drop over the 19cm.
Optimizing aerodynamics and speed is not as simple as looking at a single element like aerobar height – it is a combination of aerodynamics, sustainability and pedaling efficiency. While setting up the aerobars as low as possible to maximize aerodynamics may sound logical, once past a certain point, it can actually increase drag while inhibiting muscle recruitment and power. A professional fit with a qualified and experienced fitter with the right technology and knowledge can help you explore the range of equipment, technique and positioning options needed to find the right balance of aerodynamics, power and sustainability for you.
Ian Buchanan and Dean Phillips are co-owners of Fit Werx, Road & Triathlon Cycling Specialists. Fit Werx has locations in Waitsfield, VT and Peabody, MA and offers cycling and triathlon products, specialty bicycle fitting and analysis services, consultation, and technology research. Fit Werx can be reached in VT at (802)496-7570, in MA at (978)532-7348 or through the Web at www.fitwerx.com.