Frame Geometry and Handling – Energy and Power Use Breakdown*
% of Total Power Consumption:
Frame Geometry and Handling: Intangibile.
The bottom line: A bike’s geometry and design combines with your position on it plays a large role in how comfortable you are on the bike, how stable the bike is, and how well it handles. Your weight distribution on the bike and how well the frame’s geometry matches up with your body and use are the major players in this. Other items like frame stiffness and vertical compliance can also play substantial roles in how the bike tracks over the ground and handles. You want to be on a bike that inspires you with confidence while cornering and going in a straight line.
How to go about minimizing its negative effects: Get properly fit to guarantee correct frame geometry and weight distribution for your riding style and use. Buy your frame based first on how well it fits and then concentrate on choosing technology that can help the bike track the ground and maintain traction optimally. Like with rolling resistance, more vertically compliant frames (especially suspended designs) will track the ground better and maintain better traction than stiffer frames.
*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:
From touring to crits and triathlon, there are many different frame geometries, each designed to work best in a specific use. Many of these changes are subtle, like criterium bikes having steeper head tube angles and higher bottom brackets for sharper handling and more cornering clearance than a standard road frame. However, in some cases the differences are substantial. Comparing versatile road specific geometry with dedicated TT/Triathlon geometry is such a case. Road geometry actually has more in common with mountain bike geometry than TT/Triathlon geometry. For this reason, you will find an explanation of some of these differences and why they are done below. These two frame concepts are as different from each other just as a sports car is from a luxury touring sedan. However, both are designed for performance and comfort in their own domain and both have their place.
Why is an aero frame different and why is it designed the way it is designed?
Almost a full 50% of your energy is spent overcoming your body’s own air resistance, never mind the air resistance from your bike. An aero position maximizes the rider’s aerodynamic efficiency without compromising power output and breathing efficiency.
Compared to a traditional road geometry, a properly fit forward geometry aero bike allows for three main things to be accomplished:
1) It places the rider into a far more aerodynamic position by lowering the rider’s profile and reducing their frontal surface area.
2) It preserves key body angles (hip to torso and arm to torso) for proper muscle recruitment and efficient power while simultaneously taking load off the muscles by allowing the body to be supported more by the skeleton through the proper use of aerobars.
3) The frame’s geometry is adjusted to compensate for the forward weight distribution of the aero position and to improve handling and stability in this position.
The difference between a road bike and triathlon bike geometry can be broken down into 2 categories:
1) Geometry changes to adapt to changes in body position.
2) Geometry changes to help the bike’s handling and stability while riding in the aero position.
1) Geometry changes to adapt to changes in body position from a road position.
These geometry changes are made to preserve strong body angles and muscle activation while riding while simultaneously allowing for the proper use of aerobars in place of the drop bars found on road bikes.
Steeper Seat Tube Angle (further back bottom bracket): On a road bike, as the upper body is made lower, the angle between the torso and the ankle at the bottom of the pedal stroke becomes more acute than the 90 degree angle that has been shown to be optimal for most riders. For most riders, once the hip angle becomes smaller than about 90 degrees the hamstrings no longer fire as early or as efficiently, they may lose comfort because their range of motion/flexibility (especially at the hamstrings) will be exceeded, and they may no longer be able to breathe as efficiently as their wind tunnel becomes folded over on itself against the legs. In order to allow an open hip angle to be preserved while in the more forward aero position the bottom bracket is placed further back in relation to the saddle on a forward geometry aero bike and thus the seat tube becomes steeper and more forward, often between 76 and 78 degrees instead of the 73 degree angle found on most road bikes.
Shorter Top Tube: Unlike a road position where the upper and lower arm are only about 10-15 degrees from creating a straight line, in an aero position, the lower arm is made perpendicular at a 90 degree angle to the upper arm. Aerobars also place the rider’s hands further away from the handlebar than drop bars. In order to compensate for both of these aspects, the top tube on an aero bike is made shorter (usually about 2cm). Shorter stems are also often used to reduce the length of the cockpit and to further accommodate for this.
Shorter Head Tubes: As the rider is brought forward and down on the bike, their handlebar position will need to be lower. Furthermore, most aerobars sit 1.5-2” above the handlebar, which means the base handlebar should be placed lower as well. In order to allow for this substantially lower handlebar position, the head tube length is reduced from what would be found on a road bike.
Designed for Aerobar Use:
At its most basic level, when building a strong forward aero position, you want to preserve many of the hallmark strong body angles found in a good road position. However, simultaneously you need to build a support structure to comfortably hold the rider as their pelvis is rolled forward into a lower and more aerodynamic position than they would normally be able to hold while riding their road bike. In order to accomplish these two goals, the rider’s weight must be supported by the skeletal system in the aero position instead of the muscle system as required by a road position.
The mainstay of this skeletal support structure is the aero handlebar.
Aero handlebars are designed to redistribute weight that would otherwise have to be supported by the rider’s abdominal, arm and back muscles onto the skeletal system. Unlike your muscles, the skeleton doesn’t get tired or fatigue quickly when supporting your weight. As the body rotates forward into an aero position, the pelvis needs to rotate forward with it and the rider’s weight is redistributed forward. A properly set-up pair of aerobars creates a strong support system with the torso by allowing the upper bone of the arm (humerus) to be supported on the bone, not the muscle. This allows the bones of the skeletal system to support each other, like studs and headers in a building, and reduces the load on the muscle system.
In a road position, the muscles of the abdomen and in the arms must support the rider while seated, this limits how low the rider can go while still being comfortable. The aero position lowers the rider and rotates them forward. Without aero bars supporting the rider’s weight, this rotation would force the abdominal, arm and back muscles to support the rider. The requirements on the muscles would simply be too much for the rider to be able to maintain the position for any amount of time. Luckily, aerobars were invented and when set-up properly can redistribute the load from the muscles to the skeletal system.
2) Geometry changes from a road geometry that help and aero bike’s handling and stability while riding in the aero position.
Further back bottom bracket (longer front to center and wheelbase): By placing the bottom bracket further back, the front to center and wheelbase on an aero bike are lengthened in order to accommodate for the much greater amount of weight placed on the front of an aero bike. The front to center measurement is the distance from the center of the bottom bracket (where the front gears/crank bolts on) to the front axle (where your front wheel attaches). Lengthening these dimension has the same effect as skiing on a longer ski, it reduces quick steering response some, but adds much needed stability. Note that head tube angle and the rake of the fork you use affect the bike’s handling as well.
Slacker Head Tube Angle: Some, but not all, aero bike manufacturers use slacker head tube angles to lengthen the front to center (bottom bracket to front axle) to add stability. While diminishing Criterium bike like razor quick handling, this can make the bike easier to ride in a straight line and more stable and predictable. Keep in mind, other variables like weight distribution, wheelbase and the fork rake effect handling and stability characteristics too.
Longer Fork Rake: Actually, this is more of a dream than a reality. Many aero frames would handle better and benefit from forks with more rake. Sadly, especially with 650c wheels, the reality is that there is usually reduced fork rake as most 650c forks are just cut down from molds for 700c forks and thus actually have a shorter rake. Maybe someday…
Shorter Chainstays: Shorter chainstays are used on aero bikes for two main reasons. 1) Shorter chainstays pull the rear wheel closer to the bottom bracket and seat tube which can help to shield the rear wheel and improves aerodynamics. 2) Shorter chainstays bring the rear wheel under the rider’s weight more which helps to balance out the weight distribution issues caused when the rider is pulled forward and down into the aero position. Weight distribution is a real concern on an aero bike as it effects everything from the pressure point of air (which directly effects handling and stability) to the center of gravity (which also directly effects handling and stability).
650c Wheels: While 650c wheels are certainly not required for a well designed forward aero bike, they were made popular because of them. 650c wheels lend themselves to aero bike design as the diameter is smaller than a 700c wheel, which allows for shorter chainstays which helps to balance out the weight distribution issues caused when the rider is pulled forward and down into the aero position. As a side note, there is virtually no aerodynamic advantage between 650c and 700c wheels. What 700c wheels lose in surface area, they gain in reduced head tube length.
Bent or Cut-Out Seat Tube: These design elements are also often used in order to help pull the rear wheel more under the rider to help rebalance weight distribution. Are you sensing a pattern here? By bending the tube around the rear wheel or cutting out tube material the rear wheel can be tucked under the rider more. This also shields the rear wheel and often can be used to improve aerodynamics and compliance (comfort) of the tube.
Bottom Bracket Drop: Bottom bracket drop is the distance from the bottom bracket height to the imaginary line between the two dropouts where the wheels attach to the frame. TT frames will often have a lower bottom bracket height than a road bike equivalent. A lower bottom bracket reduces cornering clearance some, but can enhance stability by lowering the center of gravity of the bike and rider.
These changes, when combined with a properly fit bike, all add up to an aero bike being very stable and confidence inspiring while minimizing aerodynamic drag. However, an aero geometry frame will not handle quickly or have the cornering clearance of a road frame. Different goals, different geometry.