While it may seem redundant to say, most indoor bikes do not move. Please, stop and think about that for a moment. We are not talking about forward motion or distance, of course an indoor bike (or an outdoor bike on a trainer) does not move forward. But what about the other motions that are involved in cycling.

When a rider is out of the saddle, most riders sway the bike from side to side a bit. Why does this happen? Generally, it is the mechanical reality of the situation due to applying extreme power to each pedal. Since the pedals are not on the centerline of the bike, applying a large force to the right pedal will, physically speaking, apply a rotational force that pushes the top of the bike to the right and the bottom of the bike to the left. Without this counterbalancing motion, the wheel would kick out to the side. By swaying the bike in the opposite direction, the amount of force that can be applied to the pedals is increased without crashing.

The second primary aspect of swaying the bike is that it allows the rider to engage their upper body (especially core and arms) into the movement which increases power.
Take a moment and watch some of the pros race, they only have about a 12 degree sway; less than most avid riders. This is due to their efficiency and power.

The second motion that we want to consider is the flexibility of the bike frame. I learned a great deal about this from Kirk Frameworks Co. (http://kirkframeworks.com). There are three basic types of frame flex: Vertical, Torsional and Bottom Bracket/Drivetrain. It is important to understand that these different types of frame flex are more or less related to each other.

Vertical Flex
A frame with the proper amount of vertical flex will have a very smooth ride. More importantly, it also offers better control and handling.

Lateral/Torsional Flex
Torsional flex is when the head tube and seat tube are twisted relative to each other, and thus are no longer in the same plane. All frames, regardless of material or quality have a good deal of torsional flex. Try this sometime — sit on your bike with it pointed straight at a mirror. With your butt on the saddle and your hands on the brake hoods, push your hands to the left and your butt to the right. Watch the frame twist.

Bottom Bracket/Drivetrain
When a rider pushes down on the pedal he also, for better or worse, pushes sideways. This is because the pedal is off to the side of the bike and is not in the bike”™s centerline.

When the rider pushes down with the right foot the bottom bracket flexes to the left. When the bottom bracket is pushed to the side, it stays there until the force that was holding it there is released. So at the top of the pedal stroke the bottom bracket starts its sideways move and at the bottom of the stroke it returns to neutral. In returning to neutral it applies that returned energy to the drive train and then to the road. This flex and return smoothes out our power transmission to the ground, making acceleration smoother and optimizing traction.

Frame Flex

With the exception of a few new bikes on the market, most indoor bikes do not provide movement side-to-side. Because of this limitation, riders must emphasize relaxation when riding and allow gentle upper-body movement. Attempting to maintain a still upper body can place the spine and surrounding muscles at risk from the forces being generated by the legs.

Originally posted 2019-02-08 21:41:46.

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