ATC: What do the terms BDC and TDC mean in relation to cycling?

Bike Coach Rick Schultz answers one reader question about what the terms BDC, or bottom dead center, and TDC, or top dead center, mean with regards to cycling and pedal stroke. A short answer to that question you never asked, but always wanted to know. 

A.N. of Irvine asks “I hear the terms ‘BDC’ and ‘TDC’, can you explain what these are?”

The acronym BDC means Bottom Dead Center, (aka Max Extension), and TDC is short for Top Dead Center, (aka Max Flexion). As it relates to cycling, BDC is when the cleat is at the furthest point away from you, which is also the point at which the leg is fully extended in the pedal stroke. In BDC, you are neither pushing on the pedal nor are you at the point where you are pulling back and up on the pedal. Inversely, TDC refers to when you are positioned at the top of the pedal stroke, in the “dead spot”, and are no longer pulling up or have any leverage to push.

See our previous article on the new LEOMO Real-Time Motion Analysis tool that actually calculates and scores dead spots throughout the entire pedal stroke and also further breaks these metrics down into a Power-Cadence-Dead Spot score map. Dead spots change depending on cadence and power. This 2-dimensional map shows all dead spots throughout the cadence and power range. Note: Coach Rick is now part of Peaks Coaching Group, the only coaching group to be certified in working with athletes with this new ground-breaking technology.

So what allows you to pedal through this “dead spot”? The answer is mainly the iliopsoas, also known as your hip flexors; the hip flexors are engaged at the BDC.

The next logical question is “where is BDC?” Most people would say BDC is where the crank arm points straight down – and there are many YouTube videos that show this – but this is incorrect. It would be the case if the seat tube was also vertical, but bicycles are not built with vertical seat tubes.

Adam DeVos Rally Cycling
Adam DeVos of Rally Cycling gets the stats and works on his pedal stroke during season training at the San Diego Wind Tunnel, 2016.

In fact, most modern road bike geometry sports around a 73-degree seat tube angle (as measured from the horizontal line between the dropouts to the back of the seat tube). This means that BDC is actually where the cleat is closest to the 5-o’clock position and the 6-o’clock position, all dependent upon the cyclists femur and tibia lengths.

To get close, I would just split the difference, to be accurate, you will need a motion capture video system running at a minimum of 60fps. The video is captured then output to a video file that can be analyzed with software tools such as Dartfish or Kinovea. Then true hip-knee-ankle angle (i.e., seat height) can be determined.

The next time you see someone on a road bike, take a close look at their saddle. The saddle will be positioned so that it is about equal distance between the center of the rear wheel and the center of the bottom bracket. That’s what a 73-degree seat tube angle looks like and the reason that BDC is not vertical to the ground.

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