As both a coach and bike fitter, I know that I and my fellow practitioners sometimes throw out acronyms as if the entire world understands exactly what our peculiar lexicon means.
I also know that being able to “speak the language” with your coach and/or fitting professional will help you get the most benefit from their services. With that in mind, I thought I would explain a couple of those bike fit acronyms you might have heard before but not really have understood.
These two terms in particular are vitally important if you suffer from sore knees and are looking to a professional bike fit to help address the issues that contribute to that.
BDC – Bottom Dead Center
The acronym BDC means Bottom Dead Center, (aka Max Extension). 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.
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 reason BDC is important to nail down is that BDC locates the leg in the ideal position to determine correct saddle height.
The next logical question is “where is BDC?”
Most people would assume that BDC is where the crank arm points straight down – and there are many YouTube videos that make this claim – 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.
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 between the 5-o’clock position and the 6-o’clock position (when looking from the right side, at the right leg), all dependent upon the cyclist’s 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.
TDC – Top Dead Center
TDC is short for Top Dead Center, (aka Max Flexion). TDC refers to when your foot is positioned at the top of the pedal stroke, in the “dead spot,” and are no longer pulling up or have any leverage to push.
TDC is important to measure accurately to help determine the correct knee angle. In my opinion, this knee angle measurement is perhaps even more important than the BDC measure. In my experience, most bikes come with crank arms that are too long. I see 175mm crank arms on M or M/L frames all the time.
The bike fitter can easily determine the correct saddle height based on the legs measured at BDC, but how do you account for hyperflexion of the knee at TDC?
Virtually EVERY cyclist that comes into the fit studio complaining of knee pain has crank arms that are too long. This is especially true with triathletes and those who ride TT bikes. Crank arms that are too long cause the knee in the TDC position to over-flex, which in turn causes severe knee pain.
This is especially noticeable in those cyclists who start pushing early in the pedal stroke. The knee is hyperflexed when the cyclist is starting to generate maximum power, a recipe for disaster.
The easiest solution to this issue is to swap out long crank arms for shorter ones. There are two manufacturers that make crank arm lengths down to 145mm, and numerous studies show that there is no real power output difference between 125mm and 210mm crank arm lengths.
I have swapped out many 175mm crank arms in favor of 165mm-170mm lengths. Once the cranks are installed, another bike fit is in order. 99% of the time, this has alleviated sore knees, but, of course, I always mention to the cyclist that it is really a team effort between the cyclist, the bike fitter and a medical professional (Physical Therapist, General Practitioner or Orthopedic Surgeon) if knee pain is involved.
As an aside, the new LEOMO Real-Time Motion Analysis tool 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. The Peaks Coaching Group, of which I’m now a member, is the only coaching group to be certified in working with athletes with this new ground-breaking technology.
Coach Rick Schultz is an avid cyclist who trains, races and coaches in Southern California. Rick is an engineer by trade, and in addition to being a coach, he’s a bike fitter and prolific product reviewer. He’s the author of Stretching & Core Strengthening for the Cyclist and Bike Fit 101: Your Toolset for a Great Bike Fit in the RBR eBookstore. Check his product reviews website, www.biketestreviews.com, and his coaching site, www.bikefitnesscoaching.com. Click to read Rick’s full bio.
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