An oval chainring is a necessity for any single speed bike, hands down. Let me explain.
- Maximize your leverage.
- Ease your transition.
Maximize – Between the 1 and 2 o’clock position through the 5 o’clock position for either foot is the location where the most leverage is found when riding a bike. Having the widest part of the chainring vertically for this area is the equivalent of adding 2 teeth to the chainring, essentially giving you an increased gear. Ex. a 34 tooth oval chainring will put out a power equivalent of a 36 tooth round chainring throughout this position.
Ease – Conversely, the muscle transition zone in your pedal motion occurs between the 5 and 7 to 8 o’clock position for either foot. As a result this is where your leverage is at its weakest. With an oval chainring the narrowest part of the chainring falls throughout this area, reducing the chainring to the equivalency of a 32 tooth round chainring. Essentially given you an additional decreased gear, making this area easier to pedal, and easing the strain on your knee and ankle joints and muscles as this transition takes place.
In order to recreate this effect with round chainrings, you would have to have 2 gears and a front derailleur that is ready to do some serious work. For every full revolution of your pedals you would have to shift between a 32 and 36 tooth chainring 4 times. 36 tooth at the 1 o’clock position; 32 tooth at 5 o’clock; back to 36 tooth at 8 o’clock; back to 32 tooth at 11 o’clock. Then repeat for every revolution…
Often the main argument against oval chainrings is in reference to the failed Biopace oval chainrings of the 80’s and early 90’s which lead to multiple knee and ankle issues with riders.
There was one fatal flaw with the design of Biopace chainrings that resulted in these injuries. They were designed to be installed 90 degrees off from the current ones, and that makes an incredible difference. Installed in that manner places the hardest part of the chainring (36 tooth area) directly in the muscle transition zone. So you were struggling to power through the most difficult part of your pedal rotation while your knees and ankles are at their most vulnerable.
The rationale from Biopace was that the increased momentum gained from the easier section being in the high leverage zone would virtually pull itself through the transition area. A flywheel concept of sorts. Unfortunately, mountain biking is rarely a smooth, flat, open road and generating enough momentum to achieve a flywheel effect is virtually impossible, even with road biking this would be nearly impossible. Hindsight also proves that this theory was implausible, evident in the resulting knee trauma.