Bicycle Frame Flex

This is a good article/podcast from the CyclingTips.com. 

While bike frame flex and surfboard flex may not be technically parallel concepts, there's a lot to be learned by studying both areas...

CyclingTips Podcast

Does Frame Stiffness Matter?

by James Huang

June 6, 2017

 

It has long been a goal of the bicycle industry to make road bike frames increasingly rigid in an effort to convert a greater percentage of a rider’s efforts into forward motion. Few of us can deny being enticed by that immediate snappiness of a super-stiff bike when you first push down on the pedals. Surely if less of your power is going into flexing the frame, more is going into making the rear wheel turn, right?

The idea that some degree of frame flex can be beneficial is hardly a new one. Steel bike devotees have long advocated the idea that the side-to-side flex witnessed at the bottom bracket isn’t wasted; rather, it behaves more like a spring, storing and returning energy in sync with your cadence. Nevertheless, it wasn’t until late 2014 that someone coined a widely accepted phrase to describe the theory.

Jan Heine, publisher of Bicycle Quarterly and head of Compass Bicycles, tagged the phenomenon as “planing” in his popular blog, referencing a term from the boating world when a watercraft starts to rise out of the water, barely skimming the surface and reducing hydrodynamic drag. Heine admits that it’s hardly a direct analogy, but the phrase has stuck nonetheless.

According to Heine, planing isn’t just a theory; he’s actually measured its positive effects with dramatic results in a specifically crafted, double-blind test. Almost without fail, riders in his trial were faster on a more flexible bike, and slower on a stiffer one.

Not surprisingly, that flies in the face of what the bicycle industry has touted for ages. Perhaps no one should be more resistant to Heine’s findings than Cannondale road engineering manager Damon Rinard, whose lengthy (and storied) background in the business includes lengthy stays at Cervélo, Trek, and the early days of bicycle composites pioneer Kestrel. Rinard has spent much of his career trying to translate what riders feel into measurable numbers, and while he falls short of fully supporting Heine’s planing theory, he does feel that after years of studying the topic, bicycle frame stiffness isn’t a simple matter of linear progression; if some is good, more isn’t necessarily better.

Given how long bicycle engineers like Rinard have worked to numerically characterize bicycle performance, why is so hard to prove (or disprove) the idea of planing? If some flex is good, how much is too much? Is there such a thing as an ideal level of stiffness, and is that sweet spot the same for everyone? Are there different kinds of frame stiffness? And if so, how does each one affect how a bike feels on the road?

We tackle all of this and more in this week’s CyclingTips podcast, and some of the answers may surprise you.