Stiffness and Material Behavior primer (Pt II)

August 20 — 2012

Part 2 of 3

Words by: Joe Graney, SCB Engineering

In the first installment, I'm basically saying that the stiffer your mountain bike frame is, the more predictably its going to handle on the trail. It's about decreasing the deflection the frame undergoes when its loaded. The lower the deflection (the stiffer the frame is), the better it will handle because it's not flicking around like an un-damped pogo stick. That seems pretty straightforward - but it's not the whole story of why carbon frames can feel so magical, and make the "same" frame feel totally different by switching materials.

If you think of a metal bike frame like a spring without a damper, then you can think of a carbon bike frame as a spring WITH a damper. This isn't unique to carbon fiber, or to fiber reinforced polymers, but to all polymers. That includes your tires and tubes, your grips, the foam cushion in your saddle, maybe even in your chamois. It's the nature of the material on the molecular level that makes the difference. Metals are atomic structures, aligned into crystals, while polymers (aka plastics) are molecular chains or strings. The mechanical behavior is fundamentally different.

In practical terms, think of metal as behaving like a blown shock. A blown shock feels like poo because it's just a spring, and it's not absorbing any energy, so the bump doesn't get absorbed by the damper because the damper is faulty. Carbon reinforced polymers act like both a spring and a damper, so they actually absorb some of the energy instead of transferring all of it to you. This sort of thing is pretty hard to measure accurately, especially on a whole bicycle, so it's hard to put exact figures on it. But if you've felt the difference, you might find it pretty hard to put that genie back in the bottle. Even when all stiffness measurements point us to believe that we have about the same stiffness between carbon and aluminum frames, there's a tangible difference in "feel" when riding the carbon one. Its almost like there is less buzz that goes from the trail to the rider. Look at this same figure from the first installment. The Y axis of amplitude can also be "energy". With damping, the energy decreases over time, without damping the energy stays the same.

On a suspension bike that means you'll hit a bump and that blown shock is going to keep bouncing you up and down and up and down forever (not really forever, because you, your tires, and other factors will absorb some of that energy, but you get the point)

So carbon frames can have a stiffness advantage that always feels better, and they have the vibration damping built into the material that helps absorb buzz before getting to the rider. So what's the downside? Well, that same damping characteristic that absorbs energy makes very high stresses that can cause a material fracture if this is not understood and designed for. And this is one of the reasons carbon got its bad name in bike stuff, because if you give a carbon part too much energy too quickly - things don't go so well. The dark side of carbon composites will be covered in Part 3.

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