Stiffness and Material Behavior primer (Pt III)

August 19 — 2012

Part 3 of 3 (The Dark Side of Carbon)

Words by: Joe Graney, SCB Engineering

When you cut a frame open, you expose everything.

Besides being able to see the wall thickness in different locations, one can see the surface finish internally - which is related to how well the layers of carbon are compacted together during the curing process.

When there are internal wrinkles of carbon - those plies aren't doing anything but increasing weight. On our frames, we don't use ANY filler materials or foam that stays in the frame - and our front triangles are laid up and molded together in ONE piece, instead of gluing pieces together.

Joe's Corner - Santa Cruz V10c

This allows continuous fibers to carry the load, increasing stiffness, strength and decreasing overall weight.

It's a lot of small things that all come together, getting inside the frame allows you to see what's real - and can't be hidden behind a fancy paint job.

Material behavior seems to have a lot of push and pull - no pun intended. Adding a lot of stiffness can sometimes come at the cost of fragility. Strength is the ability to withstand permanent deformation, and stiffness is the measurement used to resist elastic deformation. There's another measurement, called toughness (the opposite of fragile), which is the ability to absorb energy before fracturing. Strength and toughness are unrelated - and, at least for a bike frame, you want both. Toughness can be practically tested using a pendulum, or some kind of high velocity impact in a controlled manner, where strength (and stiffness) is measured using a constant displacement, and measuring deformation.

Practically speaking, metal bike frames don't really "need" to be tested for toughness. The result for an aluminum frame in an impact test correlates to its strength values, and the failure mode (what breaks) is often the same. For carbon bike frames though, all bets are off when it comes to toughness. Its this behavior that leads people to think that carbon frames can "explode", or "fail catastrophically" when you least expect it. Many people believe this still, even while believing that carbon frames can be strong, and stiff. The fear of the catastrophic failure is that the frame is not tough. And certainly, there are frames that are not tough out in the world.

Polymers are strain-rate sensitive materials. What that means is that they care about how FAST you hit them. Think of it like water: when you step into it slowly, it doesn't push very much. But if you jump off a bridge, it pushes back hard when you hit it fast. The stress that the material sees changes depends on how fast it gets hit, and not only by the force it sees. This is a little hard to understand at first, but it meant that when we first started making carbon mountain bike frames, we faced the challenge of defining how tough the frames would need to be. So we designed lab tests that would simulate conditions where the frame would need to absorb a lot of energy.

Imagine these scenarios: casing the front wheel into a massive double, or landing on the saddle when over-shooting the landing of a jump. Or running into a deep rut at speed where the wheel gets caught - and your frame has to absorb that energy fast without failing. And what about crashing? We figure that mountain bikes shouldn't be pretty little things you have to baby - and that crashing is part of riding - so our bikes are designed to withstand not just use, but a level of abuse as well. This is where our impact testing comes in.

Pinkbike Visits The Santa Cruz Test Lab on Pinkbike
Impact testing isn't anything new for bicycles - but the standardized testing regulated on bikes across the board is laughable when it comes to making mountain bikes that won't shatter when you look at them funny.

Since we only make mountain bikes - we tailor our internal testing standards to be specific for them - taking shock lengths and compressions into account, as well as how a frame is constrained to avoid false positives. This gets you the lightest and strongest frame - because you focus on only the areas that need to made stronger - and not throwing weight at problems that don't affect a real rider.

Our testing includes fatigue, ultimate and impact loading in a number different loading scenarios, including ones that simulate crashing, and rock strikes. After failures in the lab, we dissect the broken samples to see why the failure occurred, and what we can do to increase the strength and toughness of the frame.

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