How Hard Can You Clamp Carbon Handlebars Before They Crack?

Somebody at the shop donated these 195-gram Zipp carbon handlebars because, as you can see in the photo below, there is a little crack where the shifters were clamped. 

These bars sparked a little controversy, because one of my co-workers was adamant that the rider must have crashed, and that's what caused the crack. I said no way, somebody clamped the shifter on too tight.

So, since the bars were already ruined, I thought I might see how much clamping force Zipp bars can take before they snap.

Below lies my torque wrench, which is good for 2-15 newton meters (nm), and also the 4-bolt Dimension Brand stem that'll work as the torture device.

I started with the appropriate 6nm, just how the instructions printed on the bars told me. No cracks formed and the bars felt well-held in the stem clamp. 

Then I said screw this, and cranked 'er up to 9nm. I figured that wouldn't be enough to do any damage.

But, as I turned each bolt ever-tighter in an "X" shape to ensure equal bolt-torque conformity, I began to hear –first– a little 'tic' noise, then a hiss.

While three of the bolts reached 9nm, the forth kept spinning. Though at first the bolt was hard to tighten, it became easier at the same moment the hiss noises began. Every turn of the screw after that first break became easier, and a visible crack formed right at the bolt.

I unfastened the stem to see the damage to the bar, but it doesn't look like much. Just a little crack and a crease where the stem's face plate dug into the carbon.

Channelling 'Spinal Tap,' my next thought was to take the torque wrench up to 11nm and see if anything shattered.

Sadly, I didn't get to see how clampy the stem could get, because before we reached 11nm, the heads of two bolts snapped right off!

Not to be deterred, I extracted what was left and used the two remaining bolts to fasten opposite sides, then went right back to cranking.

As you can see, the face bottomed out against the stem and I couldn't tighten any further without risking another snapped bolt head. 

So there you have it. The tightening suggestions on carbon components actually mean something, and should be followed closely.

For fun, I decided to see if I could break the bars with just my hands – strong-man style. I worked them back and forth, switching muscle groups while the bars creaked with each effort. After a few minutes, the bars split at the original crack, where the shifters were mounted.

It's a good thing this customer stopped riding these bars! If I could snap them with my hands, I can only imagine what horror a person could do in a full-effort sprint!

I tried to get the bars to break at the stem clamp, but after the bar split at the bend it was hard to get a good grip, and I couldn't think of any way to get the right kind of leverage without mounting the stem to something.

Instead I resorted to just beating the thing against a metal railing. That finished off the crack at the bend. If you look inside you can see a big clump of glue where they must've bonded something in the factory. Obviously it didn't do much for strength.

This is what handlebars look like after beating them against pavement eight or nine times. 

And hey, if you like this post you probably are interested in performance, so why not check out this post on the top performance cycling upgrades?