Technical Q&A with Lennard Zinn – The source of a blowout; That knocking sound

By Lennard Zinn

Dear Lennard,
I recently experienced a sudden blowout of my road bike front tire that led to a crash and lots of injuries. I’m writing in an attempt to determine what went wrong and why.

Although I do not remember the crash here’s what I do know. I was with two friends and we were about three minutes into a descent and traveling at about 35 mph. My two riding partners tell me that I was in the front and we were on a sweeping turn when they heard a loud “pop” and suddenly I was down. My front tire had blown.

As a result my inner tube has about a four- to five-inch slit in it but the tire itself looks okay. The slit is near the valve and on the rim side of the tube.The day before, on a morning ride the same tire had gone flat slowly from a piece of glass that it had picked up. I changed the inner tube out on the road with a tube that been in my seat bag for at least a few months and maybe as many as 8 or 9 months. It also had two patches on it from previous repairs. I installed the new tube carefully. I inflated it, let some air out and re-inflated with as much air pressure as I could muster with my frame pump. I checked that all looked good with the bead and didn’t see any irregularities. I then rode the ten miles home with no problem.

The next day on my morning ride I used my floor pump to inflate my tires to the spec pressure -110 for the front and 130 for the back. We rode about six miles to the base of our climb and about 6.5 to the summit once again with no problem. So at this point I had over twenty miles on this “new” tube with just a few short downhills.

But then on the descent, as stated above, this front tire blew. The result was a concussion, broken collarbone, lots and lots of road rash, a touch of vertigo still hanging around 25 days after the accident and plenty of doctor bills.

Did I screw up on the installation of the new tube? Did I screw up by leaving the tube in my seat bag for too long a time and therefore installed a tube that was prone to failure?

Does this just happen sometimes and this accident is the equivalent of being hit by lightning? Was it something else altogether?
Bob

Dear Bob,
Inner tubes do get ozone cracking the older they are. One way to avoid it is to coat them with 303 Protectant. I find that old tubes often pop at the transitions from thick to thin near the valve on the rim side, and that superlight tubes are particularly prone to fail there, even when brand new. However, on old tube may deflate rapidly through a hole or crack, but even rotten tubes don’t blow and create a long rip unless there is somewhere for the air to go – it has to be able to escape rapidly from the tire for the tube to explode.

Which leads me to the only realistic conclusion I can come to – namely that you did not get all of that inner tube tucked up under the bead, even though you checked it. I imagine that there was a bit of inner tube sticking out under the bead right where the stress of you leaning over at speed on a turn pulled the tire sidewall slightly away from the rim. That was enough to allow the tube to explode. If your friends heard a loud pop and the tire had no sidewall cut, the air had to have gotten out under the bead. And that’s why you have a long rip on the rim side and not the top; even if the rip was completely on the valve side, the tube may have been twisted where it was pinched under the bead.
Lennard

Knock, knock
Dear Lennard,
I have befuddling problem with my Chorus ultra torque crankset and was hoping you could offer some advice. Shortly after I installed the cranks I heard a slight knocking sound which I discovered was the result of some lateral play in the crank arms. I had my mechanic face the BB shell and I then reinstalled the cranks, which solved the problem for about a week until the same knocking sound and lateral play reappeared. I then tried tightening the bolt at the Hirth joint slightly beyond the recommended torque, which again solved the problem for a few rides before it reappeared.

Not giving up, I figured that the BB shell might be too narrow and added a 1mm spacer, and yet again I was granted a reprieve for a ride or two before the problem returned. I figured that possibly the bolt at the Hirth joint was coming lose, but once the play develops in the crank I have checked it with a torque wrench and its right at spec. I’ve reached a point of absolute frustration and don’t know what to do at this point other than warranty the crank.
Todd

Answer from Campagnolo:
Dear Lennard,
It would be better that such requests were directed directly to us [rather than to your column]; we have a specific space in our website where we respond to about six thousand tech inquiries every year.

It is always difficult to remote-diagnose the real source of a noise, but we’ll try. Progressively eliminate all possibilities.

1) Make sure the BB shell is within these specs: min 67.2mm, max 68.8 (+/- 0.8mm) for an English BB shell, 69.2-70.8 for an Italian shell.

2) Use plenty of Loctite 222 on the threads of the BB cups, this will prevent noise, play and corrosion. Let the cups rest for 48hrs to let the Loctite solidify.

3) Tighten the central bolt to spec. A higher torque will be no problem (50, 60, 70 Nm) but a lower torque, lower than 30Nm, will generate play and wear in the Hirth serration and may damage the bolt (fatigue). Please note that the spring washer, which is part of the central bolt, will prevent any loosening, so unless another bolt was used, once properly tightened there will be no further problem.

4) Make sure the semi-circular clip is properly in place. It prevents lateral play of the crankset assembly.

5) Make sure the bearings are okay; if there’s damage (usually caused by initial improper assembly) they need to be replaced

6) There’s no other source for play, so if these actions don’t solve the problem, they should be warranty-inspected by a Campagnolo service center, and eventually replaced. But the above actions solve 99 percent of the problems.
Simone Roncali
Campagnolo

Zinn’s VeloNews.com column is devoted to addressing readers’ technicalquestions about bikes, their care and feeding and how we as riders canuse them as comfortably and efficiently as possible. Readers can send brieftechnical questions directly to Zinn (veloqna@comcast.net)Zinn’s column appears each Tuesday here on VeloNews.com.

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