Q&A - Bar Busting
By Paul Smith | Sunday, June 17, 2007 11.00pm
Q: My bike is four years old, and I cycle commute to work 20 miles per day all year round. One Wednesday morning the left hand portion of the handlebars simply sheared off at the stem! Luckily they broke while I was negotiating an underpass at slow speed. I hate to think what would have happened if it had been at speed or in traffic.
The responses I've had so far are:
1 Had the handlebars been subjected to a previous impact? (Not to my knowledge.)
2 Were there any score marks acting as a stress riser? Yes there are score marks - you can see in the photo score marks from the original stem clamp between the clamp and the light.
The handlebars are Humpert Straight Town Exclusive aluminium which are 25.4mm at the stem clamp, and the stem is a 3T Zepp which apparently has a 26mm clamp. Does this mean that the handlebars and stem are not compatible? I fitted a longer stem to get more reach in the riding position - probably something lots of people do - and I certainly in all my hunting around for components did not come across any warnings about handlebar/stem clamp compatibility - except of course being aware of standard and oversize bars, not using a shim to fit standard bars into an oversize stem, and stem bolt torque settings.
I have also now seen articles on the internet for fitting replacement handlebars which say that aluminium suffers from something called catastrophic failure, but knowing when to replace them is difficult!
Why did my handlebars fail? What is the rule of thumb for replacing aluminium handlebars? Do you think that this information/warning should accompany sales of aluminium bars or at least be better known?
Look forward to your help.
Andrew Kirkham, email
A: The advice you've been given is a mixed bag really. Previous impacts that a user is unaware of can be a problem with any bar, no matter what it is made from. We never advise buying secondhand bars purely because of this reason. Score marks and scratches on a bar's exterior surface lead to a stress riser. Bars that are not shot-peened (which produces a rougher, slightly sandpapery looking surface) tend to have a greater load path running along the surface of the material rather than through the cross-section; stress is much like electricity in that it will take the path of least resistance. A diameter of 25.4mm is perfectly within tolerance for a bar, and 26mm is spot on for a stem clamp. You say you fitted a longer stem for more reach, and looking at your pictures you've fitted a mile and a half of headset spacers too. This would indicate one thing right away: your frame is too small for you.
Articles on the internet say all kinds of things, and it is common for them to be written by people who don't have the slightest idea what they're on about but have third-hand information from other websites, loonies in forums, and self professed bike gurus who have no material or engineering knowledge at all. Which is why you've read that aluminium suffers from catastrophic failure. Catastrophic failure is a mode of failure and is not specific to a material. All it means is that when something starts to break, it does so with gusto and aplomb, failing in a massive and catastrophic manner. There are plenty of aluminium alloy handlebars out there that bend a long way before catastrophic failure occurs, depending upon their condition to begin with, the mounting/clamp quality, and so on.
When to replace a handlebar is a huge minefield. First, it is very dependent upon the bar itself, what it's made from, wall thickness, width, surface finish and so on. Then we have the stress caused by the mounting of it into the stem, the shape of the stem clamp, tightness, uniform or non-uniform hoop stresses and so on. Our simple rule of thumb has always been that silly lightweight race bars should be replaced every year, and not so lightweight bars every three years. Heavy touring bars and so on should be every five years. This is based upon good quality bars, 14 hours' riding per week, with a not hugely heavyweight rider riding through the odd big pothole.
Before you fit your new bar, take a small round file and radius the edge of the stem clamp. This means that when the bar bends under load the sharp edge of the stem clamp does not dig into the surface of the bar and cause a concentration of stress.
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