Friday, 22 June 2012

Are you fatigued?



The eagle eyed may notice something a little wrong in this picture, the handlebars have taken sick. As this bike is only just over a year old, and probably not much more than 6,000 miles on the clock it came as a bit of a surprise to me. In fact, quite a lot of a surprise at 6:30 in the morning shortly after I set off. What happened was that, when I was spinning along at around 20 - 25 mph (I don't use a computer) the left hand bar just went. I wasn't stressing it at the time, I got no warning, as a result I had a coming together of me and a parked car. Fortunately I was not seriously hurt, a bit of bruising, my helmet stopped by head being hurt as I gave the car's rear spoiler a Glasgow kiss. Frankly, just about anywhere else on my commute the result could have been unthinkable; anywhere on the main route would have been bad, I sometimes touch 40-ish on the 1 Km descent along Holland Park Avenue, mostly in close proximity to traffic [shudder].

This failure has concentrated my mind around metal fatigue, to try to work out how to avoid and survive it. After all, it's not the first time. Back in February my four year old Marin Point Reyes succumbed:



I noticed that crack around the seatpost, given that all that would happen would be the seat swinging around I rode home out the saddle for most of it. That ended up well, as Marin provide a lifetime warranty on their frames, and this was clearly DED. They supplied the current version, which is a 29er, which I sold and found an old model being sold at reduced price, everyone happy.

But now, I've been trying to find out what it is that causes the aluminium to fail (bearing in mind that the frame of the On-One is steel). The bars themselves were 3T Mutant such as these:



I contacted them to ask what alloy they were made from and discovered that they predate the relaunch of 3T four years ago. But, after a little more fossicking on the Internet, I discover that they are made from 6068 T6 Alloy. This is important, as it describes the characteristics; the first part is the alloy - 6061, 7075 are common - which provides its base characteristic of strength and weight and the second is the temper, which provides the added capability to better withstand age, flex, corrosion. This bar also has a 26mm clamp section.

So what is the difference between the different alloys? This is where I poke around the dark corners of the Internet and try to sound as if I have some idea; I don't. If someone turns up who does, I would be grateful for their opinion.

First 6061-T6. This is the most common alloy used. My Marin has 6061 bars. Wikipedia tells me that " the fatigue limit under cyclic load as 14,000 psi (100 MPa) for 500,000,000 completely reversed cycles using a standard RR Moore test", in contrast 7075 (the other common alloy is "159 MPa 23000 psi 500,000,000 cycles completely reversed stress; RR Moore machine/specimen" So, almost half as good again, right? The trouble is I know that most bike manufacturers have the weight weenie marketing inthe back of their mind. So, if something is half again as strong, they will only use two thirds the thickness, or some such. Can't find much about 6068, I assume it falls between the two.

So where does that leave me? Frankly, I don't know. I do know the rigours of my commute are rather harsh on bikes. I do know that I want to get the strongest bar which means the most fatigue resistant alloy and the largest diameter clamp. After that, I don't know how to translate that into real life, any ideas?

4 comments:

  1. I do not know much about 6068, but 6061 is preferred for bars over 7075 because it forms better. 6068 will probably have fatigue properties similar to 6061. Reducing the load on a part even a little will extend its life by a lot. A bar that is tapered in wall thickness will also be much longer lived than a straight wall bar of the same weight, just as double butted frame tubes are.

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