COMPONENT DAMAGE - CRANKS AND RINGS

There are a few common maladies associated with crank arm and ring damage.  The rings, incidentally, are the front sprockets and there will be one, two or three of them to inspect.

The most common damage I experience when considering these components is bent rings.  These components are quite fragile and can be easily knocked out of true.  It is easy to check for a bent ring.  Start by looking down on the assembly as you rotate the cranks backwards.  Does the top of the chain on the sprocket appear to wander towards the center line of the bicycle and then out again?  If so, the ring is probably bent.  Repeat this test with the other ring, once again searching for in-out wobble.  If one test presents wobble and the other doesn't, the problem is likely a bent ring.  This is not a huge issue since the rings are usually quite easy to straighten out with a little work.  However, if both rings wobble, the problem could be something worse.  Perhaps a bent or cracked crank spider.

A second concern might not fall under the damage category.  The teeth on the rings will have experienced wear.  Too much wear and the rings should be replaced.  How do you tell if the rings are badly worn?  Just look closely at each tooth.  If they appear to be pointed, they are worn.  If there is no machining marks left, they are quite worn.  And if the tops of some of the teeth are broken off, they can also be considered to be worn.

Also, compare the apparent wear of the big ring to the small.  Usually, crank rings will wear at different rates.  It is not uncommon for riders to spend more time on one ring than another.  With this in mind, there might be an apparent difference in how the teeth look.  Just one more clue to pursue when inspecting the crank rings.  The teeth on this Stronglight set look to be evenly worn, however my guess would be that the smaller ring saw more use than the big.  I might add that this kind of comparison will work with cog sets also.

Look also to the backside of the rings.  Often time the backs will be deeply scratched or lightly gouged.  This is a result of the chain falling off due to a poorly adjusted front derailleur and the missed shifts that accompany the situation.  Though the ring will still work fine, the cosmetics are most unattractive.  When you see this kind of damage, look also for ring wobble since the ring might also be bent.

I have never come across a bicycle with an obviously Busted! crank arm.  However, I have seen several Busted! crank arms that did not look to be Busted! at first.  When considering a bicycle for purchase, look closely at the crank arms for hair line cracks.  And I do mean look CLOSELY!  You might even need a magnifying glass to do this properly.  Though the crack on this lovely Mavic SSC crank set was very hard to see, the crack was indeed there.  Further use of this deficient component could and probably would spell disaster, sooner or later.

The cranks arms are asked, over and over, to do a demanding job – transmitting all of your leg power to everything else.  Good quality cranks crack just like poor quality ones do, and often times for two fundamental reasons – lack of installation/maintenance understanding and/or hard use.

If a crank set is installed, by someone who does not know how to do it properly, there is a good chance that the set will be installed wrong.  Too loose, and the cranks will have movement between their tapered fits and the tapers on the bottom bracket spindle.  This movement will create stretch and accelerate wear of the taper fit at the same time.  In a remarkably short time, the cranks will be rendered useless.  If, on the other hand, the crank is tightened up too much during installation the metal will be forced to expand as the crank is pushed up the spindle taper.  If pushed too far, the crank will actually crack.  Once again, the crank will be useless.

The long and short of this is look closely at each crank.  Are there any cracks?  And, could a cracked crank spell disaster elsewhere?  Consider this…

If the crank was installed improperly by someone who did not know what they were doing, what else was installed wrong?  If the crank broke because of rough use, what else is approaching the point of failure due to that same kind of use?  See the point?

Of course, there are other issues associated with the crank arm.  Rub marks, created by the leather Rat Trap straps, are often present.  These marks do not present a structural problem, but the marks negatively impact the bicycle's cosmetics.  In days gone bye, these marks could be polished away if they were not too bad.  Today, however, a rub mark is there to stay unless you wish to take the time to completely polish away the entire anodized surface - and that is a lot of work.

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