Tolerance

Lacey’s Panasonic DX-5000 is such a wicked bicycle, but the 42 tooth small chainring isn’t terribly forgiving on the hills.  She is plenty fast, and faster of late even, but why pretend to be a 1990s racer as a recreational rider in 2012?  I ordered new Sugino rings for the Shimano 600 cranks (130 bolt circle diameter) in 38 teeth and 48 teeth (to replace the 42 and 52 biopace jobbies that were on there).  I figured if they made a 38 tooth ring, it would fit.  I was wrong.  The 38 is so small that the crank spider arms rose above the valleys between the teeth.  Here is a look at the problem.  The arm is between the two rings in the picture below.  See it ooching above the valleys?

The chain wouldn’t fully seat when it came into contact with the arms.  See the space between the chain and the ring (just above and to the left of the open bolt hole)?  

Might work, but it made slight clicking noises with each contact and the chain and rings could wear unevenly.  I couldn’t find advice on the web and didn’t want to order a larger ring, so I clamped the crank arm in the bench vise and filed away the ends of the crank arms.  I didn’t have to remove much material to get the result I wanted, but I did remove a little more than in the picture below in a second round of filing (just enough so that the crank arm spider was below the machined lip on the ring).  

Still plenty of material on the crank arm and now the chain doesn’t contact the spider arm.  See?  The gap is gone.  Again, I later removed a bit more material than in the pictures above, but you can see below that even before the second round of filing that the end of the spider arm is now below the valley between the teeth on the chainring.  Compare it to the first picture in this post to see the difference.  Success!  Kinda odd that the 38 tooth ring didn’t fit without filing.  Don’t you think Shimano would have seen that coming and redesigned the spider ends?

Now Lacey is all set to burn it up (with a little less stress on her precious knees).  Maybe this weekend?

Take care.

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2 responses to “Tolerance

  1. Really appreciate your writing this up and taking nice pictures – I’ve just picked up an old bike I intend to ride in the hills and it came with 53/45 rings with a 21 on the back. For me that means walking, so your post is very helpful.

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