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Warped crank spider on Nervex crank

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Warped crank spider on Nervex crank

Old 03-15-20, 11:16 AM
  #1  
capnjonny 
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Warped crank spider on Nervex crank

I posted this in mechanics section but in case anyone here can help here goes


I am finishing the assembly on a 1972 Botteccia Special I have been restoring for the Bike Exchange and after trial fitting the beautiful Nervar crank I noticed that the chain rings wobble about 2 mm side to side.

Closer inspection indicates that one of the 3 spider arms is slightly bent. I am going to try to bend it back and am looking for advice on how to do it.

Has anyone had this problem before and if so how did you fix it.

Thanks, I will post pics when finished.

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Old 03-15-20, 04:49 PM
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juvela
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when the wobble is in the form of a shallow wave it is usually as you state - at the level of the spider

this is easier to tweak into alignment than when it is out at the level of the dentition

one purpose made tool for this is the LaJeunesse chainwheel straightener from Bicycle Research Products

it has a narrow slot on one end for chainwheels and a wider slot on the other end for spider arms

alternately, one can simply employ an old fsshioned monkey wrench

https://jimlangley.blogspot.com/2010...hainrings.html


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Last edited by juvela; 03-15-20 at 04:51 PM. Reason: spellin'
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Old 03-15-20, 05:19 PM
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dddd
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I fix hundreds like this, and generally use a wood dowel and 3# hammer to move the spider arms. I strike the bolt area rather than apply any bending force to the rings(s), since the latter is far more likely to break off a spider tab.

Another thing I often do first is to reposition the crank 90-degrees on the spindle, re-torque partially, then continue checking each of the remaining two positions for the one with the best true.
Often this alone offers significant improvement, and surprisingly enough I can test all four positions in about ten minutes.
Remember to re-torque fully to around 30 ft-lb after finding the best position.
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Old 03-17-20, 10:53 AM
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This is how I deal with that sort of problem but please take the time to read the whole article and then give it a try...

Grand Premio Crank Truing...
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Old 03-17-20, 12:39 PM
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This was a very common bike shop task BITD. Several ways to fix it. I always did it with the VAR or Bicycle Research tool meant for the purpose. I worked in a shop, had access to the tools, and that's how I was instructed to do it.

The main thing is to remember it's like truing wheels. Proceed carefully and methodically. If as you say it's just one arm of the spider that's bent, should be pretty simple. I'd suggest you pull the chainrings off and verify that they are flat first. Those are harder to true if they are warped.
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