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  1. #1
    He be a moose too.
    Reputation: pinguwin's Avatar
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    Aug 2004

    The secret life of a Syncros Revolution crank

    On my annual Solstice Ride, where on summer solstice I ride from sunset to sunrise. In 2008, on this ride, i was about ten miles from home when crossing against a light. I pushed off and couldn't clip in (for that matter, couldn't even find the pedal with my foot) but cars were coming so I madly pushed across with one foot. On the other side, upon extremely close and detailed inspection, it couldn't help but notice that my drive side pedal was hanging by a thin steel thread. So what to do? Break off the pedal, put it in the jersey pocket and ride ten miles home left-footed.

    Ritchey pedal there. I had about four sets of pedals warrantied for the same problem and Ritchey told me they've had 100's of thousands of pedals sold and not one other one had the problem I did. Uh-huh, sure. Glad when I got the last warrantied set that it put on ebay and bought some XTR ones.

    So, the Revolutions were replaced by an unpainted M900 pair and the cracked one was kept around. I had 40-50K miles on the Revolutions, so it wasn't that bothersome it broke. Parts eventually fatigue and it didn't fail catastrophically while under load and cause a crash.

    I had seen the following x-ray of a Syncros crank on Retrobike and was interested.

    Just a reminder what they look like. The black crank arm is steel, the silver part is aluminum and the black spider is also aluminum and bolts onto the silver part. So, recently with a metal bandsaw and Bridgeport mill, it was time for a little further investigation.

    The crank arm near the spindle in cross section. The steel is .048" thick. I believe the hole there is to allow hot air to escape during welding. If there is no place for hot, expanding air to escape, it can blow out parts of the weld, so put a vent for better welding. If this is incorrect, can someone correct it?

    This is what it looked like after the band saw. The taper isn't symmetrical since the cut was intentionally done off center. The top/outer part is the steel crank arm. The bottom/inner part is aluminum (power plate in Syncros terms).

    Ran the other side through the mill and it comes out as such. The bulge on the left is where crank arm is welded onto the cylindrical steel part in the middle.

    Tried to separate the two sections by heating them up and cooling down the aluminum in water but it didn't work. So put it in a vice to apply stronger measures but as soon as the vise tightened up, they popped apart. So here is what the splines on the steel look like.

    This is the aluminum center. It looks like it has some glue residue on it. The steel splines definitely look machined. The Al ones, not so sure about them. Could they have been formed by the steel splines by just pressing the steel part on? Maybe. The splines on both parts are rather shallow. Much less than I would have thought but have never heard of one with stripped splines.

    So there is the secret life of a Syncros crank. Any other pictures you want, let me know.
    Last edited by pinguwin; 10-24-2014 at 07:32 AM.

  2. #2
    mtbr member
    Reputation: RockiMtn's Avatar
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    Jun 2008
    very cool, thanx for sharing! hope neither of the two sets i have ever fail.

  3. #3
    mtbr member
    Reputation: chefmiguel's Avatar
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    Dec 2007
    From the obsessed for the obsessed.

    Technology dragass

  4. #4
    velocipede technician
    Reputation: hollister's Avatar
    Join Date
    Sep 2005
    Very cool

    I haven't spread enough rep points around to hit you again
    looking for 20-21" P team

  5. #5
    mtbr member
    Reputation: DoubleCentury's Avatar
    Join Date
    Nov 2005
    Wow! Thanks so much for doing this. I had wondered for a long time how they were held together and had consulted with various people in the bike industry on how it might have been done. It was hard to believe that just adhesives could hold over such a small diameter. I didn't anticipate those small splines, which do look like a form fit.

    I had contemplated replacing a spider I had which is cracked, and now I know what is involved.

  6. #6
    Phobia of petting zoos.
    Join Date
    Oct 2005
    It takes a certain kind of person to find that interesting.

    Admittedly I am one of those people.

    That's an observation, not a criticism.


  7. #7
    mtbr member
    Reputation: laffeaux's Avatar
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    Jan 2004
    Nice post. Break more components so that we can see their guts.
    Each bicycle owned exponentially increases the probability that none is working correctly.

  8. #8
    Join Date
    Dec 2012
    Syncros used green lock tight to bond their seatposts with no failures. got a headset race that is loose? green lock will do the job

  9. #9
    mtbr member
    Join Date
    Feb 2010
    Great post. Thanks for taking the time to investigate.

    Quote Originally Posted by rismtb View Post
    Syncros used green lock tight to bond their seatposts...
    Be careful when talking about red / green / blue loctite. For most people, its used as a shorthand for the three different strength varieties of loctite threadlocker. However, the "green" loctite that Syncros used is probably Loctite 609 which is a retaining compound specially formulated for cylindrical components. For example, its what I used when I pressed the dropouts back onto a shortened Manitou 1 fork.

  10. #10
    All Lefty's, all the time Moderator
    Reputation: MendonCycleSmith's Avatar
    Join Date
    Feb 2005
    Cool! I love destruction for scientific purposes...

    All I can add is, correct, that small hole is indeed a heat vent for the welding portion of the construction.

    Thanks for taking the time to share
    This is a Pugs not some carbon wannabee pretzel wagon!!

    - FrostyStruthers

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