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  1. #1
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    crank arm square taper streching

    Hi

    I bought Paul Comp. cranks and SKF ISO bottom bracket. When I put the arms on the spindle, there is a lot of taper showing. By measuring the chainline, the crankarms need to go 5mm (!) further down on the spindle !!

    Will tightening them to the correct torque slide the arm 5mm deeper on the spindle ? I guess not ... It seems like 2 mm would be a better guess. What is your take ? How many turns does it take to reach the 35Nm from bolt tightened only by hand ? Thread pitch is 1mm, so one turn would be 1mm on the spindle - once again - I dont think you need 5 full turns of the hex key to tighten the arms ?

    Robert

  2. #2
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    That seems ok. You need some space otherwise the crank bolt will bottom out before the crank seats on the spindle.

  3. #3
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    Robert sound like you need a narrow spindle, 5mm of showing spindle is pretty normal..

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    Quote Originally Posted by patineto View Post
    Robert sound like you need a narrow spindle, 5mm of showing spindle is pretty normal..
    Yes, could either get a narrower spindle BB (something 10mm narrower that what you now have) or get something like a Phil Wood BB that is adjustable and then you can dial in the chain line and/or get an offset spindle if you need chain stay clearance.

  5. #5
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    I measured precisely, the crank needs to go 4mm during tightening (for 44mm chainline). I contacted Paul and they said that when installing new cranks on a new bb the arms will go additional 3mm during tightening, I should be OK. I'll report back to you when I install them next week on my new build
    take care !

  6. #6
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    How many turns does it take to reach the 35Nm from bolt tightened only by hand ? Thread pitch is 1mm, so one turn would be 1mm on the spindle - once again - I dont think you need 5 full turns of the hex key to tighten the arms ?
    Absolutely use a torque wrench, or you'll risk damaging the crank arms.
    Ride it around the block for a bit, then re-torque things. then see what your chain-line looks like.

  7. #7
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    Of course I will tighten the bolts with torque wrench. I never had an intention to tighten cranks in number of turns.

  8. #8
    dru
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    Bikerbo, what Paul's cranks did you buy?

    Their website says the Mountain Cranks have a chainline of 52 mm (using a 111 mm spindle), yet you want a 44 mm chainline.

    What length is your SKF spindle?

    Drew
    occasional cyclist

  9. #9
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    Paul road crankset

    44mm chainline with 111mm iso bb. i have 111 mm SKF ISO bb

  10. #10
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    It's surprising how deep the spindle sinks into the crank. Make sure you have the correct taper (you do, ISO on both), lightly grease the flat parts and torque it to spec. If chainline is 1 mm off, it doesn't matter.

  11. #11
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    Quote Originally Posted by Saul Lumikko View Post
    It's surprising how deep the spindle sinks into the crank. Make sure you have the correct taper (you do, ISO on both), lightly grease the flat parts and torque it to spec. If chainline is 1 mm off, it doesn't matter.

    I'd be careful about getting too much grease on the spindle. It seems hard to get a definitive answer on this one but I've never greased them and a Shimano tech rep once told me that if you touch the spindle with clean fingers you have more than enough grease there.

  12. #12
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    I find this essay on the subject to make sense: Installing Cranks by Jobst Brandt

  13. #13
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    Interesting. It seems though that most square taper crankarm failures are caused from improper installation and not from lack of grease. It seems they can be installed properly with grease (didn't know that) but they also can, and have been installed correctly for many years with no grease. From Park's site-




    Aluminum cranks typically do not require lubrication of this press fit. Aluminum by its nature is self-lubricating as it is covered with a thin layer of oxidation. Adequate torque is typically enough to keep arms from creaking.

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    I will be using a small amount of grease. Both Paul Comp and SKF recommend using it. And the crankarms are very, very beefy around the tapers. No risk of bottoming them out either, as Paul designed those cranks to have 5mm of tapers "available" when fully tightened.

    Take care

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    Quote Originally Posted by bikerbo View Post
    I will be using a small amount of grease. Both Paul Comp and SKF recommend using it. And the crankarms are very, very beefy around the tapers. No risk of bottoming them out either, as Paul designed those cranks to have 5mm of tapers "available" when fully tightened.

    Take care
    The risk is not in bottoming them out ... The risk is in developing a micro-crack within the arm by over stressing the interface of spindle to arm..

    I've seen this happen to a few pair over the years ... The best looking crack was in a set I loaned to a guy so he could test out a new X-Ray machine ... Neat picture !!

    I didn't know they were cracked, but they were.

    Others were found via dye-penetrant ... If a square taper develops a creak, and tightening only stops the creak for a short period of time ... The arm is probably cracked.

  16. #16
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    Well, if Paul Comp says to grease - grease I will. The square tapers in the arms also have big rounded corners not to create stress points where fatigue cracks can start. Also the aluminium they use is not the stiffest one - its 2024 and it has about 10-15% elongation rate. I don't know for sure, but I guess most of crankarms are 6xxx or 7xxx series alloys with worse brittle cracking endurance

  17. #17
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    J.B. Weld, it seems like a very similar question as the "which way spoke heads should face?" in wheelbuilding: reputable manufacturers and mechanics say one thing or another, but the important thing is that the work as a whole is done correctly.

    Bikeabuser, were the cracked cranks installed to the proper torque with grease and immediately removed to inspect the crack? The reason I ask is that it could have been caused by excessive torque (at installation) or insufficient torque (during riding). I feel that the cause has not been isolated enough to draw conclusions.

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    Never EVER Grease the tapers, they need to be install dry and totally clean..

  19. #19
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    Quite a few reputable manufacturers and mechanics disagree.
    Then again, quite a few reputable manufacturers and mechanics agree.

    We arrive at plus-minus-zero.

    I'd say do it either way you please. If you have trouble, it's most likely something else gone wrong than the presence or lack of grease.

  20. #20
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    Quote Originally Posted by Saul Lumikko View Post
    J.B. Weld, it seems like a very similar question as the "which way spoke heads should face?" in wheelbuilding: reputable manufacturers and mechanics say one thing or another, but the important thing is that the work as a whole is done correctly.

    Bikeabuser, were the cracked cranks installed to the proper torque with grease and immediately removed to inspect the crack? The reason I ask is that it could have been caused by excessive torque (at installation) or insufficient torque (during riding). I feel that the cause has not been isolated enough to draw conclusions.
    Sorry, no clue how it happened ... The one that was X-Rayed was a junk pile pickup, and the others were items I checked for friends.

    But, if I had to guess, I'd say they were over torqued, or as a second guess ... Ridden while loose.

    The grease vs no grease is a new one to me ... I've never used grease when installing this old stuff, and my Raleigh (bought new) Edge cranks have been off/on many times over the years.

  21. #21
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    Quote Originally Posted by Saul Lumikko View Post
    If you have trouble, it's most likely something else gone wrong than the presence or lack of grease.
    On that, I totally agree.
    Some people have no clue what a torque wrench is, and this is a high stress item that needs to be properly torqued.

    Click, click ... What's that ? ... Ah, screw it, keep pedaling

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