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Old 02-20-06, 07:57 PM   #1
joshben
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Square taper crank loosening/errosion/rounding

hi there...i'm a longtime reader, first-time poster...yada yada...

anyways, i've got a bit of a problem

my left crankarm (and possibly my right one, i can't tell yet) has started to come loose/develop rotational play recently...when i first noticed it, last wednesday, it was pretty bad, not to the point of "flopping about in the wind," but to the point where i didn't feel safe riding on it, and i don't exactly have the greatest sense of self-preservation. fortunately, i had a 15mm socket wrench with me, (for the rear wheel) which happened to be the right one for my crankarm bolts. so i tightened both (the right was also rather loose) and was on my way...so then on thursday, i was cleaning my bike and found that the left one had once again loosened up a bit (over about 20 miles of riding)...this time, since i was cleaning the bike anyway, i removed both arms, and after cleaning them, reinstalled them, making sure both were on pretty damn tight...once again both felt secure...now this morning, i tightened up the left again (as is becoming routine), and, while it feels like it made an improvement, there is definitely a bit of play in that arm (something like 2-3 mm at the end of the arm, if that makes any sense)...

now i do realize that this topic has come a few times before:
Chronic crank looseness
Loose cramk arm!
loose / stripped crank typical when riding fixed?
however none of these really gives me enough information, so i'm trying to see if anyone out here knows a bit more that they could tell me. specifically, how i managed to cause this, if there is any cure, (that doesn't involve permanently bonding it to my BB spindle--does an aluminum foil shim really work?) are left arms more prone to this, are fixies (particularly brakeless ones) more prone to this, and, if it can't be cured, how long do i have before i'll need to replace the arm.

and lastly, if it makes any difference, here's some background info:
-it's on a brakeless fix
-i ride ~80-100 miles a week
-i've used the cranks for about 4 months
-i installed a new BB about 2 months ago
-they are a Sugino double chainring model from sometime in the 80's
-i do a fair amount of skid-stopping
-i weigh ~160#, and usually carry ~5-15# of whatever
-i clip in with my left foot first
-i start up with my left foot forward, and skid with my left foot back
i think that's everything relevant, and probably some extraneous crap, too

any replies will be greatly appreciated...hell, i'll be grateful if anyone even reads all that
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Old 02-20-06, 08:18 PM   #2
sch
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Tapers male and female should snug up tight so that there is no wobble in crank at all when the crank is put on and torqued up. First things: look at the bolt and make sure it doesn't look stretched anywhere along the threads. These are not high grade bolts, and under torque the threads can ooze. Second, as noted put the crank on torque it up and remove the bolt. Crank should stay tight with bolt off, if it slides off the taper on the crank is hosed, replace crank. If it stays tight wobble the crank by hand against each of the 4 flats and each of the 4 corners, there should be no wobble. If it wobbles crank is caput, replace. Finally look at the end of the BB axle, it should not be flush or protrude beyond the washer flat on the inside of the crank. If it does the crank est fini, replace, there is no way to tighten the crank if the BB axle is proud of where the washer sits.
Steve
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Old 02-20-06, 09:33 PM   #3
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Quote:
Originally Posted by sch
Tapers male and female should snug up tight so that there is no wobble in crank at all when the crank is put on and torqued up. First things: look at the bolt and make sure it doesn't look stretched anywhere along the threads. These are not high grade bolts, and under torque the threads can ooze. Second, as noted put the crank on torque it up and remove the bolt. Crank should stay tight with bolt off, if it slides off the taper on the crank is hosed, replace crank. If it stays tight wobble the crank by hand against each of the 4 flats and each of the 4 corners, there should be no wobble. If it wobbles crank is caput, replace. Finally look at the end of the BB axle, it should not be flush or protrude beyond the washer flat on the inside of the crank. If it does the crank est fini, replace, there is no way to tighten the crank if the BB axle is proud of where the washer sits.
Steve
And if yours pass all those tests but still don't want to stay tight, I've had luck with a little bit of blue (removable) Loctite on the threads. Ideally it shouldn't be necessary and makes for a gummy removal when you WANT to remove them, but it works.

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Old 02-20-06, 10:17 PM   #4
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By now, my guess is that your cranks have been ruined. The play will have distorted the square sockets and they will never fit correctly again. The cranks are Al and the bottom bracket spindle is very hard steel. Guess which wins if the fit isn't correct?

Be aware that the recommended installation torque is VERY high, much more than you would possibly expect unless you use a torque wrench. Proper installation is to put the crank on and torque the bolt to spec ONCE. After that, leave it alone and it should work fine until you remove it intentionally.
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Old 02-20-06, 10:58 PM   #5
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it's also possible that the new BB and the old one had different slightly different tapers (iso/jis).
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Old 02-21-06, 02:49 AM   #6
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ISO and JIS tapers are the same angle, they just have a different length and size at the tip. The vast majority of cranks with fit fine on both, but will sit at different widths.

Once the crankarm comes loose and you continue to ride on it, you can pretty much write off that crankarm. The much, much harder steel spindle with a sharp 90-degree corner will completely bugger the hole, making a tight fit impossible from that point on. There are multiple levels of security in attaching the crank, in order of importance:

- bolt-tension: use a TORQUE-WRENCH! You really need a long 10-12" handle to torque the crank-bolt down to proper-tension of 25-35 lb*ft. Forget those wimpy 6" double-sided Park wrenches, you end up tightening the bolt to only 1/2 of the proper tension.

- blue Loctite: will help keep the bolt from loosening in the event that you didn't get it tight enough the first time. At least it'll keep the crank-arm from wobbling off right away and getting chewed up. But it's NOT a fix for improper tension to begin with.

- lockwasher: replace the flat-washer with a tooth-lock or wedge-lock washers. These provide the sharp-edges to firmly grip the bolt. Similar to the wedge-lock nuts used on the solid-spindles with male threads.

- safety wire: drill a small hole in the hex-head of the bolt. Tighten bolt to proper-tension with Loctite, and locking washer. Insert safety-wire through hole in bolt-head and wrap it around the crank to keep it from spinning off.

Last edited by DannoXYZ; 02-21-06 at 03:29 PM.
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Old 02-21-06, 04:33 AM   #7
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So far every answer has been very good. The fact is that you said the cranks are 80's vintage. Did you buy them used? They might just need replacement as was mentioned. Your spending a lot of time and energy on 20 year old cranks that sound really worn out. Good luck

Tim
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Old 02-23-06, 12:48 PM   #8
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thanks a lot for the responses, sorry to bring it back up again after a couple days of inactivity...

so, yeah it looks like i'm gonna have to get new cranks...and i think i'll also be getting a torque wrench to use this time around...so when i install the new ones, approximately how tight should they be, 25 ft-lbs? 30? 35? more?
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Old 02-23-06, 01:45 PM   #9
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Just from mentally picturing how long my wrench is and how hard I torque crank arm bolts down, I would say 30 to 40 foot pounds is a healthy number.

The taper's job is to actually slightly deform and stretch the aluminum cranks around the spindle. It's a bit like putting on a snug-fitting sock compared to a loose one. You want the fixing bolts to be tight enough to properly stretch the tapers into place, but not so tight that the crank arm fractures from the stress or the bolt shears from tension. As long as you have reasonable hardware and have prepped it properly, you can really go to town on those bolts with no adverse effects. Problems with tapers are almost always a result of having the bolts too loose, which is easy to do when people are used to tightening fasteners on things like brake levers and water bottle cages. With crank arm bolts, you generally want to torque the bolt past "tight" until you feel it noticeably jam into place as the crank is stretched as far as it can over the spindle.

Make sure you clean all of the surfaces involved well to get dirt and crud out. Having any solid objects in the tapered joint will either damage the cranks or make it difficult to impossible to effectively tighten the bolts. There is some debate, but I would recommend lightly greasing the taper surfaces and the threads of the fixing bolt before assembling. Some people will tell you that the grease will make the taper wedge together too tightly, but this is not true. You need all the tightness you can get short of splitting the crank or snapping the bolt (both unlikely), so grease can only help you get there.

Another thing to note is that it is a good idea to hold the opposite crank while you are tightening the fixing bolts. If you use the crank that is being tightened to back up the wrench, you are theoretically pressing it onto the taper at a slight angle and may not get the best results. Reach through the frame and use the crank on the other side for wrenching so the force on the crank being tightened is purely inwards.
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