Crank arm coming loose?
10-09-18, 02:50 PM
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Crank arm coming loose?
Hi,
I'll give a little backstory before I get to the issue. In the hope of losing weight I have come back to my former hobby, cycling, and since I'm pretty broke I invested some money into a used MTB and decided it would be alright for some basic commuting and some longer trips. I'm not into any serious mountain biking or anything like that, I just figured it was a decent price.
Straight away I started having issues with the crank arms and after buying new crank arms and trying to fix it up, having to deal with incompetent dealers that had no clue I then decided to buy a whole new bottom bracket + crankset. I went for a BB-ES 300 BB and Shimano Alivio Octalink crankset, figuring it would be better since it had steel integrated crank arms and was built sturdier. I put it all together, carefully greasing the right parts and following a thorough guide. It's been running fine but after some trips out on the bike I felt the non-drive side crank arm give, it isn't much but it was enough to make me worried. I think I might not have tightened it enough, and as I don't own a torque wrench I have to admit I'm unsure how much is too much, I don't want to overtighten. I know a lot of people would probably just say "Buy a torque wrench" but I don't have the money right now, and I'm honestly fed up of how complicated things have become. I find myself more worried about the cranks than anything else, it's ruining my cycling experience - which I normally enjoy immensely.
I have seen some videos of people measuring torque using fish scales and suitcase scales - could that work? I have been thinking of doing that. I'll take the crank arms off tomorrow and inspect, I hope it won't be ruined after such a short time. I just bought the whole thing.
Thanks in advance from a tired bike newbie.
I'll give a little backstory before I get to the issue. In the hope of losing weight I have come back to my former hobby, cycling, and since I'm pretty broke I invested some money into a used MTB and decided it would be alright for some basic commuting and some longer trips. I'm not into any serious mountain biking or anything like that, I just figured it was a decent price.
Straight away I started having issues with the crank arms and after buying new crank arms and trying to fix it up, having to deal with incompetent dealers that had no clue I then decided to buy a whole new bottom bracket + crankset. I went for a BB-ES 300 BB and Shimano Alivio Octalink crankset, figuring it would be better since it had steel integrated crank arms and was built sturdier. I put it all together, carefully greasing the right parts and following a thorough guide. It's been running fine but after some trips out on the bike I felt the non-drive side crank arm give, it isn't much but it was enough to make me worried. I think I might not have tightened it enough, and as I don't own a torque wrench I have to admit I'm unsure how much is too much, I don't want to overtighten. I know a lot of people would probably just say "Buy a torque wrench" but I don't have the money right now, and I'm honestly fed up of how complicated things have become. I find myself more worried about the cranks than anything else, it's ruining my cycling experience - which I normally enjoy immensely.
I have seen some videos of people measuring torque using fish scales and suitcase scales - could that work? I have been thinking of doing that. I'll take the crank arms off tomorrow and inspect, I hope it won't be ruined after such a short time. I just bought the whole thing.
Thanks in advance from a tired bike newbie.
10-09-18, 03:02 PM
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It is quite possible the crank was not fully seated in the splines and loosened because of this. An inspection of the splines on the crank
should show if this was the case: the splines will be partly 'boogered up'. If so just take a knife and carefully trim off any loose shards
of metal and next time tighten them more firmly than you think necessary and then some. I have had this happen in the octalink era.
Also had it happen with 8-9-10 spd cassette lock rings, until I got a torque wrench.
Unlike the era of sq taper cranks and dodgy metallurgy in the crank fixing bolts, when it was possible to pop the head off the fixing
bolt, Shimano fixing bolts are stout enough to take any practical torque you can place on them. But a scale approach can give you
an idea of where you are at torque wise which should be in the 30-35 ft lb range.
should show if this was the case: the splines will be partly 'boogered up'. If so just take a knife and carefully trim off any loose shards
of metal and next time tighten them more firmly than you think necessary and then some. I have had this happen in the octalink era.
Also had it happen with 8-9-10 spd cassette lock rings, until I got a torque wrench.
Unlike the era of sq taper cranks and dodgy metallurgy in the crank fixing bolts, when it was possible to pop the head off the fixing
bolt, Shimano fixing bolts are stout enough to take any practical torque you can place on them. But a scale approach can give you
an idea of where you are at torque wise which should be in the 30-35 ft lb range.
Last edited by sch; 10-09-18 at 03:09 PM.
10-09-18, 03:39 PM
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It is quite possible the crank was not fully seated in the splines and loosened because of this. An inspection of the splines on the crank
should show if this was the case: the splines will be partly 'boogered up'. If so just take a knife and carefully trim off any loose shards
of metal and next time tighten them more firmly than you think necessary and then some. I have had this happen in the octalink era.
Also had it happen with 8-9-10 spd cassette lock rings, until I got a torque wrench.
Unlike the era of sq taper cranks and dodgy metallurgy in the crank fixing bolts, when it was possible to pop the head off the fixing
bolt, Shimano fixing bolts are stout enough to take any practical torque you can place on them. But a scale approach can give you
an idea of where you are at torque wise which should be in the 30-35 ft lb range.
should show if this was the case: the splines will be partly 'boogered up'. If so just take a knife and carefully trim off any loose shards
of metal and next time tighten them more firmly than you think necessary and then some. I have had this happen in the octalink era.
Also had it happen with 8-9-10 spd cassette lock rings, until I got a torque wrench.
Unlike the era of sq taper cranks and dodgy metallurgy in the crank fixing bolts, when it was possible to pop the head off the fixing
bolt, Shimano fixing bolts are stout enough to take any practical torque you can place on them. But a scale approach can give you
an idea of where you are at torque wise which should be in the 30-35 ft lb range.
10-09-18, 03:45 PM
#4
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Octalink has a depth to the splines , so the arm bottoms out.. maybe bring an 8mm allen wrench
in the tool kit so to not leave them too loose, too long to the point of damaging the arm splines irreparably..
hopefully that has not yet happened.. it could..
in the tool kit so to not leave them too loose, too long to the point of damaging the arm splines irreparably..
hopefully that has not yet happened.. it could..
10-09-18, 03:48 PM
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Sorry to hear that you have more then one incompetent dealer in your area, maybe if we knew where you live one of us might have a recommendation for an experienced and motivated shop.
Crank arm tightening has quite a history here on this (and other) forum. Some searching in the archives should give you vastly more opinion. My opinion is to install the crank arms with correct torque and to revisit the tightness after an initial few rides. It is this second step that, IME, is not done and leads to the left arm (nearly always just the left) coming loose. Once the arm is ridden when loose the damage is usually done and the arm is likely to never stay tight even with periodic checking. I explain it as this. The arm is generally made of aluminum, which is softer then the steel BB axle. (And every Alivio crank set I have worked on/touched is aluminum, not steel). So when the arm is allowed to wiggle on the axle the axle acts as a auger and opens up the arm's hole (that the axle fits within). Once the hole is distorted the slightest amount the arm will most likely continue to wiggle on the axle. IIRC the torque level for the retaining bolts is between 30 and 36 ft lbs. Most consider this to be a pretty high amount of force when using a "L" hex wrench, especially if it's only 6" long.
One way to set a crank arm retaining bolt to the right range of tightness is to use a shop with a torque wrench for only checking/tightening your cranks. Many will do this for free (if you bought the crank from them) or very little cost. It's in the shop's best interest to be good to you, someone who in time might very well want a nicer bike. Andy
Crank arm tightening has quite a history here on this (and other) forum. Some searching in the archives should give you vastly more opinion. My opinion is to install the crank arms with correct torque and to revisit the tightness after an initial few rides. It is this second step that, IME, is not done and leads to the left arm (nearly always just the left) coming loose. Once the arm is ridden when loose the damage is usually done and the arm is likely to never stay tight even with periodic checking. I explain it as this. The arm is generally made of aluminum, which is softer then the steel BB axle. (And every Alivio crank set I have worked on/touched is aluminum, not steel). So when the arm is allowed to wiggle on the axle the axle acts as a auger and opens up the arm's hole (that the axle fits within). Once the hole is distorted the slightest amount the arm will most likely continue to wiggle on the axle. IIRC the torque level for the retaining bolts is between 30 and 36 ft lbs. Most consider this to be a pretty high amount of force when using a "L" hex wrench, especially if it's only 6" long.
One way to set a crank arm retaining bolt to the right range of tightness is to use a shop with a torque wrench for only checking/tightening your cranks. Many will do this for free (if you bought the crank from them) or very little cost. It's in the shop's best interest to be good to you, someone who in time might very well want a nicer bike. Andy
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10-09-18, 04:29 PM
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Sorry to hear that you have more then one incompetent dealer in your area, maybe if we knew where you live one of us might have a recommendation for an experienced and motivated shop.
Crank arm tightening has quite a history here on this (and other) forum. Some searching in the archives should give you vastly more opinion. My opinion is to install the crank arms with correct torque and to revisit the tightness after an initial few rides. It is this second step that, IME, is not done and leads to the left arm (nearly always just the left) coming loose. Once the arm is ridden when loose the damage is usually done and the arm is likely to never stay tight even with periodic checking. I explain it as this. The arm is generally made of aluminum, which is softer then the steel BB axle. (And every Alivio crank set I have worked on/touched is aluminum, not steel). So when the arm is allowed to wiggle on the axle the axle acts as a auger and opens up the arm's hole (that the axle fits within). Once the hole is distorted the slightest amount the arm will most likely continue to wiggle on the axle. IIRC the torque level for the retaining bolts is between 30 and 36 ft lbs. Most consider this to be a pretty high amount of force when using a "L" hex wrench, especially if it's only 6" long.
One way to set a crank arm retaining bolt to the right range of tightness is to use a shop with a torque wrench for only checking/tightening your cranks. Many will do this for free (if you bought the crank from them) or very little cost. It's in the shop's best interest to be good to you, someone who in time might very well want a nicer bike. Andy
Crank arm tightening has quite a history here on this (and other) forum. Some searching in the archives should give you vastly more opinion. My opinion is to install the crank arms with correct torque and to revisit the tightness after an initial few rides. It is this second step that, IME, is not done and leads to the left arm (nearly always just the left) coming loose. Once the arm is ridden when loose the damage is usually done and the arm is likely to never stay tight even with periodic checking. I explain it as this. The arm is generally made of aluminum, which is softer then the steel BB axle. (And every Alivio crank set I have worked on/touched is aluminum, not steel). So when the arm is allowed to wiggle on the axle the axle acts as a auger and opens up the arm's hole (that the axle fits within). Once the hole is distorted the slightest amount the arm will most likely continue to wiggle on the axle. IIRC the torque level for the retaining bolts is between 30 and 36 ft lbs. Most consider this to be a pretty high amount of force when using a "L" hex wrench, especially if it's only 6" long.
One way to set a crank arm retaining bolt to the right range of tightness is to use a shop with a torque wrench for only checking/tightening your cranks. Many will do this for free (if you bought the crank from them) or very little cost. It's in the shop's best interest to be good to you, someone who in time might very well want a nicer bike. Andy
10-09-18, 04:34 PM
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I was worried putting too much force. Had been reading too much online and people were quite particular, I find using a long pipe with my allen key allows me to provide a substantial amount of force. I brought a regular ratchet with me and tightened up within a few minutes of feeling any looseness.
10-09-18, 05:13 PM
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10-10-18, 10:14 AM
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One 'fairly simple' approach might be to use a scale designed to be stood on to weigh people, place under crank, use your
pipe+8mm allen wrench in a horizontal position and use a large enough piece of wood of the right length between the 30cm
mark on the pipe and the scale and as you press down on the pipe at the 30cm mark you can read the weight directly and
hence the applied torque. A 7-10cm sq piece of wood should be stable enough, and a small V in the top will cradle the pipe.
Of course you will have to apply counter torque to the crank, which can be a problem depending on the crank/pipe angle.
pipe+8mm allen wrench in a horizontal position and use a large enough piece of wood of the right length between the 30cm
mark on the pipe and the scale and as you press down on the pipe at the 30cm mark you can read the weight directly and
hence the applied torque. A 7-10cm sq piece of wood should be stable enough, and a small V in the top will cradle the pipe.
Of course you will have to apply counter torque to the crank, which can be a problem depending on the crank/pipe angle.
Last edited by sch; 10-10-18 at 10:18 AM.
10-10-18, 04:17 PM
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Apparently I can only post 5 times every 24 hours.
I decided that it's all too complicated for me. I went ahead and bought myself a torque wrench, I found a fairly decent deal on a 1/2" that goes from 28 to 110nm I believe it is. They didn't have the right right Allen key socket/set thing so I had them order one that should arrive tomorrow. A lot of people on here say I'd have to go for at least 35ft lb, so I imagine I'd have to set it at 50NM? Anyways. I took the crank arm off and I'd say it looks like there's no real damage. I'll try to post some pictures. Live and learn, I guess.
I decided that it's all too complicated for me. I went ahead and bought myself a torque wrench, I found a fairly decent deal on a 1/2" that goes from 28 to 110nm I believe it is. They didn't have the right right Allen key socket/set thing so I had them order one that should arrive tomorrow. A lot of people on here say I'd have to go for at least 35ft lb, so I imagine I'd have to set it at 50NM? Anyways. I took the crank arm off and I'd say it looks like there's no real damage. I'll try to post some pictures. Live and learn, I guess.
10-11-18, 06:12 AM
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The torque spec for crank arms is around 30 ft/lbs. That's a lot compared to other bicycle fasteners, That's more than you are likely to produce if you are using a 6 in. Allen key. Since you don't have a torque wrench, imagine a 30 lb. weight hanging on the end of a foot long wrench.
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10-11-18, 08:19 AM
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The torque spec for crank arms is around 30 ft/lbs. That's a lot compared to other bicycle fasteners, That's more than you are likely to produce if you are using a 6 in. Allen key. Since you don't have a torque wrench, imagine a 30 lb. weight hanging on the end of a foot long wrench.
