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# Torquing a crank bolt without a torque wrench.

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# Torquing a crank bolt without a torque wrench.

03-06-14, 03:16 PM
#26
SquidPuppet
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Originally Posted by fietsbob
yea your weight is reduced,and transferred to the wrench handle ,
so the % of your weight on the scale would be less..

then the length of the wrench handle in feet or inches or Meters
and your weight on the end of that handle in pounds, Kg.

deduced by how much less is on the Loo Scale ..
I make no claims to be able to perform the mathematical calculations necessary, cetainly not my field. But once a number (lbs) was arrived at, as well as the correct lever length, I think it would just be a matter of loading the lever while simultaneously montoring the unloading of the scale until acheiving the correct number.
03-06-14, 04:39 PM
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@Gnosis, thx for that, and to make it even easier, how about just keep it all in metric? For my own edification mostly, that would be like this:

68kg = 68kg*(9.81N/kg) = 667.08N

(it's 9.81 because that's the gravitational constant corresponding to earth's mass)

50 Nm / 667.08N = 0.07495m ~ 7.5cm

Or in my case, since I am almost exactly 100kg,

100kg * 9.81 N/kg = 981N

50Nm/981N = 0.50968m ~ 5.1cm

Or if my 100kg self needs T Nm of torque, I need to apply all my weight to a T/9.81 or approximately T/10 cm lever.

Or if your M kg self needs T Nm of torque, apply all your weight to a T/(M*9.81) meter, or approximately 10T/M centimeter lever.

(Note my example of 100kg, 50Nm works out again to 10(50)/100 = 5cm)

Also, for all the haters out there, with your "force is a vector" and "gravity is only down", just use a ratchet, and don't lean on the wrench handle when it's vertical, torque it up approximately horizontal, and let lim(x->0)sin(x)/x=1 take over. (i.e. pushing straight down on a nearly horizontal wrench handle (like within +/- 15 deg) makes no practical difference.

Last edited by CbadRider; 03-07-14 at 09:35 PM. Reason: Removed comment that violates forum guidelines
03-06-14, 04:41 PM
#28
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If the wrench is or can be made 1-foot long,you can hang the correct weight on it directly,that will get you in the ballpark.But if you don't have a scale.......

On paper,dialing your torque wrench to 30 pounds or a 1 foot wrench with 30 lbs on it is the same thing.

Last edited by Booger1; 03-06-14 at 04:48 PM.
03-06-14, 04:43 PM
#29
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Originally Posted by SquidPuppet
I make no claims to be able to perform the mathematical calculations necessary, cetainly not my field. But once a number (lbs) was arrived at, as well as the correct lever length, I think it would just be a matter of loading the lever while simultaneously montoring the unloading of the scale until acheiving the correct number.
Yes, you can do the same calculations as above and hang an object of a known weight (gallon of milk? can of paint? cinderblock? small child?) at the appropriate length along the lever arm. Hanging by a rope or bungee would also help to pin down the exact point where the force is applied.

03-06-14, 05:00 PM
#30
SquidPuppet
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Originally Posted by cny-bikeman
Problem is determining lever length, and I would think it would be easiest to push down on the wrench, which means you would check for the amount of decrease in the weight. .
That was my plan. :-)
03-06-14, 05:08 PM
#31
AnkleWork
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. . . Also, for all the haters out there, with your "force is a vector" and "gravity is only down", just use a ratchet, and don't lean on the wrench handle when it's vertical, torque it up approximately horizontal, and let lim(x->0)sin(x)/x=1 take over. (i.e. pushing straight down on a nearly horizontal wrench handle (like within +/- 15 deg) makes no practical difference.
Yes, the truth is frequently hateful and you have good reason to hate it.

Last edited by CbadRider; 03-07-14 at 09:36 PM. Reason: Edited quoted post
03-06-14, 05:21 PM
#32
j814wong
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Originally Posted by AnkleWork
Yes, the truth is frequently hateful and you have good reason to hate it.
Who wouldn't be hateful at gravity at times? It seeks to murder our legs on steep climbs. Then again, it also keeps us rollin'
03-06-14, 05:48 PM
#33
dweenk
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I have never used a torque wrench on a square taper crank. I tighten the crank down to the point where it seems stable and then I go for a ride. We are pretty flat around here, but there are a couple of short steep hills. I come back home and check the crank bolts. If they are too loose, I do it again. Rinse and repeat.
03-06-14, 06:32 PM
#34
02Giant
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I am extremely accurate at guesstimating 80 ft lbs with my 18" breaker bar...if that would ever do you any good.
03-07-14, 05:58 AM
#35
Homebrew01
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Originally Posted by hueyhoolihan
by all means try it, hate to see a good physics education going to waste. OTOH, i use the 1 grunt, 2 grunt, or occasionally the 3 grunt metric...
I use the exact same method .... It has been very reliable over the years. With experience, I now have fractional grunts.
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03-07-14, 06:45 AM
#36
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Although I entertained the torque workaround above I also have never used a torque wrench for a square taper. You would have been very challenged to find a torque wrench in most bike shops in the 20th century and almost none before the 90's (tension gauges showing up only slightly earlier). If you don't get them tight enough the cranks will click when ridden to tell you to wrench some more. The crank bolt wrenches are all roughly the same length for a reason. Long enough to get sufficient torque, short enough to not overdo it.
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There's no such thing as a routine repair.

Don't tell me what "should" be - either it is, it isn't, or do something about it.

If you think I'm being blunt take it as a compliment - if I thought you were too weak to handle the truth or a strong opinion I would not bother.

Please respect others by taking the time to post clearly so we can answer quickly. All lowercase and multiple typos makes for a hard read. Thanks!
03-07-14, 07:38 AM
#37
rydabent
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To do the job correctly, you need a torque wrench. As someone pointed out for \$10 you can get one at Harbor Freight. This is a case where the type of person that posts here, probably does a lot of his own maintence. It is also a case where a person can and should buy the necessary tools as they are needed.

Never, I say never trust a mechanic that tells you he is totally accurate "by feel".
03-07-14, 08:32 AM
#38
WK95
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Originally Posted by rydabent
To do the job correctly, you need a torque wrench. As someone pointed out for \$10 you can get one at Harbor Freight. This is a case where the type of person that posts here, probably does a lot of his own maintence. It is also a case where a person can and should buy the necessary tools as they are needed.

Never, I say never trust a mechanic that tells you he is totally accurate "by feel".
I read some reviews of it and some people said it would over or undertorque before making a clicking sound. Some even said that they found the inaccuracy to be more than the listed +14% which worries me.
03-07-14, 08:51 AM
#39
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Yes, the truth is frequently hateful and you have good reason to hate it.
Are you calling me fat? I would be offended, but (a) you said it cleverly, and (b) I am fat.

Well played, sir.
03-07-14, 09:17 AM
#40
cny-bikeman
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Originally Posted by rydabent
1. To do the job correctly, you need a torque wrench.

2. As someone pointed out for \$10 you can get one at Harbor Freight.

3. Never, I say never trust a mechanic that tells you he is totally accurate "by feel".
1. Agreed, and I did not say otherwise

2. ..and it's worth perhaps that much. Note the above post regarding a 14% error.

3. I would agree, but I've not met the mechanic who would make that claim.
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There's no such thing as a routine repair.

Don't tell me what "should" be - either it is, it isn't, or do something about it.

If you think I'm being blunt take it as a compliment - if I thought you were too weak to handle the truth or a strong opinion I would not bother.

Please respect others by taking the time to post clearly so we can answer quickly. All lowercase and multiple typos makes for a hard read. Thanks!
03-07-14, 09:24 AM
#41
Hendo252
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Originally Posted by cny-bikeman
Although I entertained the torque workaround above I also have never used a torque wrench for a square taper. You would have been very challenged to find a torque wrench in most bike shops in the 20th century and almost none before the 90's (tension gauges showing up only slightly earlier). If you don't get them tight enough the cranks will click when ridden to tell you to wrench some more. The crank bolt wrenches are all roughly the same length for a reason. Long enough to get sufficient torque, short enough to not overdo it.
Ditto.

on a bike, the bigger the bolt, the less you need a torque wrench. Crank bolts are one of the biggest bolts on your bike, so just have a go. It will be fine.

small screws are different... Like stem attach screws on a carbon fork... Where you could do some damage. Use a Ritchie torque key and you are good to go.

But square-taper crank bolts? Just make it Gud-n-tite and go ride.
03-07-14, 10:08 AM
#42
AnkleWork
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Are you calling me fat? I would be offended, but (a) you said it cleverly, and (b) I am fat.

Well played, sir.
Man, that's twisted. At least I didn't call you a hater . . . come to think of it, I didn't call you anything.
03-07-14, 12:20 PM
#43
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I've used a wrench and scale to torque fasteners in situations where I couldn't fit a torque wrench and didn't have a suitable adapter handy. It's also a good way to verify the calibration of torque wrenches. Measure the distance and the force being sure the force is applied perpendicular to the arm and in the plane of rotation. Do it if you're confident, don't if you're not.

In most cases, having a legit torque wrench does make it a lot easier though.
03-07-14, 12:42 PM
#44
cyclist2000
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I use a torque wrench on my square tapper bolts because I never seam to tighten them enough.
03-07-14, 01:34 PM
#45
rpenmanparker
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@Gnosis, thx for that, and to make it even easier, how about just keep it all in metric? For my own edification mostly, that would be like this:

68kg = 68kg*(9.81N/kg) = 667.08N

(it's 9.81 because that's the gravitational constant corresponding to earth's mass)

50 Nm / 667.08N = 0.07495m ~ 7.5cm

Or in my case, since I am almost exactly 100kg,

100kg * 9.81 N/kg = 981N

50Nm/981N = 0.50968m ~ 5.1cm

Or if my 100kg self needs T Nm of torque, I need to apply all my weight to a T/9.81 or approximately T/10 cm lever.

Or if your M kg self needs T Nm of torque, apply all your weight to a T/(M*9.81) meter, or approximately 10T/M centimeter lever.

(Note my example of 100kg, 50Nm works out again to 10(50)/100 = 5cm)

Also, for all the haters out there, with your "force is a vector" and "gravity is only down", just use a ratchet, and don't lean on the wrench handle when it's vertical, torque it up approximately horizontal, and let lim(x->0)sin(x)/x=1 take over. (i.e. pushing straight down on a nearly horizontal wrench handle (like within +/- 15 deg) makes no practical difference.
Basically you're right, but I would take small issue with expressing 68 kg as equal to 68 X 9.81 N/kg = 667 N. I think what you really mean is ma = F or kg X 9.81 m/sec2 = N. Not exactly the same thing. 1 kg generates a certain number of N when acceleration (gravity) is applied, but kilograms can't be equal to any number of Newtons. Nit-picky I know, but WTH.
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03-07-14, 02:44 PM
#46
Looigi
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1 kgF x 9.8N/KgF = 9.8 N

or

F= M x A = 1 kg x 9.8 (m/sec^2) = 9.8 (kg x m/sec^2) = 9.8 N
03-07-14, 03:44 PM
#47
Gnosis
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Originally Posted by rpenmanparker
Basically you're right, but I would take small issue with expressing 68 kg as equal to 68 X 9.81 N/kg = 667 N. I think what you really mean is ma = F or kg X 9.81 m/sec2 = N. Not exactly the same thing. 1 kg generates a certain number of N when acceleration (gravity) is applied, but kilograms can't be equal to any number of Newtons. Nit-picky I know, but WTH.
Robert, though you’re correct in stating F = ma, neither has RubeRad erred, as any impeded mass experiencing a rate of gravitational acceleration yields the object’s weight in Newtons of force just as force via the equation F = ma is also in Newtons via the commonplace SI (Standard International) units.

As concerns your presumption that the numerical value of kilograms cannot be equal to the numerical value of Newtons…

Newtons of force will precisely equal mass in kg when the rate of acceleration = 1 m/s/s:

F = ma

1 kg * 1 m/s/s = 1 Newton of force

Thus, RubeRad’s use of a 9.81 m/s/s/ rate of gravitational acceleration multiplied by a mass of 68 kg yields a force of 667.08 Newtons, which when converted to pounds of force is just under 150 pounds. This area of physics more often expresses the equation as follows:

W = mg

Whereby, Weight is in Newtons, mass is in kg, and gravity is in m/s/s, but it yields precisely the same outcome as writing F = ma, as the units are precisely the same for both cases, but ‘g’ is used (instead of ‘a’) so the rate of acceleration is known to be that of gravity.

When RudeRad stated he has a mass of 100 kg, his mass multiplied by the earth’s rate of gravitational acceleration yields a weight of 981 Newtons, which converted to pounds equals 220.6872237 pounds. This is long established basic physics.
03-07-14, 04:08 PM
#48
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oopx, I converted wrong. Turns out I am 112kg.

But also, the discussion is correct above that "68kg=...=667N" is sloppy handling of units. On the face of it, it is wrong simply because kg!=N. But numerically it turns out to be ok because the missing unit conversion factor has a numerical value of 1.
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