I was curious , and need some help . Can anyone tell me how much weight would it take to start deflecting 3.125 x ,375 D.O.M. V/S 3.125 x .500. D.O.M tubing. In other words just how much stronger is the 1/2" wall versus the 3/8's wall with a given length of 30"


Thanks Chris:D

that's some beefy shit man... what are you going to use that on?

TTT:D

it sounds like you are making axle tubes, i would guess that the weight difference for that length couldn't be more than 5 to 10 pounds, on an already heavy axle, is 5-10 pounds worth the piece of mind?

Originally posted by heep86
it sounds like you are making axle tubes, i would guess that the weight difference for that length couldn't be more than 5 to 10 pounds, on an already heavy axle, is 5-10 pounds worth the piece of mind?

Hey newbie, read the post, and don't answer if you do not have the correct answer.
He does not ask about the weight of the tubing, he is asking how much weight before the tubing starts to deflect.

well, with some simple math i can tell you that there will be 27.4% more material with the .500 wall. I'm not sure that that translates into 27.4% more strength though.

hope that helps!

Originally posted by TechGuru
I was curious , and need some help . Can anyone tell me how much weight would it take to start deflecting 3.125 x ,375 D.O.M. V/S 3.125 x .500. D.O.M tubing. In other words just how much stronger is the 1/2" wall versus the 3/8's wall with a given length of 30"


Thanks Chris:D

If you pulled that .375 wall from my post about getting a larger OD tube. I messed up. I meant to say something like .625-.700 wall and 3.25 tube. so you could turn the tube down to fit the housing without loosing strength from your original want of .500 tube wall.

for any tube, you can figure all this out yourself.
all you have to do is figure out the Moment of Inertia of the section.
take the outer diameter of the tube (d) raise it to the 4th power, and multiply by .049. Do the same with the inner diameter. subtract the number you got for the inner diameter from the number you got from the outer diameter and that's your Moment of Inertia. (I)

basicially: .049*OD^4 - .049*ID^4 = I

looke up the yield strength (called Fy) and the Elasticity (called E) for the steel the tube is made from.

I always multiply Fy by 0.55 as a saftey factor.

then figure out your load that you will be placing on the tube (P)
and how far out from the support you plan on putting that load (L)

once you figure out what your load is, multiply it by 1.33 to account for the additional force of impact.

wanna know how much weight your tume can handle?

it's just: 2*I/OD*Fy/L

wanna know how much it deflects?

it's just: P*L/(E*I)

that's pretty easy huh? :)


somebody check my numbers, it's early and I'm hung over.

Based on it's moment of inertia, the thicker wall tubing is 18% stronger and 18% stiffer in bending, and is 27% heavier.

Gawd damn I thought I was good at this shit, uh I need to go back to remedial physics :

Sundowner, how did you derive the .49 and a saftey factor of .55 in other words where did these # come from? The basis it seems good for determing the overall given strength of a given tube inner and outer tube but how about a specific material such as this formula with the difference of EREW v/s D.O.M v/s various grades of cro moly? Obviously the cro moly would be more resistant to deflection but how? (E)

Twadler , nope just trying to figure some math here

Donovan Thanks for making it somewhat simpler but what does that transfer into ft. lbs to the moment of elasticity the 18%

Some are going to say why who cares about .125 diff. , its just that I am turning twards building axles to what I need at a given weight with the same strength , to most its stupid but I would like to know why and how much?:D

the .49 is an idealized number.
the actual equasion is 1/4*Pi*r^4
work in the diameter instead of the radius and you get .49*d^4

.55 is the allowable design stress in steel used for the constuction of highway bridges as specified in AASHTO design manuals since the slide rule. I use it as a matter of course, and I haven't killed anyone yet!:D

as far as the material properties, the distributor should know that, or you can look it up in American Standards and Testing for Materials texts (ASTM)

Originally posted by TechGuru

Sundowner, how did you derive the .49 and a saftey factor of .55 in other words where did these # come from? The basis it seems good for determing the overall given strength of a given tube inner and outer tube but how about a specific material such as this formula with the difference of EREW v/s D.O.M v/s various grades of cro moly?Obviously the cro moly would be more resistant to deflection but how? (E)



The different grades of steel have nothing to do with how much the tube deflects because they have the same modulus of elasticity (E). The physical dimensions determine the stiffness of the tube and thus how much deflection there is at a given load. The material choise just determines how far the tube will bend before it yields and takes a permanent set or failure

now what about the same thickness tube, just different size?


1.75 vs 2.0????

Originally posted by Sundowner
for any tube, you can figure all this out yourself.
all you have to do is figure out the Moment of Inertia of the section.
take the outer diameter of the tube (d) raise it to the 4th power, and multiply by .049. Do the same with the inner diameter. subtract the number you got for the inner diameter from the number you got from the outer diameter and that's your Moment of Inertia. (I)

basicially: .049*OD^4 - .049*ID^4 = I

looke up the yield strength (called Fy) and the Elasticity (called E) for the steel the tube is made from.

I always multiply Fy by 0.55 as a saftey factor.

then figure out your load that you will be placing on the tube (P)
and how far out from the support you plan on putting that load (L)

once you figure out what your load is, multiply it by 1.33 to account for the additional force of impact.

wanna know how much weight your tume can handle?

it's just: 2*I/OD*Fy/L

wanna know how much it deflects?

it's just: P*L/(E*I)

that's pretty easy huh? :)


somebody check my numbers, it's early and I'm hung over.

ya what he said. i want that guy on my team.

Originally posted by RudeZuk
now what about the same thickness tube, just different size?


1.75 vs 2.0????

If you are comparing 2.0" x .120 wall to 1.75" x .120 wall, the 2" tubing will be 53% stronger and stiffer. Changing the diameter makes more of a difference than changing the wall thickness.