need input on shock hanger design (8 Viewers)

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eleblanc

eleblanc

Joined
Jul 4, 2006
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I'm working on something and need to ask your thought on a shock axle hanger like i designed here on the picture. I'm worried even if it is tight it could eventually rotate on the axle tube. Then i thought that if the U-bolt is fully threaded i doubt once tight it would rotate, since the therad would grip on the tube.

What you all think?
shock-1_resize.jpg
 
You got some mad skills boy!

I think it'll rotate since you won't be able to tighten the two nuts well enough. The flat piece of metal will keep bowing as you tighten the two nuts.
 
alia176 said:
You got some mad skills boy!

I think it'll rotate since you won't be able to tighten the two nuts well enough. The flat piece of metal will keep bowing as you tighten the two nuts.
Click to expand...

Yeah, and even with proper pressure it may eventually rotate, i'm going to scrap this design and think of something else.
 
Your design is great why not weld some corners on to the round axle and use a square U bolt.. it wont rotate then ......
Just a thought.....
 
I want to avoid welding! that is the catch!
 
I don't think that it's a tall order at all. Have a look at the lower mount on a late model Ford pick-up's rear axle. Copy that shape and tighten the snot out of the U-Bolt and you're golden!
Or just get some of those!
 
ntsqd said:
I don't think that it's a tall order at all. Have a look at the lower mount on a late model Ford pick-up's rear axle. Copy that shape and tighten the snot out of the U-Bolt and you're golden!
Or just get some of those!
Click to expand...

Picture?
 
eleblanc said:
Picture?
Click to expand...
Could not find a picture so I made a qwikie SW model & saved a jpg off that.
I have not heard of them rotating, but I can't claim a lot of experience with them either. It is important to consider the application. So they eventually rotate on the rear of a softly sprung 1/2ton pick-up when used hard. I don't see a less frequently used stiffly sprung 1/4t rated trailer working them nearly as hard.

No argument that welding them would be more secure, but as stated by the OP that isn't possible and if something like these are built robust enough I'm not convinced that it's needed.
shckmnt.jpg
 
I'd be more afraid the extra stud lenght on the u-bolt would just break-off from the presure..my opinion from use of u-bolts...
 
For the original design to work the U-Bolt will have to be at least a 1/2" and more likely a 5/8" U-Bolt (since those are the ID's of common shock bushings and their sleaves). While common u-bolts are not made from exceptional material, to shear off one that large would take a force in the range of catastrophic damage to such a light (even loaded) trailer. Necessarily trailers are stiffly sprung. Should the mount fail it's not a trip killer, just a nuisance.

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Looking at it from a pure numbers perspective:
I'm going to guess that the trailer is in the 2000 lbs. loaded range. As a rough dynamic RoT of 3X static weight, that means that the max dynamic loading that shock mount is likely to see is ~6000 lbs. That means that the max shear stress on the U-Bolt is:
1/2" = 30,558 psi
5/8" = 19,557 psi

If the U-bolt is assumed to be no better than SAE Grade 2 then the shear strength of the bolt is ~32,490 psi resulting is a Factor of Safety (FS) of 1.66 when using the 5/8" size. Not good enough, minimum FS should be at least 2.

If the U-bolt is SAE grade 5 then the shear strength is ~52,540 psi. That results in a FS of 1.7 for the 1/2" size, NFG, and a FS of 2.69 for the 5/8" size. That makes a G5 (or better) 5/8" U-bolt the only acceptable solution.

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However, I'll stand by what I said in the first paragraph and that is that generating that kind of load, given the other suspension features that are involved, but ignored for the calcs, will be catastrophic or nearly so to the rest of the trailer.
 
Crikey ! I take my hat off to you on your engineering maths. Did you factor in how many times that sucker will concertina in a off road situation. A lot of trailer suspensions fail when used on corrugated roadways.
 
To do true Fatigue Life calcs we need to know the actual loads, they're too much of a pain to do using guesstimates. As it is, the FS, in part, allows for some offset for fatigue.

Have to keep in mind what the trailer's total weight and intended use is, then build it just a wee tad stronger than what the most extreme use would require. Anything more stout usually means that it's working against itself because now it's own weight is a significant factor.
 
It is alot more complicated then that. first off yes we need to know the load but no, we cannot use the load + weight of the trailer into the calculation. The factors are; tire compression depending on tire and pressure in the tire. Spring rate of the leaf, and resistance rate of the shock. You assume the shock is totally rigid, that is correct when you will bottom the shock. The shock is a monroe and the bushing ID is 10 mm for a 3/8" pin. So i assume that the maximum resistance due the to shock will NOT shear or bent the pin, depending if monroe is assuming the pin is fix on both side or only one side. also the bushing will reduce dynamic of the load and will mostlikely prevent it from shearing but cause it to bend and assuming the assembly arounf the axle tube does not rotate.

Also in your analysis, you are not assuming there are 2 shocks (i know load will not be equal on both sides) and that the top and bottom pin will receive the same amount of loads.

But again this is useless has i am concerning about rotating the mount, in order to propely simulate this i need to find friction of the assemly around the tube when the 2 bolts and torque at a specific number and considering that my plate will bend. Once this is known, and the maximum resistance caused by the shock (without bottoming it) it is possible to calculate reaction tangientiel to the tube. With that and friction we can find out if it will rotate.

I didn't took time at all to run this into my FEA but will do.
 
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All of those variables mentioned would serve to reduce, in various different ways, the actual loading on the lower shock pin. I intentionally ignored them and constructed a simple worst case model such that my calcs were conservative.

I would rather build and try it than do FEA on it. Unless FEA is a regular part of your daily job GIGO comes to mind. The one thing that I might do different on your original model would be to use a coupler nut instead of the two nuts.
 
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