Quote:
KD7NAC_07FJ previously said:
Post 490 and 49 pages - whew - made it finally. Edit: 492 & 50 by the time I got it posted geesh
First I have no budges or cracks, moderate wheeling, slow & steady - only as fast as necessary.
After reading the first 20 some pages I began to see a pattern with the lifted rigs. From what I can tell the vast majority have after market bumpers with 886 (or equivalent heavy) front springs.
Totally unrelated to the bulging (not knowing about it) but because of one curve I drive around ever day from work I swapped out my 886's with 885's to eliminate some tire hop issues. Now the whole vehicle responds instead or the front end feeling like a pogo stick.
I have a Road Armor bumper with Warn 9.5 winch and I have too small scrapes at either corner where the bumper meets the fender. If the flexing of the frame/body/bumper combination were causing enough stress to budge and crack the structure I would expect to see some deformation of the fender in that area on any vehicle. So far no-one has mentioned any and mine is only paint scraping which can be resolved by cutting back that corner.
Also you should see a crack in the lower lip of the fender but I haven’t seen any pics of that area from any affected rigs. If someone can take some that would help.
I borrowed someone's pic to illustrate some directional forces that are occurring.
At first glance this is a compression of the structure. There could be some extension pointing to cyclic stress but the only thing I can think of that would cause this activity would be bouncing across a washboard surface which would repetitively flex the front clip independent of the rest of the structure. Whether the duration is long or short I cannot determine. But again there should be equal cracks elsewhere. Whenever we find a crack in an airplane we look for its mirror (in the structure not side to side).
I am going to get one of my AOG engineers to look at these pictures to see if he can give an idea where these forces are coming from. So far I haven't seen anything that definitively points to the cause. I will be doing some forklift flexing at the hanger tonight to see if I can determine any other contact/stress points but I’m not leaning toward a flexing problem but more of the repetitive annealing issue BellyDoc is referring to but I'm not a stress engineer. I'll post up what he has to say. |
With respect, I disagree with your analysis of the loading along this rib. Although the rib was DESIGNED to accept a compression (and dissipate the energy by blowing out in the lateral direction), I do not believe that this is what is happening.
The body mounts to the frame at multiple locations. There is a mounting point behind the front wheel well and another in front. There are multiple other mounting points, but because the body is so LOW PROFILE through the hood region compared to the rest of the cab, this region can be modelled as if it were a cantilevered beam. The fixed base of the beam is the cab section. The beam is being pushed up and down at it's end by the forward body mount. Like this:
When the frame flexes upward on one side, it quickly compresses the rubber bushing of the body mount which is only 1.5" tall (and becomes stiff with much less than 1" of travel) and then it deflects the fender panel on that side upward. On the other side, the frame can flex down away from the body without pulling it because the bushing BELOW is much longer and more compressible. This is why people can measure the twist of the bumper relative to the grill!!!
The top of the flexed body panel is crushed into compression. The bottom is stretched. This changes the conformation of the crumple zone folds and hardens the metal. The only place for it to go is lateral. Then later it gets too stiff for further flex, and cracks.
There is no compressional load. That would require a front end collision and air bags would deploy.
