Thanks for the heads up Kurt. This is fascinating. I just went down and inspected mine too. What I found on mine is that there isn't any crack there, but if I look down from the top, I can find a subtle buckling in the sheetmetal, bowing inward toward the engine compartment on both sides. Just as everyone else here is showing, it's at the crumple zone closest to the firewall.
I'm running the ARB bumper with winch.
I have 3" of suspension lift and 35" tires.
I've carved crap out of the way to make the wheels clear all the way around, but in deep compression, I can still just get the top of the tire to kiss inside the well. It's a rare event, and I don't think that it imparts any force of significance.
Irrespective of bumper choice, the 4500 pound vehicle has all the ingredients necessary to flex the frame a bit when it gets into a twist. Although the body mounts are rubber, they still impart a force. If the bumper is brought up against the body, I'm sure it can impart even more force, but I don't think it's a deal breaker. The beauty of physics is that there is no "immovable object", there are just limits to the sensitivity of measuring tools.
This is flex fatigue. Note that the crack is happening ON TOP. Ask yourself how to apply a one-time force necessary to get the metal to TEAR at that location. It would have to be bent OVER something with down pressure in front and behind, while up pressure is applied beneath ... or some equivalent mechanism to get this to be a point of bending. That was the thinking in guessing about wheels hitting inside the well. If the body is supported in front and behind, and the wheel pushes up in the middle, that would explain it. Unfortunately there are crumple points that are much closer to the place over which the wheel touches, and if you push up inside the well at the point of contact, you can flex the actual well, without moving the stiffer support box above. The wheel wouldn't be able to do it.
That's NOT what is happening here. The twisting motion of the frame in offroad driving conditions is only partially being damped as it is transmitted to the body. The rest of the body has a lot more stiffness because of the height from floor to roof. The engine compartment is much flatter and therefore has less vertical stiffness. The body is getting "flexed" up and down (and probably in TWIST) just like the toe of a well broken in running shoe. This bending effect is amplified at the level where the low profile engine zone meets the high profile body zone. High flex meets low flex.
When metal is bent back and forth, it ANNEALS. This is a process of work-hardening that uses heat and compression to pack the crystal structures closer together. Annealing trades ductility for hardness. This is why medieval blacksmiths heated the steel bar red hot and hammered it thin before sharpening it into a sword. This is why bending coat hanger wire back and forth eventually warms it and then snaps it.
What you're seeing is an annealed flex point which eventually got too brittle to flex, and subsequently parted.
I think that the bumper may contribute to this to some degree, but my guess is that you've uncovered a design flaw that could become evident in any FJ after the right number of twisty trail runs. I'll guess that it happens sooner if the bumper design transmits more bending moment, but like the coat hanger... big bends or little bends... it eventually parts.
Ok... if that's the diagnosis, then what's the management?
My guess is that if you try to weld some sheet metal over that area, you'll just move the flex point to one of the welds and repeat the cracking process. You'd also de-engineer the crumple mechanism to some extent. I doubt it'll mean a thing if you actually wreck, but when your passenger is killed and their lawyer gets ahold of the investigator's report, you could be doing a lot of explaining. Realistically, for me, though... I doubt this would be a problem in the face of all the other massive offroad modifications I've got going on. When they get in underneath and see where I've actually cut and welded on structural points, I think the body issue would be pretty trivial.
My first thought was actually to go the other way. If this is where it's going to flex, let's not fight it, let's actually free it up to do what it has to do. I was thinking about actually completing the fracture by grinding out a quarter inch gap and then affixing a sandwich in the gap made of two pieces of sheet steel and a layer of urethane. However, I don't see that as being any better than the welded repair in the scheme of liability (which bothers me a little bit) and urethane has a habit of squeeking (which actually bothers me more). In the end, this seems like more work than it's worth!
Therefore, my plan will be to let it happen and then just leave it. If there has to be flexion at that point, then thats what it will be allowed to do. I looked at all the parts anchored onto the body forward of that location on both sides, and absolutely none of it would suffer. There's the radiator (which is attached to everthing else by flexible hoses) the battery (attached with wires) and some various other tubes and wires that I can't see having a problem with additional flex. The air system is rubber. My ARB compressor is on top of the wheel well, but its connected with wires and hoses.
If there's annoying flex and rattle after that, I'll weld on some supports and destroy the flexion point... and I'll just see what happens.
Homedadr, my bumper design is a lot narrower along the width of the vehicle than the ARB (which basically wraps around the front fenders as far as I can see). Mine stops leaving the approach angle open at the wheelwell. I want to check for rubbing but find no evidence of it, even with our crappy Toyota paint. Where in your opinion is the impact being made? Maybe photochop an arrow or something in your pic so I can think this over better. Thanks for your homework.
EDIT: Therefore, my plan will be to let it happen and then just leave it. If there has to be flexion at that point, then thats what it will be allowed to do. I looked at all the parts anchored onto the body forward of that location on both sides, and absolutely none of it would suffer. There's the radiator (which is attached to everthing else by flexible hoses) the battery (attached with wires) and some various other tubes and wires that I can't see having a problem with additional flex. The air system is rubber. My ARB compressor is on top of the wheel well, but its connected with wires and hoses.
If there's annoying flex and rattle after that, I'll weld on some supports and destroy the flexion point... and I'll just see what happens.
It's only body metal.
Sound reasoning as always. Also an excuse at some point to get cool fiberglass parts. Surely I'm too deep with this rig to worry about it now lol, as are you Bellydoc from your list of mods. I would like to say though, I have at least several thousand miles offroad in my FJ which I've been using hard in Moab and other rough spots for a year and a half and I have none of the pre-crack distortion you describe. I feel very lucky. I'm sure there are a number of factors here contributing to the annealation in that spot, which I agree to be happening.
Thanks for the heads up Kurt. This is fascinating. I just went down and inspected mine too. What I found on mine is that there isn't any crack there, but if I look down from the top, I can find a subtle buckling in the sheetmetal, bowing inward toward the engine compartment on both sides. Just as everyone else here is showing, it's at the crumple zone closest to the firewall.
I'm running the ARB bumper with winch.
I have 3" of suspension lift and 35" tires.
I've carved crap out of the way to make the wheels clear all the way around, but in deep compression, I can still just get the top of the tire to kiss inside the well. It's a rare event, and I don't think that it imparts any force of significance.
Irrespective of bumper choice, the 4500 pound vehicle has all the ingredients necessary to flex the frame a bit when it gets into a twist. Although the body mounts are rubber, they still impart a force. If the bumper is brought up against the body, I'm sure it can impart even more force, but I don't think it's a deal breaker. The beauty of physics is that there is no "immovable object", there are just limits to the sensitivity of measuring tools.
This is flex fatigue. Note that the crack is happening ON TOP. Ask yourself how to apply a one-time force necessary to get the metal to TEAR at that location. It would have to be bent OVER something with down pressure in front and behind, while up pressure is applied beneath ... or some equivalent mechanism to get this to be a point of bending. That was the thinking in guessing about wheels hitting inside the well. If the body is supported in front and behind, and the wheel pushes up in the middle, that would explain it. Unfortunately there are crumple points that are much closer to the place over which the wheel touches, and if you push up inside the well at the point of contact, you can flex the actual well, without moving the stiffer support box above. The wheel wouldn't be able to do it.
That's NOT what is happening here. The twisting motion of the frame in offroad driving conditions is only partially being damped as it is transmitted to the body. The rest of the body has a lot more stiffness because of the height from floor to roof. The engine compartment is much flatter and therefore has less vertical stiffness. The body is getting "flexed" up and down (and probably in TWIST) just like the toe of a well broken in running shoe. This bending effect is amplified at the level where the low profile engine zone meets the high profile body zone. High flex meets low flex.
When metal is bent back and forth, it ANNEALS. This is a process of work-hardening that uses heat and compression to pack the crystal structures closer together. Annealing trades ductility for hardness. This is why medieval blacksmiths heated the steel bar red hot and hammered it thin before sharpening it into a sword. This is why bending coat hanger wire back and forth eventually warms it and then snaps it.
What you're seeing is an annealed flex point which eventually got too brittle to flex, and subsequently parted.
I think that the bumper may contribute to this to some degree, but my guess is that you've uncovered a design flaw that could become evident in any FJ after the right number of twisty trail runs. I'll guess that it happens sooner if the bumper design transmits more bending moment, but like the coat hanger... big bends or little bends... it eventually parts.
Ok... if that's the diagnosis, then what's the management?
My guess is that if you try to weld some sheet metal over that area, you'll just move the flex point to one of the welds and repeat the cracking process. You'd also de-engineer the crumple mechanism to some extent. I doubt it'll mean a thing if you actually wreck, but when your passenger is killed and their lawyer gets ahold of the investigator's report, you could be doing a lot of explaining. Realistically, for me, though... I doubt this would be a problem in the face of all the other massive offroad modifications I've got going on. When they get in underneath and see where I've actually cut and welded on structural points, I think the body issue would be pretty trivial.
My first thought was actually to go the other way. If this is where it's going to flex, let's not fight it, let's actually free it up to do what it has to do. I was thinking about actually completing the fracture by grinding out a quarter inch gap and then affixing a sandwich in the gap made of two pieces of sheet steel and a layer of urethane. However, I don't see that as being any better than the welded repair in the scheme of liability (which bothers me a little bit) and urethane has a habit of squeeking (which actually bothers me more). In the end, this seems like more work than it's worth!
Therefore, my plan will be to let it happen and then just leave it. If there has to be flexion at that point, then thats what it will be allowed to do. I looked at all the parts anchored onto the body forward of that location on both sides, and absolutely none of it would suffer. There's the radiator (which is attached to everthing else by flexible hoses) the battery (attached with wires) and some various other tubes and wires that I can't see having a problem with additional flex. The air system is rubber. My ARB compressor is on top of the wheel well, but its connected with wires and hoses.
If there's annoying flex and rattle after that, I'll weld on some supports and destroy the flexion point... and I'll just see what happens.
It's only body metal.
BellyDoc,
Thanks for the thorough analysis. I agree in most parts with your statements.
The flexing causes the final cracking and fatigue of the metal sheet. This is for sure not caused by a single impact. But I still do believe that the tire causes the upward bucking of the rail. A flex would cause a rather constant torque load along the rail, rather than a point load at this crumble zone. Let me know what u think about this theory...
This is flex fatigue. Note that the crack is happening ON TOP. Ask yourself how to apply a one-time force necessary to get the metal to TEAR at that location. It would have to be bent OVER something with down pressure in front and behind, while up pressure is applied beneath ... or some equivalent mechanism to get this to be a point of bending. That was the thinking in guessing about wheels hitting inside the well. If the body is supported in front and behind, and the wheel pushes up in the middle, that would explain it. Unfortunately there are crumple points that are much closer to the place over which the wheel touches, and if you push up inside the well at the point of contact, you can flex the actual well, without moving the stiffer support box above. The wheel wouldn't be able to do it.
That's NOT what is happening here. The twisting motion of the frame in offroad driving conditions is only partially being damped as it is transmitted to the body. The rest of the body has a lot more stiffness because of the height from floor to roof. The engine compartment is much flatter and therefore has less vertical stiffness. The body is getting "flexed" up and down (and probably in TWIST) just like the toe of a well broken in running shoe. This bending effect is amplified at the level where the low profile engine zone meets the high profile body zone. High flex meets low flex.
This is what I've been thinking. It would be an interesting test to back off on the nuts on the forward most body mounts (near the bumper attach points) and measure if the front clip rises up at all (as measured near the front turn signals). This would indicate the kind of stress BellyDoc is talking about. This would be easy enough to check. I still believe it is caused by something out of kilter after installing lifts.
__________________
Grok FJ
"Every man is born as many men and dies as a single one."
Martin Heidegger
"Wars begin where you will
but they do not end where you please"
Take what man makes and use it,
But do not worship it,
For it shall pass.
If to do were as easy as to know what were good to do,
Chapels had been churches, and poor mans cottages, princes’ palaces.
I can't see anything through all that dirt. Do you think you could give that thing a bath and retake the pics???
Quote:
homedadr previously said:
ok pal...now I'm going to take fender photos...you have 13 posts here...I've been wheelin for many many many years....the guy said he did NO FLYING...I ASSume you are looking for a class action on this....Toyota did a damn good job on a first year run truck....I DO take mine flying and have NEVER bent a body part doing so...the friggin bumper is the problem...it needs room for FRAME FLEX!!!!!!
first photo.....nice smooth crush points
second photo...the stupid screw that DOES hit when flying...notice it's bent to hell on mine
third photo...body mounts NOT chopped...cause you don't need to with 285's even if you rock climb AND fly
I, too, have stress cracking in the exact same area, and it is growing quickly. Anyone with an aftermarket bumper (or who uses lights on the stock bumper) knows how much vibration is present in the body of the FJC when driving. My guess is this stress cracking is due to the excessive frontal vibration present in the vehicle due to extremely soft / tall body mounts designed for flexibility. While a stiffer suspension and additional weight on the frame may increase the vibration of the front end, I, in no way, believe it would be solely responsible for this type of stress cracking alone.
Quote:
BellyDoc previously said:
This is flex fatigue. The twisting motion of the frame in offroad driving conditions is only partially being damped as it is transmitted to the body. The rest of the body has a lot more stiffness because of the height from floor to roof. The engine compartment is much flatter and therefore has less vertical stiffness. The body is getting "flexed" up and down (and probably in TWIST) just like the toe of a well broken in running shoe. This bending effect is amplified at the level where the low profile engine zone meets the high profile body zone. High flex meets low flex. I think that the bumper may contribute to this to some degree, but my guess is that you've uncovered a design flaw that could become evident in any FJ after the right number of twisty trail runs.
I know a lot more about fighter jets than I do about trucks and I'm also (uhhh) drunk right now, but what CruiserLarry and BellyDoc said makes a lot of sense to me. Which means this will probably happen to every FJC that gets flexed a lot eventually, it's just that you guys and Air2Air reached the metal fatigue / breaking point before everyone else. (Hope you guys will pardon me for the partial quotes.)
the reason I came to the conclution of bumper problem is I've seen the Warn and I own the ARB...
1.the ARB gives lots of room for play...the first gen.Warn gives none...
2. mine..even with 1+ inches of play.. still pushes on the front body hard enough to cause noticable damage to the body and wheel wells(see photos)
3. the original poster said he rock crawls but does not fly
4. he also stated he has a Warn bumper with no play room
5. he also put his truck in flex on a ramp in the driveway...causing enough flex to push the bumper into the body...AND CLOSE the seem where it "ripped"
I do fly mine..I crawl...swim...I baja hard on washbourd trails and have smashed the wheel into the wells hard enough that if a problem was there ...it would surely have reared its ugly head by now...
by simple prosses of elimination I figure the bumper pushes on the sheet metal just enough..over time.. to tear the metal where it is designed to bend....
what I have never seen is a stock...even flying ... truck have this problem....it's always with a tight fitting bumper....
Hey Doc..your All-pro is pretty damn tight too ...is it not???
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