Articulated Chassis Flex? (1 Viewer)

[C6H12O6]

My last trip out, I climbed up onto a stump to see how flexy things would get and I jumped out of the rig to take a look (safe?) and just had to open the back upper gate. Ok, I needed batteries for the camera. I noticed the gate didn't seem to be lined up perfectly while the rig was flexed up. It still opened and closed, but the latch dragged quite a bit. Normal? Probably normal but stupid to attempt?

Everything appears to be normal now that I'm back on level ground. I'm just a little worried about body flex after the new rear quarter panel.

 

[LexissLX470]

That is perfectly normal, try to jack one end of your car in the air, or drive one tire up a ramp and the chassis actually twists and bends under the pressure, your doors, trunk, hood shouldn’t align bc he car is supposed to flex in that situation. If ur rig was soo rigid that you had no flex, it would literally be like driving a Shoe or a Brick.... no good for any on or off road activity.... Now mods like sway bars, strut bars ect are built to give cars less body roll and flexibility to improve handleing, but for what we want out of our trucks u want a lil flex here and there.

 

[OTEP]

Yep, that's normal. Also happens to other rigs. Don't worry much about it.

 

[C6H12O6]

Thanks. I thought it was ok, and everybody I was with didn't seem to be concerned in the least, but I'm still a little spooked from that ridiculous repair.

 

[Kalawang]

Interesting that it's a normal thing. I've felt mine flex once or twice and had made a note to do something about it in the future if I ever get through the more urgent fixes.

I would have expected that body flex was a bad thing. Would anybody care to explain why the body should be allowed to twist under some conditions ?


Kalawang

 

[Hayes]

Definitely not my area of expertise, but I would expect that a truly rigid frame would be more difficult to engineer and builld.

I would also speculate that any weak links in the system would encounter more stress if everything was perfectly rigid--the flexing distributes the stress throughout the frame and chassis?

Talking out of my butt here.

Hayes

 

[OTEP]

IF things didn't flex, they'd just get bent or break.

 

[CreeperSleeper]

Thanks. I thought it was ok, and everybody I was with didn't seem to be concerned in the least, but I'm still a little spooked from that ridiculous repair.

Oh, second guessing me I see... You should be glad I didn't look at you with a dead straight face and say, "Wow, that sucks... time to bring it back to the bodyshop." or "Just follow that kid with the solid axle swap in his mini-truck... that root on the passenger side will fix it.";p

 

[DirtyHarry]

If ur rig was soo rigid that you had no flex, it would literally be like driving a Shoe or a Brick.... no good for any on or off road activity....

Not exactly. The suspension is supposed to handle the flexing, not the chassis. Some rigs are designed to have chassis flex as part of the suspension (the Unimog comes to mind) but typically this is something to be avoided. Some will still occur since the body is mounted to the frame with bushings and there is some chassis flex and bushing give with that much weight twisted up, but ideally the suspension would handle all of the articulation, not the frame.

At least 80s are a lot better about this than Ford pickups.

 

[jcolegrove]

concur w/ dirty, which is why on every new car or truck introduction, it is much hyped about X% increase in torsional rigidity or other measure of stiffness, like degree twist per 1,000 pounds, etc.. it's a GOOD thing.

more stiffness allows suspension to be calibrated more precisely since the pickup points don't move.
also makes for a better ride, in that secondary quivers are minimized when hitting a bump
also can actually make for a smoother ride in that the suspension can be calibrated more softly since it is the only thing giving, rather than compensating for the give in the chassis.
it's not true if it doesn't flex it'll break-- c'mon! the weight of the vehicle is nowhere near enough to make something 'break'-- all that would happen is the suspension bottoms, and then the vehicle lifts on whatever side is fully compressed.

with that said, the flex is normal and happens on all cars, just depends on what degree.

 

[Nomis]

Chassis flex iis probably a factor in the cracking damage on our C Pillars:
https://forum.ih8mud.com/showthread.php?t=42740&highlight=pillar+crack

 

[Kalawang]

The comments of Jcolegrove and dirty harry as well as of Hayes lead me to the tentative posture that body flex may be a necessity for comfort. I'll accept that body flex is more likely to be more apparent in LWB vehicles than SWB's such as the 40 series and the army jeep. Neither of those are all that much fun on long trips but are very reassuring off road.

Would it therefore be more correct to say that body flex is a natural result of LWB and any engineering would be more to control the degree of flex as against the loss of comfort ?


Kalawang

 

[Rusty Phillips]

Not exactly. The suspension is supposed to handle the flexing, not the chassis. Some rigs are designed to have chassis flex as part of the suspension (the Unimog comes to mind) but typically this is something to be avoided. Some will still occur since the body is mounted to the frame with bushings and there is some chassis flex and bushing give with that much weight twisted up, but ideally the suspension would handle all of the articulation, not the frame.

At least 80s are a lot better about this than Ford pickups.

that picture gives me the shivers

 

[C6H12O6]

Oh, second guessing me I see... You should be glad I didn't look at you with a dead straight face and say, "Wow, that sucks... time to bring it back to the bodyshop." or "Just follow that kid with the solid axle swap in his mini-truck... that root on the passenger side will fix it.";p

I wasn't really second guessing you, exactly. It's just that nobody really said much about it. I wasn't totally sure if that was because nobody wanted to be the one to tell me my new quarter panel was about to sheer off like that pink plastic panel that keeps popping off my Barbie Jeep.

 

[Nomis]

that picture gives me the shivers

Pictures of Fords have a tendency to do that.

 

[jcolegrove]

kalawang....you're correct in posturing that all else being equal, LWB vehicle will have a more flexible chassis. can be demonstrated with any long and short pencil, steel rod, peice of wood, etc...
it is also true that it is more difficult to build a body on frame vehicle to be stiff than a unibody of same dimensions. again, think of bending a welded metal box vs. a welded metal flat frame. this is why unibody vehicles are inherently stiffer-- that is not to say 'tougher' or stronger, just stiffer. another comparison-- an airplane- incredibly stiff in bending resistance one way, but try to bend it in another, it's very very weak and crumbles. it's not 'tough' or 'strong', but it is stiff.
i assume your thought of comfort gains are related to off-roading, where any additional chassis flex is just that much more articulation and wheel travel?
certainly, there are no comfort gains on roads and flex actually decreases on road comfort.

 

[CreeperSleeper]

I wasn't really second guessing you, exactly. It's just that nobody really said much about it. I wasn't totally sure if that was because nobody wanted to be the one to tell me my new quarter panel was about to sheer off like that pink plastic panel that keeps popping off my Barbie Jeep.

I was just messing with you... I didn't say much because I didn't think of it as a big deal, but then again, I used to wheel with Cherokee's. When they flex NONE of the doors open!

 

[Kalawang]

kalawang....you're correct in posturing that all else being equal, LWB vehicle will have a more flexible chassis. can be demonstrated with any long and short pencil, steel rod, peice of wood, etc...
it is also true that it is more difficult to build a body on frame vehicle to be stiff than a unibody of same dimensions. again, think of bending a welded metal box vs. a welded metal flat frame. this is why unibody vehicles are inherently stiffer-- that is not to say 'tougher' or stronger, just stiffer. another comparison-- an airplane- incredibly stiff in bending resistance one way, but try to bend it in another, it's very very weak and crumbles. it's not 'tough' or 'strong', but it is stiff.
i assume your thought of comfort gains are related to off-roading, where any additional chassis flex is just that much more articulation and wheel travel?
certainly, there are no comfort gains on roads and flex actually decreases on road comfort.

My thoughts on comfort gains were actually more concerned with passenger comfort on fast travel through unpaved twisty roads.... and also paved. Driver's comfort is pretty much a given as he has the steering wheel to hold on to and can anticipate the vehicle movements. Likewise with front passengers who have a grab bar as well as a clear view of the road being traveled. Rear seat passengers in a 40 series tend to be thrown against the ceiling though, and in the case of the 80, they get tossed from side to side and tend to divest themselves of recently consumed food. I've had two victims of this and it does tend to make the side of my 80 unpleasant.

I suppose I'll just have to drive more slowly.


Kalawang

 

[PHAEDRUS]

You guys are forgetting one very important thing to remember.
Steel in any form is just a super cooled liquid, it is physically enpwered to flex and most of the time return to its original position.
There is a reason however you dont see any 80's running around without doors on the trail.
the realignment of the door back to the veh afterwards could be interesting.
Dave

 

[DirtyHarry]

There is a reason however you dont see any 80's running around without doors on the trail.

Because the doors are heavy and a PITA to take off?

There is no reason why alignment would be a bigger issue on an FJ80 than the multitudes of Jeeps, FJ40s, Samurais, and Toyota pickups running around on the trail without doors (other than the hinge mechanism itself).