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Old 09-04-2010, 10:20 PM   #1
SomeDude451
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Excessive Frame Flex/Fatigue

I'm trying to determine where to draw the line in regards to frame flex. I know these trucks can flex the frame when offroading. I have no issue with that. However, mine will twist when driving on the street. During normal street driving, while going through a turn I can see the bed twisting a good inch or inch and a half in relation to the cab. I can also see the bed move fore and aft independently when going over bumps. It's like the frame where the cab and the bed meets is weakened. This is the second truck I've had like this and I really don't remember the other one flexing and moving around this much. I personally haven't abused it in the last two years that I had it, but there's no telling what the previous owner did to it, and i have my suspicions that it was used a tow vehicle before I got it.


Are there any ways to test for frame fatigue? How much is to much frame flex? Am I just being paranoid?
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Old 09-04-2010, 10:32 PM   #2
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That is a lot of twist! Have you checked the frame for cracks?
What do your body mounts look like?
Are they tight?
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Old 09-04-2010, 10:59 PM   #3
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I haven't looked at the frame with a magnifying glass or anything like that, but every time I've been under the truck it looks fine. A bit of surface rust here and there and some dirt, but other wise in decent shape. I'll have a good look tomorrow at the frame and the body mounts tomorrow.
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Old 09-04-2010, 11:19 PM   #4
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check the body mount bolts and rubber mounts. the bed is mounted directly to the frame and the cab is on rubber mounts.
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Old 09-04-2010, 11:53 PM   #5
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I would bet your body mounts are shot before the frame. I've never heard of any Broncos/150's having frame problems, even with all the flexing.
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Old 09-05-2010, 12:05 AM   #6
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gm frames, on the other hand....
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Old 09-05-2010, 08:48 AM   #7
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I checked it out today. The rear section of the frame, particularly on the driver side is more corroded than the front, but is not what I would call excessive. It's a twenty year old truck that originally lived in California. Some surface rust and a bit of pitting. The body mounts look fine to me. They're not rusted, the bushings are a bit compressed, but not horribly. I checked what I could of the frame and aside from the noted rust, I didn't see any large defects or cracks. Also, unlike the Broncos the F150s only have four body mounts. Two for the core support and two for the cab. Both of mine for the cab look the same.

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Old 09-05-2010, 02:01 PM   #8
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I checked it out today. The rear section of the frame, particularly on the driver side is more corroded than the front, but is not what I would call excessive. It's a twenty year old truck that originally lived in California. Some surface rust and a bit of pitting. The body mounts look fine to me. They're not rusted, the bushings are a bit compressed, but not horribly. I checked what I could of the frame and aside from the noted rust, I didn't see any large defects or cracks. Also, unlike the Broncos the F150s only have four body mounts. Two for the core support and two for the cab. Both of mine for the cab look the same.

Really only 4 total it should be 6; 2 on the core support and 2 on the left side of the cab and 2 on the right side of the cab. Try to replicate the frame flex by jacking the front of the truck up and leave the rear on the ground see where it is flexing from out side of the truck.
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Old 09-05-2010, 02:08 PM   #9
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That looks new still!lol I don't understand where you would get 1-1 1/2" of flex or twist from with a frame and mounts that looks that good.I would check the bolts on the box and see if they are loose or missing.
theramsey3 has a good idea to jack it up and look for anything that might be giving you the problem.
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Old 09-05-2010, 03:14 PM   #10
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You would think there would be more, but the Ford shop manual I have confirms that there are only four total for F150s. It's dark out right now, but I'll try to get out the jack tomorrow and see if I can make it twist.
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Old 09-05-2010, 03:19 PM   #11
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You have two bushings at the rear of the cab on top of the frame rails.
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Old 09-05-2010, 11:24 PM   #12
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When you jack it up, take a picture. My white F150, aka rusty, didn't even flex that bad. :

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Old 01-17-2011, 11:40 PM   #13
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This is probably a stupid question, but is your frame flexing or are you suffering from body roll in turns? if your frame was flexing that bad you would have cracked your fiberglass shell. I suggest, that during a turn, you look at the top of your tailgate corners in relation to your bedsides. if your frame is flexing that much you should see a huge gap between your shell, tailgate and bedsides. After you eliminate that, check your rear sway bar bushings and rear leafspring bushings.
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Old 01-18-2011, 12:25 AM   #14
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Quote:
Originally Posted by SandStallion View Post
This is probably a stupid question, but is your frame flexing or are you suffering from body roll in turns? if your frame was flexing that bad you would have cracked your fiberglass shell. I suggest, that during a turn, you look at the top of your tailgate corners in relation to your bedsides. if your frame is flexing that much you should see a huge gap between your shell, tailgate and bedsides. After you eliminate that, check your rear sway bar bushings and rear leafspring bushings.
he is talking about an F150
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Old 01-18-2011, 05:02 AM   #15
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I have seen some gnarly frame flex out of the F-150s personally. What configuration is yours? Single cab long bed? My dad's old 86' (now my sisters) had flex like no tomorrow. But you also have to look at how long those frames are without much in the way of supports. If you are truely worried I would see about fish plating the sides of the frame to beef it up. But that would make for a lot of work for something that is not causing you a ton of problems. I would throw a nice new set of body mounts on the truck and see if that helps any.
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Old 01-18-2011, 08:10 PM   #16
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Quote:
Originally Posted by SandStallion View Post
This is probably a stupid question, but is your frame flexing or are you suffering from body roll in turns? if your frame was flexing that bad you would have cracked your fiberglass shell. I suggest, that during a turn, you look at the top of your tailgate corners in relation to your bedsides. if your frame is flexing that much you should see a huge gap between your shell, tailgate and bedsides. After you eliminate that, check your rear sway bar bushings and rear leafspring bushings.
Both. The body rolls as you would expect a truck to, and the bed flexes independently of the cab. The bed also moves along a vertical (pitch) axis when going over bumps. Leaf spring bushings look ok. Not amazing, but not horrible. The front and rear sway bars are in a trash pile somewhere.

Quote:
Originally Posted by cajunrebel View Post
I have seen some gnarly frame flex out of the F-150s personally. What configuration is yours? Single cab long bed? My dad's old 86' (now my sisters) had flex like no tomorrow. But you also have to look at how long those frames are without much in the way of supports. If you are truely worried I would see about fish plating the sides of the frame to beef it up. But that would make for a lot of work for something that is not causing you a ton of problems. I would throw a nice new set of body mounts on the truck and see if that helps any.
Supercab shortbed. It hasn't gotten any worse since I've had it. It's enough that I keep an eye on it, but not enough for me to do something about it just yet. It's an old, tired truck. It's not really surprising at this point. Also, I'm lazy and it still goes from A to B.
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Old 01-19-2011, 04:22 PM   #17
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I have a bronco stripped to the frame right now. I moved it with a tractor/front end loader by lifting one frame rail at the front. While rolling on the back wheels over little bumps at slow speed, I noticed alot of flex. This is with no engine, trans or tranfercase. This Bronco has very little rust. Now I know why the bodys always crack at the rain gutter.
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