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Fishtailing F-550

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Maybe some of you guys can help me out with this one...  I have a 2012 F-550 2WD single cab dump truck.  While driving in a straight line, it is perfectly fine, but once you move the steering wheel, it feels as if the rear of the vehicle is not following the front.  I've made sure the trackbar is correctly torqued, I've made sure the rear u-bolts are tight, and that the locating pins are in place.  I've had the alignment checked, which is fine other than the camber being slightly positive, but within range.  I've followed it down the road, and it doesn't appear to be dog tracking.  It doesn't matter if you're turning left or right, the feeling of the rear end coming around is the same.  The fleet manager is telling me it's never been in an accident.  I'm assuming there are no frame issues, thinking the alignment figures would eliminate that.  Any ideas? 

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Assuming all F-550 trucks are equipped with that big huge open diff (no locker) is there any slight chance of the diff side gears binding up?

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I have had some 19.5 tires that give that feeling. Usually it's got cheap tires on it. try swapping rear tires and rims if another is available. I have had trucks that were scary to drive replaced new tires and concern gone.

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I am leaning towards the tires being the issue, as it has some pretty recent Hercules on it.  I've used Hercules quite often in the past with no issues, but they are cheap tires.

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Any chance they mixed bias ply and radial tires front to rear?  

If not, they different plys?  Load Range E in front with Load Range C in back? 

 

Joe

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No.  They're all Gs.  Lug in the back, rib in the front.  The same fleet has another truck with 19.5s here for some body work...  I'm going to swap rear tires, and check to make sure the rear wheel bearings are adjusted properly, as I just noticed yesterday that the shop that looked at it before we did replaced the rotors.

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I have a 2006 F450 4x2 crew cab. I have lived with that driving on ice shit since I got the truck with 1600 miles on it. As tires wear it gets decent, new tires and it sucks! I bought Michelins last round and they are as bad as Sumitomos and the OE Continentals and the replacement Contis. Traction treads on rear had to get more miles before they settled but those were Sumitomos and had deeper tread siping than the other brands so the issue may not have been the tread type. ZERO wear or lost motion in steering. Zero yield of track bar. Toe settings more and less than spec. tried. Tire pressures increased and decreased. I increased caster. No looseness in rear leaf spring eye bushings. The problem is negligible when heavy trailers are on and that is what it is for so it stays hooked to a trailer most of the time.

When I drove it back from tire store after getting the Michelin highway treads on it I could prop knee against the wheel on a decent road and it would do it with NO motion of steering wheel. It reminds me of my motorcycle riding days and riding over a bridge with grating deck.  I have had someone drive this truck and I followed and watched and it sure looked like the rear was wiggling on pavement with no motion of tire/wheels relative to body.

 

It is odd that some never mention it. I have driven customer vehicles as bad as mine and it was not a concern on RO.

 

Side note: The caster increase did make it generally drive better and seems to give a touch more steering precision.

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I swapped out the brand new Hercs with 50% worn XDS'.  It's 1000 times better.  I also ended up replacing the pissing rear hub seals at the same time, but I didn't check the wheel bearing torque first.  We're going to leave the Hercs on the truck I swapped with to see how it handles... 

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I've used Hercules and Sailun tires for at least the last 10 years, and have never had an issue like this.  I always give people the option of Michelin and Hercules and let them decide.  Mind you, the Hercules drive tire that is my go to is a different model that the one used on the truck I was dealing with. 

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Glad you figured it out. We have this issue from time to time and its always related to getting new tires on it. Bed type and load also play a pretty big part. We have about 70-80 F550s ranging in model years 2004 to 2017

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I used to work for a guy that had around 12 tow trucks, from 1 ton up to 50 ton. Whenever we put new drive tires on the 3 and 5 ton stuff it did that. The deck trucks were especially bad empty,less noticeable when loaded. After 3 to 5 k is put on the tires it seemed to go away. He ran mostly Hankook stuff, the odd Hercules or double coin. My theory always was the "square" edges needed to wear off all the tread blocks. I don't think it was the carcasses as we had a few sets of ours re-capped and they did it as well. The more aggressive the tread the worse it is.

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I picked up the donor truck from the bodyshop yesterday, which now has the suspect tires on it.  It now behaves the same way, perhaps not as bad, but this is a bucket truck, and is substantially heavier than the dump truck the tires came from.

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I have gotten used to it and it bugs me the most if I haven't used the truck in a couple or more weeks. I think the complaints would be greater but some people wouldn't notice or drive like crap already and the vehicle runs all over the road they way they drive in something with no issue.

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