Hey guys, I have been a member for a while but I am new to this part of the forum and truck so please bear with me.

I just picked up a 2009 Screw Fx4 with the 5.5' bed and it is smooth as glass riding unloaded, but when I hooked up to my 16' flatbed tandem axle trailer (about 1900 lbs) I am getting a bad vibration at about 55 mph. When I say vibration I mean the feels like an unbalanced tire. I towed this trailer last week with my 03' and there was no vibration that could be felt. The truck has a 2" spacer level on the front and after a lot of reading it seems like the rear pinion angle may be out of line when the truck squats in the rear. Ive read that leaf spring shims may fix my problem but I'm having a hard time visualizing their installation orientation and what angle they should be.

My truck also has the factory 2" receiver, but im not sure if that should change anything.

Thank you ahead of time for taking the time to read through my post and offer any suggestions.

 

part of it is just bad ford design in my experience. i have a 16 foot enclosed trailer i tow with anywhere from 2-4k in it. both my 04 f150 and my 13 f150 have a vibration that shakes the passenger seat so bad i can see the head rest bounce back and forth. didn't have the problem with a 04 z71 1500, 01 f250, 04 dodge 2500 of 05 f250. All same trailer, all same load. none had problems except for the f150s.

 

Well that definitely doesnt sound good. Is your f150 stock height or modified?

 

04 was leveled with 33s load range e. 13 is a boost stock. Shook with stock tires, but I don’t recall it being super bad the last trip with load range e. That was also 4-5 months ago. It’s odd too because it was really only felt in the passenger seat. It made my wife ill one trip but I was fine. Who knows.

 

That is odd. I just had new load e tires put on and a new tech did the balancing. Maybe it's just out of balance but it only shows up when towing.

 

Could be driveshaft is slightly unbalanced and with the extra torque needed it shows up when loaded.
Or could be a wheel bearing starting to go out and again extra weight on the rear (or less weight for the front) is causing you to feel this issue when loaded.

Or like you said, could be simply a tire is out of balance or maybe you need a little more pressure in your tires when towing.

 

Thank you for pointing out the possibilities. I'll be sure to go through them all just to be safe.

I was looking at the different weights on my wheels earlier today while I was tacking the exhaust and I noticed 2 wheels didnt need any weight, one had one small weight, and the other has at least 3 oz. I'm thinking that is my problem tire and the new guy didnt know what was going on.