Trailer and truck bounce

What is the loaded and wet weight of your trailer? What is your tongue weight?

Drive at 55....... Just kidding.

Have you scaled your rig to see what your actual weights are? I'm no expert, but it sounds like you may have too much tongue weight and or enough bar to handle what you have.

I have an Equalizer 4 point sway control hitch, and I tow 7000 lbs with my 3500. When I first set it up, I was only interested in sway control, and not so much in weight distribution since a 3500 towing 7000 lbs is a bit of an overmatch IMHO. It worked OK for me, but I would occasionally get into the same situation you described on some stretches of undulating road in my area. I dialled in more weight distribution, and it greatly improved the tendency to porpoise.

Just my experience. Hope this helps.

Concrete hwys are made with expansion joints and they seem spaced just right so that while towing with certain combos it's bouncy. My setup is the same way and I even had jerking with my previous 3 trailers of which one was a 5er.
There's a few stretches on I=5 in Oregon that rattle me good.

Harmonics and because the wheel base. Spacing if the pavement joints, your suspension compression, tire PSI, shocks (they also have a natural harmonic), speed, etc all lined up

Once that harmonic synchronization happens...the amplitude will go ever higher if dwelled there


Speed up or slow down are the only things a driver can do

Back home...changing tire PSI, increase/decrease WD spring rates, etc in order to change the setups harmonic point(s)

Stiffening everything moves the harmonic frequency higher

If the designers of roads and highways would change the spacing of those joinys... that would also help because there wouldn't be a constant frequency...
It would not be at ONE frequency

An example...ho to a softly sprung truck, with ride quality tire PSI

Push the TV sideways with your foot on the bumper

As it comes back over center...push it again and again

Notice how it will naturally continue with LESS push from you. Just time it to that harmonic

Then air up the tires to max PSI...and repeat

It will be at a higher frequency and you won't be able to get it moving as far side to side

I agree probably harmonics of your set up just by luck conform to the frequency of the roadway at 60 mph. If the truck shocks are original consider replacement. Possibly add shocks to the trailer if you will be using this roadway frequently. May not be a cure unless the existing is fully shot... but should help.

“If the designers of highways would change the joint spacing...”
It’s hard to believe, but some states still have typical joint spacing. WSDOTis one of the dummies who still do this.
The first road I concrete paved was in NM, over 20 years ago. NMSHTD varied panel joint spacing back then. It only takes a little variation to keep the harmonics from starting.
Also doesn’t help that cut concrete also swells or curls at the cuts.

US 69 hiway is a trucking route between I-44 in ne OK down to DFW. It has section of concrete surfaces that my wifes 1500 chevy pulling a 18' 10k car hauler empty or loaded just kills me. It bucks hard.
However I can run my 16k gvwr 36' 6800 lb empty triaxle GN stock trailer over the same roads with her truck and its super smooth. (no I don't use her truck with this trailer when loaded but I do have to pony the empty trailer sometimes between sale barns/pasture loading pins/etc.)

Freeway hop...as we call it since man started pulling trailers mainly on concrete hiways. When I hauled for a living sometimes I would route around concrete hiways for that reason.
There is no single "thing" that will fix all combinations and there are many "things" that causes it. Most folks find trial and error remedy solutions worked for them.

To help reduce the hop/jerking, speeding up/slowing down is best option. About 10 mph either way makes a big difference. I have also noticed that while staying in the lane, but keeping to the edge, one side or the other, helps too. However, you are more likely to get a tire puncture, while running on the lane edges.

Jerry

time2roll wrote:
I agree probably harmonics of your set up just by luck conform to the frequency of the roadway at 60 mph. If the truck shocks are original consider replacement. Possibly add shocks to the trailer if you will be using this roadway frequently. May not be a cure unless the existing is fully shot... but should help.

X2
Bilstein’s on the TV good shocks on trailer.

I had the same problem with mine. Ram 2500 towing 36’ x 10k TT. I put 4 bilsteins and added a pair of stable loads. Stable loads engage the overload springs on the truck and stiffens it significantly. The ride is still good. I really can’t tell a difference on most roads. They are easily taken out of service for times you don’t have a load. On concrete hwys that I could barely run 45 on I can now comfortably run 60. If your F150 has P tires upgrade them to LT’s. That can help a lot too.