F150 porpoising under Travel Lite 625SL
Hello, I have just purchased a 2015 F150 (20K miles) and put on a 2017 Travel Lite 625SL (1,300 lbs). The truck sits proudly loaded and only squatted an inch or two after the installation of the camp...
Forum Discussion
Putting my engineering hat on (it's been a while since I took Engineering Dynamics but I'll do my best...) this could likely be a combination of too low of tire pressure coupled with bad shocks creating a two spring oscillating harmonic system.
I'll start with the tires first... Ever see one of these scrapyard guys or cardboard guys with a tiny Ford Ranger beaten to death loaded to the max with springs maxed out cruising on the freeway? What you will see is that the tires are bulging and acting as the "suspension" of the truck because the springs are completely overloaded. Even though 40 psi is pretty high, the fact that they could go up to 50psi suggests they aren't at their max rigidity. The tires are now acting as the "springs" in the suspension system.
Next, taking on the suspension of the truck itself, your springs may very well be just fine to hold the load of a camper but if the shocks are blown you will have an undampened spring system causing the springs to oscillate between compressed and uncompressed longer than they should.
The combination of overloaded/under inflated tires and blown shocks is the creation of a two spring system where certain resonant frequencies between the two springs can cause an amplification of the normal harmonic motion in the two systems greater than their individual parts. The result could be the bucking and porpoising you are experiencing at seemingly random times.
To picture this even better picture a flat bed two truck with a car strapped on the bed by the tires. As the flatbed bounces down the freeway there will be some bounces that will be so great that they will cause the suspension of the car on the flatbed to also react. Sometimes these oscillations will hit at their resonance frequency at the same time and cause a far more dramatic "bounce" observed in the car than the actual bump that the truck felt. Even a small bounces in an undampened two spring system can produce some surprisingly large osculations because of the harmonics of the system coming into play.
That being said, the tires and suspension in your truck are always going to be a two spring system as the tires will always have some level of compression and the springs obviously are designed to compress. However, if sufficiently inflated for the load, the compression of the tires will be minimal and of course the springs in your suspension should be dampened by your shocks.
Where I would start... Start with increasing your tire pressure to the max rated recommended pressure on the sidewall for the max load. This might be enough to cause the tires to stop acting as a spring and eliminate the "unnecessary" spring in your two spring system. Next, go get your shocks looked at and make certain they aren't blown somehow or possibly just overloaded (if they are maxed out they won't sufficiently dampen the springs causing the same problems). Replace with upgraded shocks designed for the loads you are going to be carrying.
Also, though not likely a culprit in this scenario, the frame of the truck is technically a "3rd" spring in the system. Take a look at your taillights in your rearview mirror and you may be able to observe the frame flexing as you drive down the road. My F350 Dually flexes about two or three inches that I can tell. Unless your truck is super overloaded, the flex in the frame should be within normal limits.
Note... I'm not a suspension expert so don't take my explanation as gospel. I'm just using my textbook knowledge and trying to come up with an explanation based off of my understanding of what could be happening.
Best of luck!
I'll start with the tires first... Ever see one of these scrapyard guys or cardboard guys with a tiny Ford Ranger beaten to death loaded to the max with springs maxed out cruising on the freeway? What you will see is that the tires are bulging and acting as the "suspension" of the truck because the springs are completely overloaded. Even though 40 psi is pretty high, the fact that they could go up to 50psi suggests they aren't at their max rigidity. The tires are now acting as the "springs" in the suspension system.
Next, taking on the suspension of the truck itself, your springs may very well be just fine to hold the load of a camper but if the shocks are blown you will have an undampened spring system causing the springs to oscillate between compressed and uncompressed longer than they should.
The combination of overloaded/under inflated tires and blown shocks is the creation of a two spring system where certain resonant frequencies between the two springs can cause an amplification of the normal harmonic motion in the two systems greater than their individual parts. The result could be the bucking and porpoising you are experiencing at seemingly random times.
To picture this even better picture a flat bed two truck with a car strapped on the bed by the tires. As the flatbed bounces down the freeway there will be some bounces that will be so great that they will cause the suspension of the car on the flatbed to also react. Sometimes these oscillations will hit at their resonance frequency at the same time and cause a far more dramatic "bounce" observed in the car than the actual bump that the truck felt. Even a small bounces in an undampened two spring system can produce some surprisingly large osculations because of the harmonics of the system coming into play.
That being said, the tires and suspension in your truck are always going to be a two spring system as the tires will always have some level of compression and the springs obviously are designed to compress. However, if sufficiently inflated for the load, the compression of the tires will be minimal and of course the springs in your suspension should be dampened by your shocks.
Where I would start... Start with increasing your tire pressure to the max rated recommended pressure on the sidewall for the max load. This might be enough to cause the tires to stop acting as a spring and eliminate the "unnecessary" spring in your two spring system. Next, go get your shocks looked at and make certain they aren't blown somehow or possibly just overloaded (if they are maxed out they won't sufficiently dampen the springs causing the same problems). Replace with upgraded shocks designed for the loads you are going to be carrying.
Also, though not likely a culprit in this scenario, the frame of the truck is technically a "3rd" spring in the system. Take a look at your taillights in your rearview mirror and you may be able to observe the frame flexing as you drive down the road. My F350 Dually flexes about two or three inches that I can tell. Unless your truck is super overloaded, the flex in the frame should be within normal limits.
Note... I'm not a suspension expert so don't take my explanation as gospel. I'm just using my textbook knowledge and trying to come up with an explanation based off of my understanding of what could be happening.
Best of luck!
