i do, the 710 from pretty much anywhere to where it meets the 405 in los angeles is freaking horrible. I will drive 8 miles to the 110 just to avoid it, everything after the 405 to long beach is wonderful tho because they put down asphault instead of concrete strips. New shocks helped me some, but it still sucks.

 

The suspension in a vehicle can oscillate. The sprung mass, the leaf springs, and the shock absorbers form a system that has a mechanical resonant frequency where "inputs" to the system (i.e. upward force from the unsprung mass - wheels and tires and axle) at a certain frequency or frequencies will cause the system to oscillate rather than simply dampen out the input.

It is the mechanical equivalent of an oscillator in electronics.

You can develop an intuitive understanding this way:

The suspension system is at "rest" when it receives an upward force from the unsprung mass (tires and wheels and axle going over an expansion strip) and the system absorbs the input, and goes up and then comes back down, then rebounds a bit. Suppose just at the moment that the system is rebounding, it receives another upward force from the unsprung mass (goes over the next expansion strip). Instead of the up and down motion dampening out, the second force is perfectly timed to ram the system up, reinforcing the upward motion and the cycle repeats.

The system will oscillate at a characteristic frequency. The particular frequency depends on the specifics of the suspension (leaf springs and shocks and sprung mass) and the various physical dimensions of the system. When you drive over expansion joints at just the right speed, the inputs are at the characteristic frequency of the suspension system and all hell breaks loose.

For good handling, designers are always after low unsprung mass. Less input force (Force = Mass x Acceleration) to the system. Less force for the suspension to have to deal with. Hence mag/alloy wheels.

But, on my F-150 XLT Sport, I have relatively large wheels and tires (285/60-17), plus I have the 3.55 LSD, all of which amounts to a large unsprung mass. And with the long wheelbase and heavy duty shocks, and no load in the bed, it just adds up to a bad combination.

It'd be nice if it didn't do this, must be hard to solve the problem for certain wheelbases. I know a Chevy long wheelbase owner in Vegas who complains about the same thing on I-15.

You may recall seeing bigger commercial trucks doing this on freeways. Usually not the 18 wheelers, but medium sized two or three axle rigs running empty. Sometimes you see empty flatbed trailers hopping up and down like mad, or empty hopper trailers doing it. (Maybe that's why they're called hoppers? - no, just kidding)

 

Well, this bouncy problem is now bothering me even more now that I picked us a 1971 longbed Chevy C-10. I've dropped that thing about 5" up front and 4" in the back and it rides like dream.

Now it does have a coil springs rear end, which makes a difference...I know. But still! I have Eibach front springs and maybe they just don't work well with the rear stock leafs lowered. It is bothersome that a 30+ year old TRUCK that has been lowered (does not even have new shocks yet) can ride nicer than my 2001 F-150.

I'm thinking about lifting mine up just a bit again, in order to get some of that stock feel back after having put the replacement springs in place.

 

Three things that will help.

1) Make sure your rear tires are not over inflated

2) Install a better shock, Rancho, Edelbrock, or Bilsteins

3) Run a tire with a softer side wall, Michelin

4) Put some weight in the box

5) Velvet Ride Shackels simulate a load in an empty box

Do the first three and there will be no rear end bouncing.