Advice on 1985 FLC Daycab Bouncy Ride???

[Papa Rick]

I bought a 1985 FLC Spring Ride Daycab that I use to haul logs for a living. Here is my question: At certain speeds my Truck has a bounce in it, not a vibration but a bounce up and down kinda like riding a horse. It does this at 45 to 50, and 65 to 70. It does not matter if it is going up 45 to 50 or coming down 50 to 45 etc., it will still do this.

I even had a mechanic drive it to see if he could figure out what was causing this, and he didn't have a clue.

I had new steer and drive tires put on in hopes of this curing this, but no help. Even had the Truck jacked up on blocks and ran it to see how the tires were running, and nothing.

One other thing that it does, it only does this if I am either bobtailing, or unloaded??

Any advice would be greatly appreciated, as this is really getting on my nerves.

[GMAN]

Have you checked your shocks?

Called a harmonic vibration in the suspension. Your truck is spring ride correct. The spring equalizers in between the rear springs may be worn out and might need to be replaced. Also have your front spring shackles checked out for wear those do wear out and need replacing. The other thing it could be maybe is a worn tracking bar or link to keep the rear ends in alignment. The reason why it only does it empty or bobtail is fully loaded the tires act like a shock absorber and absorb the impact. I drove a 73 International cabover that was the same way unless I was heavy. Empty that thing bucked like a bronco heavy road OK.

[Part Time Dweller]

Bad shocks is a good place to start looking. Also, the springs could be shot, especially if they are original. Also check the rear cab mounts yo nmake sure they are tight.

[Papa Rick]

Should a good 3 Axle Alignment Truck Shop be able to find the cause?

[Part Time Dweller]

I would think so. They usually check everything before they do the alignment.

[special k]

All the above tips are good but you may have to live with it. A friend of mine has a 94 Pete 378 that has done that since new. The closest thing to an answer they got from Pete was that sometimes the frame will get into a harmonic bounce when empty. Older Int's actually had a deeper belly on ther frame under the cab /sleeper and some guys say that was to damp out the harmonics. On the Pete they also suggested he might have to change the frame rails. As his truck was out of warranty he decided he could live with it.

[Papa Rick]

I really appreciate the suggestions everyone has given. I am taking it to the shop tomorrow to get the 5th Wheel rebuilt, and will have them check all of the above mentioned items. This is such a great site to get help from.

The reason that I believe something is wrong is due to the fact that this bouncing up and down has gotten worse as time has went on. It was there when I bought the Truck in Oct of 06, but has gotten steadily worse.

Question: I had a Mechanic check my front shocks (Back is Spring Ride), and watched as he unbolted the top of the shock and then pulled it all the way out. It stayed out (Extended), and I commented that the shock was bad, but he told me that no if the shock went right back in then it would be bad.

He reason was that a bad shock will not stay extended out, but would just collapse back in. Anyone know about this.

[roadranger]

Ain't that easy to test a shock - easiest thing is to replace them every couple years, The machine to test them isn't that common. You can feel them for temperature after a run to make sure they are all equally warm to hot but if they all wear equally might be hard to tell when they are gettin' weak? Also your driveshaft might be gettin' out of balance? I'd think a log truck would be hard on the driveshaft and a hard start in deep reduction might twist it just enough to go out-of-balance and each new "incident" would find it easier to tweak it worse?

[Part Time Dweller]

You can heat test the shocks with a laser thermometer. Come in off the road and shoot each shock with the temp gun. The colder ones are not doing as much work, which means they are no good.

Roadranger is right, this procedure won't tell you the true condition of the shocks, just if you have any that are worse than the others. The hotter the shock, the more work it is doing.

But what you are describing to me sounds like a harmonic vibration in the suspension from the springs or something. Does it go fron to back or back to front. Also on the shocks on the Spring ride trucks the only place they have them is on the front axle to help keep the steering in check. On my truck it turned out to be in the spring equalizers binding up from being worn and also there was a track bar link bushing bad. That led to the same condition you have.

Yes, he is correct in the way a shock works. On a truck they are a lot stronger than a car. The springs on a truck and car are to absorb the impact. Without the shocks the truck would bounce back up to it's original position very quick. The shocks slow the process down to make it a nice smooth transition back to there original place. When you see a car going down the road and it hits a dip in the road then bounces up and down for a while, it needs shocks. Shocks do not raise or lower your truck or car if it's sitting low, that's the springs job.

If your wheels were out of balance your tires would show this very quick.

My guess it's the drive shaft.

And by the way, it's called Harmonic Frequency and everything has there limit but I doubt that it's the problem. During world war II they discovered this when army men marched across a bridge. The whole bridge started swaying back and forth and almost collapsed. Now, when the army walks across any bridge they are told to walk without rhythm.

[allan5oh]

Could be as simple as tire balance as well as blown shocks.

I once saw a trailer with both, the whole thing was bouncing down a smooth road. Wow.

Blown shocks do not cause a wheel to bounce up and down, they cause the whole car or truck to bounce up and down. If the wheels were out of balance and bouncing that much they would show signs of wear immediately, it's something else. Just ask a tire professional and they will tell you the same.

[allan5oh]

Yes the whole trailer AND wheels were bouncing up and down. That's why I thought it was wheel related. Not much else to cause a vibration like that on a trailer. The trailer wheels were coming OFF THE GROUND.

[Papa Rick]

The shocks were totally gone, got them replaced and it took 50% of the up and down bounce out. I believe 25% is from the left steer tire that took a beating while the shock was blown. The tire man told me that due to the shock being so bad that the tire had to act as a tire and shock at the same time.

It did not get all of it out, but a major improvement. I am going to get the springs and bushings checked out in the future.

Thanks to everyone who helped me on this matter.