I have a 1985 FLC Daycab with Spring Ride Suspension. I am hoping to find some people who drove these and had a problem such as I have.

When I am loaded, there is not a smother riding truck I have ever been in, but when I am empty I get a up and down vibration, (it is like riding a horse real fast, up and down, not side to side), at 2 different speeds 35 to 40 and 65 to 70.

It only does this when it is empty, with or without the trailer.

So far to date I have had or done the following:

New steer tires put on.

New drive tires.

All tires and wheels balanced. I even had them jack the drive axles up and watched the tires spin and noticed no up or down movement, or vibration.

It is not in the transmission as it will do it even if you take it out of gear when it starts, and you cannot fill it in the gearshift.

I took it to a Alignment shop to have the 3 axle alignment done, as I thought this could be the problem, but the Tech told me that if one of the axles were out of alignment, it would not cause it to hop up and down, and that he had never heard of one doing this.

When I spoke to the previous owner, he told me that he could never get that out. (Wish he would have told me that prior to purchase)

He then told me that he had to buy 3 sets of steer tires in 5 years, and there did not have many miles on them. This is a logging Truck and Trailer, not more than 60,000 to 70,000 per year.

When I had the steer tires checked after I bought it, they had less than 30,000 miles on them. Heck they looked like they had 5,000 to 6,000 on them.

They put a machine with a wheel on it on each steer tire, and both of them, were only hitting half of the tire they were dancing from side to side.

Sorry for the long post, but I have spent over $5000.00 to try and find the problem, and no help yet.

Any suggestions would be greatly appreciated.

I'm not a mechanic. If it was a car I would say you needed new shocks.

Is it a four spring suspension or walking beams? I have no experience with the 4 spring but, with walking beams a slight up and down hop seems to be the nature of the beast.

Would shocks cause this only at two different speeds, and only empty? I am not questioning your advice, just trying to reason this out in my mind.
I believe the previous owner had this done, and no change he said.


I appreciate any advice.

Yes, 4 spring suspension.

If you have a spring ride Tractor and a Air Ride Trailer they work against each other so you get a bumpy ride. It's not as noticeable when you're loaded.

Both Tractor and Trailer are spring ride

If your bounce is on the front axle then you could have several things.

Out of round tires. Have tires cut true and balanced.

Bent rims. Replace rims.

Loose wheel bearings. Tighten or replace bearings.

Check king pins.

Bent wheel hub. Replace hub.

I had a truck that bounced like you say. I had new wheels, new tires, tires balanced. Nothing helped. I finally pulled the rims off and ran a couple lug nuts back on to hold the drums in place. I turned the hubs by hand to check the run out of the wheel mating surface.

One hub was out of round. I replaced it, and no more steering shimmy or bounce.

You could also have a brake drum severely out of balance.

I have seen a new truck do this. All rims, tires, and hubs were replaced. Did not help a thing. If you were in the sleeper at 35 MPH you would almost get thrown out. This was a cabover Mack.

Good luck

I am working on a old Cab over all spring ride. I had a problem with one of the front spindle nuts not letting the wheel bearing tighten up while adjusting. I took it for a ride not knowing the wheel bearing was quite loose. It bounced like a SOB. I got it back to my shop and dug back into it and found the drivers side wheel bearing real loose. Found out I had a burr on the spindle not letting me tighten the wheel bearing properly. Got it fixxed and no more bounce. My bounce was at about 35mph, didn't have the gonads to go any faster to see if it came back at 60 lol

Have The driveline angles checked w/ trailer empty Have them ck slip yoke for being to long or to short

jack up rear end and check u joints could have on binding ( spin wheel slow by hand

** There are only 2 shocks on a spring suspension ....Steer axle only**

Question any vibration in steering wheel or in your butt
( steering wheel most of the time front end.... Butt ... rear suspinsion