Front end 65mph shimmy coasting while towing.

[IdahoDoug]

Subject says it all. Towing with the vehicle level (airlifts in the rear coils), I can do a gentle coast (not fully off the gas) on a downhill and get a fairly significant front end shimmy. The instant I touch the brakes, it stops. The instant I get on the gas it stops. It's not felt in the steering wheel like a classic front end shimmy where the wheel rotates back and forth - feels like the entire front axle is going up and down. The only time the truck has ANY shimmy is while towing. Otherwise it is the smoothest 80 I've ever driven. Clearly the front end is either higher or lower while towing at speed but I can't think of a way to determine this. Clearly, because it does not shimmy when just carrying passengers.

It's going to be a weird one, which is why I posted a dedicated thread. It's been there for years but suddenly got worse this towing season and I replaced both the tires and wheels and it's still doing it. Stock Michelins, stock alloys.

First thing I checked was the front driveshaft in terms of tightness and slack. Put the truck on a level surface in neutral with parking brake on so the shaft is not accidentally held tightly by the vehicle and cannot get any appreciable movement of the shaft either twisting, longitudinal or grabbing tightly and shoving up and down. Seems remarkably tight and has been recently lubed.

I'm going to check the wheel bearings, knuckle nuts, and rotate the tires but the lack of steering movement tells me it's not a "single side" thing like a tire out of balance. I'll also be able to remove the shaft and lock the center diff and see what that does. Could be the front drive shaft has been out of balance for 200,000 miles and now age has generated some slack so it's wobbling. Almost like a front axle shaft is bent and making the front axle housing bounce up and down as a unit.

So, anyone else experience this? Did a quick search and nothing jumped out as similar where either braking or accelerating instantly stops it. That's a big clue I think - just can't ID it...

DougM

 

[-Spike-]

Sounds like you do a lot of towing, but I'll start with the obvious- how's your trailer weight distribution? Is the tail wagging the dog? Always the same trailer? Always loaded the same?

 

[IdahoDoug]

Yeah, same trailer. No sagging, and it handles the trailer well. I have airlift poly bladders in the rear coils - an arrangement I've had on 2 80s over a combined 320,000 miles and it has worked well. Tows level. One thing you can't tell though is what air resistance is doing to the vehicle's angle. But nothing is a crazy angle or I'd notice from the driver's seat or one of my buddies over the years would have said something. Assume it's a pretty level ride.

DougM

 

[flow]

my first thought is that your caster angle is being lessened by the rear end squatting while towing. when you get back on the brakes, the weight transfer increases your caster and the shimmy goes away.

edit: sounds like you are confident that it is towing flat. but I believe that the simplest explanation is usually the right one. maybe the low caster angles are just exacerbating another problem.

 

[-Spike-]

Try towing a different trailer, see if things change, might be an aerodynamic thing. Try adding or subtracting tongue weight. Also, try adding some weight in the back of the cruiser, without the trailer, see if you can duplicate the problem that way. If not, it's probably the tail wagging the dog, the truck is fine.

 

[IdahoDoug]

Good call loading the back without the trailer. Will save a bunch of time. I don't think it's going to be a simple load wagging the dog that's the issue however and the trailer tows straight and stable. Tapping the brakes or leaning on the gas pedal very gently instantly stops it and doesn't change the vehicle's attitude.

Doug

 

[Pin_Head]

I was thinking caster angle too, caused by the trailer loading, but it seems strange that you don't feel it in the steering wheel. This makes me think that it may not be a real shimmy where the wheel is oscillating. Maybe something like the Birfields vibrating from wear on the star/bell or the spindle bushings? This would fit the deceleration symptom as it would change the loading on the drive shafts.

 

[Eicca]

Mine does something identical, albeit no trailer involved and it kind of comes and goes. I heard somewhere that under the right conditions springs can amplify even the slightest out-of-balance tire vibrations...

 

[psfpilot]

With the weight on the rear, perhaps a slightly loose wheel bearing is suddenly more noticeable when the trailer's tongue weight lifts the front end. The airbags will keep the vehicle level, but I can't imagine that they would reduce the downward moment of the trailer tongue which causes a corresponding upward moment upon the front axle.

 

[80t0ylc]

Can you get an observer in another rig to ride beside you on the interstate? They can be looking at the axle to see if anything is visibly shimmying. You could communicate with walkie-talkies so they would know the exact time for them to be watching. You might have them look at more than just the axle. They could also give you feed back on attitude.

 

[Eicca]

They could also give you feed back on attitude.

My attitude deteriorates fast when the vibrating starts, let me tell ya.

 

[IdahoDoug]

Some good calls here on diag and suggestions to check. Thanks - great stuff. I have not gotten back to looking at the truck after finding the shafts tight and good. Like the point that the front end may have quite a bit less weight on it, which jibes as coasting always means we're heading downhill which means the trailer tongue is pressing down harder. Hmmm.

The axle comment is a good one, too. Would make sense that I would not feel that in the wheel - would make the axle housing shimmy up and down without a particular wheel shimmy (which I'd feel in the wheel). These birfs are nice and tight but I wonder what else in the front axle would allow radial movement. Such as the drive plates. Such as the spindle bronze bushing. Hmm - nice input on that one, PinHead (feels funny saying that kindly...heh..)

DougM

 

[greentruck]

One other thing, but it's a long shot. Trunnion bearings?

Don't think they've been much of an issue, except if mishandled during an axle overhaul (doesn't seem to be the issue here.) Might cause shimmy under differing angles on the front axle hauling the trailer that would be unnoticed without the tow.

However, I'd think the trunnions would give steering feedback -- and that doesn't seem to be the case.

 

[rove_mcanus]

If the trailer has mechanical brakes then you should check that out. It could be exasperating a pitching problem with the vehicle. Something like a busted or weak shocks/springs.

 

[IdahoDoug]

Good stuff.