Car shaking between 55-60mph under full load

[sjonnie]

Yesterday I was towing a 2,500lb trailer and had a canoe and two kayaks on the roof, so the car was working hard. However, at 65mph the car ran great, but with that load, it's hard to get up hills and the speed drops to 55-60mph. Of course the car is under full boost at this point. When the speed drops below 60mph, the car starts to shake badly. If I let up off the gas and drop the boost to 1-2psi the shake becomes less although the speed also drop obviously. If the hill isn't so steep, the car can drive up it at full boost at 65mph with no problems and little to no shaking. The car ('02) has new(ish) tires, new suspension, new(ish) front drive shafts and 194K miles Why the shake only at particular speed under high load? It's doesn't matter which gear it's in, 5th, 4th or 3rd the shake is the same so it much be a drive line issue? There's no shake a 55-60mph normally, only when the engine is pulling hard. At any other speed under full boost the car doesn't shake at all

[Ocean Racer]

Wheel balance and alignment.
That speed is a sweet spot for it.

[billr99]

Wheel balance and alignment.
That speed is a sweet spot for it.

Agreed. This chassis seems to have a thing for that particular range in regard to wheel balance. Have them balanced again and by someone who knows what they are doing. A sloppy job is all it takes. Of course, you could have also thrown a weight so a quick walk-around might reveal that.

Good luck,

Bill

[sjonnie]

Wheel balance and alignment.
That speed is a sweet spot for it.

I guess I can check wheel balance. Alignment was done last week after I replaced control arms. Why would a wheel out of balance only shake under high engine load?

[Astro14]

also check engine misfire and CV joints....

[billr99]

I guess I can check wheel balance. Alignment was done last week after I replaced control arms. Why would a wheel out of balance only shake under high engine load?

You said it "shook less" under coasting. Obviously, an out-of-balance situation is speed dependent mostly; but if you are putting more force (i.e. as in the physics use of the term) into a rotating mass that is not balanced, that increased force will "throw" the unbalanced weight just that much more. I'm sure there are formulas and such that would show it.

With all this said though, I'd have to modify my response in concurrency with Astro in regard to the halfshafts. You say you recently replaced your A-arms. Did any of that work affect the shafts, possibly pulling them slightly out of the diff, for example? Did you pull the uprights to change the arms and if so, were all the torque values checked? Now that we know that your arms were recently done, it does open up some possibilities and it might be wise to go back over the work to make sure all is as it should be.

Good luck,

Bill