Car, vibrates at 37 and 42 mpg, 1994 Voyager

I've noticed something strange about my 1994 Plymouth Voyager, 3L,
plain vanilla model.

I bought Pep Boys Future 2000 tires? Probably because it's one of the
few with 2 ply sidewalls.

Anyways, I rotated the tires and then over the course of months, would
notice a vibration at 37 mpg which would go away immediately if I
accelerated just a couple of mph.

So I had all the tires rebalanced. And the problem is still there,
except at 37 and 42 mph.

So I wonder, could the previous back tires have some sort of defect,
like an out of round that a balance could not detect and certainly not
the Pep Boys' mechanics?

Or is there something else going on, like a strange mis-alignment. But
then why only at 37 and 42 mph?

The car has been inspected and the front-end is supposedly fine.

I am going to go to the dealer but I do wonder.

The shakiness is like a tire out of balance. And around the same speed
leads me to believe something strange with one of the front tires.

I guess I could rotate the tires back to the original positions and
see what happens.

Any thoughts?

P205/70R14 are the tires now on the car, not P195/75R14, that are
originally suggested. Someone suggested better handling, except in
snow, with these slightly wider tires? Maybe. I think it affected
mileage somewhat, maybe different size or maybe lower rolling
resistance since Pep Boys are Cooper tires which have lower
resistances often.

Regret not getting cheapo Goodyear with just 1-ply sidewalls but
probably better mileage. I reckon the penalty is about 10% which now
is a bit, not much, but more money for gas. I wanted the safety of the
rare 2-ply sidewalls for stability but I drive mostly slowly so that's
not really a problem as if I had heavy loads and head to do lots of
curves and so on. The car is really quite stable for a van with the
2-ply and bigger tires, so that is correct.

 

A tire that has a broken cord can still be balanced, the best way to have
your tires checked is by doing a road force balance, that will tell if the
tire is any good or not, If your inner CV joints are worn out that will
cause a vibration on acceleration and go away as the vehicle is not under
a load
Glenn Beasley
Chrysler Tech

 

How were the tires balanced???

On one of the newer computerized low-speed machines??

I did have a problem at one time where the newer low-speed machines
said the tire and rim were balanced, but they were not.

See if you can find a shop that does "high speed" balancing with the
older style machine.

I would also suspect that since you have two resonance problems at two
seemingly unrelated speeds, that you have a problem in more than one
tire / rim combination.

Just to be sure, take a very close look at the rims. It is possible
that you have a slight distortion in one or more of them.

 

In the meantime, the front tires were rotated back to the rear, and
vice versa, to see if it were the tires indeed. The vibrations have
stopped so it is the tires. I did notice a slight vibration at 45 mpg.
I suspect that a bad tire even on the rear wheel might do a little
vibrating?

The right axle was replaced seven months ago and the inner (?) CV
joints were probably inspected. I was told the axles are now in good
shape so can I assume the CV joints, inner or outer?, have been looked
at. Although it's possible that maybe they just have to go. The
vibrations have been pretty much on the clock at 37 and 42 so I
thought it's unlikely the CV joints would be so predictable.

What may have happened is that the tire went over a curb too sharply
which bent or broke a belt? I made a mistake. Cooper tires (Pep Boys)
have higher rolling resistances often, so poorer gas mileage. In the
past, some Cooper/Pep Boys tires have had belt problems and recalls.
Not mine in particular but I wonder now.

Thanks for yours and all the comments.