Jeff Lester wrote:
> Quick run down:
> 89 Suburban
> 350
> OD auto
> 2WD
> *new* BFG AT’s (30×9.5/15)
> 50k miles
> *bad* vibration front and rear between 60 and 75
Did the vibration start immediately when the new tires went on? Are
these on aftermarket wheels, with non-stock offset, etc? Did you put in
a "lift kit" when you put on the tires? Tell the truth.
Wheel/tire assemblies must be round AND balanced. They are 2 different,
almost un-related features.
Problem 1. Balance machines don’t really hold the wheels correctly.
Tire balancing machines must locate (center) the wheel/tire assembly
(WTA) off some reference. Typically their arbor uses a big tapered cone
that plugs into the center hole. Because the arbor is a taper it
catches an edge or chamfer on the wheel hole. Your stock wheels were
probably centered by a machined >>diameter<< on the hub. So the WTA is
balanced while centered from a suspicious surface, but are centered on
the car by a different surface. Change in centering = change in
balance. How much? How much did the centering change?
Problem 2. Aftermarket wheels use the wrong reference to center the
wheel on the car. If you are using aftermarket wheels, they may try to
center off the wheel studs, with a huge center hole to fit multiple
vehicles. Honda is one car manufacturer that uses the studs to center
the wheels, but most don’t dare try.
At 70 mph your tires are revving about 670 rpm. The driveshaft is
spinning about 2000 rpm (670 X axle ratio). If it’s an overdrive
transmission in high, the engine is revving much slower than the
driveshaft. If you know someone with a portable vibration analyzer you
could measure something in the interior and see if it is shaking at 670
cpm (wheels and tires), 2000/4000 cpm (driveshaft/liftkit angle) 1400
cpm engine.
If you can almost count the individual shakes, it’s probably wheel
speed. If you can hear a deep noise and the individual shakes are
happening so fast the vibration feels more like a buzz, it is probably
driveshaft. You can use your computer to make a wav file with different
frequencies so you can judge the audible frequency.
> I suspected U-joints, so my mechanic looks at
> them. They are questionable, so he replaces
> them. Vibration *still* there. We examine the
> rear tires with the rear jacked and under power.
> The wheels and drums look reasonable true,
> but the tires have approximately 1/8" to 1/4"
> radial runout and 1/4" to 3/8" lateral runout. On
> his advice, I went back to the tire store (NTB).
> By the way, what is reasonable for radial and
> lateral runout?
in the 70′s Chrysler said 3/32 inch (about 0.09 inch) was the limit for
police cars intended for high speed pursuit. You won’t find many tires
with less than 1/8 inch runout.
- Hide quoted text — Show quoted text -
> Seasoned manager took the truck for a test drive
> after they rebalanced the tires. I prefaced their work
> with my mechanic’s discovery. The manager
> reported that his guys, "said the tread was a
> smooth as glass, and only one tire needed a
> *slight* rebalance." He indicated that his test
> drive revealed to him a problem with my transmission.
> Please don’t get me wrong. I suspect I received
> the runaround, but you know what they say….
> "Never say never!"
> Is this transmission theory valid? The truck is
> in great shape, with the exception of this vibration.
> I *need* to know… this vibration is driving (forgive
> the pun) me nuts.
> Jeff
> Houston, Texas
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