Ride quality and type of pavement?

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Hey fellas,
Try to figure this one out. Wife's truck, 98' Sonoma SLS, 4.3L/4L60e, 3.42's, 34K miles, I hardly ever drive it. Stock 16x8 aluminum wheels,
Firestone Firehawk SS20's with less than 10K on them.
Wheels are perfectly balanced, vehicle is aligned, all front end components are tight, suspension components all check out.
The truck rides as it should EXCEPT when you're driving on newer asphalt and only between 60-65mph. On a freshly paved, smooth road it feels like both rear wheels are out of balance. Can't feel it in the wheel so much as you can feel it thru the seat in the lumbar area. On an average road it rides fine.
It's weird as hell, you can be driving on an older section of road, everything is fine, then you hit a construction zone with new roads and it feels like your inside a paint can on the mixer! End construction zone, back to older road, everything is back to normal. If I get it up to anything above 70mph the shimmy is gone.
Anyone have a similar experience? What was the culprit?
Doc
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Hello Doc,
Some kind of resonance at the "sweet spot" speed range generated by the combination of the new road surface (not fully finished, as in needs more steam rolling), the tires (as in tread design and pressure), suspension and frame of the vehicle whereby the combination of all of the above causes the tire/road contact area to be reduced enough to affect drivability? (Slightly similar to a vehicle hitting the rumble strips on the shoulders of a highway?) Gee, I don't know -- just drive at 70 mph, Doc...
Regards, Franko

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""Doc""

i've had the same problem in the past. the only causes i found for it was mud in the rims or the the road repair was slightly uneven. caused by the guy on the roller not packing the surface right. as time goes on and cars smoth out the new spots you will find that it goes away.
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I guess that it could be the new road condition as S.S.I.N. stated in his post. Especially if on any other smooth surface you don't feel it. My 99' K1500 would act this way on some road surfaces that were generally smooth. Boggles the mind as you would think that all would be good on a smooth road, but I saw similar issues as you are. I recently replaced the factory Good-Year tires with a set of Yokohama Geolanders not because of the ride but it did need tires. I don't notice the shimmy now hardly at all. I do know that in some radial tires there is shifting of the belts as the tires are used. I'm attributing my shimmy issue to that.
Brian
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P.S. I think that why you don't feel this same phenomena on a passenger car is due to the truck being so light in the rear end.
My 2 cents.
Brian
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Try removing the tire and wheel assembly:
Roll it and listen for noise on the inside. If you hear any noise at all. it's the talc and rubber balled up which gathers into a mass at the speed you mention.
Are you using a good computer balancer? Sometimes a bubble balancer will show this, where a computer balancer won't, as a lack of repeatability.
Refinish King

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Inline...................

Did that to check for mud inside the rims...................

Yeah I did that too, no noise!

The shop I take it to uses a computer balancer, dunno if it's a good one or not.
Doc
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I would try rebalancing the tires on a Hunter load force balancer - Discount Tire has them (i think each store has only one, they only use it if you specifically ask for it) as do most Cadillac/Buick dealerships.
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Markeau wrote:

What makes this machine so special? Does it cost any extra to have your tires balanced on it?
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The machine has a Roller in the back, when you set the Machine to do a Road Force Balance, while the machine spins the tire, this roller will push down on the tire with a lot of force and simulate the tire driving down the road. It can then measure if there's something wrong with either the tire or the wheel. This is something most Balancing machines out there CAN'T DO! You can balance a tire perfect, and the tire or wheel can still be screwed up. It also will do a better Balancing job. Especially if you want to do a Dynamic Balance and have No weights on the outside. Usually you'll end up with a Static Balance, but on the Hunter it's easy to do a Dynamic and have both sets of weights on the inside. Nice when you have some 20's 22's, or well whatever nice Aluminum wheel you happen to have.
You can when road forcing have the Machine tell you where to have the tire located on the wheel to get a lower Roadforce reading. Sometimes it'll make a huge difference. Other times, the Tire is just BAD and needs to be replaced.
You can then get your Results Printed out.

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"JBDragon" <joewald at bigfoot dot com> wrote in message

Off to discount tire I go, hi ho hi ho hi ho.................................
Doc
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Not to mention:
It can give you up to 12 configurations of mounting, to eliminate tire induced pull phenomenon.
Refinish King
"JBDragon" <joewald at bigfoot dot com> wrote in message

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It puts up to 1500 lbs of force on the tread.
Refinish King

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Model #975.
Refinish King
PS Make sure the bearings are good. They only last approximately 100 tires.

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had this happen to me twice on 2 different occassions,where the tread plies were beginning to separate internally, an air bubble developed between the plies at a certain speed range and dissipate if i went faster or slower than that speed (just like your situation, it wouldn't shimmy real bad but you could feel it, and only on smooth roads), couldn't figure what the hell was going on (tires and suspension checked out okay) until the outer casing finally split and a big rubber bump popped out through the treads. these were radial tubeless passenger car tires, 2 different brands (i forgot what brands) and it surprised the hell out of me the first time it happened (when the internal ply popped out the truck went bump, bump bump big time)...after putting on the spare and new tire, everything a-ok..
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That sounds exactly like what I'm experiencing...................thanks for the insight.
Doc
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The Hunter would have found it early!
Coats is releasing a laser version, less roller wear. The loaded cycle goes slower than the balance cycle on a Hunter.
Refinish King

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Doc,
Just a thought, maybe your shocks are the culprit?
Brian
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I wish, shocks are fine.
Doc

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This might be out of left field... but I'm wondering if it's possible that the rough road imparts enough vibration to dis-allow any resonance from building up. Whereas the smooth road DOES allow a certain resonant vibration to form at a given speed.
What do you think?
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