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  #1  
Old 07-03-2006, 07:42 AM
dcarrb dcarrb is offline
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Intermittent vibration

My car has always had the "old Suby shakes" to a very minor degree at certain speeds but in the past several days I've experienced several instances of unusual vibration.

First time: Last week, the afternoon I had my tires rotated; gently accelerating from about 40 to 65 MPH, suddenly I felt a vibration and thought maybe my guy had forgotten to tighten the lugs on one of the wheels. Smoothed-out, though, and I proceeded to drive another 20 or so miles on a winding country highway between 40 and 60 MPH without incident. All lugs tight. Noticed nothing out of sorts in my routine driving over the following days.

Second time: Yesterday, on a freeway, cruise control set just shy of 80 MPH for a 15 or 20 mile stretch, out of the blue came a minor shudder that vanished as suddenly as it started after 10 or 15 seconds. Felt like a rough patch of asphalt. A few miles down the road, same deal; just enough to get my attention. The third time, a few minutes later, the vibration seemed a bit more pronounced and I killed the cruise and slowed to 65-70. At that she settled-down and I proceeded to drive another 200 miles at normal highway speeds and on stop-and-go city streets without noticing anything unusual.

This sure feels like a balance issue, and it's one heck of a coincidence that it started on the day I had my tires balanced and rotated, but how could a balance problem (wheel/tire or driveline) come and go? The car has been running beautifully, the transmission shifting smartly. She's smooth as silk to 80 and beyond. (Edit: Also, there are no clunks, thunks or any other obvious indications of drivetrain slop.) The only other thing I've noticed in recent weeks is that in my travels to and from Atlanta, the tranny has kicked-down on a couple of uphill grades where I don't recall it doing so before (w/ cruise set), but that didn't concern me. Ummm... should it?

Suggestions appreciated. Thanks.

dcb

Last edited by dcarrb; 07-03-2006 at 11:28 AM.
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  #2  
Old 07-03-2006, 01:43 PM
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Check the ball joints and tie rod ends.

With the tire off the ground try to shake the tire grabbing it on top & bottom then front & back.

First is a balljoint test, the other is the tie rod test.
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  #3  
Old 07-03-2006, 05:21 PM
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Ain't it a beeotch? Here's what happens: you have two or more tire/wheel combos that just happen to be fairly acceptable in their present location. You get a rotation and now you have a 'come and go' vibe.

What would happen if, while the car had all four tires in the air, you positioned all four valve stems at 6:00 - how long would they stay at 6:00?

Now, take two or more tires that are slightly out of balance, have some run-out, have a molecular-density variation in the tread rubber, the rims are tweaked or any combination of the above. Sometimes you might get lucky and 'all the stems are at 6:00' but at other times, due to turning and differences in circumference due to wear or inflation, they aren't. So now 'a little bit here, a little bit there' builds up into quite a noticeable vibration (known as 'tolerance build-up'.) As you drive farther down the road they go back 'out-of-sync' and the vibration reduces/disappears.

The fact that you just had them rotated points out that you just unwittingly had one or two of the worst offenders placed at the front position(s).

So, now that you know what's going on, how do you fix it? If your tires are worth spending money on you really, really need to go to this page and find who in your area has this balancer available: http://www.gsp9700.com/pub/search/findgsp9700.cfm
Go here for more info: http://www.gsp9700.com/index.cfm

If your tires are junk, get some new ones and have them balanced on this balancer. It costs more but it is worth it! The difference will amaze you. I bought four new tires from Tire Rack and found one of them was beyond balancing properly (I have this machine at work. ) I called them and told them I was using this machine and one of the tires was reading 37#. The only question asked was if I wanted the replacement shipped to the same address. Lucky for me the tire showed up the next day! What do you think would have happened if I told them I bought four tires from them and now I have a vibration?
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  #4  
Old 07-03-2006, 05:47 PM
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BTW - this balancer starts at around $9000. I'll bet a dollar to a donut that every tire store owner/manager knows about it.The stores that don't have one are too dang stupid/cheap (funny how stupid and cheap so often go together) to realize the benefit of having one available and are only concerned with grabbing your $7.50 -$15 per wheel for balancing on a machine that costs around $2000-$3000 and will be done in 15 seconds. If you complain about a vibration they'll balance them one or two more times before they begin the song and dance that preludes your frustrated departure. Seen it too many times.

Ya, this machine is expensive and it takes quite a bit more time to balance four tires properly - sometimes the tires and wheels need to be marked, dismounted and re-mounted. So be prepared to pay more - and get more.

Normally I don't recommend customers in the shop but this is one time that I do, unless you are absolutely comfortable with the guys doing the work. Here's why: when the tire/wheel ass'y is mounted to the machine a roller is placed against the tire tread with 1400 lbs. force. The ass'y is then rotated and deflection is measured. Then the ass'y is spun at high speed. At this time a screen appears showing the amount of deflection, in pounds or kilos. A lazy person will let higher numbers flow past. Even Hunter (the manufacturer) has some 'loose' figures. I work on Lincoln-Mercs and while Hunter will tell you 30# on a Navigator tire/wheel is acceptable, I'm here to tell ya that anything over 15# will be coming back to haunt me. Then again, many established Lincoln owners are the epitome of the 'Princess and the Pea'. So, if 15 lbs. is discernable on a Navigator tire/wheel at, say...70 mph, how many lbs. will it take for your tookus to feel 'shaken and not stirred' on tires half that size? Personally, I'd shoot for under 10 lbs. with 15 lbs. max.
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  #5  
Old 07-03-2006, 05:54 PM
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I'm no princess...even at 100mph+ "on the track". I bubble balance my tires and never notice any vibration.
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  #6  
Old 07-03-2006, 06:54 PM
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That's just my point, some they do, some they don't.

But I am smart enough to know that static (bubble) balance doesn't compensate for all issues.

What the heck, I've got nuttin' else to do at the moment, and since nobody asked ( ) let's have a balance class:

Picture a stick, divided perfectly in half by a nail that secures it perpindicularly to another stick, such:

---------|---------

Now, at each end of the horizontal stick is a weight, equal on both ends:

o---------|--------o


When then horizontal stick is spun it spins evenly, no flutter. If the sticks were held at 90° to their above position, there would be no kinetically-induced energy as both ends are in balance. This is 'static' balance, what is addressed by 'bubble balancing'.

Now place one weight on top of the stick and the other weight below, as such:

°------------|-------------○


What happens when this is mounted at 90°? Nothing. The weight at the each end is identical and equidistant from center.

What happens when the stick is spinning? Each weight will try to escape, that is each weight will, due to centrifugal force, try to reach outward as far as possible. This will cause the stick to wobble, as such:

(%---------|---------%)

This portrays dynamic imbalance (to the best of my textual/graphic ability) and I'll guarantee that if you ever drive a car with four properly balanced tires it won't feel anything like a car with four bubble-balanced tires.
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Last edited by Beav; 07-03-2006 at 07:16 PM.
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  #7  
Old 07-03-2006, 06:56 PM
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..........
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Last edited by Beav; 07-03-2006 at 07:17 PM.
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  #8  
Old 07-04-2006, 05:06 PM
oab_au oab_au is offline
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Bubble balancing is for push bikes.

Even motor bike rims are getting too wide for Bubble balancing. The wider the rim, the more, dynamic balancing is needed.

Harvey.
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  #9  
Old 07-05-2006, 06:04 AM
dcarrb dcarrb is offline
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I appreciate the input, guys.

Beav, I was surprised when I dialed-in my zip and found three shops with that rig within one mile of my office; two dealerships and a quick-lube joint. My tires, stock-spec Cooper Lifeliner all-season (hopefully not junk), on original rims, have about 8000 miles and could pass for new, so I'm not ready to toss 'em and will give one of these places a go. Incidentally, the shop that rotated and balanced my tires used a "high-speed" balancer. These guys have serviced my vehicles satisfactorily for years but only in the past 9 months or so began selling tires. That might be telling.

David, I'd be surprised to find fault with a ball joint or tie-rod end. The car tracks well and has no tendancy to follow ridges or wander. Tread shows no obvious abnormal wear. In 20 years of driving Subys I've never had a suspension component replaced (unless you wanna count rear wheel bearings). In the Road Salt Belt that might be a different story.

Thanks again.

dcb
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Old 07-05-2006, 10:35 AM
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Quote:
Originally Posted by dcarrb
In 20 years of driving Subys I've never had a suspension component replaced (unless you wanna count rear wheel bearings). In the Road Salt Belt that might be a different story.
I had to replace 1 ball joint on my SVX so far.

Our Justy had over 300,000kms and the ball joints were still good as new.
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  #11  
Old 07-05-2006, 11:09 AM
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The shop I was using in Hackettstown has that machine and I have gotten lots of wheels balanced there but no more. It depends on WHO is running the machining and placing the weights as to how well the job gets done. It was always an "okay" balance at 60mph but traveling a little faster brings out the lack of attention.

I have also found a shudder from front axles that were worn. This showed up in very slight bends of the highway around 80 and would come and go. The other item is excecessive play in the tie rods or rack. The free play in the steering wheel, car not running and steering wheel unlocked should not exceed 5/8" movement at the OD of the wheel. The SVX has a very tight steering system.
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  #12  
Old 07-05-2006, 11:51 AM
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Ah Beav, I am no much better a graphic but I thought I would give it a try. I like this subject, statics and dynamics.


Now, at each end of the horizontal stick is a weight, equal on both ends:
0
|------------|
.........^.......0

So in the picture above,
please ignore all the little "." as these are required to get the graphics to line up,
the "^" is the bubble balancer, the "|-----------|" is the tire on the rim and it is sitting on the bubbler balancer. The "0" is the unbalance location on the tire. The unbalance on the top is equal to theunbalance on the bottom and they are 180 deg apart form the center of rotation. The tire is in static balance. The first of the unbalances is on the top or the outside of the tire when mounted on the SVX and the other is on the bottom or inside of the tire mounted on the SVX. The tire is in static balance.

This is great for slow speeds and narrow width wheels/tires BUT it does not spin. When spinning the unbalance produces a force not accounted for here.

When the tire spins, it produces a couple. The wheel is still statically balanced or even single plane dynamic balanced but not two plane dynamic balanced.

<---0
......|------------|
...............^.......0 --->

The "<---" and the "--->" represent forces due to the spinning of the wheel.

Call the top one "A" and the bottom one "B".

Lets say that A is equal to B and is 1 gram. Lets say you got a 16" rim and the weights go at a radius of 8" from the center of rotation.

Then the unbalanced force at A is 8 gr-in and the force at B is 8 gr-in AND the static unbalance and single plane balance is ZERO because A and B are opposite.

So lets get crazy and say the that A and B are 1000 gm-inches. This is still statically balanced and single plane balanced and I would not even stand next to the balance machine when it was running.

Take care too,
John
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  #13  
Old 07-05-2006, 12:08 PM
dcarrb dcarrb is offline
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Quote:
Originally Posted by svx_commuter
I have also found a shudder from front axles that were worn.
I'm familiar with the clack-clunk of shot front outboard CV joints, and both my front half-shafts are only at about 35k, but in my mind's eye I envisioned a joint periodically not wanting to articulate freely and causing the entire axle to sort-of go into orbit around its normal axis, sending a shudder through the driveline and steering. I don't even know if this particular scenario is possible...

The steering feels completely secure from the driver's seat but I'll give everyting a tug from underneath and see if there's more give than seems proper.

dcb
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Old 07-05-2006, 03:56 PM
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Almost impossible to feel vibes from a halfshaft unless it's under power/acceleration. Remember, they spin 3.5 times slower than the driveshaft and are small in diameter. It's difficult to be out-of-balance by a great amount when the diameter is so small. CV shaft vibes will come from bad inner joints. I have never seen or read of an outer joint causing a vibration - noise yes, vibe no.

John - Lots of good info here: http://hunter.com/pub/undercar/index.htm
and here: http://www.gsp9700.com/pub/technical/index.cfm
Particularly interesting is how much road force is generated at various speeds by slightly out-of-balance tires.

Harvey - Ever ride a Vespa at high speed? Gyroscopic forces on a ten inch tire at 80 mph is one thing - try coupling that with a dynamic imbalance of the front wheel on a six lane highway during rush hour... (in this case that smilie needs to be a LOT bigger)
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Old 07-05-2006, 04:14 PM
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i would just like to say the GSP 9700 rules but the rest of hunter's equipment sucks
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