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  #16  
Old 01-14-2006, 01:55 PM
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A tire/wheel can be in static balance, but not dynamic balance. Mathematically dynamic balance means the angular momentum vector is not coincident with the axis of rotation, which will make the assembly want to wobble, and wobble can induce shimmy without adequate damping.

That's why Mercedes high caster suspensions have steering dampers.

Dynamic unbalance became more critical as the aspect ratio of tires was reduced (larger width to sidewall height ratio) and wheel width increased to match the wider tires.

Many think that the old on-the-car spin balancers offered dynamic balance. NOT TRUE! They only statically balanced the assembly.

Dynamic balance requires the tire to be balanced across it's width. An assembly can be in static balance, but not dynamic balance, but not vice versa. In order to achieve dynamic balance weights are required on both sides of the wheel, except for rare cases.

I have one wheel/tire on my Cosworth Vega that read zero-zero without weights. We even rechecked it. My guy said he's seen that before, but it's very rare.

Duke

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  #17  
Old 01-14-2006, 04:01 PM
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Can rotors cause vibration at speed?

While on balance, vibration and front end components, I recently installed a new set of front rotors - Balo - on my 1990 300E 2.6 (95K). I was careful to thoroughly clean the hub so as not to leave any residue that might cause uneven setting of the new rotors. While installing one of the rotors, I noticed what looked like a "V"-shapped piece of the metal missing from one of the interconnecting "webs" (probably not correct term). The edges where the piece was gone were very smooth - not jagged like they might be if a piece had been broken out. Thought the missing piece might have been machined out to achieve balance. After installation, I noticed a very slight vibration around 55+ mph that had not been there before. I don't believe this is tire related because I had the tires re-balanced, then a new set mounted and road-force balanced. The pedal also does not feel quite right - a barely perceptible bit of pulsation at times, almost as though there is a VERY SLIGHT warp of the rotor. Again, I thoroughly cleaned the surfaces and was careful to torque everything to spec. Is it possible for a rotor to be out of balance? Is some metal machined from the web during the manufacturing process to achieve proper balance of the new rotor? I discussed this with someone who said he has been involved with NASCAR and he said he has never heard of a rotor being out of balance or of how to check the balance on a rotor.
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  #18  
Old 01-14-2006, 04:21 PM
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Oliver,
Ithink you got a defective rotor.

Duke,
If you say the weights have to be on both flanges of the rim, I will have to disagree with you. Your theory may be correct and scientific, but for all practical purposes weights on one flange are all that is necessary even for folks like me who like to open it up across the Mohave Desert on the way to Vegas. The only person I know who might be concerned with your theory is my neighbor who lives 20 miles away. When he runs his car it is around 600 MPH and he has special tires that are Kevlar rapped.
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  #19  
Old 01-14-2006, 06:07 PM
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oliverb

It s not unusual to have a rotor with some of the ventilating vanes machined off at the end.
That's an indication it was necessary to achieve balance.
Autozen could still be correct though.
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  #20  
Old 01-14-2006, 08:16 PM
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Quote:
Originally Posted by autozen

Duke,
If you say the weights have to be on both flanges of the rim, I will have to disagree with you. Your theory may be correct and scientific, but for all practical purposes weights on one flange are all that is necessary even for folks like me who like to open it up across the Mohave Desert on the way to Vegas. The only person I know who might be concerned with your theory is my neighbor who lives 20 miles away. When he runs his car it is around 600 MPH and he has special tires that are Kevlar rapped.
It's not a theory. It's basic physics.

If you don't mount weights on both sides the assembly is probably not in dynamic balance, which may or may not cause a problem.

There is insufficient room between the wheel drop center and brake caliper on my Cosworth Vega to mount stick on weights, and I don't want to marr the outside of the wheel with weights, so I only have them statically balanced with aluminum wheel clip on weight on the inside flange. Since the wheels are only 6" wide with 205/60R-13 tires, dynamic balance has never been a problem. Dynamic unbalance is be indicated if the steering wheel wiggles right-left a bit at certain speeds.

As wheel and tire width go up, dynamic balance becomes a greater issue.

On my 190 I have the tech mount stick on weights on the wheel drop center just inside the outside face. Modern equipment has probes that provide the machine with the distance of the weight planes from the wheel center plane and also the radius of the weight from the axis of rotation. Both these dimensions are required so the machine can calculate the required weight for dynamic balance.

Most who have not been exposed to an engineering course in dynamics do not understand dynamic balance. The angular momentum vector conincident with the spin axis is the easiest way to describe it, but from a practical point, if the steering wheel doesn't wiggle back and forth, any dynamic unbalance present is not sufficient to cause problems. In the case of Mercs, this is true IF the steering damper is okay. If not, get ready for some serious front end shimmy.

Duke

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  #21  
Old 01-14-2006, 09:20 PM
LarryBible
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I have to agree with Duke on this one. It is certainly possible to get lucky with a static balance using weights only on the inside rim. In my experience if you are using Michelins or Kumhos you might get lucky because I have found that these tires typically require a very small amount of balance weight.

Even in a case of a tire that requires a small amount of balance weight if you don't DYNAMICALLY balance both planes the wheel will have wobble and on a nimbly suspended car such as most any MB built in the last 30 or 40 years you will very likely get a vibration.

Static balance will keep the wheel from hopping up and down and is better than nothing, but a proper two plane dynamic balance will almost always eliminate vibration.

autozen, since you only use weights on the inside rim, you may very well lose money to your friend. If you are only using your spin balancer for static balance you are doing no better than can be done with a careful bubble balance job.

Have a great day,
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  #22  
Old 01-24-2006, 07:04 PM
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Peter, I also agree with Duke on this one.

As a person who has been involved with balancing jet engines for 25 years, the proper way to balance an automobile tire and wheel assembly is by the use of a dynamic balancing machine.

"Dynamic" in this sense is the combination of "static" and "couple" unbalance. Static unbalance is a single force acting through the center of gravity of the object that effectively displaces the C.G. from the axis of rotation. It can be corrected by a single weight placed in the plane of the C.G., or, in the case of an automobile wheel, by two weights on both flanges, at the same angular location.

Couple unbalance is an equal and opposite force acting on the object. The C.G. remains on the axis of rotation, however, the principal intertia axis is not parallel to the axis of rotation. A wheel that "wobbles" on a spindle would be a perfect example of couple unbalance.

All automobile tire and wheel combos exhibit a combination of static and couple unbalance. Naturally, the static unbalance component is the most important part of the balance because the forces transmit straight into the spindle. The couple unbalance will produce less force because it is dependent on the distance between the couple legs. So, the forces acting on an 8" wheel will likely be greater than those acting on a 6" wheel.

A modern tire balancing machine can sense both static and couple unbalance and provide the correction required to achieve "zero" unbalance. Please note, however, that these machines are relatively crude and do not have much capability of seeing unbalance that is less than 2 oz.-in. The machine has software that automatically displays "0" when the machine gets below this "magic" number.

Just for comparison, some of my jet engine components weight 250 lb. and the balancing machine is capable of reading the unbalance down to .1 oz-in. in each of the two correction planes. A similar machine, if used on an automobile tire, could measure and display readings down to .02 oz.-in. in each of the two correction planes.........fully 100X more sensitive than the machine in the tire store!
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  #23  
Old 01-24-2006, 07:49 PM
LarryBible
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Quote:
Originally Posted by autozen
Oliver,
Ithink you got a defective rotor.

Duke,
If you say the weights have to be on both flanges of the rim, I will have to disagree with you. Your theory may be correct and scientific, but for all practical purposes weights on one flange are all that is necessary even for folks like me who like to open it up across the Mohave Desert on the way to Vegas. The only person I know who might be concerned with your theory is my neighbor who lives 20 miles away. When he runs his car it is around 600 MPH and he has special tires that are Kevlar rapped.
autozen,

I hope you don't think that I'm trying to be combative about this. I am just trying to show you the difference in static and dynamic balance. For years I got by with static balancing using a bubble balancer. My first 240D in 1977 gave me fits the first time I tried to run wheels balanced on a bubble balancer. That's when I became acquainted with dynamic balance and I am a huge believer. I feel that it is absolutely mandatory on modern cars with nimble suspensions such as the 123 and newer MB cars.

The article explains this well and has diagrams that will help show the difference.

http://www.gsp9700.com/pub/technical/4202T/6GLOS003.cfm

Have a great day,
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  #24  
Old 01-24-2006, 09:55 PM
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I appreciate all you guys being considerate of my professional standing and not flinging me up against the wall. It turns out I am in total agreement with the concept of dynamic balance. I misinturpreted Duke's comment about two flanges to mean two spots on the flange. At my age brain f*rts come far too often. I'm well aware that wheels with KPI need static balance to eliminate front wheel tramp and dynamic balance to eliminate wobble from the wheel changing direction along its axis. Wheels that always remain in the same plane only need static balance.

I pride myself on reading posts very carefully and picking out details that others miss. I guess I blew it on this one. Sorry guys.
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  #25  
Old 01-24-2006, 10:01 PM
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Quote:
Originally Posted by autozen

I pride myself on reading posts very carefully and picking out details that others miss. I guess I blew it on this one. Sorry guys.
........ahh, when you get to be our age........all is forgiven.
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  #26  
Old 01-24-2006, 10:07 PM
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As an addendum: I do not use a bubble balancer. I merely commented on a former shop owner turned Snap-On dealer who commented he could match a high speed balancer with a bubble ballancer. I have a Sun high speed balancer and A snap-On hand crank balancer. I really like the Snap-On machine and have loaned my Sun machine to another shop.

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