Balancing a wheel?

[Admiral Akbar]

Does rotating speed of the assembly have any effect? If an assembly is dynamically balanced at X RPM, is it possible that the assembly may exhibit imbalance and/or vibration/oscillation above or below the balance RPM?

Absolutely.. Tire will grow based on RPM.. The imbalanced point could mode in out out based on RPM but the weight used on the wheel don't move... Max

[ederdelyi]

>>Absolutely.. Tire will grow based on RPM.. The imbalanced point could mode in out out based on RPM but the weight used on the wheel don't move... <<

True, but that's not the only reason that a rotating assembly can show different balance points with RPM. The RPM selected for dynamic balancing is most often a mean point in the normal operating range of the assembly. Above and below that point the assembly may show signs of imbalance, as long as the magnitude of the imbalance at the lower or higher RPM does not reach problematic levels or if the assembly will never see the RPM required to induce the condition it's not a problem.

Underspeed or overspeed certain assemblies and the results from imbalance can be quite dramatic.

[Ken R]

Remember how they used to balance car tires in the old days (before machines)?  They'd spin the wheel up to 60 to 70 mph with this gadget attached to the rim.  The gadget had thumbwheels in line with the axle.  The balance guy would adjust the thumbwheels with one hand, his other hand on the fender to feel the vibration.  I watched and watched and never figured out what they were doing.  
Some of the techs would put his hand on the fender and raise his index finger; I was told that the feel was more sensitive that way.  
Others would throw a few drops of water on the fender and watch them wiggle with the vibration.  

I do remember that at certain rpm, the vibration would resonate with the car more than other rpm.  At 50 it might shake significantly, but at 70 it smoothed out.  I imagined that the amount of shake due to out-of-balance was the same throughout the rpm range (more or less with centrifugal forces) but that the car body had a resonance that exaggerated the vibration.  

[Dan89flstc]

>>Got me to thinking<<

While yer at it ...

Does rotating speed of the assembly have any effect? If an assembly is dynamically balanced at X RPM, is it possible that the assembly may exhibit imbalance and/or vibration/oscillation above or below the balance RPM?

The rotating speed can have an effect on the unbalance of the wheel/tire assembly, since when the speed of the wheel increases, the centrifugal force can change the shape of the tire, a good example is a dragster or funny car tire that grows as speed increases.

These are the type things that manufacturers test, so that as long as you stay within the speed rating of the tire, the unbalance should stay within a safe limit, assuming the tire was properly balanced.

I have seen some rotors shake while accelerating in the balance machine, usually at about half way up to balance speed.

I`m not sure what is happening there, as I`m not a rotor dynamics engineer, but they seem to steady out just above those speeds. We do have a minimum speed we must spin the rotor, well above the speed they seem to shake at.

Depending on the type of rotor the unbalance may change as RPM increases, long rotors that can flex will be affected much more as the speed changes.

Most jet engine rotors remain pretty stable as speed increases, because they are fairly rigid.

[Dan89flstc]

Anyone ever balance  the wheel before mounting the rim strip, tube and tire? Was wondering if it was worth it. Was just messing around with a stock one in the man cave for the heck of it. Actually was surprised how far outta whack it was.

Sorry for the severe HI Jack of your thread...I doubt there is any value in balancing the wheel before the tire is mounted, because if you balanced the wheel by itself, then mount the tire and re balance, you will still be using the same correction method.

However, there is not harm in finding where the wheel is heavy and mark that spot, then when you mount the tire you would put the heavy part of the tire 180 degrees out.

I never could remember if the dot on the tire was at the heavy spot or the light spot.....

[ederdelyi]

Dan,
I wasn't really asking if they do ... I know that they do. I was merely pointing out that dynamic balancing is most effective within a RPM range, and that an assembly can exhibit undesirable characteristics when operated outside of that range. The physics and vector analysis is quite complex and beyond the scope of this thread.

Yep, gas turbines, "jet" engines, helicopter rotors, and turbo props run at either a fixed or narrow RPM range. Put your hand on the airframe or engine nacelle and feel the vibration during spool up. At some point it will be quite pronounced and smooth out as the R's reach normal operating speed. If the mechanism were capable of reaching the high speed harmonic without self destructing it would again start to show the signs as it approached the resonance point.

Interesting stuff. Thankfully, most street vehicle wheels don't have the mass and rotational speeds to be a big problem with the balancing methods commonly used.

[ederdelyi]

>>I never could remember if the dot on the tire was at the heavy spot or the light spot.....<<

For most, it's the point 180* from the heavy spot on the tire, and the dot should align with the valve stem or the heaviest point on the wheel. There were a couple of tire manufacturers that did the exact opposite, I don't recall which ones off the top of my head. I know this 'cuz I remember having to check with the maker as there was some confusion with those tires. Just proves the old adage ... a$$ume you are right and you are likely wrong. 

[Admiral Akbar]

I was merely pointing out that dynamic balancing is most effective within a RPM range,

If you balance something perfectly, it won't vibrate regardless of RPM range.  The issue is that the nothing is perfect and the supporting structure may have a resonate frequency that can amplify this imbalance.. The another way is for the item under dynamic conditions to flex and therefore go out of balance.. External forces can also create vibrations, air, oil whatever.. but in this case the force is external and the balance is still good..

I seem to remember having this argument with VTN

Max

[ederdelyi]

Perhaps "balance" is the wrong term to be using. On a simple disc that has no flex or external forces applied, in theory, what you say is true. This could be argued to the extreme ... real world assemblies do flex, external forces are applied, and they go in and out of resonance as the RPM varies. I've seen it first hand, I'm sure you have too.

Is it a true dual or a 2 to 2 with a xover? Not worth arguing about as far as I'm concerned.  

EDIT: Considering Frank's (VTN) background I don't doubt that you two likely butted heads a bit on a subject like this.

[FLTRI]

...If you balance something perfectly

I don't think this can be accomplished in real life, omly ion theory. Just like there is no perfectly straight line, etc
Bob

[rbonner]

Just got back from mounting up new rubber and punching in a new set of wheel bearings on my 79 at my engine guy's shop.  I don't have a bearing puller...  Then Leo and I between gun talk discussed this topic static vs dynamic and most of the time in their shop they just put them up on the truing stand and start playing.  But he's a land speed record guy and the go-fast bikes all get spun. 

Then he mentioned the shop who has the equipment that does his dynamic work...  OH OK well we'll take these over to spin them.  Cool.  They are a metric go fast shop; I'll just tell them this is a 200 MPH Harley..  That will make for interesting conversation.  Like Oh you drop it out of an airplane etc...

BOB

[FLTRI]

Here's an informative article by Yokohama tire company addressing all types of balancing:
http://www.yokohamatire.com/pdf/tsb-TireBalance-12803.pdf
Bob

[FSG]

 Reply #38     

[Admiral Akbar]

EDIT: Considering Frank's (VTN) background I don't doubt that you two likely butted heads a bit on a subject like this.

Yep it was a coupla buttheads, butten' heads.. Liked VTN, he made me think...

Thanks to the additional info both Bob's.. On the Yokohama sheet you can kind of tell the guys first language wasn't engrish.. Max