What makes tires go out of balance?

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It never fails, I can have a new set of tires installed on my vehicle and before it's time to replace them, they have to be rebalanced because one or more of them start shaking from becoming out of balance. I'm a fairly mechanically inclined guy and I pretty much understand the mechanics behind just about everything automotive related, but I can't for the life of me figure out how a tire that was once perfectly balanced becomes unbalanced over time. I know it isn't because one or more of the wheel weights came off because they're all still there. This phenomenon makes no sense whatsoever....
 
Just had this done for the first time on the 2012 Camry that had 20k miles on the RT43. Had them balanced at Discount Tire for $8 each. He said normally they charge $10 each but would give me a break. I had these tires mounted at a Ford Quick Lane. From what I've read these tires have a history of needing rebalancing.
 
Seems to be a tire dependent phenomenon. I have had some tires that that would need rebalancing by about the 2nd rotation and others that didn't get shaky until they were worn out and it was time to replace them. On the ones that had to be rebalanced, once was all it took however.
 
oddly enough, I have never had to rebalance bridgestone tires. I had one particular set of Turanzas from around 1999, which were installed on a vehicle with 36,000 on it. When I sold it with 112,000, those tires were still smooth as glass with 1/3 of the remaining tread, no feathering, no cupping. The rubber was harder by then and wet traction was not as good, but they were still predictable and controllable in the rain. I usually don't get that high of mileage out of tires, 40-50 depending on the tire, but have yet to need to rebalance.

That said, I did have one batch of new firestone LE2s that were installed with 3 of them being defective. The installer replaced all 4 two weeks later and they lasted until a rear-end accident totaled my step-daughter's suv.
 
If you imagine mounted tires needing a certain amount of weight to counter the heavy spot on the opposite end, then as the tire wears, it would seem to me that weight ends up being too much in relation to the weight of the tire.

It seems to me that a tire would naturally need to be rebalanced at some point.
 
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I think I remember capriracer saying tire diameter changing with age( not just due to treadwear). If that's so then the balance point would surely change.
 
Accelerate hard and sometimes the tire will spin on the wheel. I've had it happen on multiple vehicles which could necessitate a rebalance.
 
Originally Posted by RhondaHonda
Accelerate hard and sometimes the tire will spin on the wheel. I've had it happen on multiple vehicles which could necessitate a rebalance.
I want to mark my tires/rims with a marker and test this out!
 
There are many reasons but one of them is uneven density of the rubber material. It may have needed 1-2 oz of weight to properly balance when the rubber tread was thick but, as the tread wears down uniformly around the tire, the non-uniformity of the density of the rubber, throws it out of balance.



Ray
 
I feel that since most of the spinning mass is at the tread surface that as it wears away it changes as it gets lighter, and any residual imbalance, or run-out will worsen over the miles as the tire hit hits pavement with centrifugal force, with vibration you can't even feel wearing the tire out of round, which by itself will make a tire unbalanced. Other than that I have no idea? Maybe if you have a tire that takes a lot of weight initially to balance, that might have a slight effect on wear spots, since even though the vibration is gone, the light/heavy spot of the tire are still there, not taking into account the imbalance from the wheel.
The wheel balancer only spins the tire at around 100 rpm, not the 800+ rpm it sees flying down the road. A huge difference in centrifugal forces on the tread surface..............
 
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Originally Posted by RhondaHonda
Accelerate hard and sometimes the tire will spin on the wheel. I've had it happen on multiple vehicles which could necessitate a rebalance.


Heck thats horsepower! Made so much my wheels are misaligned. What a story for Saturday morning at the diner.
 
There may be 2 things going on.

First, no matter what, a tire is always going to develop at least a bit of irregular wear. It might be a very small amount. It might be huge. It just the nature of the beast.

Why? Because tires are never round and completely uniform. There is always some amount of runout and force variation. In fact, every tire has a variable pitch sequence (tread pattern) - with the exception of some low speed tires such as garden tractor tires.

There is also always some amount of imbalance - even after the tire is balanced.

Put these altogether and these imperfections cause irregular wear - which in turn can cause not only more imbalance, but increased force variation.

Second, because tires grow, they may not grow evenly. They may develop some amount of imbalance and force variation.

Put these 2 together - then add the starting amount of imbalance and force variation - and the result is a tire that requires rebalancing.

But I think most of what happens when a tire develops a vibration late in life is because of irregular wear caused by mis-alignment - and by misalignment, I mean anything less than perfect.

Ya' see, there is no such thing as perfect alignment. Every time you turn a corner, the steering geometry changes and the toe changes a small amount. That's called Akerman (variable spelling). There is one and only one speed for which this akerman is perfect - so, of course, that rarely happens.

Then there is also suspension travel. Suspensions don't move straight up and down. There is either some sideways movement or some angling, or both, taking place.

Did we talk about bushings? They also deflect a bit - even steel ones!

All this turning and suspension travel adds to the irregular wear.
 
I took my car in to Discount Tire Friday and they rebalanced and rotated all 4 tires. The shaking was still there. So I took it back and they rebalanced all 4 tires again. The shaking is still there. So now I think I either have a tire that has something wrong with it, like a slipped belt, or it has become out of round, or I have a CV joint going bad. I don't know what else could be causing this issue...
 
Originally Posted by CapriRacer
... In fact, every tire has a variable pitch sequence (tread pattern) - with the exception of some low speed tires such as garden tractor tires.

...




Another common exception includes semi-truck tires. The pitch sequence is often eliminated for the purpose of achieving more even wear over the LONG life of those tires.
 
Originally Posted by grampi
I took my car in to Discount Tire Friday and they rebalanced and rotated all 4 tires. The shaking was still there. So I took it back and they rebalanced all 4 tires again. The shaking is still there. So now I think I either have a tire that has something wrong with it, like a slipped belt, or it has become out of round, or I have a CV joint going bad. I don't know what else could be causing this issue...

Could be anything, like even a brake rotor or something else that had been changed, or needed to be like shocks. Not enough info to make a guess. I would start with new tires assuming you are dealing with old/cheap tires.
 
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In addition to the above,

Running over/into curbs or jamming tires into parking stops can bend/distort the steel belts. Leading to a shake. Even a big pothole can do this.

Quite often the abuse a tire takes is directly related to the shake.

Also, rims can bend ever so slightly. Not necessarily the bead area either
 
These tires are 70K rated tires with 48K on them. Tread looks good yet, so they're not even close to needing replacement (for tread wear anyway)...
 
I'm taking the car into my mechanic next Friday. I have run out of things to try, and he can rule out or identify a problem with the CV joints/axles...if those are good, I would say it's a tire problem...
 
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