Re: Aftermarket wheel shaking

["Rudy Consolacion" <rconsolacion@kumhotech.com>]

Chris Hlubb wrote:

> Hi guys,
> I have a 98 A4 2.8 Q and just put a set of 17" TSW Evo on. I was 
> initially having a 
> problem with one of the wheels and found that it was balanced 
> incorrectly, that was fixed. 
> Now, at about 65 mph I feel a tight shaking coming from the front end of 
> the car. I can't 
> figure out what's causing it. I have had some Audi enthusiasts check it 
> out this weekend at 
> a meeting in Washington, D.C. The car is smooth until this speed and is 
> at its worse 
> between 65-80 mph. I have only taken beyond that once and it seemed to 
> die off a little. 
> I'm afraid to take it any higher with this shaking. I am hoping that it 
> is caused by alignment. 
> The car is slightly out of alignment because of the new wheels and that 
> is noticed through 
> the strong pull to the side when stopping or letting go of the wheel. 
> The wheels do have 
> the hubcentric rings on them so I'm out of idea's. Please help!
> Thanks,
> Chris Hlubb

If the vibration goes away at higher speeds, then the problem is with 
the tire.  Tire manufacturers have force variation limits for radial 
and radial first harmonic.  Believe it or not, bad tires do leave the 
factory.  You'll notice a high radial force variation right 
off the bat assuming the wheel/tire assembly is balanced.  It will 
feel like a flat spot vibration.  A radial first harmonic vibration 
shows its ugly head in a certain speed range (hence, a harmonic).  
This problem is amplified with low aspect ratio tires since radial 
spring rates approach 1000 pounds/inch and any tire anomalies are 
easily transmitted back into the suspension. 

A lazy tire engineer's fix is to lower tire inflation. Try 26 psi and 
see if it reduces or goes away.  This lowers the spring rate which 
will allow you to determine if the suspension is suspect.

Regarding the strong pull, the first thing to check is tire 
inflation. You can feel a 4 psi difference from side-to-side.  If 
that's okay, then it's the tires.  Tire manufacturers have spec 
limits for conicity.  Tire conicity is defined as a rolling tire that 
literally becomes coned-shaped which results in a side or lateral 
force.  Conicity results from variations in tire construction from 
side-to-side.  The most common problem is off-center belts which 
varies the radial spring rate from serial to non-serial (or inside to 
outside) side.  The tire industry tends towards negative conicity or 
a pull direction to the right in the US.  This is done so that if you 
happen to let go of the steering wheel or doze off, the vehicle will 
naturally go to the right (and crash into a guard rail or ditch) 
instead of into oncoming traffic.  A quick way to determine if 
conicity is the culprit is to swap the front tires. If they're 
directional, don't worry about.  If the pull changes direction or 
goes away, then it's the tires.  If not, then it's the vehicle 
alignment.

Then there's the question of RSAT or Residual Self-Aligning Torque.  
But that's another can of worms.

If the tires are in question, I would return them for another set.  
People pay too much money for high-performance tires to have to put 
up with bad uniformity.  I design these things for a living and can't 
stand it when the factories build sh*tty tires. 

HTH!
Rudy C.
Akron, Ohio
87 5ktq
97 a42.8q
83 928s
70 dodge challenger
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