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Discussion Starter · #1 · (Edited)
First, I'm not claiming that this is a generic fix for the vibration and wandering that some owners are experiencing. Just passing the info along. I have an A5 with 245/40R18 Pirelli P6 Four Season tires. The car had a tendency to wander slightly at hiway speeds above 50 mph and in the 62-67 mph range I had a slight, but noticeable, vibration in the steering wheel. My vibration was not severe like some have reported. Between the 62-67 mph vibration and the 50+ mph wandering, the wandering was the most annoying. The car also had more understeer on curvy roads than I expected. More than my 2002 A4. While checking the tire pressure in my motorcycles I decided to check the A5. The tire pressures were 32 psi for all four tires. The sticker in the door indicated that the pressure should be 36 psi front and 35 psi rear so I bumped it up accordingly. I've got approximately 150 miles since doing so and the wandering tendency is completely eliminated and the vibration is gone as well. The steering response, while still having an understeer, is what I would expect from a nose heavy car and is less than the A4. That 3-4 psi difference seems to have had a significant positive impact on the road feel of the car. The effect is actually quite similar to my experience with an underinflated motorcycle at normal highway speeds. Running tires down 3 psi on my bikes has a significant impact on cornering feel. Running lower pressure is good at track speeds of 120 to 160 mph but not so great at 70 mph. So, you might wanna check your tire pressure. The lower the aspect ratio of the tire, the more impact running a low pressure will have on feel.
 

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Just checked my tyres now as well (just in case)...
Luckily all OK..... got to admit it was a pain in the arse trying to figure out which was the correct pressure from the sticker (as there were 3 different categories).

Got to admit, I wouldn't have put wheel vibration down to under inflated tyres, but then what do I know!!! I would have thought it was wheel balancing, but at least I'll know for next time.
 

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Discussion Starter · #3 ·
Again, I'm not claiming that increased tire pressure is the solution for all A5 vibration problems and I'm not an automotive tire expert. However, I would assume that an automobile tire responds similar to a motorcycle tire running low pressure, although the dynamics of steering and cornering are very different. For motorcycles, running low pressure means that the sidewall of the tire can deflect more than the design intent unless heat buildup and centrifugal force from high speed (100+ mph) counteract the reduction in sidewall stiffness. This deflection is experienced in two primary ways: (1) the handle bars can shake when running in a straight line with very light or no pressure on the bars {similar to steering wheel shake?} and (2) the bike steering feels mushy when leaned over going around a corner {similar to understeer?}. That said, I can make a case for lower than design pressure, and the resulting sidewall deflections during driving, having an impact on steering wheel vibration, wandering, and understeer. The degree of impact is dependent on a number of issues (outside temperature, tire temperature, tire rpm, sidewall strength, ect). All I can say is that running the Audi recommended pressure rather than the pressures set by the dealer cured my problems as best I can tell.
 
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