Oscillating vibration at high speeds...

[pbastian]

I have a 2000 Boxster 5-speed which I track (~5-6x a year). I just installed new rear rotors and pads, new front pads (with fairly new rotors), and brand new Hankook Ventus V12 EVOs. Rims are 18s. This is my second set of Hankooks. I was running Michelin PS2's before.

I've been struggling with vibrations at higher speeds (+70 MPH). I believe the vibration is coming from the rear because I feel it in the seat, not the steering wheel. I do not know if this can be misleading, sort of like referred pain can lead you the wrong way. Steering wheel is steady, though. With this most recent set of tires, the vibration comes and goes, on about a 2-second interval, at speeds above about 75 mph. When I hold speed with cruise control, you can definitely feel this. This is true at any gear, and with or without the clutch engaged. I do not have warped rotors, as there is no issue with braking. I found another thread on another forum that describes this perfectly, and amazingly, the OP on that 2012 thread also was using Hankook Ventus V12 Evo's. There was some discussion about the quality of Hankook tires on that thread. So, I am reconsidering going back to the Michelins after this set wears out. Would an out of round tire cause this? 

I get that this is a process of elimination. The new tires just came back from being mounted and RF balanced, but I did not witness the RF balancing myself. I may have to get that redone just to be sure. And I don't believe the oscillating nature of the vibration is something I would expect from a tire balancing issue, but I could easily be out of bounds on that.

With the previous set of Hankooks, I was there for the RFB. My rims had the following runouts:

Driver front 0.026, no weights added
Passenger front 0.024, 0.5 oz added to inside rim
Driver rear 0.034, but corrected to 0.014 after indexing the tire, weight added 
Passenger rear 0.023, added 2.5 oz to outside rim

I have zero experience with whether this is a lot of runout, or just ok, but the tech said that the driver rear runout was barely in spec before indexing the tire and getting to 0.014. It was the only tire he indexed. The rims have been worked on in the past to repair damage.

Subject to a rebalancing, I want to get opinions on what else might cause the vibration other than tire related (incl. pressures, balancing, etc)...I'm imagining what else back there could cause this. Things like suspension and sway bars and bushings are unlikely, as all this was upgraded in last two years. Motor mount is 3 years new, and I just had semi-solid transmission mounts put in earlier this year. Exhaust is tight and intact.

Would bearing be an issue? It's got a new bearing on one rear side (installed earlier this year), but I didn't also replace the other at the same time. I've had bearings wear out before, and until now, have always noticed a worn bearing by the sound, vs. a vibration of any kind. But now that I am tracking the car, I am curious if that accelerates bearing wear (I live in South/Central TX).

Hoping some of you experienced folk out there can chime in...

[flyingpenguin]

Hi Peter, the Hankooks you are using have a tendency to "cup" a bit when they are standing for a while. I have found that the issue is worse with lower psi's and in colder ambient temps, but goes away after a few miles of driving as tire temp goes up and centrifugal force rounds them out.

I run these tires on 3 cars and apart from this issue I like them a lot.

On your Boxster keep the cold pressures around 35 psi and see how that affects the vibration.

Joost

[pbastian]

This issue has been solved. Took the wheel assemblies to a wheel refinisher. They straightened 3 of the 4 (both rears, 1 front), and remounted tires on their Hunter GSP9700. Vibration is gone.