Generally adding pressure in the front...

I just got back from my Indie for an oil change, and got some advice on a vibration in my front end and wanted to run it by you folks here for input.

Since I got my new Michelin summer tires early last Spring, I had a vibration in my front end that start at over say 50 mph that smooths out around 95 MPH. I first noticed this with my new tires I had installed at the same time I took off my winter Pirellis (I hate them), so I immediately took them back to the tire shop and they re-balanced them telling me they were out (from two days before). The vibration was substantially lessened but I still have it even today. I know, I should have been more aggressive with them but I was stupidly busy at the time.

At the middle of last summer, my indy put the front tires on a balance machine and it came up zeroes. Today, he put them on a Hunter with road pressure and one wheel was 1/4 ounce off. This did smooth it out considerably despite being so little weight.

Now the mechanic really thinks the issue is the front control arms. I am not sure what you call these, but they are the fat arms that are lowest, and shoot out perpendicular to the car - not angled. He showed me the bushing on the inside part of the arm and he could put his screwdriver in between a sliver of rubber and the metal. He said this is normally bonded and if he can do this, there must be more wear inside where it counts. So... he thinks the vibration is from these arms and not the tires. I am not so sure but I can't argue with seeing the rubber delamination.

These arms are original on my 180K miles chassis. What do you think? The tires or the arms? If the arms, why didn't I notice it with my winter Pirellis? I just hate spending the big bucks to replace the arms and get a realignment if it is a defect in one of the tires.

... on the control arms do wear.
180,000 miles is enough to wear them down.
But, it's something that your tech can check by checking to see if there's any movement of the wheel when the suspension is unloaded.
He can also disconnect the control arm from the wheel hub and check for wear.

Also some tires have manufacturing blemishes which make them vibrate even when they are properly road-force balanced.

Happy Boxstering,
PEdro

Pedro Bonilla
1998 Boxster 986 - 311,000+ miles: [www.PedrosGarage.com]
PCA National Club Racing Scrutineer - PCA National HPDE Instructor - PCA Technical Committee (Boxster/Cayman)

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Racecar spelled backwards is Racecar

"Racing is life. Anything before or after is just waiting" ... Steve McQueen as Michael Delaney in "LeMans"

"If you wait, all that happens is that you get older"... Mario Andretti

"Being second is to be the first of the ones who lose" ... Ayrton Senna

first. When playing around with tire pressures I found I could induce a vibration similar to what you describe by overinflating the front tires a few psi.

But check all 4 tires and be very sure they have the proper cold tire pressures. I have found with the factory pressures recommended for my car: 29psi (front) and 36ps (rear); there is no vibration provided of course the tires/wheels are properly balanced.

If the tires pressure proves to not be the cause of the symptom at least you know and it costs you nearly nothing but a few minutes of your time.

Sincerely,

MarcW.

It seems very odd that this vibration came when you changed tires. Maybe one of the tires is not round and is out of shape, sort of like a flat spot. This is going to be way out there, but another possibility, which did happen to me once, is to make sure the front tires are on the front and the rears are in the back. It was a while ago, but I can't recall how I knew something wasn't right.

I always put about 4 pounds more in the front than spec'd. This dials out the understeer. I will give it a try..... rain this weekend so may take a few days. I have a strange neural disfunction in that I can't remember absolute numbers - really it runs in my dad's side of the family - so I can not for the life of me remember what my tire spec is. So I have it written in my manual. I do know I run about 4 pounds higher. I found when I used to track my car that 2-3 pounds over spec dialed out the understeer to perfectly neutral on Michelins and I always go a bit harder simply because I can't tell a handling difference and my tires seem to wear slower. I also had a few pounds to the back tires also but I don't think they are vibrating.

I will post my results when I run a test in few days. Thanx. Boy, you really know how to fuel a neurosis.

... will increase understeer (less grip up front).
Lowering the pressure increases grip and reduces understeer.
In the rear, more pressure - less grip - looser rear end (more oversteer).
Happy Boxstering,
Pedro

Pedro Bonilla
1998 Boxster 986 - 311,000+ miles: [www.PedrosGarage.com]
PCA National Club Racing Scrutineer - PCA National HPDE Instructor - PCA Technical Committee (Boxster/Cayman)

---

Racecar spelled backwards is Racecar

"Racing is life. Anything before or after is just waiting" ... Steve McQueen as Michael Delaney in "LeMans"

"If you wait, all that happens is that you get older"... Mario Andretti

"Being second is to be the first of the ones who lose" ... Ayrton Senna