deliriousga

Yep. This should make it an obligation to be at SITM, right? The wife wouldn't want me to withold payment now, would she?

deliriousga

Two beers to Andrew.

You know the old acronym KISS (no, not Knights In Satan's Service your mom used to tell you)? The one your dad used Keep It Simple, Stupid!

Saturday: I took her down the road hitting the problem speeds to see if I could feel anything that would help figure it out. I was going to WalMart alone so it was a good time for it. Couldn't feel anything to give me clues, and I didn't have any time to investigate further.

Sunday: Had to go again to Wally World and get some pictures processed. I get to the car and go to put my laptop in the passenger seat when I see the rear, passenger side tire is almost flat. Down to 10psi. I drove across the street to a gas station to fill the tires. Since I just had the tires balanced I had not been concerned with the pressure, but I went ahead and checked it on the other tires just in case while I was pumping. 30PSI ALL THE WAY AROUND! After having thoughts I should not have about the people at Kauffman Tire (especially the one who said "oh, I work on Porsches all the time, race them, I'll make sure it's right". That was a few minutes after I saw him at the door and he asked "what kind of Porsche is that?" I'm sure we all have one of these) I filled them to 36 front, 44 rear.

After filling the tires, I took her up to 120mph and no shake or vibration. Yesterday she got to 130, still no repetitive shake or vibration. Just occasional movement that I believe is from the wind changes at that speed
.

Now I just need to fix the tire before whatever got in it comes out. Hopefully tomorrow night.

Thanks for all of the ideas!

mark kibort

hey, what am i, chopped liver? my post was on the first page of replies!

"sounds like balancing or out of round tires. they can be balanced, but still have vibration at certain speeds.

see if the vibration goes away with controlled braking, and braking while turning. could be wheel bearings, but that is a more high frequency vibration, like hummmm.

mk"

mark kibort

wait a minute, are you saying the 10psi tire caused the vibration, and you are upset with 30psi all the way around? nothing wrong with 30psi around the car, but 36psi and 44psi is way to high. thats max pressure. generally, most street cars will want 32-34psi on all 4 corners and, depending on wear patterns, you can play from there. 44psi is way to high, unless your main goal is MPG.

Mk

James-man

Gee Mark, with the cost at the pump these days, MPG is getting up there priority wise.

What are you California folks paying for premium these days? $3.29 9/10 per gallon?

James-man

I've been watching this thread with some interest.

For me, I could feel the steering wheel shimmy a bit at normal speeds (55-65mph). Thinking out-of-balance being the culprit, I took the fronts off for balancing. In the meantime, I rotated the rears forward, and put on an extra set of wheels with pretty worn tires in the rear (this configuration was license to spin tires).

Eventually, I got around to pulling off the very worn rears and putting back on the balanced pair in the rear. It was getting dark and that was all I had time to do. Anyhow, I still have some steering wheel shimmy at 60+.

Is it remotely possible that I balanced the wrong wheels? Recap: I balanced the fronts & put them on the back, but didn't balance the rears when I moved them to the front. Steering wheel shimmy before and after.

Is it possible for out of balance rears to cause steering wheel shimmy? Doesn't seem likely does it? How likely is it that all four wheels got out of balance in less than 6k miles?

I drive the car at least once a week so the tires shouldn't be flattened from parking.

In mountain turns, the car is still enjoyable, but on long flat straights it is nowhere near as comfortable as it was 5-10 years ago.

The rear has significant road noise, but can this cause shimmy? Alignment? Anyone?

Thanks.

deliriousga

Sorry Mark, but Andrew put the beers on it. You were there first suggesting tires. THANK YOU MARK KIBORT!!!!

Now, for the psi you were talking about. I assumed the listed pressure in the manuals were what they needed to be, not just a maximum. I've played with it a little and I can take it down to about 37psi before it starts to shake again. After that, the shaking starts at around 110mph and it's minimal. If the pressure goes down more, the speed that the shaking starts goes down too and it shakes more as you get faster. I was mainly upset because I had specifically asked for my lug nuts to be put on a certain way, which wasn't done, then the tire pressure was not what the manuals called for.

I read several previous threads about tire pressure and saw that several people had lowered their pressure for better handling like you're talking about. I'm not as concerned with mpg, I just want good handling and no shaking at high speeds. The only drawback is I was going 90mph on a back road on the way home tonight and didn't notice it because it was driving so smooth.

So, is there something else that could cause the shaking at lower pressure? Just wondering if another problem could be disguised until the pressure drops.

AO

Let's see if I can get a couple of more beers out this one...

John, I surprised that it made that mich difference. But when you lower the tire pressure, your are effectively placing a greater load on the sidewall of the tire. When you pump them up, you're now running more on the flat/center of the tire.
Given your description, you probably have a tire with a slight sidewall defect or you have a wheel that isn't quite round - but the latter is less likely as you had these done on hunter machine.

Since you have the luxury of having the Boxter too, I will again suggest you do some wheels swaping in an attempt to locate the culprit. If the tires are as new as you say you should be able to get a replacement for the defective one(s) for darn near free after they pro-rate the warranty.

Start with the fronts. Go ahead and run them at the lower psi to see if you still get the shake. If you still have the shake, replace the wheels and move to the rear. Once the shake is gone, then try to narrow it to right or left.

I just looked up in the manual (44-01), and it states minimum cold pressure for 15" and 16" wheels should be 36 psi for fronts and 44 psi for rears (assuming max pressure allows 44psi), but for 17" wheels (and I assume 18" too) it's 36 psi all the way around.

goliver

John,

The condition that you describe is very likely a tire that has seperated. Do as Andrew suggested and isolate the particular tire and try and get it replaced under warranty.

Regards,

mark kibort

If you got a vibration, its the tires. its only when i get flat spots to i get vibrations. in the old days, wasnt it nice to put on new goodyears and have it run smooth as a cat? well, thats the problem.
I know you probably didnt do the test of the wheel and tire trueness, but i think you will find the tire is a bit out of round, either by age or by a bad fit on the install. (sometimes a re-install can fix it)

usually its a tire manufactured defect that is the toughest to deal with. its so slight, that the tire shop will probably not stand behind and fix it, and it still drives you crazy.

Mk

by the way, we are still at 2.70 a gal for premium. glad our sessions are only 30 min each. i usually run 5 gals per session and 7 gals in the race. so, 46 bucks in gas and $280 for entry fees. not bad for a full race weekend!

Mk

deliriousga

I'm going to do the tests with swapping tires around with the Boxster to see if one or more of the tires have problems. I don't know what will happen warranty wise since I don't have a receipt (PO's PO bought the tires), but maybe it's a good excuse to put some REAL tires on her. I'll be getting into the full testing on the 16th. I have too many things going on this weekend to even think about it.