Tire problem?

dipso · 06:56 PM

Can somebody help me. I have a 75 911S. It has never had any problems at high speed until now.
I had 205/50/zr/16 tires all the way around. I recently changed the rears to 225/50/zr/16. I have 6" and 7" rims I think.
Ever since I changed the rears the car drives terrible about 90. I can barely keep it in the lane and it feels like a pinball. I just had it aligned and new torsion bars (22) in the front done today. It still drives like **** at 90, that's the magic number apparently. I used to have no problems up to 120,130.
Is there any truth to needing to replace all four tires at the same time, has anybody only replaced the rears and not had any problems.
The front tires are at least 5 years older than the 2 week old rears.
The tires are goodyear eagles,I only put a few thousand miles a year on the car so the tread is still decent in the front.

dipso · 09-24-2007, 07:49 PM

anybody........anybody........

PatrickB · PatrickB's Garage

YES!

Scott (450knotoffice) posted a thread about this not to long ago. It can be quite annoying until your new rears are really broken in ~500 miles? It also turned out that his camber and toe was a bit off, front & rear. The new rubber in back only exacerbated this situation!

HarryD · 09-24-2007, 10:41 PM

Quote:

Originally Posted by dipso

Can somebody help me. I have a 75 911S. It has never had any problems at high speed until now.
I had 205/50/zr/16 tires all the way around. I recently changed the rears to 225/50/zr/16. I have 6" and 7" rims I think.
Ever since I changed the rears the car drives terrible about 90. I can barely keep it in the lane and it feels like a pinball. I just had it aligned and new torsion bars (22) in the front done today. It still drives like **** at 90, that's the magic number apparently. I used to have no problems up to 120,130.
Is there any truth to needing to replace all four tires at the same time, has anybody only replaced the rears and not had any problems.
The front tires are at least 5 years older than the 2 week old rears.
The tires are goodyear eagles,I only put a few thousand miles a year on the car so the tread is still decent in the front.

Hi,

Tires over about 5 years old can be too hard and should be replaced to get the maximum performance. This may be part of your problem.

Steve@Rennsport · 09-24-2007, 11:02 PM

This is quite common, especially when the fronts are worn a bit and the rears are replaced.

One end of the car has excellent grip & the other end has FAR less due to full-thickness treads, mold release on the tires, and the rears are not scrubbed in. Depending on how much front tread is remaining, you may not ever get the car to behave like it did previously.

In almost all cases, I prefer to replace all 4 tires at the same time just for these reasons.

Paulporsche · 09-25-2007, 05:04 AM

Your car looks nice. Do you have S or SC rear flares? If you have S, I'm surprised to hear you can fit 225s under there. What is your ride height? As the others said, let the new rears wear in a little. New tires seem to almost always have this problem, although in your case, you should probably change the fronts now too. Conventional wisdom lately is to change tires @ 6 yrs anyway, even if there is tread remaining.

Rot 911 · 09-25-2007, 05:07 AM

I don't think 5 year old tires in the front and new in the back are going to give you the problems you describe. Maybe a difference in traction on curves, but not acting squirrly going in a straight line. I suspect either the alignment was not done right or you have a defective new tire.

widgeon13 · 09-25-2007, 05:15 AM

Quote:

Originally Posted by Paulporsche

Your car looks nice. Do you have S or SC rear flares? If you have S, I'm surprised to hear you can fit 225s under there. What is your ride height? As the others said, let the new rears wear in a little. New tires seem to almost always have this problem, although in your case, you should probably change the fronts now too. Conventional wisdom lately is to change tires @ 6 yrs anyway, even if there is tread remaining.

I concur on the question of what flares are on the car, I just don't see how 225/50X16 will fit on narrow body car, can we have some close up pics.

I had a squirrelly situation on my 74 some years ago but it was from not having the same speed rated tires on the car, and it was scary to drive, only took a minute or two at 60MPH to know it was not going to work out.

tonythetarga · 09-25-2007, 06:02 AM

You mentioned new torsion bars and an alignment, but what about a corner balance? Higher speeds will make an unbalanced car difficult, at best, to drive straight.

dipso · 09-25-2007, 06:12 AM

Thanks for all the advice on the tire situation. I think I am going to replace the front now. I did a little research last night and here are a couple of interesting articles regarding the porsche and its tires.http://www.carbibles.com/tyre_bible.html
http://www.tirerack.com/tires/tiretech/techpage.jsp?techid=26

Here is a picture of the rear of the car , I didn't have any flares added to the car but I did have some work done to the fenders.
Instead of the stock concaving on the fender well I had it worked so now it is convex. I think its called "rolling" not sure.

dipso · 09-25-2007, 06:35 PM

The ride height in the rear is 24", Ride height in the front is 23 1/2".
I just measured the tread depth and there is an 1/8" difference between the new rears and the old fronts. Also I checked the DOT#s and the fronts are 5 years old.The (new) rears are 2 years old. I think the combination of tread depth difference and age difference means new front tires tomorrow.
Thanks for all the input .

Uber-fanatic · 09-25-2007, 08:28 PM

OMG-What a beautiful car...and those Fuchs!! I have nothing to add to the tire discussion, but did you have the wheels refinished? and if you did, by whom? I want to take mine there tomorrow, present them a picture of your car, and say "make 'em like that".

Goth · 08:55 PM

Did you check the tire pressure on all 4 tires? Too much air in the tires will make it a hand full in a straight line. Tire shops love to put 45 lbs in them.

Corner balance off usually causes the steering to pull to one side & makes the back end want to come around under straight line hard braking at high speeds. This does not sound like the symptom you described.

I also agree with the scrubbing in thing on tires new to your car, but I think this would be less of a problem in a straight line, and felt more when taking corners.

Hope this helps...

EDIT: Forgot to tell you how nice your car looks! Sweeeeet!

HarryD · 09-25-2007, 09:07 PM

Quote:

Originally Posted by dipso

The ride height in the rear is 24", Ride height in the front is 23 1/2".
I just measured the tread depth and there is an 1/8" difference between the new rears and the old fronts. Also I checked the DOT#s and the fronts are 5 years old.The (new) rears are 2 years old. I think the combination of tread depth difference and age difference means new front tires tomorrow.
Thanks for all the input .

I am assuming you are measuring to the top of your fender arches. If so, it sounds "wonky". Typically, most folks aim for a Euro height of 25" front/24.5" rear. That gives your car a slight forward rake. Your setting has you with the front higher than the back. Not a good thing.

Paulporsche · 09-26-2007, 09:30 AM

Ditto Widgeon13. Make sure the fronts are the EXACT same tire (except for size) as the rear.

Also, per Harry D the front ht should be about 1/2" higher than rear. For your tires, something like 24 1/2"R and 25F would be more like it. Although, maybe in CA you can go lower because of a lack of potholes, although you have to be careful of the tires fouling the arches under corning. Some guys do like the lowered front look, but that's too "Funny Car" for me, and you should probably get your headlights reaimed then. Did you measure straight up thru the center of the wheel to the arch lip?

dipso · 09-26-2007, 02:54 PM

Thanks for the compliments on my car, it's nice to hear.

Anyways about the rims, I do them myself. Mothers, elbow grease and beer. Don't try with cocktails, you'll never get all four done at the same time. They look even better now, that photo was with chrome center caps and chrome lugs. I recently changed to polished alloy centers and lugs(purchased through the host).

About the height. If the front fender is 1/2' higher than the rear fender,through the center, (I think it is?) and the front measurement is 1/2' lower than the front that means the front is 1' lower than the back.

If a circle has 360 degrees(inches) and the front is 1" lower that means it has a 1 degree rake forward. Correct? I just did that in my head so it could be wrong. That sounds about right though.

burgermeister · burgermeister's Garage

+1 on new fronts. Even same type tires could act different than the old ones - manufacturers make many changes, and 5 years is a long time....

mkossler · 09-26-2007, 05:12 PM

Quote:

Originally Posted by dipso

Thanks for the compliments on my car, it's nice to hear.

... snip!

If a circle has 360 degrees(inches) and the front is 1" lower that means it has a 1 degree rake forward. Correct? I just did that in my head so it could be wrong. That sounds about right though.

The rake forward would actually contribute towards positive caster, which should serve to stabilize the tracking behavior, not make it worse (in case that's what you were thinking). Could you check to make sure that the front and back tires both have the same performance rating (H, V, Z, or W)? A common source of the symptom you describe is differing stiffness / deformation rates of the sidewalls from front-to-back. Softer sidewalls in the front especially will create the behavior you describe. It can be quite unsettling.

Best of luck! I think your decision to match the tires is a wise one.

dipso · 09-26-2007, 06:32 PM

Wel,well,well.
After new torsion rods, a ball joint, alignment, new turbo tie rods and spacers and a trip to the tire shop, it seems all I needed all along was new front tires.
Severe cupping on the inside of the drivers side and minor cupping on the passenger side.
I don't know why the new rears amplified this, but maybe more lean was put on the fronts with the 1/2" height raise in the back,205 to 225. Maybe the back was cupped as well and they were in unison? I dont know.

O.K. Back to my original post, YES, there is some truth to buying four tires instead of two trust me on this one.

P.S. I know the height is what caused this little problem but I like it.

Thanks for helping, people.

450knotOffice · 450knotOffice's Garage

Dipso, as PatrickB has already said, I went through this EXACT scenario a year ago and it lasted up until just recently. My car handled great on old Bridgestone S03's, one of the best tires made. I eventually wore out the rears and decided to buy new tires for the rear only. I bought the same tires, but I decided to go with 245's instead of the standard 225's (on eight inch wheels). Immediately I noticed that my car had been transformed in a very BAD way. The car became so unsteady and squirrelly, especially at speed or during corner entry, that it caused me to lose confidence in it - big time. Within a week, I decided that it had to be coming from the sidewall squirm caused by the fatter tire on the wheel so I convinced my shop to take back the 245's and swap them for the original sized 225's. Well, things only got slightly better. The car still exhibited what I considered to be scary handling traits.

Anyway, eventually I told Tyson Schmidt about this (he's one of the most respected Porsche mechanics/drivers on this forum) and asked him what might be causing this. He immediately zeroed in on the fact that I had replaced the backs and had kept the old fronts. He said the back end, with the new tires and new rubber, had totally different grip characteristics and tread block squirm issues than the front, which had old, hard, worn out rubber with very little tread compared to the new back tires. He said that my situation was a classic example of the back steering the front like a fire engine and was a very good demonstration of why he always recommends replacing all four tires at once rather than the fronts and backs separately.

Now, in my case, as Patrick said, I also had some toe and camber issues which made the situation worse. However, I believe Tyson when he says that tires on these cars should always be replaced as a set of four and never in pairs only.

By the way, for your situation, you mentioned that you had 205's on the rear 7's. Now you have 225's on those same 7's. You will definitely get more sidewall flex than you had before. This is certainly being felt by you, at least to some extent anyway, and when added to the new rears was most likely causing you car to feel significantly less sure-footed.