New Rear Tires Makes Front End Nervous??

[DCOP]

I just replaced my old Dunlop SP40's 225-50-15 with Dunlop 9000's. Now the handling seems different. The front end seems twitchy and overly sensitive at highway speeds. The front tires are almost new, Dunlop 8000's. 205-50-15. Couldn't find 8000's for the rear. The tread pattern is different from front to rear but I have always had the same sizing and didn't notice this before. Tire pressure is ok and shocks seem fine. Any suggestions?? Thanks Darryl

[Paulporsche]

Some thoughts:

1.Could just be the difference between the 8000 and 9000. Different tread pattern, compound, level of stick...

2.Often new tires feel a bit squirrely. Maybe in this case the rear is somehow affecting how the fronts feel. See what happens after a few more hundred miles.

3.Is the size of the new rears the same as the old? If smaller dia they might push the nose up a little.

I'm leaning toward thought 1.

[rvanderpyl]

Did you check your air pressures and make sure the shop didn't change the fronts? Were your front tires rotated at the same time? Any other work done, ie alignment, new balance etc

[tcar]

You said you kept the same tire size.

Generally, mixing tread pattern, tire models (even if same mfg.) is not a good idea, they can have very different handling.

Drive conservatively for several hundred miles, see if it improves.

May have to get 9000's for front, too.

[ToddM]

huge no no

Even the same exact tire by the same manufacturer can behave differently over the lot code

Reduce your pressure in the front and/or increase your pressure in the rear

[john walker's workshop]

15" tires usually have wimpy sidewalls. that's your problem most likely. try more air pressure, or return them as crappy and get stiffer ones.

[rusnak]

Never mix tire brands, models, etc. I think it has to do with sidewall stiffness, which pitches the car on one or two axes.

[burgermeister]

+1 never mix & match, especially on a car with as little understeer as a 911.

The cornering compliance of the 2 tires needs to match (the wider tire with more pressure in it will make the rear less in this case), or the front needs to have larger cornering compliance than the rear. The other way around (rear has more compliance than front) can create an oversteering car.

Tire cornering compliance is a function of tread and sidewall design (and tire width, aspect ratio, pressure, and load)

As tire manufacturers don't publish such data, unless you have a pal in tire & wheel systems at a car manufacturer, your best bet is to keep the same tire front and rear.

Then there is the problem of absolute grip, which may also be different between different tires. The problem is a different one, but with the same end-swapping potential.

[DCOP]

Thanks all for the quick responses. I'll try the air pressure suggestions but I didn't care for the Dunlop 9000 "V" goove tread pattern when I received them from Tire Rack. It is a lot different then the standard tread design of the 8000's on the front. But as you know 15" staggered tire size matches are getting hard to find. This poses 2 new questions. If I switch brands (to find matching models) what would you recommend for my 7" & 8" x 15" wheels? Also if I run 225 x 50 on the rear should I run 205 x 50 or 205 x 55 on the front? I was also thinking of 245 x 45 on the rear if I can find them? The car does have SC flares on the rear.

[dipso]

I had the same problem, exactly. New rear tires and the front end was all over the place.
I replaced the fronts and all the problems went away.
Something about the age difference in the rubber compounds from old to new.
I noticed my front tires were a little cupped too. It seems to me that whatever wear was happening on the front tires was being balanced by corresponding wear on the rears. When I changed to brand new rears the front end was all out of whack.
I know it is not cheap but I think you should replace all four at the same time.