Same size front and rear tires - balance issues?

Z06C5R · 03-26-2008, 07:26 AM

Just bought a 944, and it's going to need new tires all-around: The fronts are worn and the rears are way too big for the rim width. I'll have to double check, but I'm pretty sure the front and rear rims are both 7 or 7.5". I've noticed that these cars usually have wider rims and tires in the rear, which I'm sure is to make the car more tame... But if I were to run the same size tire both front and rear - say 225/50/16 - would it develop an unsafe amount of oversteer? I'm pretty good with car control and I do prefer a car that's slightly loose, but of course theres a fine line between controllable and unsafe when we're talking road use.

Tidybuoy · 03-26-2008, 07:51 AM

My '88 has 225/50/16's all the way around. I have the turbo phone dials which are 7" in the front and 8" in the rear. The car is fine to drive as is and does not have an unsafe amount of oversteer.

However, the factory calls for 205/55/16's in the front and that is what I plan to do next time around. The wider fronts tend to grab every dip in the road at low speed and I feel that I have to really pay attention on rough roads - I did not have this problem with the original 215/60/15's that were on the car.

It also depends on the tire. I have Potenza RE70's which are fairly firm riding.

Vern

Aufgeladen944 · 03-26-2008, 07:54 AM

I think it would understeer if you put wider tires up front, not oversteer. As in, it would plow ahead more, not slide out at the rear-end.

I just got new tires. They get put on next Tuesday yay!

JustinL · 03-26-2008, 08:05 AM

Quote:

Originally Posted by Aufgeladen944

I think it would understeer if you put wider tires up front, not oversteer. As in, it would plow ahead more, not slide out at the rear-end.

Wider fronts reduce understeer/increase oversteer. 944s have some understeer from the factory so dialing some of this out is a good idea from a performance perspective.

I run 245/45/16 on my n/a and 255/40/17 on my turbo at all 4 corners for my street tires.

Z06C5R · 03-26-2008, 08:11 AM

Wider tires equate to more surface area in contact with the road, thus more grip. So were all 4 tires the same size, grip produced by the tires would be the same at all 4 corners. Therefore, the suspension tuning and inherent dynamic properties of the car would shine through. I know the 944 has an almost perfect weight distribution, so I wouldn't expect it to be very extreme in either direction. Of course thats all theoretical - the fact that it's very common to see wider rims and tires on a low-hp RWD car tells me that there's some degree of oversteer built into the car. But as long as it's not going to be a tail-happy monster, I'll be fine.

I'd test the balance myself, but I don't really trust either set of tires I have on it now to give me an accurate feeling for the chassis' properties.

Dantilla · 03-26-2008, 08:23 AM

Most 944s came from the factory with the same size tires all the way around.

The later cars were a bit wider in the rear to induce a bit of understeer, which is considered safer for the average driver. Pretty much every car sold today has mild understeer from the factory.

A 944 is most fun to drive with equal tires front/rear. Beautifully neutral handling. Really fun to let the tail drift out now and then, with complete control.

I have owned, autocrossed, and raced several 944s. While they look good with wide tires in the rear, that really messes up their wonderful handling.

Same size all the way around is the way to go. I currently race an early '85 with 225/50/15 Hoosiers on all four corners.

ryan951s · 03-26-2008, 08:24 AM

I run 225/40/18s front, 275/35/18s rear on the street on an '89 Turbo, and it definitely understeers!

At autocross and on the track I run 245/45/16s all the way around and the car is much better balanced. It's very stable through the slaloms and not tail-happy at all. The tail will come out with the throttle (and it'll come WAY out if you kick the turbo in at the wrong time!), but the car is so well balanced that you really have to be doing something stupid to get it to spin.

Dave L · 03-26-2008, 08:29 AM

Here are the stock tires sizes that came on the 944 series.

http://www.cantonia.com/wheel_offset.html

FWIW I run 225/245 on 16inch 7&8 rims for the street and 225/50's on 15x7's for the track

Aslet · 03-26-2008, 08:44 AM

Same size all around on the NA is factory setup, and a great ride. The more powerful turbo and S2 have bigger in the rear, but this is also since they have the power to use the extra traction for acceleration.

Z06C5R · 03-26-2008, 08:56 AM

Alright, sounds excellent. 225/50/16 all around it is!

924RACR · 03-26-2008, 10:24 AM

The cars are inherently balanced, as alluded to previously, not just in feel/handling but more fundamentally in weight. Except for the turbos/S2's, they don't have enough power to really throw things out of balance with throttle-on oversteer, so there's no need for bigger tires in the rear.

An added bonus - when properly maintained, this balanced chassis also means very nicely balanced tire wear, even on-track at 10/10ths.

Aufgeladen944 · 03-26-2008, 01:00 PM

Ahh mine is the turbo. Perhaps that's what I confused with.

Sorry!

Tervuren · Tervuren's Garage

Quote:

Originally Posted by Z06C5R

Wider tires equate to more surface area in contact with the road, thus more grip.

Not true - more surface area in contact with the road results in lower pressure on the contact patch - less grip. The advantage of wider tires is very complex - but basically this.

When you work the tires hard - you get heat, too much heat, you lose grip. Wider tires can disperse more heat, and having less contact pressure also means there is less friction(less heat) per area. A narrow tired car can be faster for one lap, but wider tires will win races.(hence, any auto crosser will swear wider rears slow them down) The rear tires have to deal with acceleration braking and cornering. This is the real reason a lot of cars have wider rear tires than front. Also, narrower tires will have more lateral grip, but less longitudal. The rears are not under the same lateral strain as the fronts, so having more braking/acceleration traction is a distinct advantage.

Tires also have performance dynamics where too much weight and the level of friction begins to disappear. Hence why newer much heavier cars are getting wider and wider tire sizes. The wider a tire, the more difficult to keep the entire contact patch connected as the car's suspension and body moves.

I have 205/50's front and 255/40's rear on one of my cars, sport suspension, and it has no trouble inducing oversteer whether I run it with 205's on cookie cutters all around, or the 17" wheels.