rsabeebe

i'm surprised someone didn't pipe in with "about an inch". i guess i'm that smartass.

i'm running 19" Fikses on a C4S, so i have no specific comment on 17s to 18s. my 18" Twists are sitting in a basment in MD. i did just switch from 19" Ruf reps to the Fikses and the new tires made a big improvement is overall ride quality. as many have said, the tires will make a large portion of the difference and the overall weight of the respective 17s and 18s. you could certainly find 18s that could potentially be lighter than some 17s. it's not a given that one will be lighter than the other.

nile13

I can agree with your other points. I can not agree with this one. As a matter of fact, I strongly disagree.

The idea of going from 205 to 255 is not just to get ultimate grip. It is to balance the car and dial out some understeer that was built into a street car for reasons that I'd be happy to discuss. You are confusing turn-in with "steering precision". There is no steering precision in an understeering car and 993 out of the box with 205/255 combo is just that.

If you need to be enrolling with F1 team in order to feel the difference in understeer on a 993 with front 205 vs. 225 tire... ummm... I can guarantee you that you will feel understeer at very low speed in regular daily street driving. And understeer is dangerous. Oh, and another things. I have not found a 993 steering to be precise with any particular tire/wheel/suspension combination, and I've driven quite a few on street and autocross courses. It's a car controlled by the right foot. The tires and suspension can help a bit with basic characteristics of the car. The actual precision in turns comes from the right foot, both in turn-in and coming out and we are talking about fairly low non-F1 speeds. I'll wait for track people to chime in and agree/disagree on this.

p993c4

I’m not a track person, but I suppose 4WD is additional parameter in this equation (referring to the earlier message, #32), especially in wet conditions, right?

Cupcar

Man there are so many variables here rim width, wheel weight, wheel diameter, tire manufacturer and type of tire, tire size that change the feel, ride, adhesion and look.

Having done this type of change on many cars over the years, Porsche and non-Porsche, my impression is that one can say GENERALLY that going to a larger diameter wheel but keeping the tire section width, rim width and overall diameter constant, the larger diameter wheel combination is stiffer and therefore has crisper steering feel, more edgy handling at the limit and a stiffer ride.

orangecurry

You seem to have gone off on a tangent. I wouldn't dream of telling anybody that their car that I've never seen understeers or not - I didn't even mention it.

What I said was that you will notice the difference through the steering between a 205 on a 17x7 vs a 225 on an 18x8.

Depending on the relative tirewall stiffness, this may or may not be the result you hoped for.

orangecurry

I just need to point this one out - please read what I said:

I said 'becomes an issue' not 'feel the difference'.

nile13

If understeer is not an issue for you at street speeds, you are a much better driver than me (and probably 99% of other drivers).

Understeer is a huge problem for anyone going 5 miles faster than needed coming onto or off of a highway ramp, for example That's a very street speed. I, personally, started autocrossing many moons ago because I scared myself on the street several times - a mid-engined car had a significant understeer followed, by a snap-oversteer. Of course in a rear-engined 993 this is even more so. But, hey, if that is F1 territory for you or you are capable of overcoming understeer by sheer driving skill, you win in life. Me? I went to 225/255 combination on my street 993s.

nile13

Absolutely correct. I should say that my suggestions are purely regarding C2s on the street, this is what I'm mostly familiar with. From my limited experience with C4/C4s they tend to understeer even more than C2. Thus, they need some conscious effort to try and make them more neutral via suspension and tire choices.

vincer77

I would disagree with you here. Why then are cars designed by OEMs to inherently understeer? Because it is safer than a car that oversteers.

Also, 5mph more than needed may not be highway speed if your need for speed is great. My car has no safety issues related to understeer on a highway ramp unless I am trying to double or more the posted speed.

On a related topic Nile, we had been conversing about the Conti DWS I had on the car and the dislike I had for them because it felt like the car was tail happy on turn in. If you recall, I was running Michelin Pilot Sports A/S with the older chevron-pattern tread in front and the Conti on the rear. I was attributing the handling response to soft sidewalls on the Contis. You suggested that it might be the mixed tire combo. Well, you were correct.

I bought a set of 10-spokes which came with some half worn Pilot Sports A/S with the newer "straight" tread pattern. I put these on the front while I had one of the rears straightened, so I still ran the Contis on the rear, also straight tread pattern. Made a huge difference in the turn in behavior.

I am now running the Michelins all around and loving it. Even though the rears have the older, chevron pattern tread (don't ask).

As far as the 18" rims are concerned, for those that have piped in, no one seems to complain about a harsher ride. That is the biggest factor that has prevented me from going to 18"s.

orangecurry

Seriously? Nahh c'mon you are pulling my leg.

Tens of millions of 993 owners have driven safely on 205/255 x17 since the beginning of time. If it was an issue, then they'd all have been killed to death, and Porsche sued of course.

Understeer is the opposite of dangerous - all manufacturers since, ooh the 1960s? have deliberately made all cars understeer as most drivers have less accidents with that setup.

callipygian 911

The front/back relationship of tire widths does affect understeer/oversteer but, to quote the great Steve Weiner ...

If you're nearing or exceeding the limit of grip with these cars with any regularity, it seems like conventional wisdom suggests a tuned or at least refreshed suspension should, at the minimum, have adjustable (anti) sway bars. All the suspension experts I've spoken with seem to agree on this point. Any tire width combination can be compensated for with adjustable sways.

Car manufactures tend to set up their cars to understeer out of the factory, because it's considered safer than a car that oversteers or is perfectly neutral. It is easier for an untrained driver to regain control over an understeering slide than on oversteering one.

nile13

That's not so, in my opinion. The only diference is - you see what you're going to hit in an understeering car.

More seriously, oversteer is easier to control if you know what you are doing. Understeer, while uncontrollable, makes you scare yourself and reduce speed, hopefully before you kill yourself. This is part of the reason understeer is built in by most manufacturers. In a 993 it's more dangerous than in FWD front engined car because understeer leads to snap-oversteer after the driver starts yanking the steering hweel and hitting brake pedal.
May I humbly suggest a simple exercise? Please don't do it on the off-ramp, do it on an autocross course or skidpad. Drive at just above normal speed in a corner. The car will push. Lift off the throttle. See what happens.

At least this is how I show my students two concepts. One - slow down before going into the corner. The other - don't lift. Both are simple and completely counter-intuitive for those of us with years of bad habits from street driving.

My point - you hardly need "F1 speeds" to experience understeer and feel the effect.

nile13

Sure. You are welcome to argue this with someone else.

nile13

If that was the case, all cars would run on the same tire with the same pressure and just adjust the bar (theoretically).

Adjustable bar does a lot to help in moving balance between car's ends. But... there are at lest 4-5 factors - bars, springs, shocks, tire width, tire pressure/sidewal stiffness. Far, far from everything can be solved with adjustable bars. If you consider more esoteric things, like complete difference in behavior in turn-in, and coming out of the turn...

vincer77

I am familiar with the snap-oversteer of a 911, and if you consider trying to negotiate a 35 mph curve at 60 mph normal speeds, then yes, you can easily get a 911, or many other cars, upset in the process. Good friend of mine who is a good driver (not as good as I thought though) had an S2000. Took a 270 degree on-ramp a bit too fast and nosed it into a lamp post on the inside of the turn. This was due to oversteer.

Always slowing down for a turn - but is still a temptation to want to tap the brakes at times. When I had the quirky handling with the other tires, it was actually great time to develop a technique to steer that big ole butt around a corner.