rich beebe

I'm curious whether owners of the C2S, with 275 or 285 rear tires, feel as though their cars tend to understeer more than they'd like. I own a 93 RS America and I'm trying to determine whether the combination of 225/40-18 front tires and 275/35-18 rear tires will create any unwanted understeer situation. A friend of mine, and RSA owner, brought this to my attention today. I'm currently running 225/40-18 fronts and 265/35-18 rears right now on 18" Fikse rims, but I'm about to change to 18" Ruf rims and I really wanted to put 275s in the rear. The 265s fit without any problems and I now know 275s will fit too. This would be a 'street' only setup, as I have the stock 17" Cup rims for autox/track. Any thoughts? Thanks.

H20NOO

I'm running 225/40 18F and 285/30 18R Bridgestone S02's on Fikse FM/10 wheels. I have no suspension modifications whatsoever (yet).

When I autocross, the car demonstrates excessive on-throttle understeer and feels a bit unbalanced. It will also rotate (oversteer) heavily when trail braking but I can't maintain oversteer when I apply the throttle. It will almost immediately push the front end depending on how aggressively I apply the right pedal.

All that said... On the street, local twisty roads, and other mid-speed driving conditions I find the car to be very well balanced. I've not run it on a "real" track yet.

I think most of the understeer can be dialed out with a bit of negative camber and a good alignment. I've had some luck achieving better balance (in auto-x) by running 32-34# of air in front and 42-44# in the rear.

Good luck!

MC

Robert Henriksen

Like MC says, it's not just f/r rubber ratio, but alignment, suspension, air pressure, presence of a strut brace, etc.

My experience:

225/45-17 F, 255/40-17 R
RoW M030 suspension
0.6 negative front camber, 1.0 negative rear
front strut brace

I'm very happy with the combination! It's a bit oversteery, *IF* I don't nail my turn-in and dial in too much steering to try & get in to the apex. But as long as the nut behind the wheel is tight, it kicks butt.

That said, I find it hard to worry too much about +/- 10 mm of rubber *for street driving*. If I drove my car hard enough on the street to need an extra 10mm of rubber to balance the handling, I'm driving too damn fast! I buy Ray Calvo (tm) RE-730 type tires for street use, too, for exactly the same reason.

rich beebe

MC,

Thanks for your thoughts on my tire combination question. I have no doubt that your car understeers quite a bit, during autoxes, with 225 fronts and 285 rears. I ran my street tires at our last autox, and with 225s and 265s, my car handled the same way. The 275s would strictly be for street use and not run to any limits.

Robert,

I totally agree with your thoughts on street driving and, if anything, the extra 10mm of rear tire is not going to help the balance of my car. I've raced SCCA, I've been a driving instructor at Summit Point and I have a good bit of autox experience and I agree that driving to/at any limits should be done on the track or autox course. I guess I just like the look of a big tire in the back. For autox and track, I'd also go with either a 225 front and 255 rear or 235 front and 265 rear combination.

Thanks.

E. J. - 993 Alumni

Rich,

I run 225s on the front and 275s on the rear when I go to the track with my hoosiers. Stock 95 - pretty aggressive camber 1.6 front, 1.8 rear. I love this setup. I rotate the car all day long. Of course you are talking about street, so I might go a little smaller. I run 225s and 255s on the street.

You should try VIR.

E. J.

rich beebe

E.J.,

When you say you can rotate your "car all day long", are you referring to on-throttle oversteer or under trailbraking? With 275 rear Hoosiers, I'd be shocked if you could rotate that car with on-throttle oversteer. So, your car doesn't tend towards understeer through the corner with applied throttle? If not, you must have a great suspension setup and a good feel for your car. Thanks E.J.

I checked all the clearances again last night and it looks like the 275s are going to be very close. With 48.3mm offset, on a 9.5" rim, I'd be pretty close to the oil line and trailing arm. Looks like I should play it safe and run 265s.

As for VIR, I've raced there a few times, full course. Wonderful layout and terrific facilities. I've been invited to instruct there by the local BMW club (I'm a past president of the NCC chapter here in MD/VA/DC), but I can never find the time to go. I should try harder. <g> The other place that really impressed me was Watkins Glen. What an awesome place to race for the first time. It struck me as very 'professional'. I remember waking up early on Sunday to find track sweepers out cleaning the track and thinking, "Wow. They're doing that for us (SCCA)."

H20NOO

Rich,

Since you are a driving instructor I'll offer a couple other observations... I've driven many other cars and motorcycles at Road Atlanta, Roebling Road, Sears Point, Thunderhill, Buttonwillow and Laguna Seca.. but never a 911. (I regret never driving Summit Point during the 12 years I lived in northern Virginia..)

I find this car to be far more sensitive to weight transfer than any other car I've driven. Simply stated, with 60+ % of the weight in the rear it should be prone to understeer more than most other cars. I've actually found that running a full tank of fuel and leaving the spare tire, tools, etc.. under the hood reduces understeer at autoX events. Probably changes the weight bias by 2 or 3%.

Under braking, I've never driven a car (on street rubber) that turns in quicker. It also holds a line very well with neutral throttle. But, I've learned to modify my driving style to accomodate the very different handling charactaristics of the car. Basically, I try to do most of my turning under braking and then squeeze it hard once I'm pointed in the right direction. This can be difficult in long or double apex corners.

Trail braking is a great way to keep my car turning but doesn't work well in uphill turns! Oddly enough, downhill turns (where I can keep the front wheels loaded) are the perfect scenario for the car and uphill turns (where I can't keep the front loaded or worse yet - must apply throttle) are the worst! All good fun just the same. Sorry if I got a bit carried away...

I love driving my 993 and look forward to making some suspension and alignment changes. I think they will improve grip but fundamentally, the weight x-fer issues won't change.

Driving instructor suggestions always appreciated!

Regards,

MC

H20NOO

One more thing... My car is a narrow body model. I have experienced a little tire rub in the rear when the suspension is fully compressed. I'll likely replace the rears with 275's when the time comes. If you provide an e-mail address I'll send you a photo with the 285's. They DO look great!

MC

Craig in Texas

Don't underestimate the value of tire pressure in controlling understeer, especially in autocrossing. I'm still experimenting with my 993, but I've done a lot of autocrossing in my 300ZX. By reducing the cold pressure 4# in the rear tires I can eliminate the Z car's heavy understeer in tight corners and induce much better rotation of the rear.

E. J. - 993 Alumni

Rich,

Sounds like you have some experience so here goes a short version. MC is correct in what he said. Our cars require a different style. Since you've been to VIR, I will use that for my example. Turn 1 I understeer BADLY. So I trail brake a lot and as I approcah the exit, I leave my right foot on the floor and stab the brakes pretty hard with the left foot to eliminate the understeer - works great.

I can't think of any other turns where I understeer though. I drive pretty hard (about 90% all the time) so I am at the limits most of the time.

As Craig says, try your pressures. I use 33 35 cold with the hoosiers.

There are some other methods to eliminate the understeer too, but I don't really recommend them. You could carry too much speed into most of the turns, certain to cause oversteer. Then you could lift in the mid of the turn, also certain to cause oversteer. Finally, you could turn in way too early. All of these will give the desired effect.

Seriously though, our cars will understeer some. No, I am not oversteering on the throttle (not enough power) and no, I have the stock 030 suspension with an aggressive alignment.

The more time you spend with the car on the track, the more you will grow to love the handling. The understeer is not that bad.

YMMV

E. J.

rich beebe

I appreciate all your thoughts on this understeer issue. I was originally only looking at this from the standpoint of a wider rear tire to front tire relationship and only under street conditions (fast sweepers, rain, etc.). I agree with what you guys are saying about the car and what it does on the track, but I'd try not to use a 225/275 tire combination on the track/autox course, to avoid the exact problems we are currently discussing. I appreciate all your thoughts on the car though, as I'm new to the 911 and it's obviously different than all the BMWs I've driven on the street/track/autox. I used to autox/track an E30 M3 and it was wonderfully balanced. Turn-in was amazing and it had the best on-throttle oversteer of any car i've driven so far. We currently also own an E36 M3 and turn-in on that is very good, although it tends towards slight understeer. Driving with so much weight is the rear is a new experience
for me. As you said Matt, this car is very sensitive to weight transfer. Your idea of leaving the spare tire, tools , etc. in the trunk should work very well, increasing the weight on the front end, where it needs it. I'd also say that the advantage of a full tank would probably outweigh the penalty of the additional weight. Although, the gas tank certainly is a lot higher than the tools, spare tire, etc. I'm looking forward to learning how to make this car fast out on the autox course next year (basically quit autoxing when i went racing a few years ago and I'm looking forward to some autoxing again. I just couldn't fit them both into the schedule.). Maybe I'll even head down to VIR again at some point.

So E.J., do you really think lifting in a corner will help the car rotate?