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Many years ago - I think 1990 - I got to spend an afternoon at the Del Mar race track watching the Nissan team's Geoff Brabham and Trevor Harris workout suspension setting for qualifying.
At the time they had I believe Bosch instrumentation and after a few laps Brabham would come in and the data was gone over and the decision was something like (making up this, can't recall):
"Let's change the front spring rates up 50 pounds, add 30 pounds of cross weight to the right front, up the front damper bump a click and increase toe on the left rear 0.050 inches and add +1 degree rear wing".
Then Brabham went out and was faster.
So the broad brush strokes of conversation here only scratch the surface of what teams were doing and understanding 20 years ago - today it is even more complicated.
I believe what mere mortals can do is whats being discussed
get the basics right and then tweak w/ the adjustable stuff
basics: spring rates, ride height, shock valving, wheels, tires, chassis setup
tweeks: sways, tire pressures, shock settings(if you have them)
Thanks, Josh. I've been jockeying with the same high-powered machinery for the last 5-6 seasons. Have been able to win the last 3 after going to 275/15 Hoosiers on 15x10" wheels (to bring this back to the topic somewhat)
Interestingly, adding extra 33% (!) of unsprung weight per corner vs. 255/13 Hoosiers on 13x8.5" wheels has been the answer to going faster. I never expected that much more grip and there's been a number of discussions on this with CSP Miatas for a couple of years until all the fast national competition went to 275s. What is truly odd is that the rest of suspension setup remained unchanged and the car is still pretty neutral.
Wow! No kidding? That's certainly incontrovertibe evidence in favor of wider tires and shorter sidewalls. It's amazing what a difference this makes. Very observable and measurable from the driver's seat and the finish line.
I guess the performance advantage from the wider tires was more the than plenty to offset the extra unsprung weight you were carrying.
Originally Posted by Bill Verburg
I don't know how this thread go here form the original post but my opinion on the original Q is that foor normal driving he'll feel little of no difference
...
Bottom line for street use go w/ whatever floats you boat for track use 18s all the way
Anyone who enjoys a nice curve at speed would probably notice a difference, including the OP. While stuck in traffic, I would agree: no difference, for sure. The strength of aircooled 911s is primarily cornering ability (they're not too hard on the eyes either), not straight line acceleration. Hopefully, everyone who drives one enjoys a good corner.
Not to be contrarian, but there is an argument in favor of 17 inch wheels at the track, especially if you're not near the top of your racing class (me): if you like to skid around and learn the balance of your car extremely well at the limit, it is much easier and safer to do so with narrower tires and less grippy compounds (so you can be at the limit of grip at much lower velocities). They're also a lot cheaper to replace than 18" racing tires. After all, my budget is a long way off from launching my own F1 team, and tire costs can add up quickly for us mortals.
Still, when Bill Verberg, Cupcar, and Steve Weiner post, I very often get the feeling I'm in the presence of greatness.
Wow! No kidding? That's certainly incontrovertibe evidence in favor of wider tires and shorter sidewalls. It's amazing what a difference this makes. Very observable and measurable from the driver's seat and the finish line.
I guess the performance advantage from the wider tires was more the than plenty to offset the extra unsprung weight you were carrying.
Anyone who enjoys a nice curve at speed would probably notice a difference, including the OP. While stuck in traffic, I would agree: no difference, for sure. The strength of aircooled 911s is primarily cornering ability (they're not too hard on the eyes either), not straight line acceleration. Hopefully, everyone who drives one enjoys a good corner.
Not to be contrarian, but there is an argument in favor of 17 inch wheels at the track, especially if you're not near the top of your racing class (me): if you like to skid around and learn the balance of your car extremely well at the limit, it is much easier and safer to do so with narrower tires and less grippy compounds (so you can be at the limit of grip at much lower velocities). They're also a lot cheaper to replace than 18" racing tires. After all, my budget is a long way off from launching my own F1 team, and tire costs can add up quickly for us mortals.
Still, when Bill Verberg, Cupcar, and Steve Weiner post, I very often get the feeling I'm in the presence of greatness.
At legal or near legal street speeds, no way any one will notice much if any difference
17s have some advantages: generally shorter and generally lighter, sometimes as low a profile
but are generally not available in the lowest profile and wider sizes
Hoosier has the best selection of 17" sizes. For a 993 their 245/40 & 275/35 x17 on 8.5 & 10 x17 wheels would be a very nice way to go but 245/35 & 285/30 x18 on 8.5 & 10 x18 will still outperform them at most tracks on most 993s that are set up to use them. A stock 993 is so soft and floppy that there is little benefit from really sticky rubber
Few other tire companies have any selection of 17s in a profile below 40
One thing with 17" wheels is it theoretically becomes harder for a driver of lesser skill to master the available additional performance of a low profile tire, so 17" wheel with higher profile tire may produce better times for some depending on the track.
I edited the pics from the BF Goodrich article to show more clearly the difference in working rubber in a high vs. low aspect ratio tire. The low aspect ratio tire has more of its contact patch working at full efficiency so when it lets go, well it lets go all at once.
Also the pneumatic trail that makes the steering light at the limit of adhesion operates in a narrower envelope and theoretically harder to feel.
One thing with 17" wheels is it theoretically becomes harder for a driver of lesser skill to master the available additional performance of a low profile tire, so 17" wheel with higher profile tire may produce better times for some depending on the track.
I edited the pics from the BF Goodrich article to show more clearly the difference in working rubber in a high vs. low aspect ratio tire. The low aspect ratio tire has more of its contact patch working at full efficiency so when it lets go, well it lets go all at once.
Also the pneumatic trail that makes the steering light at the limit of adhesion operates in a narrower envelope and theoretically harder to feel.
that's a good point, R tires in general provide very little warning that they are going to let go, part of that is illustrated here, street tires, even Max perf street tires give you tons of warning.
Wow! No kidding? That's certainly incontrovertibe evidence in favor of wider tires and shorter sidewalls. It's amazing what a difference this makes. Very observable and measurable from the driver's seat and the finish line.
I guess the performance advantage from the wider tires was more the than plenty to offset the extra unsprung weight you were carrying.
Unsprung weight, having to cut up the car to fir the new rubber and slower acceleration since the new tires are overall about 1.2" taller. A lot of negatives, that's why it took a long time to decide.
I actually would love to pick Bill's and others brain on this as I've been thinking about it for a while and only changed my mind based on my own empirical evidence. So I ran 255/40-13 A6s on a sub-2,100 lb car. Went to 275/35-15 A6s. Since one was run on an 8.5" rim and the other on 10" rim, the barrel distortion effect of the sidewalls is fairly small. Also, A6's sidewalls are so stiff that with, literally, 0 psi of tire pressure, you can't visually tell that there's no air in the tire until you jack one side pf the car up.
If we look at the numbers, 255/40 has a sidewall that is 102 mm tall. The 275/35 has a sidewall that is 96.25 mm tall. So we are talking about 6% difference. In view of the slip angle discussion above, does it explain the fact that I ended up with the same alignment, swaybar, shock rebound adjustment and tire pressure setting on 275/15s as I ran on 255/40s? Or should I try some adjustment that I'm not thinking of right now?
Mike.S, the merest of mortals.
PS. I'm a so-so driver. I'm a better instructor. One of my students won Nationals two weeks ago, one was second. They have not been students for a while and they can beat me ike a rented goalie, but I was their first instructor. I'm immensely proud of that, more so than any of my own driving.
Many years ago - I think 1990 - I got to spend an afternoon at the Del Mar race track watching the Nissan team's Geoff Brabham and Trevor Harris workout suspension setting for qualifying.
At the time they had I believe Bosch instrumentation and after a few laps Brabham would come in and the data was gone over and the decision was something like (making up this, can't recall):
"Let's change the front spring rates up 50 pounds, add 30 pounds of cross weight to the right front, up the front damper bump a click and increase toe on the left rear 0.050 inches and add +1 degree rear wing".
Then Brabham went out and was faster.
So the broad brush strokes of conversation here only scratch the surface of what teams were doing and understanding 20 years ago - today it is even more complicated.
Similar to what I'd brain picked on one of the 935s (Fiitzpatrick, circa 1982 ! Sears?), and definitely pre-data. These guys at this level just knew what to do.
Similar, was cool hanging around the Garretson shop with their 935, and being the beyond-total novice when I was 18-19 asking stupid questions about setup.
Also remember one year when Mears went from nowhere to the front in a Zakspeed Capri/Mustang @ Sears. Klaus Ludwig was the #1, Mears was a guest driver. Wholesale changes that just plain worked the next session.
And the outcome from all this, for non-track guys, is to keep the car as original as possible, use Porsche's suggested tires, and enjoy the car as often as possibe. When you talk with track guys, read this thread firstly, and pretend that you know something...
Pros are at such a high level it is like comparing a T ball player to a Babe Ruth. Without instrumentation and the knowledge of how to use it most drivers end up lost until they have the experience to interpret the feel of the different changes and this takes a lot of track time and experimentation.
But, without any knowledge of what may or may not happen then it would take forever
So, I went to the books (and still can't feel what is happening most of the time):
To get driver perspective go to Mark Donohue "The Unfair Advantage" and "Aj Foyt" with William Neely, those will whet the appetite and are fun to read.
Then, for less fun reads but the real meat in the burger, go to Milliken & Milliken "Race Car Vehicle Dynamics", Dixon "Tires, Suspension and Handling", all of Carroll Smith's "...To Win" books, Haney "The Racing and Performance Tire"
09-15-2014, 02:42 PM
