On an S8: Dunlop SP8000 vs Bridgestone S-03 Pole Position (long)

[ryoung]

First ... my priorities:

I was looking for a tire with the most direct connection possible between the road and the steering wheel, as wide a tread as possible within the limits of the front fender well, but while still maintaining the outside diameter of the OE 245/45-18. The tire choice quickly narrows down to only the 255/35-20. Even a 305/20-19 will fit in the rear, but anything above 265 is too wide up front, and mixing sizes would only generate more understeer. Thankfully, the roads here in SW Pennsylvania are pretty good, except in the winter when potholes begin appearing, but I plan to run winter tires on the OE wheels during the snow season.

I also have a philosophical reason for going from a 45 to 35 series tire with it's stiffer sidewall. A tire is a spring just like those steel coils at each corner, and the overall spring rate of the car is

1/rate(car) = 1/rate(steel spring) + 1/rate(tire spring)

In the very extreme case of F1, more than 80% of the overall spring rate of these thoroughbred racecars is determined by the tires and less than 20% by the "real" steel springs. The important consequence is that the shocks absorbers are not able to damp most of the motion of the tire! IMHO, the tires should have as high a spring rate as possible, allowing a softer coil spring which can be properly controlled by it's shock absorber. [OK, sorry I got sidetracked.]

A. Why the Bridgestone S-03 Pole Position?



Though the tread patterns of the S-03 and SP8000 are very similar, what attracted me to the S-03 was that

1. The center rib is continuous, versus slotted/cut on the SP8000.

2. The outer four rows of tread blocks are all linked together at full tread depth on the inner side of the block (visible in the photo), versus completely free standing on the SP8000.

The result of all of this is that the tread on the S-03 should be very stable with minimal squirming around during cornering, be very responsive to steering inputs, and be more predictable under hard braking. Also, track tests at TireRack.com show that in both wet and dry, these tires were the fastest "max performance" tire when compared to the Michelin Pilot Sport and the Pirelli PZero Rosso. [BTW, I would be interested in seeing a comparison between the S-03, the Pirelli PZero Yellow, and the Goodrich g-Force T/A KD.]

More subjectively, I'm impressed with the technical innovations Bridgestone claims for their new tire, where the SP8000 has been around for some time and is getting a bit dated. Bridgestone has undoubtedly learned a thing or two from their F1 campaign, and the S-03 is now their flagship tire. Because of the Firestone tire recall fiasco, I expect Bridgestone is sparing nothing to insure that their best tire really is the best.

Last, but not least, I had a sidewall fail on an SP8000 on the first 90+oF day after about 4 hours at 80 mph. The tire was definitely properly inflated (I checked before leaving on this trip), it was otherwise undamaged in any way, but both Audi and Dunlop said it was my fault and wouldn't replace the tire. Both claimed I damaged the tire hitting a curb, even though there were no scuff marks on either wheel or tire. Not the way to win friends and influence people.

B. On to Handling:

First, I'm not going to push the car/tires on the street as I would at the track. Streets don't have safe run-off areas, but also because of rear tire pressure. I set street tire pressure as a compromise between balanced handling, the desire to keep the maximum amount of tread on the road to maximize treadlife, and some kind of reasonable ride quality. The result is probably lower rear tire pressure than many run on the street. I usually ran front/rear 38/32 with the SP8000's; but since the low profile S-03's tires contain 10% less air volume, I started off with 42/35. Looking at the tires, though, this was definitely too much; and I eventually backed off to 38/34.

... these Bridgestone S-03's are definitely different.

The biggest change is that after turn-in, I no longer seem to be making ANY minor steering corrections on most road surfaces. Once you turn the wheel and get aggressively into the corner, that's it ... the car just goes where it's pointed. I never really had any complaints with the SP8000's (except for the understeer, which is not the tire's fault), the S8 handles very well, and I never seemed to notice the very small steering corrections that I was apparently always making. Now that those corrections are absent, the difference is very dramatic.

A subjective feeling is that the S-03's seem to generate grip with less slip angle than the SP8000's. This is a pure seat-of-pants opinion, but I do seem to be cranking in less steering wheel when pushing the car thru familiar corners (instrumentation would be necessary to confirm). One possible explanation ... The Bridgestone dealer where I bought these tires was not able to mount them on the wheels with his tire machines! The sidewalls were too stiff, and he had to send them out to a race prep shop where they have special equipment for mounting low profile race tires. Steering wheel input with the S-03 is directly transmitted to the tread with minimal sidewall flex or tread squirming, of this I have no doubt. I wouldn't say that a 4000 lb Audi S8 is tossable, but it's getting close.

Finally, the reaction to steering inputs feels EXTREMELY linear, also without seeming to be upset by any minor road surface irregularities. Put it all together, and the S-03's inspire much confidence. Maximum lateral g's in a corner? Who knows? But I would trust that's it's easier to get there with the S-03 than the SP8000.

C. Braking:

The OE brakes on my S8 were never even able to active the ABS system. The SP8000's had more grip than the brakes, which is not the way it's supposed to be, and this was the main reasons I upgraded the front brakes to the 6-pot Alcon's with braided SS brake lines. As a result though, my subjective opinion of braking performance won't be directly comparable to other A8/S8's.

With the SP8000's, threshold braking did require a bit of finesse, but this was probably as much a result of the Alcon's as the tire. Get on the brakes hard with the Ferodo DS2500 "street" pad, and if not careful, you're immediately into the ABS. With the S-03's, you can activate the ABS probably just as quickly, but something feels different, and modulating the brakes at the threshold of ABS is much easier to achieve. Unless it's the very stable tread and sidewall under these conditions, I'm not sure what's really causing the difference. A side "benefit" -- the S-03's make a rather pleasant sounding, deep rumbly growl under very hard braking. Again, the S-03's inspire much confidence.

I obviously didn't try any trail braking near cornering limits on the street. But where you're combining cornering and braking, I expect the S-03 to be very predictable and much easier to control than the SP8000. A comparison of 100-0 stopping distances with the OE brakes/tires versus the Alcon's and S-03's would probably be very interesting; I wish I had this quantitative data.

D. Creature Comforts:

The ride with these 35 series tires is certainly firmer, but I think it seems "worse" than it really is. Compared to the SP8000, the S-03 definitely makes a very audible thump going over tarstrips, cracks in the road, etc; and I think I hear the bump as much as I feel it. When I focus and try to ignore the audible clue, the ride seems only slightly firmer than with the SP8000. I would expect that many would find this "thump" even more objectionable than the slightly stiffer ride. Similarly, over textured or grooved concrete surfaces, the S-03's are noticeably noisier, though certainly not objectionably so to me.

E. And in the wet ...

Being a reasonably sane person on the street, I probably don't drive more than 8/10th's in the dry; and 5/10th's in the wet. On the S-03's, I feel like I could corner at 8/10's of dry speeds ... in the wet. Even driving well past my comfort level, the car just grips like I'm on a dry road. At least on the street, pushing any harder is too scary. On the track in the rain, I couldn't hardly ask for more (see link).

One last caveat: I'm comparing two sizes of tire, 255/35-20 versus 244/45-18; certainly not apples-to-apples. I would expect that all the differences would hold up using the tires of the same size, but the difference might not be as great in some cases.

Enough for now?

[ryoung]

Time to be honest.

Until I put the S-03's on my S8 over the weekend, I had not attempted any maximum effort braking in over two months, when I ran at the June Mid-Ohio track event. As I wrote the braking comparison between the SP8000 and the S-03, I actually downplayed the difference. Since it was more than I thought could possibly be explained by the more stable tread on the S-03, I thought my memory from Mid-Ohio had faded somewhat and/or was playing tricks on me. In fact, the difference in ability to modulate braking between the SP8000 and the S-03 is HUGE! This has been bothering me all day, until I remembered a meeting I had with a race tire engineer two years ago, and I dug out my old meeting notes. I believe that #1 below explains the amazing braking performance of the S-03; I'd bet that this tire is very very stiff in the longitudinal axis.

.................................

The following is much condensed summary of a conversation I had with the race tire engineer who was responsible for the construction and compound design, and provided track support, of the tires used for six cars at the 24 Hours of LeMans. Two of "his" cars finished 2nd and 4th. Anyway, among the many things we discussed was the effect of sidewall stiffness on Formula car performance and handling.

Our engineer reminded me that there's stiffness in three different axis, and they all need to be varied individually to obtain the optimum compromise. He emphasized that maximizing the size of the contact patch is critical, and that he'd examine the patch by watching various tires from underneath thru a glass plate under various cornering and downforce loads.

1. Longitudinal axis, the "easy" one: Since extremely high power is not a consideration with my car, stiffer is better to improve braking response. Too soft, and the tire "winds up" under heavy braking. When it reaches it's limit, suddenly all braking forces are transmitted directly to the contact patch, modulation becomes difficult, and the wheel can lock up.

2. Lateral axis: Softer makes the tires more forgiving; but too soft can result in the inside of the tread on the outside tire lifting during corning, reducing the size of the contact patch, and requiring more static camber. Stiffer improves feedback to the driver.

3. Vertical axis: Since the tire is also a spring, too soft results in a spring rate that can't be controlled by the shock. Too stiff and under heavy cornering the tire will tend to tip like a solid barrel, rather than the outside sidewall collapsing before the inside sidewall lifts, and again reducing the size of the contact patch. Soft in the vertical axis requires less static camber, which is the opposite of what's required for a tire soft in the lateral axis.

Our helpful engineer said there's obviously no way to predict the effect on lap times in advance. I can just barely imagine trying to optimize an F1 tire under various conditions, with downforce varying by a ton or more depending on cornering speed. In any case, you can be sure that no two race tire manufacturers have chosen the same set of design criteria - with the result that each tire requires experimenting with camber, tire pressure, springs, and shock adjustment to achieve the best performance.

.................................

BTW, this is the first message where I reply to myself.

[Meteor910]

Randy - Thanks for posting your excellent tire comparison. I look forward to your wet weather follow-up, if it ever rains again! I'm running the SP8000's on a 60-day old A8L.

Given your considerable experience with the SP8000's on your S8, did you ever notice the wet weather problems with the tire that Kjell A commented on a few days ago?, i.e., sudden hydroplaning, unpredictable, etc.? He didn't like the tire at all because of this.

BTW, given that you have them on a S8, I say I like your new wheels. They would be a bit too much for the personality of an A8. I would agree with the comments that painting the rivets black would improve the look to my eye. To me, the chrome "dots" on the black rim are a distraction and draw attention away form what is otherwise a pleasing combination. Just my view.

May your S8 enjoy a well-balanced diet of "Bimmers"! Ken

[ryoung]

Please remember that my "considerable" experience with the SP8000 is only eight months, 11,000 street miles plus about 200 miles at Mid-Ohio in the dry. For a while, it did look like we were going to have some light rain at Mid-Ohio, but no luck (if rain at the track is luck). Unfortunately, I never tried Kjell's simple and excellent suggestion to just push the car/tires a bit in the rain at low speeds in a large parking lot. [I absolutely will do this with the S-03's.]

So with this background, the few times I drove my S8 in the rain would all be considered very normal driving ... picture an old lady in a Saturn. I did experience hydroplaning several times on the Interstate at about 65 mph, but I can't confirm that the problem was with the tire. At that speed, any tire will hydroplane in standing water if it's deep enough, and I have no idea how deep that water was. Other than that, driving the SP8000 in the rain was uneventful and "normal". Kjell is right, though, in that the SP8000 is an older tire design, and rain tire technology has advanced quite a bit in the last few years. I wouldn't throw the SP8000's away, but once they're worn out, you'll serve yourself well by upgrading to a state-of-the-art tire.

Also, Kjell is absolutely correct about the SP8000 in snow. This is more than just "not recommended", don't do it. I only drove my S8 on snow covered roads once, and very carefully. All the 3-letter abbreviations where thoroughly tested that afternoon. AWD, ABS, EDL, ASR, EBD, and ESP all worked as advertised, kept the instrument panel lit up with warning lights, and thankfully kept me on the road; but it wasn't fun. An advantage of the 20" wheels is that I've freed up the "old" Avus wheels for dedicated winter tires.

[ttuling]

When I tried driving the A8 on SP8000s around a wet corner at speeds which are comfortable and precise in a 30 year-old understeering pig French car on MXV4s, the Audi entered a four-wheel drift followed by the front wheels regaining grip with the back wheels still sliding out. This gave both me and the ESP something to do.

On Michelin Pilot Sport tires, the A8 exhibits no such strangeness. And they've improved the car in most of the ways experienced by ryoung with S-03s (a superior tire to the Michelin by most accounts), especially by doing away with the need for small steering corrections when cornering -- never the sign of a quality car the way I see it -- and offering greater grip in all directions.

You probably don't need to go as far as the S-03 or Pilot Sport to improve substantially on the SP8000.

[Bollinger]

In F1, the tire profile is mandated. They would like to use less sidewall (and therefore get less spring effect) except that it is against the rules. Their setup is far from optimal.

Anyway, good post.