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Performance of GS with Std vs Cup Tires vs Z51

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Old 12-06-2017, 08:47 AM   #1
JerryU
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Default Performance of GS with Std vs Cup Tires vs Z51

There are often questions raised on the Forum about the performance of the Grand Sport with standard Michelin Pilot Super Sport tires versus most published performance data that employed Pilot Sport Cup 2 tires. Also there are questions about the performance benefit of the Grand Sport's wider Michelin Pilot Super Sport tires with the same but narrower tires on the Z51 that has similar suspension.

The January 2018 issue of Car and Driver presents comparative data:

With the Standard Michelin Pilot Super Sports they state max lateral "g" at 1.13 and stops from 70 mph in 136 feet.

With the Z07 package and it's carbon-ceramic brakes with Michelin Pilot Sport Cup 2 tires the max lateral "g" is 1.18 and 70 mph stops take only 129 feet.


The article compares the Grand Sport with the 718 Boxster and Cayman.

Although not in the same article, Car and Driver in a 2016 comparison with a Mustang showed a C7 Z51 achieving 1.05 max lateral "g" and stop from 70 mph of 149 feet.

Bottom Line: Wider tires are very effective and probably were also the major factor in the 5% reduced stopping distance with the Z07 package in the Car and Driver test. The main benefit of the carbon-ceramic brakes is when racing/Tracking where they eliminate any fade at high temperatures.

The 9% shorter braking distance with the wider standard Michelin Pilot Super Sport tires on the Grand Sport vs the Z51 is achieved with the wider tires and larger brakes.

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Old 12-06-2017, 03:57 PM   #2
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Interesting, I have wider front 265, rear 295 Cup 2's on my Z51, I wonder how that compares.
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Old 12-06-2017, 07:04 PM   #3
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Interesting, I have wider front 265, rear 295 Cup 2's on my Z51, I wonder how that compares.
FWIW. I remember Jim Hall's comments about the need for wider wheels not just tires. Found this interview of the very innovative Hall in the CanAm series! (Google is amassing- just put in the right words and up came this ~1999 interview!)

"So Firestone told us their tire was built to run on a 7-inch wide rim. I had built some modular wheels and we could vary the width by what mid-section parts we used. We'd run the tires they gave us on a 7-inch rim and maybe an 8 and a 9 too. The wider rims made the sidewalls stiffer and istead of running a 0.95 average G you'd run a 0.99. So of course we put them on 9-inch rims.

Well, we'd go to the next race and the Firestone guys would look at them and say, 'We didn't design those tires to run on a 9-inch rim. We better build a new tire.' They'd add an inch to the tread. They'd do them quick too. They can build prototype tires in a hurry. In a couple of weeks they'd have another tire. And we'd run it on a 9-inch rim and a 10 and maybe an 11. And we'd find it was better on a wider rim so it would happen all over again.

In a matter of less than a year we went from those narrow little Dunlop Green Spots to a tire that was almost double the width. And our single-speed torque converter just couldn't handle all that grip. We immediately went to a two-speed transmission so we could get more torque multiplication off the slow-speed corners.

So that's what happened with me on tires. In the middle 60s, probably 64/65, Firestone took us from less than 6 inches of tread with up to 12 inches."

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Old 12-07-2017, 03:15 AM   #4
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My understanding is that the GS's suspension (Z06) is not the same as a Z51 so tire size is not the only factor.
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Old 12-07-2017, 03:58 AM   #5
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My understanding is that the GS's suspension (Z06) is not the same as a Z51 so tire size is not the only factor.
Yep, the tuning is different and the front sway bar sure looks different, almost like a solid axle! And I agree, because of the effect on transient performance on a race track that is a factor. Track times on specific courses will be affected but not, IMO significantly for the two performance measures I picked to compare.

Doubt the differences have any affect on braking. Typically on a steady state skid pad not sure stiffness is a factor. The roll bar size may keep the wider tires stay flatter. Recall Jim Hall saying a roll (anti-sway) bar doesn't make the car faster in a turn, you just know sooner if you're going too fast and might be off the road (or some such words! )

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Old 12-07-2017, 06:28 AM   #6
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Of course wider tires make a big difference. I'm willing to bet 90, maybe even 95% of the performance difference between a magride Z51 and a GS with z07 is the tires.

And yeah, Z51 brakes are pretty adequate, especially for one stop. I doubt they were overpowered at 70mph.
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Old 12-07-2017, 08:15 AM   #7
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^^

The difference between stopping distance Z51 to GS of 9% was probably as much if not more of a factor of the wider tires as the larger GS front brakes.

The 13% shorter stopping distance Z51 and Z07 was also a factor of the cup tire contact patch and tread compound as the carbon-ceramic brakes. The tests were no doubt made with warm pads and not the very hot pads resulting from racing where they are are significantly better.

Last edited by JerryU; 12-07-2017 at 08:25 AM.
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Old 12-07-2017, 08:33 PM   #8
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^^

The difference between stopping distance Z51 to GS of 9% was probably as much if not more of a factor of the wider tires as the larger GS front brakes.

The 13% shorter stopping distance Z51 and Z07 was also a factor of the cup tire contact patch and tread compound as the carbon-ceramic brakes. The tests were no doubt made with warm pads and not the very hot pads resulting from racing where they are are significantly better.
I'm guessing it was 99% tires and 1% brakes. Z51 brakes have enough power to lock up the wheels at 70mph. 1% is because maybe they had more initial bite or something.
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Old 12-09-2017, 09:07 AM   #9
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Several years ago I read an article on MT or in C&D about a similar experiment were they switched tires and wheels between a 5th gen Camaro Z28 & a Camaro SS 1LE, without noticing any major handling or braking performance improvement, the conclusion was that if you go wider also was recommended to fine tune your suspension to go with it, similar results were obtained by GM while test driving their Z28 side to side with the S.C. ZL1, even that the ZL1 had hp advantage and higher top speed without the additional aero parts of the Z28, it could not catch the Z28 while wearing the wider Z28 shoes, because the suspension tuning and brakes were not the same. Their lap times differences were not posted, just their comments published. Interesting!
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