Consolidated 991RS thread
07-29-2014, 08:00 PM
#1141
Back on topic, it's interesting that the load ratings on the previous RS tires were not increased over the GT3 to cope with the extra aero. This car's jump of 20% in tire load ratings seems to confirm some rumors we've heard previously- that it's going to produce lots of downforce, perhaps up in Viper ACR territory. That car likely dropped over 10 seconds on the 'Ring due to aero (the 2010 model's best of 7:12 is still impressive). If this is similar downforce, while tire grip and power are up... I'd think lap times will be surprising as a result, a much bigger gap than previous RSs. I wouldn't be surprised if they intend take a serious run at the GT-R's time, currently sitting on a best of 7:08...
Absolutely, thanks, and always...
Absolutely, thanks, and always...
07-29-2014, 08:11 PM
#1142
Rennlist Member
If the GT3 RS tires are the same size as the 918, maybe Porsche will offer the magnesium wheels that are available. Of coarse, at $32,500 (cost on 918) that would make for an expensive option. All the option prices on the 918 seem to be inflated so maybe they will offer them at a more reasonable cost on the RS. PCCBs and magnesium wheels would be a nice combo. Wishful thinking maybe.
07-29-2014, 08:22 PM
#1143
Rennlist Member
Back on topic, it's interesting that the load ratings on the previous RS tires were not increased over the GT3 to cope with the extra aero. This car's jump of 20% in tire load ratings seems to confirm some rumors we've heard previously- that it's going to produce lots of downforce, perhaps up in Viper ACR territory. That car likely dropped over 10 seconds on the 'Ring due to aero (the 2010 model's best of 7:12 is still impressive). If this is similar downforce, while tire grip and power are up... I'd think lap times will be surprising as a result, a much bigger gap than previous RSs. I wouldn't be surprised if they intend take a serious run at the GT-R's time, currently sitting on a best of 7:08...
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07-29-2014, 09:53 PM
#1145
Gearing will have nothing to do with the tire OD change- it's easy to adjust ratios to get whatever speed in gears you'd want.
The change to 21s is going to be driven primarily by grip. It's not an accident that as cars get more powerful the tire diameter tends to go up. Think drag racers, funny car, top fuel, even F1 for some extreme examples. Part of the reason is to extend the length of the contact patch, which does improve grip, particularly at speed, where friction usually drops off.
The best tire models include speed, because rubber's friction is also a function of how long it's in contact with the road. This is because it's a viscoelastic material, meaning "flows" into the roughness of the road to grip, and as speed increases it has less time in contact with the road to flow. By increasing the OD of the tire you increase the length of the contact patch, thereby increasing the time the rubber has to flow. A long contact patch also reduces the slip angle a tire operates at, increasing the tires responsiveness in the process.
Porsche themselves have used progressively larger diameter rear tires- the RSRs, for example, for half a decade used a tire over an inch taller than the Cups to increase grip. Unfortunately in motorsports wheel diameter has been frozen at 18" for nearly two decades to limit costs, so an increase in sidewall height was required to get this larger OD, slowing the rear tire's response. This leads to a number of undesirable traits that must be countered, including slower turn-in and both transient and steady state oversteer. It would be far better to run a short sidewall and get quicker response.
Porsche seemed to prove this by winning Le Mans with 20" wheels before they were banned, but they aren't banned on the street, so as power and grip climb to ex-Le Mans levels it's no surprise we're seeing larger wheels. It is heavier and more expensive, the large, low profile tires requiring more segments in the multi-piece tire molds, costing more to make. However on a high power car that both needs the grip and has the horsepower to spin big wheels you should see better overall performance for those penalties.
Two other things caught my eye looking at those tires:
First, the Q2 2014 estimated launch date. Seems to suggest we were originally supposed to have GT3 RSs on the road already, as I think we already know.
Second, the tire load ratings: 108 rear, or 2205 lbs, and 99 front. Much higher load ratings than the other cars, and with tires higher is not better. In fact all else being equal you generally want to use the lowest load rating you can- they increased load rating for a reason.
The load rating is up is up 221 lbs vs the 918's identically sized rears, 276 lbs from the GT3.
The front load rating is up as well: an increase of 353 lbs from the GT3, and 188 lbs from the 918. In fact the RS's fronts now have nearly as much load rating as the GT3's rears!
Unless you think RS is getting lead bumpers, this seems to point to only one thing: a huge amount of downforce at speed. Back of the napkin, between 500 and 600 lbs more than the GT3 at Vmax.
That'll help those 'ring times...
The change to 21s is going to be driven primarily by grip. It's not an accident that as cars get more powerful the tire diameter tends to go up. Think drag racers, funny car, top fuel, even F1 for some extreme examples. Part of the reason is to extend the length of the contact patch, which does improve grip, particularly at speed, where friction usually drops off.
The best tire models include speed, because rubber's friction is also a function of how long it's in contact with the road. This is because it's a viscoelastic material, meaning "flows" into the roughness of the road to grip, and as speed increases it has less time in contact with the road to flow. By increasing the OD of the tire you increase the length of the contact patch, thereby increasing the time the rubber has to flow. A long contact patch also reduces the slip angle a tire operates at, increasing the tires responsiveness in the process.
Porsche themselves have used progressively larger diameter rear tires- the RSRs, for example, for half a decade used a tire over an inch taller than the Cups to increase grip. Unfortunately in motorsports wheel diameter has been frozen at 18" for nearly two decades to limit costs, so an increase in sidewall height was required to get this larger OD, slowing the rear tire's response. This leads to a number of undesirable traits that must be countered, including slower turn-in and both transient and steady state oversteer. It would be far better to run a short sidewall and get quicker response.
Porsche seemed to prove this by winning Le Mans with 20" wheels before they were banned, but they aren't banned on the street, so as power and grip climb to ex-Le Mans levels it's no surprise we're seeing larger wheels. It is heavier and more expensive, the large, low profile tires requiring more segments in the multi-piece tire molds, costing more to make. However on a high power car that both needs the grip and has the horsepower to spin big wheels you should see better overall performance for those penalties.
Two other things caught my eye looking at those tires:
First, the Q2 2014 estimated launch date. Seems to suggest we were originally supposed to have GT3 RSs on the road already, as I think we already know.
Second, the tire load ratings: 108 rear, or 2205 lbs, and 99 front. Much higher load ratings than the other cars, and with tires higher is not better. In fact all else being equal you generally want to use the lowest load rating you can- they increased load rating for a reason.
The load rating is up is up 221 lbs vs the 918's identically sized rears, 276 lbs from the GT3.
The front load rating is up as well: an increase of 353 lbs from the GT3, and 188 lbs from the 918. In fact the RS's fronts now have nearly as much load rating as the GT3's rears!
Unless you think RS is getting lead bumpers, this seems to point to only one thing: a huge amount of downforce at speed. Back of the napkin, between 500 and 600 lbs more than the GT3 at Vmax.
That'll help those 'ring times...
07-29-2014, 10:02 PM
#1148
Burning Brakes
Back on topic, it's interesting that the load ratings on the previous RS tires were not increased over the GT3 to cope with the extra aero. This car's jump of 20% in tire load ratings seems to confirm some rumors we've heard previously- that it's going to produce lots of downforce, perhaps up in Viper ACR territory. That car likely dropped over 10 seconds on the 'Ring due to aero (the 2010 model's best of 7:12 is still impressive). If this is similar downforce, while tire grip and power are up... I'd think lap times will be surprising as a result, a much bigger gap than previous RSs. I wouldn't be surprised if they intend take a serious run at the GT-R's time, currently sitting on a best of 7:08...
Absolutely, thanks, and always...
Absolutely, thanks, and always...
07-29-2014, 10:11 PM
#1151
Extrapolating based on things done for compliance rather than performance will lead you to incorrect conclusions. Look towards competition without limits (Le Mans before the 18" limit, hypercars, etc) to see where unconstrained development actually leads.
07-29-2014, 10:14 PM
#1153
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07-29-2014, 10:20 PM
#1154
07-29-2014, 10:22 PM
#1155
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