<3mph

Couldn't agree more that the RS operating costs sound like they could be astronomical, probably f-car league, especially if tracked heavily. Ironic, given that philosophically the RS is diverging towards a track-purposed car and away from the GT3 so much more this iteration. Looks like they're upping the exclusivity and making the RS a car fewer track enthusiasts can enjoy. Certainly out of my budget range and truthfully not what I'm looking for, making me happier that I went with the GT3!! Even on the GT3, the price of many basic options (brakes, garage door opener!, paint) adds up to way more than I'd previously imagined paying.

As for MPSS tires, I'd love to hear your thoughts once you swap, since that is the direction I'd like to go too. In fact, seeing as I'll be receiving my GT3 in the winter (mostly rain here, vs snow), I may even want to start on MPSS's and leave the MPSC2 for the warmer weather.

BTW, off topic, I know, but has anyone seen a Dunlop tire on a GT3 yet?

Macca

I think the Dunlop Maxx thing on the GT3 was a enigma. They never have surfaced on any 991 GT3 fitted from factory anywhere in the world I have seen. I wonder what happened. Either the tyre wasnt as good as they thought or more likely Dunlop wouldnt drop their pants as far as PAG asked them too?

I agree. I think the 991 GT3RS target market is the Ferrari 458 SP. The GT3RS will be priced cheaper but IMO will be the quicker and more suitable rack tool. The track day running costs however on this generation RS will be far closer to Ferrari $$$ than previous generations. The 991 GT3 is already one step up but the RS will be a magnitude more I suspect in both brake/tyre/wheel terms. I just love the 459SP and would have one in a heart beat but like many here its not in my pay grade - and more specifically running it with a 50/50 road/track mix isnt in my pay grade either (its expensive enough tracking an old 993, now theres the new GT3 - my wife will be buying more shoes!).

<3mph

My wife is cut from the same cloth!

Mike in CA

Macca, it does seem counter-intuitive but the Road&Track article I linked to above says the following. BTW, this isn't the first time I've read this....

" Merely increasing the width of a tire doesn’t increase the area touching the pavement. It just makes it a wider, shorter patch."

If correct, then moving from a 245 to 265 tire, or from a 305 to 325, gives a wider contact patch but overall doesn't put more tire in contact with the pavement. I suspect that's one reason why (among others), over the years tire diameters continue to increase as chassis engineers search for ways to put more rubber in contact with the road.

I'm not a chassis engineer either so maybe someone else can shed more light generally on this subject.

Macca

there were two factories operating at the time. Adam factory and Eve Factory! .....there were also some manufacturing defects, a few broken in transit, a couple lost and a number stolen, some go left out in the sun too long and some exceeded their shelf line....but they all bloody well like shoes!

<3mph

Hi Mike, I'm no engineer either (more like plumbing for me).
But as I read it, the article is speaking of increasing width but maintaining constant overall tire diameter.
In the GT3 vs RS comparison, the overall tire diameter is not constant, but is also increased (for the RS) in addition to the width.

Background:
Increased diameter does indeed increase contact patch area (as the article stated). Tire diameter increases since the absolute sidewall height increases as section width increases (245--265) given a constant profile (35), for the front tires for example. Recall:

Sidewall height=width*profile.

Tire diameter=(sidewall height*2) + wheel diameter. (Where wheel diameter on the front is constant at 20". On the rear the increase in overall tire diameter is even greater since the wheel diameter also goes up from 20 to 21".)

Therefore, the article is speaking about a different theoretical situation than the comparison of the following two tire dimensions.

GT3:
Front - 245/35ZR20
Rear - 305/30ZR20

GT3 RS:
Front - 265/35ZR20
Rear - 325/30ZR21

http://www.tirerack.com/tires/tirete...e.jsp?techid=7 has some illustrations which helped me.

My apologies if I misread your post and I answered a different question than you were asking with things you already knew!

Mike in CA

Hi <3mph, informative post, and no apologies necessary. We're all learning here, at least I'm trying to!

Clearly this whole subject is complicated and nuanced. Macca originally brought up the issue of what possible reason Porsche could have for going to a 21" rear wheel/tire combination over the 20" wheels on the 991 GT3, which are in turn an increase over the 19's on the 997 GT3. The suggestion I was trying to make was that, along with other factors, one important part of the equation for getting more rubber in contact with the road while maintaining or decreasing tire profile, seems to be to go to larger diameter wheels.

Porsche, along with other manufacturers, apparently feels the trade offs of added weight and expense are worth it and that the benefits are real. I'm having trouble believing that the development team decided to put 21's on the rear of the new RS just because they thought they would look better! Whether it's worth it to the owner who drives the car on the street or track, and ends up with reduced and/or very expensive choices in replacement tires, is another matter entirely.

<3mph

Mike, aha, I understand your point! And it makes perfect sense to me now. The chart Macca posted clearly confirms what you say: the increase in width from 325 to 345 mm (with constant 20" rear wheel) would only increase diameter from 27.2" to 27.7"; whereas going to a 345 mm width 21" wheel increases the tire diameter by another inch to 28.7--and a much bigger gain in contact patch too presumably.

Agreed, the fact that they put these tires and wheels on the 918 seems to suggest a significant enough benefit is obtained (beyond looks, which I actually think look pretty hot on the 918). I consider that car to be a bit of a no-expense-spared exercise, and so evidently the trickle down of technology to other "lesser" models does not appear to come cheap either. That's why the Macan costs more than a comparable CUV--it's the 918 look-alike steering wheel!!!

As a newbie, I'm more than happy to learn from you--thanks for your posts!

Macca

Mike. I think you have nailed it. It must be the size of teh fornt and rear contact patch from an engineering point of view followed by a high top speed whilst utilising a lower gear set (i.e. the increased diameter allows lower gears to be used for track work without decreasing the overall Vmax of the car).

I believe those are the two benefits of the RS wheel choice and I guess we will kow for certain shortly!

TRAKCAR

Straight to the track it goes when you get tired of all that racing hassle and hard work :-) Just buy tires even at $1000.00 a tire this is cheaper than arrive and drive full service Boxter racing....

Weird?!

How do you explain that to the 918 guy that just wrote a $1,000,000.00 check?

So I can get a Fiat 458 driving experience without have to be seen in one and it costs and breaks down less? That's not all bad for many people.

With enough power more bigger = more better? Maybe all the RS is, is sexier big butt (I Like, I cant lie), non poser seats, lighter, re-geared, more down force, more power? If it out laps a 458 what may it cost?

I think they should give me one for free, but I can see some value here

I should be going to Paris in October

TrackFan

A 21" rear tire is stupid for the RS. I'm not sure what Porsche is thinking? I don't even like 20", or even 19" now that I think about it.

Race cars use 18" tires. Look at the Cup car. Is it slow in the turns?

Nizer

U be Sebring test dummy - makes total sense. Enough of this cushy Ring stuff; time to see if it really holds together

TRAKCAR

Test dumbass. Got it.
I give it 30 track days, so 2 months after it arrives from ED after the summer.
First stop Manthey?

Zucc

Normally I would agree with that, then I think about the centerlocks on the GT3's.

Petevb

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...