Issue with mounting 18" wheels on GT3 explained
09-22-2014, 12:39 AM
#31
Three Wheelin'
I did, and was completely inline with the discussion until paradocs brought up the "bling" discussion again.
The OP stated that another issue was due to the Brembo kit. Does the whole shebang (cup parts included) need to be swapped, or could the braking system alone be swapped to make 18s work? I went through the same a$$ pain in '09 when I got a GTR and there was nothing in the 20" segment outside of the experimental 20" R888s. So came the months of talking with others on 19" options that would fit the brake systems. Fortunately the market caught up and more options became available.
In the end, I'd like to know if swapping for cup parts is really necessary. Maybe for now?
The OP stated that another issue was due to the Brembo kit. Does the whole shebang (cup parts included) need to be swapped, or could the braking system alone be swapped to make 18s work? I went through the same a$$ pain in '09 when I got a GTR and there was nothing in the 20" segment outside of the experimental 20" R888s. So came the months of talking with others on 19" options that would fit the brake systems. Fortunately the market caught up and more options became available.
In the end, I'd like to know if swapping for cup parts is really necessary. Maybe for now?
09-22-2014, 12:46 AM
#32
Race Director
Is this accurate, though? I don't think that a larger diameter wheel, per se, creates a larger contact patch. It's more likely that, all other things being equal, a larger diameter wheel will be associated with a lower aspect ratio / shorter sidewall tire. It's this shorter sidewall that leads to less tire rollover and crisper, more responsive handling. But within reason--a 23" wheel with a 15-series (aspect ratio) tire on a Porsche would be ridiculous. This setup would be heavy, with hardly any compliance. All that unsprung weight would compromise acceleration, braking and handling. In fact, I wouldn't be surprised if performance is already starting to get compromised at a 20" diameter. I really think it's fashion and marketing dictating these choices. Porsche is in the business to sell cars more than anything else, and Porsche knows that wheels are a significant aesthetic part of cars these days. The general public (including a good portion of Porsche customers) thinks that larger wheels = better performance. They like fancy wheels, and the bigger the diameter, the easier it is to showcase the aesthetics of the wheel. Same thing with Ferrari and the 458.
I was VERY impressed when I saw the new-generation Viper at the NY auto show a couple of years ago: 18" wheels up front, and 19" in back. A nod to performance first, and bling second.
Want further proof? What diameter wheels do the highest-performing race cars in the world run on? 13" for F1.
I was VERY impressed when I saw the new-generation Viper at the NY auto show a couple of years ago: 18" wheels up front, and 19" in back. A nod to performance first, and bling second.
Want further proof? What diameter wheels do the highest-performing race cars in the world run on? 13" for F1.
The use of 13" wheels in F1 is unrelated to them being superior in performance. It's a legacy rule that was originally implemented to limit the size of brake rotors, unnecessary now because rotors are restricted by their own regulations. In concert with Lotus, Pirelli tested 18" wheels and tires at Silverstone this year, and would like to go even bigger. One problem is that a switch would require a redesign of F1 suspensions, since the current cars rely on sidewall flex as a major component of suspension compliance.
BTW, while F1 cars use 13" wheels their tires have very tall sidewalls and the wheel tire combo overall is quite tall. Why? Because designers want the largest possible contact patch, which can be achieved only with a larger diameter tire.
09-22-2014, 01:51 AM
#33
Former Vendor
Actually contact patch is not affected by wheel diameter. Even tire diameter does not directly affect contact path.
Contact patch area is calculated as the corner weight minus sidewall support, divided by tire pressure.
Diameter is not part of the equation, except to the extent it alters sidewall support. In general larger diameter wheels have lower profile tires which tends to increase sidewall support. For this reason larger diameter wheels tend to decrease contact patch area.
By feel, these low profile tires are very responsive. But they break away more abruptly, with less warning than with higher profile tires.
Contact patch area is calculated as the corner weight minus sidewall support, divided by tire pressure.
Diameter is not part of the equation, except to the extent it alters sidewall support. In general larger diameter wheels have lower profile tires which tends to increase sidewall support. For this reason larger diameter wheels tend to decrease contact patch area.
By feel, these low profile tires are very responsive. But they break away more abruptly, with less warning than with higher profile tires.
09-22-2014, 02:07 AM
#34
I don't have any expertise here, but I will point readers to a similar discussion regarding the 21" 918 and RS rears. Mike referenced an article in R&T in post #1089 https://rennlist.com/forums/991-gt3/...thread-73.html and Petevb contributed a very informative post further along #1110, but the discussion went on a number of pages about this very topic.
Just for interest.
Just for interest.
09-22-2014, 02:09 AM
#35
Race Director
Actually contact patch is not affected by wheel diameter. Even tire diameter does not directly affect contact path.
Contact patch area is calculated as the corner weight minus sidewall support, divided by tire pressure.
Diameter is not part of the equation, except to the extent it alters sidewall support. In general larger diameter wheels have lower profile tires which tends to increase sidewall support. For this reason larger diameter wheels tend to decrease contact patch area.
By feel, these low profile tires are very responsive. But they break away more abruptly, with less warning than with higher profile tires.
Contact patch area is calculated as the corner weight minus sidewall support, divided by tire pressure.
Diameter is not part of the equation, except to the extent it alters sidewall support. In general larger diameter wheels have lower profile tires which tends to increase sidewall support. For this reason larger diameter wheels tend to decrease contact patch area.
By feel, these low profile tires are very responsive. But they break away more abruptly, with less warning than with higher profile tires.
http://www.roadandtrack.com/motorhea...t-tire-patches
Last edited by Mike in CA; 09-22-2014 at 02:37 AM.
09-22-2014, 02:29 AM
#36
Race Director
I don't have any expertise here, but I will point readers to a similar discussion regarding the 21" 918 and RS rears. Mike referenced an article in R&T in post #1089 https://rennlist.com/forums/991-gt3/...thread-73.html and Petevb contributed a very informative post further along #1110, but the discussion went on a number of pages about this very topic.
Just for interest.
Just for interest.
09-22-2014, 02:51 AM
#37
Former Vendor
Not to be disagreeable, and with all due respect - the guy writing that article has "incomplete" understanding of the subject at hand.
He dismisses the effect of tire width on grip.. This is factually wrong as anyone with actual track experience can attest.
Then correctly understands that width does not create a larger contact patch, but somehow fails to see that diameter similarly does not affect contact patch area??????
Hey, it's on the interwebs and they couldn't say it if it wasn't true
He dismisses the effect of tire width on grip.. This is factually wrong as anyone with actual track experience can attest.
Then correctly understands that width does not create a larger contact patch, but somehow fails to see that diameter similarly does not affect contact patch area??????
Hey, it's on the interwebs and they couldn't say it if it wasn't true
09-22-2014, 03:18 AM
#38
...now I'm thoroughly confused. Chuck, you've done brilliant write ups on other topics. Would you (or anyone else) be willing to do something on this too? I've looked around on RL, and there are so many conflicting views on this that I don't know what to think. Even if this is quite theoretical, I'd be interested to learn more about this. Thanks!
A few other threads for starters:
https://rennlist.com/forums/racing-a...act-patch.html
https://rennlist.com/forums/racing-a...ler-tires.html
A few other threads for starters:
https://rennlist.com/forums/racing-a...act-patch.html
https://rennlist.com/forums/racing-a...ler-tires.html
09-22-2014, 07:54 AM
#39
Race Car
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Not to be disagreeable, and with all due respect - the guy writing that article has "incomplete" understanding of the subject at hand.
He dismisses the effect of tire width on grip.. This is factually wrong as anyone with actual track experience can attest.
Then correctly understands that width does not create a larger contact patch, but somehow fails to see that diameter similarly does not affect contact patch area??????
Hey, it's on the interwebs and they couldn't say it if it wasn't true
He dismisses the effect of tire width on grip.. This is factually wrong as anyone with actual track experience can attest.
Then correctly understands that width does not create a larger contact patch, but somehow fails to see that diameter similarly does not affect contact patch area??????
Hey, it's on the interwebs and they couldn't say it if it wasn't true
The most consistent ideal Ive seen is that and increase in width and diameter simultaneously are the only way to assure a bigger possible contact patch.
I think the article properly draws a link between the shape of the contact patch and how its give and take. Wider tire heavier weight shorter (albeit wider) patch. More mid corner speed but perhaps less exit speed and worse braking. I think the only consistent that can be drawn from the track is that there are no consistents. Every car driver and setup can be vastly different.
Last edited by wanna911; 09-22-2014 at 08:10 AM.
09-22-2014, 03:49 PM
#40
Race Director
I'm confused too. My understanding was that wider tires allow for an increase in lateral grip because the contact patch is made wider but not as long. I also understood that increasing the diameter of the tire elongates the contact patch and actually allows it to increase in size. That makes sense to me because with a larger diameter the arc of a circle at any given point along the circumference is "flatter" (that's a simple physical fact) and given equal tire pressure and sidewall stiffness it should therefore be possible for more rubber to be in contact with the ground.
I'd definitely like to have more clarity around this and have someone explain why the above physical facts about circles don't hold true with tires.
I'd definitely like to have more clarity around this and have someone explain why the above physical facts about circles don't hold true with tires.
09-22-2014, 04:07 PM
#41
Rennlist Member
you forget tire flex and also sidewall travel effects. very short stiff sidewalls have counter effect on the level of grip mid corner, otherwise all people would run now on a 0.2" thick tires on 26" rims. 
grip performance in a straight line (like during braking) and mid-turn are not nearly same things.
grip performance in a straight line (like during braking) and mid-turn are not nearly same things.
09-22-2014, 04:19 PM
#42
Race Director
you forget tire flex and also sidewall travel effects. very short stiff sidewalls have counter effect on the level of grip mid corner, otherwise all people would run now on a 0.2" thick tires on 26" rims.
grip performance in a straight line (like during braking) and mid-turn are not nearly same things.
grip performance in a straight line (like during braking) and mid-turn are not nearly same things.
It hard for me to believe that every major performance car manufacturer is going to larger diameter tires just for bling. In a competitive environment where everyone is looking for a performance edge, why would everyone be sacrificing performance just for looks? There has got to be more to it than that.
Obviously this is a complex subject; I'd like to understand it better.
Last edited by Mike in CA; 09-22-2014 at 04:34 PM.
09-22-2014, 08:33 PM
#43
Rennlist Member
Here is an interesting calculator ... can't verify it is 100% accurate but assuming it is correct, gives some results and allows user input to test various tire /wheel combos. One of the output parameters is a calculated contact area.
http://bndtechsource.ucoz.com/index/...alculator/0-20
Finally, I stumbled upon this analysis a while back when I was researching traction dynamics. His analysis does not vary diameter, but he presents some interesting findings based on Avon tire company data and relationships between weight and tire pressure.
http://www.performancesimulations.co...on-tires-1.htm
http://bndtechsource.ucoz.com/index/...alculator/0-20
Finally, I stumbled upon this analysis a while back when I was researching traction dynamics. His analysis does not vary diameter, but he presents some interesting findings based on Avon tire company data and relationships between weight and tire pressure.
http://www.performancesimulations.co...on-tires-1.htm
08-24-2017, 07:51 PM
#44
Instructor
Bringing this topic back up.
Is there a chance, that the new 991.2 GT3 will have different lower control arms? I would assume no.
Has someone already converted his GT3 and upgraded with the necessary CUP parts to fit 18 inches?
Is there a chance, that the new 991.2 GT3 will have different lower control arms? I would assume no.
Has someone already converted his GT3 and upgraded with the necessary CUP parts to fit 18 inches?