Would 19" tires perform better than 21"s on a P85?

[derekt75]

In the thread Let's say you drive it like you stole it.
bellwilliam suggests that the 21" wheels have inferior performance to 19" wheels:
Let's say you drive it like you stole it. - Page 3

Is this true?


thanks,
Derek

 

[bellwilliam]

something to clear up: I was saying smaller diameter WHEELS. assume same width wheel, same width tires, same tire compound.

1. A larger diameter wheel and tire package pushes the majority of its extra weight further outward away from the hub. This increases the mass that must be accelerated and stopped rotationally, affecting braking and acceleration.
2. smaller wheels / tires packages are lighter (again assuming same wheel construction), thus lower unsprung weight. this is usually pretty significant weight drop. To rip off a famous racer, "weight is the enemy, and unsprung weight is worse."
3. while a lower profile tires is a performance advantage (with larger wheel / lower profile tire combo) in old days (early 80's Porsche 911 with 70 series tire comes to mind). Tesla's 19" wheel is 45 series already.

while it is difficult to compare performance difference. even same model tires in different size often has different compound (Toyo R1R 195/50-15 comes to mind, with a cheater compound is a hot ticket on auto-x circle, that it is faster than 205/50-15 on auto-x), different speed and different load rating.


Effects of Upsized Wheels and Tires Tested - Tech Dept. - Car and Driver
not the most accurate test (15" and 16" wheels are narrower, 18" tire has lower treadwear rating than 17", 19" tire being a widest tire), but you will get a general idea.

 

[derekt75]

something to clear up: I was saying smaller diameter WHEELS. assume same width wheel, same width tires, same tire compound.

1. A larger diameter wheel and tire package pushes the majority of its extra weight further outward away from the hub. This increases the mass that must be accelerated and stopped rotationally, affecting braking and acceleration.
2. smaller wheels / tires packages are lighter (again assuming same wheel construction), thus lower unsprung weight. this is usually pretty significant weight drop. To rip off a famous racer, "weight is the enemy, and unsprung weight is worse."
3. while a lower profile tires is a performance advantage (with larger wheel / lower profile tire combo) in old days (early 80's Porsche 911 with 70 series tire comes to mind). Tesla's 19" wheel is 45 series already.

while it is difficult to compare performance difference. even same model tires in different size often has different compound (Toyo R1R 195/50-15 comes to mind, with a cheater compound is a hot ticket on auto-x circle, that it is faster than 205/50-15 on auto-x), different speed and different load rating.


Effects of Upsized Wheels and Tires Tested - Tech Dept. - Car and Driver
not the most accurate test (15" and 16" wheels are narrower, 18" tire has lower treadwear rating than 17", 19" tire being a widest tire), but you will get a general idea.

Yes, the question is:
245/45R19 on a P85 vs 245/35R21 on a P85
I'll grant you the same tire compound even if they don't come with the same from the factory, since at some point you'll replace the tires.

The Car & Driver report seemed was referring to a 7.6 second VW Golf wearing M+S tires, and as you noted, they kept using wider tires as they reduced the profile, and that adds to unsprung weight and reduces performance.
Furthermore, the C+D report measured skidpad and 0-60 times, but did not measure slalom or track times, which is probably what people are thinking of for performance. Since the conventional wisdom is that low profile tires corner better, it's hard to use the C+D data to prove or disprove that lower profile tires perform better.

I also imagine there might be some differences using a 4.2s heavy Model S as opposed to a 7.6s lightweight Golf as the forces involved in the former are much higher than those involved in the latter. (higher forces mean more likelihood of the tire sidewall deforming).


I agree that the C+D report adds some insight, but it doesn't quite answer the question:
on a Model S P85, what will perform better a 245/45R19 or a 245/35R21?

 

[yobigd20]

in either case the P85 is CRAZY fast. sorry I'm no help. lol

 

[Johan]

I agree that the C+D report adds some insight, but it doesn't quite answer the question:
on a Model S P85, what will perform better a 245/45R19 or a 245/35R21?

Define "performs". Straight line acceleration? Driving around a course with lots of sharp turns and few straights? Indy 500 track? Autocross? Everyday driving?

 

[derekt75]

Define "performs". Straight line acceleration? Driving around a course with lots of sharp turns and few straights? Indy 500 track? Autocross? Everyday driving?

I imagine that's debatable. I think "performance" refers more toward track driving than straight line acceleration. As for autocross vs. oval track, I don't know what "performance" means.
If the lower profile tires would be better on an autocross track but not as good on an oval track, I'd be interested to hear that.

Personally, I feel performance means which car would be best at getting away from having 3 stars in a GTA IV kind of reality.

 

[7racer]

In general. It will be on an 19. Ask ask track nut.

 

[Zextraterrestrial]

Interesting. What about a 245/35R19 or 30 even vs the others?
I just ordered replacement 21"tires but want a set of 19s for autox.

 

[derekt75]

In general. It will be on an 19. Ask ask track nut.

I don't know anybody that takes a 4650 pound vehicle to a track.
It wouldn't surprise me if a 3" sidewall is optimal on a 3000 pound car that a 2" sidewall would be optimal on a 4650 pound car, but I don't know as I'm not a track nut.

 

[7racer]

I don't know anybody that takes a 4650 pound vehicle to a track.
It wouldn't surprise me if a 3" sidewall is optimal on a 3000 pound car that a 2" sidewall would be optimal on a 4650 pound car, but I don't know as I'm not a track nut.

sorry about that last post...trying to type on the go with an iphone.

But I track my 4000lb GT-R...she's a fatty.

In general a larger sidewall will give you greater launch, better ride quality, no loss of grip (in general) but slightly less steering feel. They say the optimum side wall height is between 40-55.
Also, as you probably know, in general a 19" wheel will be lighter than a 20 inch. as there is less "metal" to make a 19" wheel and therefore less weight.

So theoretically, you will get the best performance with 19 in wheel with high performance tires.

 

[Calisun]

Like others have mentioned, a smaller wheels weighs less and each pound of unsprung weight that is reduced makes a big difference in pure performance environments. Look at F1 with their 13" wheels and Indy Car with 15" wheels. F1 actually had a discussion recently about moving to 18" wheels so that consumers could relate better to the tires on their car but in the end the teams killed it due to the design changes required to cope with the extra weight.

 

[derekt75]

sorry about that last post...trying to type on the go with an iphone.

But I track my 4000lb GT-R...she's a fatty.

In general a larger sidewall will give you greater launch, better ride quality, no loss of grip (in general) but slightly less steering feel. They say the optimum side wall height is between 40-55.
Also, as you probably know, in general a 19" wheel will be lighter than a 20 inch. as there is less "metal" to make a 19" wheel and therefore less weight.

So theoretically, you will get the best performance with 19 in wheel with high performance tires.

What tire sizes have you tried on your GT-R?
Have you noticed a difference between them?

 

[7racer]

What tire sizes have you tried on your GT-R?
Have you noticed a difference between them?

The GT-R is a weird anomaly. For a car so heavy, it's not suppose to perform as well as it does. Also, stock it comes with 20" wheels. A lot of the good track guys run 18"s or 19"s as there are some good race rubber for those tire sizes. I kept the stock 20"s but went a bit wider but staggered. I should have tried to stay "square" to allow me to rotate the tires as I might have induced a bit of understeer. Also, for such a big car, you need big brakes to stop it. I recently added aftermarket ceramic brake kit and kept the 20" to clear the rotors.

 

[7racer]

in regards to the cars weight...what I was trying to say about the GT-R is that maybe the weight helps with the "downforce" and grip of the tires...and hence way the car seems more confident inspiring to drive than a light weight car with aero.
Even the designer of the GT-R mentioned he couldn't get the same performance and would actually lose some if he lightened the GT-R.

I suspect that what he means is that for a small light car, you actually have to drive it faster into a corner to allow the aero to increase grip and downforce to corner...which scares the bejeebus out of us amateurs...
but if the car is heavier...there is already weight/"downforce" adding grip.

 

[derekt75]

in regards to the cars weight...what I was trying to say about the GT-R is that maybe the weight helps with the "downforce" and grip of the tires...and hence way the car seems more confident inspiring to drive than a light weight car with aero.
Even the designer of the GT-R mentioned he couldn't get the same performance and would actually lose some if he lightened the GT-R.

I suspect that what he means is that for a small light car, you actually have to drive it faster into a corner to allow the aero to increase grip and downforce to corner...which scares the bejeebus out of us amateurs...
but if the car is heavier...there is already weight/"downforce" adding grip.

The reason why I brought up weight is that a light car might apply 600 pounds of sideways force between the rim and the tire while a heavy car might apply 1000 pounds. If that force is bigger, then I'd think you'd want the sidewall of the tire to be stiffer to not deform under the tremendous shear force. The easiest way to get a sidewall that is more resistant to deformation is to make it lower profile, so that's why I think heavier cars would generally need lower profile tires for the same performance compared to lightweight cars. Either that, or they'd need stiffer steel belts in the tires.

Since most racers use lighter weight cars, I was thinking that conventional race wisdom might need some adjustment when applied to a 4650 pound beast.

I'd be shocked if the optimal tires for a Lotus Elise would also be optimal tires for your GT-R.

 

[7racer]

The reason why I brought up weight is that a light car might apply 600 pounds of sideways force between the rim and the tire while a heavy car might apply 1000 pounds. If that force is bigger, then I'd think you'd want the sidewall of the tire to be stiffer to not deform under the tremendous shear force. The easiest way to get a sidewall that is more resistant to deformation is to make it lower profile, so that's why I think heavier cars would generally need lower profile tires for the same performance compared to lightweight cars. Either that, or they'd need stiffer steel belts in the tires.

Since most racers use lighter weight cars, I was thinking that conventional race wisdom might need some adjustment when applied to a 4650 pound beast.

I'd be shocked if the optimal tires for a Lotus Elise would also be optimal tires for your GT-R.

that's true, but tires are also much more dynamic now a days and you can make up for some of the "stiffness" with increased tire pressure.

The stock GT-R comes with run flats so already a naturally stiff side wall. Surprisingly, many who track the car say the tire is actually slow. I now run R888s which are much more sticky tire...but run them almost 5psi more on the track...which counter acts the "softer" sidewall issues.

If you are really curious you can use a pyrometer to figure it ou...but for street driving, most likely over kill.