I have 20" wheels and hate them! The tires are about as thin as you are going to get on the wheel,
The Front are Ultrac - SESSANTA VREDESTEIN GIUGIARO 245/30 ZR 20
The Back are Pirreli PZERO NERO 295/25 ZR 20
I wish I had 18” wheels.

 

Yes with a taller tire you end up with gearing that will be taller and in any given gear give you less peak torque to the ground, but the power to the ground does not change. Raising the gearing can even be beneficial depending on the track you are at. Last season I routinely ran out of gearing at autocross tracks because I went with a short tire (a different car from my 911). Changing the gearing, taller or shorter, will affect your lap times differently depending on the course and car. Neither one is necessarily worse.

Power is torque * rotation rate so yes I can say that tire size on a 911 does nothing to power transfer for braking (or acceleration for that matter). The rotation rate is less so the torque goes up... When you stop the same car from 100mph to 0 the same amount of energy has to go somewhere. That somewhere is the brakes. The brakes will hit the same temperature since the same amount of energy goes into them. Maybe you should read up about classical mechanics and thermodynamics.

 

Your first underlined statement was what I set out to advise the OP. The second statement is garbage, There will be adverse affects on braking performance. A heavier/larger wheel will carry more energy, out past the center axis of the axle. Do you think formula race cars roll tiny wheels for no reason? I think you need to read up on polar moment of inertia, and moment of inertia. So ill leave a couple handy links.

http://en.wikipedia.org/wiki/Moment_of_inertia
http://en.wikipedia.org/wiki/Polar_moment_of_inertia
http://corner-carvers.com/forums/sho...ung+and+weight


"The moment of inertia of an object about a given axis describes how difficult it is to change its angular motion about that axis. Therefore, it encompasses not just how much mass the object has overall, but how far each bit of mass is from the axis. The farther out the object's mass is, the more rotational inertia the object has, and the more force is required to change its rotation rate." Enough said!

I think you will agree that a larger wheel usually adds more weight? So when do we start talking about the adverse affects on the change of the cars unsprung weight? And how that will affect the stock suspension...

Not trying to start a huge E Web fight here, just didn't appreciate your "What the hell are you talking about" tone. I think you will find exactly what the hell I was talking about described in the provided links.

 

You are not understanding. Although the peak torque may change, performance due to wheel diameter may be better or worse, depending on the track you are at. Raising the gearing means you wait longer to shift which can be a very good thing!

If you cannot understand why the braking performance will not change then I am sorry. Thanks for the links about moments of inertia, but I already know about that... For novice drivers a higher moment of inertia on the wheel can actually let you have an easier time to modulate the brakes (if you disable abs or have bad abs performance and desire to stay off the abs) since there is more time for the wheel to go from spinning full speed to full lockup. Once again, not everything is a simple answer as you wish to propose.

As far as requiring more energy to slow a higher inertia object, well, that energy figure is very very small when compared to the rest of the car. If you assume a 10lb heavier wheel/tire combo (that's a lot) that has the mass spread evenly across the wheel , then the relative amount of extra energy carried on the wheels compared to a 3100lb 911 is about 2.6%. That means your brakes, assuming you started from the same speed, will convert that much more energy into heat. That is not the same as braking distance.

Braking distance is controlled by things like how well the car maintains tire contact patch, ease of pedal modulation/abs implementation, and brake bias front/rear. The only time that tire height starts to play a role is when you have overheated the brakes and the friction of the pad on the disc is unable to lock the wheel up (or you boil the brake fluid). The tire/wheel weight will make a difference if that tiny extra bit of energy is too much and overheats your brakes or if your suspension cannot control the extra weight over bumps. Nowhere in here is your assertion of lower power transfer.

That thing about formula cars. I'm sure they don't care at all about the drag caused by taller tires Taller tires can allow you to run softer compounds since the heat is dispersed over more rubber and you have more rubber to loose without getting into issues of the rubber squirming under hard loads. Porsche upped the standard tire size relatively recently. They had run 25" tall tires for a very long time. They said that upping the tire height actually had benefits. I'm sure they were just blowing smoke

 

You sir are ridiculous.... You contradict yourself in your own statement. Saying that "If you cannot understand why the braking performance will not change then I am sorry." But then go onto to actually come up with a figure on how much less effective the braking system will be "then the relative amount of extra energy carried on the wheels compared to a 3100lb 911 is about 2.6%." Then say that the brake will convert that energy into heat, which is the number 1 thing all of motorsports try's to deal with or prevent.. Hot brakes? No problem huh??? Are you serious right now. The very fact that you can find an adverse percentage, points to some performance loss, regardless of how minor! "For novice drivers a higher moment of inertia on the wheel can actually let you have an easier time to modulate the brakes (if you disable abs or have bad abs performance and desire to stay off the abs) since there is more time for the wheel to go from spinning full speed to full lockup." Easier to modulate for a novice? No lock up? Whatever you call it, it is still a loss in braking performance!

Panici made the points and backed it with personal experience on page one...

Your talking about adding Unsprung weight, and adversely affecting braking PERIOD! Unless your stuffing it full of huge brakes! Same brakes, bigger wheels = worse braking, PERIOD. At whatever % you come up with. And being able to not lock them up easier is guess what? Still a loss in the total effectiveness of the braking system. That = performance loss of the actual system! Teach the novice how to correctly brake from day one!

Your only claim is that it may help change your gearing for a certain track.

Answer me these simple questions? 1. Can I bolt chrome 22" rims onto your track car? And what size wheels do you run on your track car?

2. Can you show me links/info to back up your claims?

 

I actually have a b.s. degree in physics and am within half a year of a PhD (chemical physics) dealing with/designing experiments that explore all levels of classical mechanics, thermodynamics, and electricity and magnetism. Car physics is a fun side project for me. The energy figure is for 40lbs of extra energy in rotational mass on the tire/wheel. That is to give you a ball park figure for what buying really heavy wheels does, not what a taller tire does. The difference in braking performance is not 2.6% worse. The extra energy in the car's movement is that much, but braking performance does not scale linearly with the energy of the car. I have already covered reasons why braking performance is not going to change from a taller tire: tire contact patch, brake bias, and the energy dissipation remaining constant are a few. I also have about a decade of training solving similar problems. Panici gave a fuzzy account of his user experience without any data. He also did not account for the change in tire compound or anything else. You cannot bolt 22" wheels on my track car because I don't have wheel clearance on the car and nobody makes an r-comp that size that I know of. It is important to keep in mind weight and height are different and you can buy heavy 17" wheels and lighter 19" than stock.

 

Yeah yeah yeah.... Answer my questions. And provide links to info that back up your statement. So in all your great wisdom you see no unsprung weight disadvantage?

 

PWR2Lbs, u need to chill man. First of all, slicing and dicing wikipedia doesn't make you 'legit'. Second of all braking isn't the only performance factor. A wider contact patch will actually more often than not allow u to brake faster vs. the extra couple of pounds accrued from heavier (not talking 22' spinners, so take it easy with the hyperbole) rims.

Not to mention later grip and launch grip - both of which generally increase (especially if it's a rwd of fwd car). I think C2junkie was just calling u out on being a stubborn try-hard (as am I), and he was making the point that increasing rim size is not ALWAYS a bad thing (in fact it can be a VERY good thing if done properly to a particular application).

In sum....RE-LAX buddy.

 

*lateral grip, not later grip.

 

In SCCA's improved touring, 240z have been pulling over 1g on the skidpad since the early 80s on 205/60-14 dot legal tires.

 

Not to bump an old thread, but I found this article somewhat informative. I'm just disappointed that they didn't have an apples to apples test of the same tire on two different wheel setups. Not the best article but has some insights.

http://www.europeancarweb.com/featur...est/index.html