Re: Imagine if it starts going the other way:

Is there a limit to the size Porsche can put on their sports cars? I can't see getting any larger than 20". My first Boxster had 16", my second had 17" and now my third has 19".

Edited 1 time(s). Last edit at 11/19/2012 10:54PM by Laz. (view changes)

Some engineering and aesthetic rationale will develop that causes the sizes to eventually drop to 11 or 12 inches.

Just look at an F1 car.

But there is an engineering rationale for smaller wheels - lower mass and lower rotating mass.

Plus better steering feel for 95% of drivers, due to higher slip angles (distorted sidewall and tread).

We have crossed the line into bling, in my opinion - and i also think some of the really large wheels look funny...

Grant.

Grant

gee-lenahan-at-gee-mail-dot-com

Very tasteful, no? [www.harmony.net.cn]

In the world of wheels, size matters to many drivers. It seems there is a push to fill the wheel well and lowering cars isn't the answer as speed bumps, driveway entries, etc. cause problems when you lower the car. So, the only way to fill the well is to pump up the size of the wheel inside the well.

To get more braking surface, car manufacturers have to use larger discs and calipers. That's why I doubt you could use 17" wheels on a 981. You wouldn't want to have Honda Civic brakes on your Porsche so you could 13" wheels..

The discs are huge, although people have complained that the rear calipers look disproportionately small. They do, but BMW must know what they're doing, and a vast majority of the work is done by the fronts anyway.
Wait 'til there are atomic powered brakes: they'll be not much larger than the wheel bearings!

Minus 40 degrees... Is that Fahrenheit or Celsius?



Edited 1 time(s). Last edit at 11/20/2012 12:37PM by Laz. (view changes)

Yes, but the Boxster has always received great marks for it's braking prowess, even with the 1997's original 16 inch wheels. Most wanted larger wheels on those early Boxsters, not to improve braking, but to simply fill the wheel well. I don't beleive any of them added 911 brakes to their cars when they increased the wheel size. Also, the Boxster has never been in the rehlm of the Hondas of the world when it comes to braking. The Boxster has always been held in very high esteem when it comes to its braking abilities.

heat. Specifically heat dissipation for prolonged, repeated use. Not one big stop, nro two. Lap after lap of 130--> 50 mph at threshold.

There are various ways to deal with this. For real racing, you do "all the below". For normal human beings however we can:

1. build huge heat sink capacity and surface area (dissipation area) via huge rotors and calipers
2. Accept that the brembos on there are plenty large as stopping devices, and condition the system to handle the heat rise. This means

2a. put high temp pads in there - 800 deg minimum; 1000-1200 true track; > 1200 HD race
2b. make sure the fluid does not boil. DOT3 = 401degF DOT 4 = 441 deg F DOT 5.1 = 518deg F Motul = 600deg F Castrol SF = hotter than the sun degF. All figures dry. Corollary is change it regularly so its dry.

Now, a rel racer probably wants bigger brakes than i have on my girly-man's car. He might need 17" tires so the rotors can fit within; and might also want the larger calipers to dissipate the heat and hold more fluid in the caliper cavity. But 19? 20?

let's look at some numbers.

I recently ran my 4000 lb Audi S6 avant (aka wagon) at VIR. The S6, like most audis, is a 911 in revers - lots of weight in front of the front axle. Teh rear brakes, under weight transfer, do not too much. So this is actually a good lab for what happens when you haul a Big Pig down from real fast to a whole lto less fast (say 115 --> 55 at the end of the back straight just before and during turn 14).

After a session (taking it overall pretty easy), and after a cool-down lap, and on a cool day ( and of course a cool driver), the front rotors were 560 degF. That's pretty hot - indicating they were what - 800+ at peak? More?

But the calipers were just under 300 deg. So that indicates that the rotor heat is the most critical - and can be dealt with by pads and rotor materials (race pads and if needed composite rotors). Or just an upsize to 17" wheels - i was running 16s on my Audi at the time - clearing the huge rotors and HP2 calipers.

It also suggests that DOT4 fluid was probably sufficient. I had 600 deg ST fluid in for safety ( and would again).

Did i mention there's a cult that upgrades hot-rod Audis to *BASE* Boxster brakes?

So, are there many circumstances that require 18s, 19s or 20s on 3000 lb cars?

For street - not even remotely close. Give up.
For track - not really, although it might occasionally be expedient.
For pro racing - they dont run stock cars anyway, so STFC?

QED.

Grant

Grant

gee-lenahan-at-gee-mail-dot-com



Edited 1 time(s). Last edit at 11/21/2012 08:43PM by grant. (view changes)

Car makers seem to be trending to larger wheels as a fashion trend almost like large watches are in too.