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Discussion Starter #1
Was just watching some show on Speed network, they were modding a
Ac*ra and even though the wheels weighed the same as stock
(I believe they went from 15 to 17) and the brakes were lighter, the dyno
read -12 rear wheel horses! :shock:

Apparently because the brake diskes are larger and the wheels now
carry most of their weight farther from the center, it's like putting on
heavier wheels (rotating mass). :sad:

It looks the shizznit but moves like a turtizzle.

Anyone for 205/80-13 ??
drum brakes ?? :weirdo:
 

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I'm glad someone knows the truth. :p
 

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12hp is a heck of a difference, what was the change in disk diameter? 10%? 20%? I suspect there's something else going on here to explain that big a change.
 

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Discussion Starter #6
Nope, they were astonished too. It was very step by step.

They did suspension, brakes, wheels and exhaust and got -7 RWHP.
thought the exhaust was bad so put the stocker on and went to -12RWHP.
summation: big wheels + big brakes improve handling and braking (and looks)
at a sacrifice to acceleration.
 

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Hard to say "big wheels + big brakes improve handling and braking (and looks)
at a sacrifice to acceleration". I don't think it was very scientific for them to reach a conclusion while testing two variables at the same time. Another hidden variable could be the wheel width, as opposed to the diameter alone. I'm willing to bet that the only diadvantageous thing here was the increase in wheel diameter and unsprung weight (assuming there was any). Increase in tire (not wheel) weight could also be a factor to consider. While I think I would ultimately agree with the results, I don't think the testing was at all scientific.
 

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Discussion Starter #8

Well, it wasn't scientific but it was pretty thought inspiring. The overall wheel diameter did remain the same, the unsprung weight of a 45 pound 15 inch wheel/tire is the same as the unsprung weight of a 45 pound 17 inch wheel tire (I mentioned the weights were the same) and that really has more to do with how hard the suspension will work not how well it dynos. I agree the tire weight could be a factor especially because it will be out at the farthest point from the hub.

Will this stop me from going 18"?.... no. :D

Will this make my sloth of a 1.8t pull from a stop even slower?...yes. :(

Will my new chip help once the turbo kicks in?...hopefully enough to compensate
but at a certain reduction in gas mileage too. :???:
 

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It's simple physics. The mass moment of inertia is moved out farther from the center of the wheel. For the same total weight, it takes more force (torque) to accellerate the wheel because more of the weight is farther out on the 'lever arm.' Because more of the weight is concentrated on the outside rather than being evenly distributed it acts like it's on a longer lever arm, reducing mechanical advantage.

Here's a visual example:
http://moon.ouhsc.edu/dthompso/gait/kinetics/mominert.htm
 

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Oh yeah.. and this is the reason I ran the smallest, lightest brakes available (9.4" solid rotors) and smallest diameter, widest rims possible (13x9") on my 81 Scirocco FSP Autocross car:
 

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:thumbup:

I wish I could run small brakes. :p
 

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Wow, 13x9's are really neat! That would def be optimal for track events. I was hoping to go smaller than the 15x7 setup for my Miata and CRX, but it's finding a track brake setup for smaller than 15's.

Hmm..that opens up another can of worms. With overall stopping dist, is it more important to run wider tires (more traction, or so I guess), or bigger rotors? With autoX, perhaps it doesn't make sense to run the fancy 4-pot big brake kits since brake fade isn't as much of an issue. Interesting that you would stop at 13's. What is the limiting factor, that you would not go with even smaller diameter and wider width? Tire and/or brake choice, or something else?

It would be badass crusing along the Passat in some 13x9's! Someone ought start a revolution in Passat modding, lol. 8)
 

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What is the limiting factor, that you would not go with even smaller diameter and wider width? Tire and/or brake choice, or something else?
Rules usually. 13" in the smallest diameter that fits over "normal brakes" (most auto-x classing requires stock brakes, unless you run Street Mod or higher). Not to mention that you'll have a helluva time finding wide tires that are smaller than 13" in diameter. But for HPDE, there are no rules on setup. :)

Personally, I'd be scared to run 9" rotors on a 3100+ car w/ R compounds. :p
 

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We actually did experience fade on the Scirocco for one event. I was letting someone co-drive the car and he was a left foot braking addict. We also didn't get our numbers far enough apart, so right after he took his run, I had to jump in and get it up to the line. He was braking so hard and so often on this high-speed course that when he came in, smoke was dumping out of the front wheels. When it was my turn to go the brakes felt pretty rubbery by the 6th corner. Ugh.

Steath is right.. in FSP we had to run DOT approved R-Compounds and 13" is the smallest diameter made. You can go smaller, but only with slicks. I also don't think you can go smaller and still clear the caliper.
 
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