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Discussion Starter · #1 · (Edited)
That is what Car and Driver found to be a difference between small / light tires and wheels versus larger / heavier ones. That is 0.1 sec better than the 0.2 sec. difference between 345 HP Carrera and 385 HP Carrera S...

Maybe, we have been focusing on wrong upgrades?
 

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Discussion Starter · #3 ·
Well, I think this is a bit different. Dropping 60 lbs. from the car will not be the same as dropping 60 lbs. from wheels. Seems that dropping 60 lbs from wheels / tires is the same as adding 40 HP!
 

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Tomasz,
I've been on the C&D site and can't find the article. Do you have a link?
Thanks,
BW
 

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The 14# per corner gain was with the +4 wheel/tire setup (19" wheels that were also 2.5" wider than the OEM 15" wheels). The +2 setup with 17" wheels was only 8# per corner heavier than OEM. C&D thought the 19" wheel/tire setup was a bit too much, and the 17 or 18" setup was the "sweet spot." Actually, I was somewhat surprised that the 56# heavier 19" setup didn't cost even more performance than it did.

"We" probably rarely consider a +4 wheel/tire setup anyway; more likely a +1 or +2. We also probably plan to get lighter forged wheels rather than cast wheels as used in the article. In short, we intend to go lighter rather than heavier; or at least try to stay close to OEM weights on the bigger wheel/tire combo.
 

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In general, for a car that weighs about 3000 pounds, you need 10 HP to lower the 1/4 mile time by 0.1 seconds.

To achieve the same effect, you need a weight reduction of about 100 pounds. But, if you reduce unsprung mass (wheels, tires, rotors, calipers etc.), the effect is magnified by a factor of 6 or 7. Assuming a conservative factor of 6, reducing the unsprung mass by 16.7 pounds will have the same effect on acceleration as reducing 100 pounds from other areas of the car.
 

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In general, for a car that weighs about 3000 pounds, you need 10 HP to lower the 1/4 mile time by 0.1 seconds.

To achieve the same effect, you need a weight reduction of about 100 pounds. But, if you reduce unsprung mass (wheels, tires, rotors, calipers etc.), the effect is magnified by a factor of 6 or 7. Assuming a conservative factor of 6, reducing the unsprung mass by 16.7 pounds will have the same effect on acceleration as reducing 100 pounds from other areas of the car.
You are directionally correct, but I think you meant rotational inertia at the wheels/tires, not unsprung mass. While there is an overlap between the two (wheels and tires are in both categories), the rotational inertia is what affects acceleration (and braking). Other areas of unsprung mass that are not rotational (control arms, uprights, etc) do not have the drastic effect on acceleration. Also, the radial distance from the axle centerline has a huge effect - i.e., the tire would have a much larger effect on roatational inertia than the axle (driveshaft, halfshaft, pick your name). So, it is hard to assign an actual general multiplier. But, again, you are directionally correct.

The moral is, as we've known, when changing wheels/tires we need to pay close attention to weight - or pay the price.

Of course there are other benefits too - unsprung weight, overall weight, gyroscopic effect, unsprung/sprung ratio, etc.

Then again, we need to keep this in context - for most of us this is primarily a street car - so those $10K carbon fiber wheels would look killer, but still wouldn't make this car truly fast.
 

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Don't forget that in comparing 0-60 times between different cars that the launch is all critical. There isn't much that comes out of the hole quicker than the rear engined 911. Discounting awd of course.

Otherwise, for the same car, to reduce 0-60 times, assuming you are not traction limited to begin with, you can get pretty close, within reason, by taking 0.3s as a fraction of say, 4.8s (our stock 0-60 times) which equates to 6.25%. Then, that fraction of our current hp is about 20 hp.

Since you're just playing with the power to weight ratio essentially, an equivalent drop in static weight of about 200 lbs would achieve the same.

This rough method loses any accuracy pretty quickly as you get outside of the range of a few tenths.
 

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In general, for a car that weighs about 3000 pounds, you need 10 HP to lower the 1/4 mile time by 0.1 seconds.
I think this is true for the first .1 second only. The original poster asked for .3 seconds, and I don't think it's 30 HP. You need more and more HP for each .1 second you want to drop.
 

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Tomasz

Intersting Post Tomasz ,I also noticed my dyno showed a significantly lower Number with the 19'S on my car as opposed to the 17's I dynoed with later .
Car came with 17's . Now I relize I spent 3K in AM wheels and tires only to slow my baby down some :( aarrhh!!
 

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In general, for a car that weighs about 3000 pounds, you need 10 HP to lower the 1/4 mile time by 0.1 seconds.

To achieve the same effect, you need a weight reduction of about 100 pounds. But, if you reduce unsprung mass (wheels, tires, rotors, calipers etc.), the effect is magnified by a factor of 6 or 7. Assuming a conservative factor of 6, reducing the unsprung mass by 16.7 pounds will have the same effect on acceleration as reducing 100 pounds from other areas of the car.
Where did the factor of "6 or 7" for a reduction of moment of inertia in the wheels to total weight reduction come from? I understand the principle but not the numerical value.

Lets say that you could reduce the weight of each wheel/tire combination by 5 pounds, times four tires is 20 pounds, times a factor of 6.5 (average) and you get total equivalent weight reduction of 130 pounds or roughly a 13 HP gain. That would be significant.
 

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Re: Tomasz

Intersting Post Tomasz ,I also noticed my dyno showed a significantly lower Number with the 19'S on my car as opposed to the 17's I dynoed with later .
Car came with 17's . Now I relize I spent 3K in AM wheels and tires only to slow my baby down some :( aarrhh!!
If you used an interia based Dyno the readings will be different with different wheels. Best to use a constant load type (eg Dyno Dynamics) else dont change the wheels.
 

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Where did the factor of "6 or 7" for a reduction of moment of inertia in the wheels to total weight reduction come from? I understand the principle but not the numerical value.

Lets say that you could reduce the weight of each wheel/tire combination by 5 pounds, times four tires is 20 pounds, times a factor of 6.5 (average) and you get total equivalent weight reduction of 130 pounds or roughly a 13 HP gain. That would be significant.
his hot rod math is still right....to knock off .3 in the quarter you need at least 30 hp [lb torque really]
 

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Discussion Starter · #15 ·
Yes, absolutely, this is hot rod math. But guys I tried to show Escort G-Timer gains after installing SRP, exhausts, etc. I just could not come up with anything conclusive. Dyno did show power gains - for sure.

My point is that, I have spend much money on power generating mods, that I should have spent on lighter 18" wheels. Likely light wheels and tires would have given me the same result in performance gains as my power mods.

Again, hot rod math, but I just never realized HOW much is there to gain by swapping to lighter wheel / tire combo.
 

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Tomasz,

I think you and perhaps C&D are oversimplifying, I haven't read the article, didn't see a link so I don't know their exact method of testing, etc. but a lot of factors go into 0-60 times. If all you did was change wheels and tires and you got a better 0-60 time you'd have to look at weight, diameter of the wheel (gear ratio) as well as the tire itself to determine which factors played a part in having a better 0-60 time. Was it always better or just they eeked out a peak time that was better? It could be due to the tire tread getting a better grip, less wheel hop will also result in faster times.

I don't think you can say shaving a few lbs off the wheels is the same as adding 40hp, it just isn't, and shaving 60lbs seems unrealistic. That's 15lbs a wheel, did someone go from a 22" truck tire/wheel combo to an 18" forged set? I know going from my Porsche wheels to my track wheels I get maybe 5lbs a wheel if I'm lucky, or 20lbs overall, which is a 1/3rd of what the original claim was in this topic.

So it seems oversimplified to me, something is missing here, need more info...
 

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Ken: I think reading the article will answer most of your questions. The intent was to see how "upgrading" the OEM 15" wheels/tires +1 through +4 (19") would change various performance and other measures. The 19" setup weighed 14# per corner more than the 15" setup, so the total wheel/tire weight gain from OEM to the +4 setup was about 56#. All aftermarket wheels used were cast from the same manufacturer, and wheel widths increased from the OEM 6" to 8.5" on the 19".
 

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Ken: I think reading the article will answer most of your questions. The intent was to see how "upgrading" the OEM 15" wheels/tires +1 through +4 (19") would change various performance and other measures. The 19" setup weighed 14# per corner more than the 15" setup, so the total wheel/tire weight gain from OEM to the +4 setup was about 56#. All aftermarket wheels used were cast from the same manufacturer, and wheel widths increased from the OEM 6" to 8.5" on the 19".
Yeah I just noticed that now upon re-reading one of your earlier posts. So I was correct then in stating that it isn't realistic to think you can drop 56lbs from a Cayman or Boxster or 911's tire/wheel setup and get that kind of 0-60 time improvement. Going to the original point, while I think there is some performance to be gained by going to a lighter tire/wheel combo for a Cayman, I think you'd get greater returns with a proven exhaust system in terms of 0-60 times for less $.
 

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Going to the original point, while I think there is some performance to be gained by going to a lighter tire/wheel combo for a Cayman, I think you'd get greater returns with a proven exhaust system in terms of 0-60 times for less $.
Yes, but you also gain in braking and handling by going to lighter wheels and tires. It was an interesting article and I too was surprised by the performance of the 19" wheels and tires. The #1 issue they did not address completely is sidewall construction. Obviously deflection has a great influence on the cornering numbers but it also plays a role in braking and accelerating. Even the same tires have varied internal construction based on size and load ratings. The sidewall of a tire is effectively part of the spring rate calculation for a given setup.
 

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Discussion Starter · #20 ·
I agree with you all. Having said that, I am now searching this site for OEM wheel weights. if anyone has them - please link. 15 lbs. per corner would be very hard to do, but I will try to at least price it out.
 
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