17" versus 18" wheels - impact on acceleration
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From: Switzerland
17" versus 18" wheels - impact on acceleration
I find the 18" wheels much nicer-looking than the 17" wheels. They fill the wheel arches better. They certainly provide better road-holding in curves, but the negative point is that they appartently do slow down acceleration times appreciably. Accelerating from 60 to 120 mph will be about 2 seconds slower as tested on a 993 with a 3.8 300 hp engine (French Porsche Magazine Flat6). Anybody have any other test figures?
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From: So Cal
Given the same diameter tires, the impact on the acceleration will be largely based on the size of footprint. The 18's as installed on a 993, have a wider footprint than the 17's and will likely cause a slightly slower acceleration times. However, I find it hard to believe that it is as much as 2 full seconds from 60 to 120. Assuming similar rubber compounds, I'd guess the difference would be a fraction of a second.
Burning Brakes
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Don’t forget weight, which during acceleration is rotational inertia, and can be substantial. An 18” wheel/tire combo weighs more than a 17” wheel/tire combo. But, like Viken’s comment about the footprint, I would doubt this would amount to 2 seconds.
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From: Raleigh NC
Even if you compared 17" and 18" wheel/tire combos of exactly the same weight, it will be easier to accelerate the 17" combo since the mass is closer to the hub. The further a given unit of mass is from the axis of rotation the harder it is to accelerate (just restating Tom's rotational inertia idea). Of course, if the 18" wheel+tire were to weigh any more (more metal, greater width, etc.), the situation would be worse. Two seconds seems a little extreme, though!
Will
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Don't get me wrong and perhaps my priorities are a little skewed, but how important is two seconds from 60-120 MPH relative to the look of the car? Go with the 18s - You won't need that extra two seconds (which does seem extreme) unless you attempt to make the evening news here in SoCal in yet another high speed chase. 
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From: Switzerland
In regards to the weight of the wheels used in the test: The 17" wheels were the standard Porsche wheels, the 18" wheels were BBS magnesium wheels and were lighter than 17" wheels. 17" wheels: 17 kilos- front, 22 kilos -rear. BBS wheels were 18 kilos front and rear.
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From: Back In RI...............
ok, let me get this straight....17 inch wheels go faster, than 18 inch wheels!...that being the case, why not mount the car to say 11 inch rims? If my math is correct....lets see...diff in 60-120 mph was 2 seconds for the 17 inchs....(faster), that being the case....lets see...ok, on 11 inch rims, 60 to 120 mph would be reduced by (1 inch=2seconds) 12 seconds!!!!!plus all the money you save not getting 18 inch rims tires etc@!!!!!...wooohooooo 
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From: Arlington, VA
I like Phil's thinking. Not only will I drop from 18's with 285s to 11 wheels with 175s, I will sell my Yamaha R6 for a YSR50 mini replica race bike with 6 inch wheels for instant "slingshot" acceleration. Go with the 18's.
Not only will I drop from 18's with 285s to 11 wheels with 175s, I will sell my Yamaha R6 for a YSR50 mini replica race bike with 6 inch wheels for instant "slingshot" acceleration. Go with the 18's.
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From: So Cal
Originally posted by 993RS:
<STRONG>In regards to the weight of the wheels used in the test: The 17" wheels were the standard Porsche wheels, the 18" wheels were BBS magnesium wheels and were lighter than 17" wheels. 17" wheels: 17 kilos- front, 22 kilos -rear. BBS wheels were 18 kilos front and rear.</STRONG>
<STRONG>In regards to the weight of the wheels used in the test: The 17" wheels were the standard Porsche wheels, the 18" wheels were BBS magnesium wheels and were lighter than 17" wheels. 17" wheels: 17 kilos- front, 22 kilos -rear. BBS wheels were 18 kilos front and rear.</STRONG>
17 kg = 37.5 lb
18 kg = 39.7 lb
22 kg = 48.5 lb
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From: Hsv AL
Check out the original UOP Shadow Can-Am cars. They experimented w/ VERY small diameter wheels for that reason, and proved to be effective in qualifying. These are the cars that dominated after the Penske / Porsche 917-30K was eliminated by rules changes.
Some of you may recall how well they ran (later versions, more extensively developed) at the fantastic Laguna Seca Porsche 50th Anniversary Races a few years ago. The reason speed record specialty cars don't run small diam. wheels is (I believe) due to the escessive tire heat generated, that can't be dissapated at constant high revs.
Some of you may recall how well they ran (later versions, more extensively developed) at the fantastic Laguna Seca Porsche 50th Anniversary Races a few years ago. The reason speed record specialty cars don't run small diam. wheels is (I believe) due to the escessive tire heat generated, that can't be dissapated at constant high revs.
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From: CA
Weight is the biggest factor! I changed my 17" Cup wheels (205/265 tires) to 18" Fikse FM/10's (225/285 tires) which are supposed to be very light.
The Fikse wheel tire combination weighed 5lbs more for each front and nearly 8lbs more for each rear. That's over 25lbs of mass added to the drivetrain and would slow acceleration AND lengthen braking appreciably.
Wheels typically weigh more than tires. A 15" wheel with a 265 tire should weigh considerably less than a 19" wheel/265 tire with all else being equal (circumfrence, diameter, etc). Air and rubber weigh less than aluminum, magnesium or unobtanium.
Anyone who has driven a vehicle with a lightweight flywheel would understand the dramatic effect rotating mass has in vehicle performance. Acceleration is quicker and engine braking is much stronger. It's no different with wheels/tires.
MC
The Fikse wheel tire combination weighed 5lbs more for each front and nearly 8lbs more for each rear. That's over 25lbs of mass added to the drivetrain and would slow acceleration AND lengthen braking appreciably.
Wheels typically weigh more than tires. A 15" wheel with a 265 tire should weigh considerably less than a 19" wheel/265 tire with all else being equal (circumfrence, diameter, etc). Air and rubber weigh less than aluminum, magnesium or unobtanium.
Anyone who has driven a vehicle with a lightweight flywheel would understand the dramatic effect rotating mass has in vehicle performance. Acceleration is quicker and engine braking is much stronger. It's no different with wheels/tires.
MC
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From: UK
Bit off topic, but I have a lot of trouble with the idea that the lightweight flywheel will have more than a tiny (i.e. proportional to the reduced total mass of the vehicle) effect on acceleration or engine braking. When the clutch is disengaged then the lghter flywheel will certainly allow the engine speed to change faster, so that might shave a tenth or two from the time to make the shift (in the right hands, not mine I'm afraid 
) but when the clutch is engaged the flywheel is a very tiny part of the mass the engine is trying to accelerate, surely there would be about as much benefit from skipping lunch
) but when the clutch is engaged the flywheel is a very tiny part of the mass the engine is trying to accelerate, surely there would be about as much benefit from skipping lunch
Three Wheelin'
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From: SF Bay Area
I replaced the stock 16 inch fuchs on my 930 with 18 inch 996 TT wheels a few weeks back. After the change I was able to pull a 0-60 in 4.60 seconds, a bit faster than with the 16 inch wheels (4.66 was my previous best). I would bet a bit is due to me getting better with the 4 speed gearbox, but clearly the 18 inch wheels did not slow the car down much (or at all). A few months back I also had a lightweight flywheel installed and I can tell you that is a huge difference. I certainly do not feel any change like that with the wheel switch. There are a lot of people who say the 18 inch wheels slow the car down, but when you ask them none of them actually have any real expereince with this. The 18 inch wheels cetainly improve the steering and overall handling. My 2 cents -
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From: Wash., D.C.
With all due respect to the French testers, their stats miss the point. We're not drag racing here, folks. On the track, we're trying to lhave as big a contact patch as possible for traction (cornering) and braking. Porsches are momentum cars, not super-fast sprinters. Once you get up to speed, you will thank yourself for that 18-in. wheel and wider tire, because the extra weight will be more than offset by better handling.
At least, that's the theory...
Lee in D.C.
At least, that's the theory...
Lee in D.C.