Boxster Tires and WheelsHigh-Performance Tires
Autocross Tires
I will probably be adding some info on autocross-specific tires at some point in the future, but for now I'd rather just refer you to The Tire Rack web-site. They have lots of very good info there. Popular autocross tires include BFGoodrich Comp T/A g-Force R1, Kumho V700 Victoracer (I just bought a set of these), Hoosier Radial A3S03/R3S03, and Yokohama A032R (actually more of a road-race tire). Tires are easily the most important ingredient in improving your car's cornering and handling. And they can bring acceleration times down too. My "links" page also has some other tire-related links.
But one thing I will say: Don't worry too much about matching the tire sizes Porsche recommends... there's lots of room in the Boxster's wheel wells to fit oversized tires. I've seen a number of people run 255/40-17 tires on the front of their Boxsters, and 275/40-17 in the rear on the stock 17" wheels! And it should be obvious that the optional 18" wheels and tire sizes work just fine, so you can follow that as a guide too. The wheels are 7.5x18" with 225/40-18 tires in the front, and 10x18" with 265/35-18 tires in the rear, and they clear the car's wheel wells just fine.
Fitting Bigger Tires:
Click here for a technique you can use to determine whether those big wheels and tires you're drooling after will fit your car without rubbing your wheel wells, before you spend a small fortune on them!Street Tires
I just read the March 1999 issue of Porsche's Christophorus magazine which has an interesting article on Porsche "Approved" tires. While I have my doubts about Porsche's motives behind "approving" only these tires, I also believe that they wouldn't recommend a poor quality tire. So following their advice is a good idea unless you really know what you're doing. The article discusses the selection procedure, and has a chart detailing the various sizes, brands and models of tires that Porsche considers suitable for the Boxster and 911.
Porsche has approved the Bridgestone Potenza S-02 N3, the Continental SportContact N1, the Michelin MXX3 Pilot SX N2 (which I have), the Pirelli PZero Direzionale and Asimmetrico N3, and the Yokohama AVS S-1Z N1. They also specify the sizes which are 205/55-16 front, 225/50-16 rear, 205/50-17 front, 255/40-17 rear, and 225/40-18 front, 265/35-18 rear. Porsche has tried to categorize these tires to compare their relative merits using a "sports," "all-around" and "comfort" rating. Only the Michelins and Pirellis are described as "sports" tires. The others are "all-around" (Bridgestone) or "comfort" (Yokohama and Continental). So choose the tire that best describes your driving style. If you get really annoyed at a harsh ride, get one of the "comfort" tires. If performance is more important to you, get one of the "sports" tires. If you can't decide, get the "all-around" tires.
If performance is your priority, I recommend reading the April 1999 issue of Automobile magazine which gave the Michelins very high marks compared with the other tires in the test. In fact, the Michelins scored highest in dry lateral acceleration (cornering) and braking. And my early autocross results seem to show that the Michelins are very competitive. The Tire Rack tests section also affirms this high assessment of the Michelin's traction. I'm really glad I got these tires on my car!
Tire Pressures
For my original Michelin Pilot SX MXX3 tires I currently use 33 psi in the front tires and 31 in the rears when they're cold, then I bleed them down to 34 front, 32 rear once they've warmed up after my first run. To read about how I determined the best tire pressures to use when autocrossing my Boxster, see my "autocross" page, or read the "chalking" paragraph just below this paragraph, and the "Wheel Alignments" section below. Note that the ideal pressure varies between different sizes, brands and models of tires and wheel alignments, and might be different for high-speed track events too. For instance, I've heard that the Pirellis seem to require much higher pressures... possibly over 40 psi in the front.
You can use the old "chalking" technique to get your tire pressures in the ballpark: You should dab a stripe of white chalk or shoe polish at several points along the outside shoulder of each tire before you run. These stripes should extend from about 1 inch into the tread area down to about 1/2 inch onto the sidewall. Then, after your first run, inspect each spot and see how much of it has been scrubbed away. If the scrubbing reaches past the tread area onto the smooth sidewall you need to add pressure. If the scrubbing doesn't reach to the edge of the tread you should lower the pressure. Repeat for each tire and each run.
Aftermarket Wheels
I haven't decided whether I really want to go this route. The biggest reason for me to buy wheels would be so I could use soft rubber-compound tires for competition. I used to always have two sets of wheels for my autocross cars (or more): One set with street tires, the other set with sticky autocross tires. That was so I wouldn't have to wear out the competition tires driving them around on the street. But one of the things that burnt me out in competition was having to haul those extra wheels and tools around, and swapping wheels back and forth all the time. What a pain. Perhaps I really don't want to deal with all of that again. We'll see. Plus, weight is important in competition, especially for wheels since they have extra inertia from the spinning forces, and the lightest wheels tend to be very expensive.
In general, Porsche wheels tend to be very light and strong, so if you merely want another set of wheels and aren't out to make a fashion statement with your wheel selection, consider buying a set of Porsche wheels. According to Porsche the "Technology" wheels, also known as "Turbo" wheels (not the same thing as the "Turbo-Look" wheels), are even lighter than their other wheels, due to hollow-cast spokes and a "friction welding" technique used in manufacturing.
If you are interested in aftermarket wheels, I'd suggest BBS, SAC, HRE, Kinesis and Fikse wheels. They are all quite light and strong. I also looked at the cool "Wheel Rack" that The Tire Rack web-site has. It is a very well done interactive tool that allows you to view different wheels on various cars, including the Boxster. See my "links" page for other wheel suppliers.
Fitting Bigger Wheels and Tires:
If you're competing with your Boxster in SCCA's Street Prepared Category, or you just want a cool street car, you can run larger wheels than the originals. The general belief is that you should install the biggest wheels that will fit for maximum grip, though there are disadvantages to bigger sizes too. Using a larger diameter wheel is not always desirable since the tires and wheels not only cost more, but also may make it harder to lower the car, and can rub if the tires are larger in diameter than the standard tires. Also, there are generally fewer sizes and models of tires available for larger wheels, particularly 18-inch and larger wheels. Extra-wide wheels and tires can also cause problems. Again, they may rub, and they increase the aerodynamic drag. They are also far more prone to hydroplaning on wet pavement, and don't stick well in mud, gravel, snow and ice.Click here for a technique you can use to determine whether those big wheels and tires you're drooling after will fit your car without rubbing your wheel wells, before you spend a small fortune on them!
Wheel Alignments
After tires, this is one of the most important modifications you can make to your car to improve its handling. For the Boxster, as with any car, there are certain limits to how far you can adjust the suspension before metal meets metal. In general, I'd follow the advice given at www.BoxsterRacing.org. (follow the "Car Setup" link). This is a good starting point.
But the ultimate way to determine the ideal wheel alignment for your car is more complicated. You should purchase a "tire pyrometer" which is simply a fast-reacting temperature-sensing probe that is used to measure the temperature of a car's tires at different points. The goal is to find the combination of wheel alignment (mostly camber) and tire pressures that result in the most even temperatures across the treads of the tires. Here are a few links with info that sums up tire pyrometer use pretty well:
http://www.bfgoodrichtires.com/tires/data/r1quide_extra.html
One thing I would elaborate on though:
For autocrossing I've used a tire pyrometer in conjunction with a "skidpad" (a circular path on pavement) to determine the best alignment. I'd simply make sure the tires were cool, then I'd drive the car around in circles in one direction until they'd warmed up (about 10 circles or so), stop, and quickly measure the tires' temperatures. I'd then make any adjustments to the alignment and tire pressures of the wheels facing the outside of the circle that the temperatures would indicate (that is, I'd make quick on-the-spot adjustments to favor more even temperatures). That would usually give me an alignment and pressures nearly optimum, but I'd then let the tires cool down, re-test and keep repeating the test until the temperatures were all even, adjusting between every test as needed. When done I'd go to an alignment shop and have the other side equalled, and the alignment settings tidied up.
That seems to be best for autocrossing, since turns are so important and I want to favor tire grip in turns over braking and accelerating. Taking temperatures after an actual autocross run tends to make the temperatures appear more even than they would be in turns only. But sometimes cars go faster if the camber is backed off a little (especially if the indicated camber is very high), since high degrees of camber cause the wheels to run at an angle to the ground when driving in a straight line, and that reduces the traction available for accelerating and braking. Probably not much of an issue for Boxsters with their rear weight bias... especially when equipped with the optional M030 Sport Suspension or aftermarket springs, since they don't require as much negative camber as the standard suspension.
Camber Plates
These are plates that attach between the tops of your struts and the point where they attach to the body, or they may mount above the body. Their purpose is to allow a greater range of suspension adjustment (wheel alignment) than the factory adjustments allow. As far as I know these are not available commercially for the Boxster yet. But they can be fabricated by a good machinist.
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