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Tiiite.com • View topic - Article interessant

Article interessant

Discussion général sur l'automobile.

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User avatarBiggs
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Joined: 12-11-2002 02:19 PM

Article interessant

Postby Biggs » 14-04-2003 01:37 PM

Une partie d'un article trouvé sur hotrod.com.

Vous trouverez l'article complet ici:


...
Q: For us non-physics guys, please explain F=uN.

A: F, the forward force that accelerates your car, is the product of "u", which is the traction coefficient for a given pair of materials (in this case rubber on asphalt) times the "downforce" ("N") of the drive wheels on the road. The width of the tire is never in this calculation. The only reason wider tires can help is they can increase the "effective u" just a little, not a lot.


Wide tires are far more "show" than "go," no matter how much our egos want to disagree with that. The bottom line is g’s, and we’ve done a lot of testing in this area. The data should speak for itself, and correspond to tests on the street. Track conditions are typically better. The lowest traction limits we’ve measured are all on front-wheel-drive cars. This is because of the weight transfer offthe drive wheels when accelerating. They’re always between .40g and .50g.

Rear-wheel drive vehicles fare better. Most pickup trucks generate between .48 and .52g, thanks to in-optimized weight distribution. Non-posi cars are again a little better. They typically generate a maximum of .50 to .55g almost every time. We tested a Ford Crown Victoria with 215mm rubber, and it would spin the right rear at .50g. Next test was a ‘66 Mustang Coupe with 195mm rubber, which spun ‘em at .53g. A Volvo wagon (195mm tires) spins the right rear at .55g. This Mustang Fastback, with a posi and BFG radial T/A tires (215 mm) pulled .54g before the changes, and with the battery in the trunk now pulls .55 to .56g. Grand National Buicks (with posi) usually got .55g, 5.0L Mustangs (225mm, with posi) get .56 to .59g. My friend Rich has a ‘69 Super Bee, good posi, 215mm BFG’s, and pulls .57g before they spin. Note that these are not big differences from the best to the worst!

Now for some wide tire data: my friend John has a ’66 Nova with 275mm rubber and a good posi, yet it pulls .53 g max, right in there with my skinny-tired ’66 Mustang. Rich also owns a Hemi Charger with 275mm rubber, which can’t generate more than .55g, which you’ll note is less than his Super Bee does on 215s. That’s because he has played the weight distribution game on the Super Bee.

Highest street tire numbers ever? Weight distribution is a player. New Z28s (245 mm) commonly pull .62g max. My friend Shirl has a ‘79 Corvette (245 mm) pulls .65g on street tires.

See where I’m going here? There’s no magic "factor of two" yet. Honestly, even a posi only seems worth .05g or so (10 percent). How many guys do we run into that think a posi will double their traction?

Want to see big improvements? Change the tire compound. My friend Dave bought some BFG Drag Radials for his 5.0L Mustang. His 225mm street tires spin at .59g every time, yet his 235mm Drag Radials consistently pulled .68g. That’s 15 percent! A co-worker brought in his NSX with 245mm race tires, and thanks to the combined help of its mid-engine layout, we were measuring .75 g launches, over and over again!


I’m not saying that wider tires would hurt, I’m just arguing that they’re far more for show than go. They wouldn’t double the traction. Or add 50 percent, or even 25 percent. The max we’re talking is probably under 10 percent. So without tubbing the car, I can probably squeak in some 245mm tires if I had to. By trying to play the testosterone factor low on this car, my best money will go towards a set of drag radials, and selectively moving weight to the rear, not tubbing the car and running 315mm street tires.

Q: What’s the big deal about g’s on launch?

G’s are inversely proportional to velocity. What that means is that peak g’s occur at very low speeds and continue to drop as speed goes up. When we’re talking traction limits, we’re talking peak g’s, and by definition, they happen at launch or soon after when hitting the torque peak in low, if gearing is not optimized. So when you want to talk tires, or traction limits, you talk g’s on launch.

Of course g’s on launch are everything to guys who only think about e.t. Blowing the launch kills e.t. Blowing the last 300 feet of the track kills trap speed. In a related sense when you want to discuss power, you tend to discuss g’s at some higher speed, because, let’s face it, even a Corolla can pull .50 g at 2 mph and chirp the tires. And a Hemi Charger can pull .55g at 2 mph—which doesn’t look like much of an advantage! But, by 30, 40, 50 mph, the Corolla’s low power/weight ratio has g’s plummeting down, so it’s only pulling .20 g at 30, while the Charger can still spin ’em at .55g at 30. Why? It had the power to pull 1.0 or more g at 2 mph, but thanks to tires was limited to the .55 g level. At the track, the Mustang consistently pulls .70g at 60 mph. This makes it clear why, on the street where the traction limit is more like .55g, the car will spin the rear tires at speeds below 70 mph.
2 strokes: Because one stroke isn't enough and four strokes waste two (many).

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