mpe488

It's common knowledge that wider tires = more grip. But I'm pretty sure it isn't as true as we would like to think. Sure, the heavier wheels/tire combo will sap whp thus making any application more traction friendly, but thats not the kind of traction we have in mind when we buy wider tires, is it?

I'm arguing that any significant improvement that we think we see, can either be attributed to another variable, or to our own imaginations.

Now, I know the actual equation for static friction force is too complicated to be used practically, but in the simplified model:

(coeff. of friction) * (Normal force) = (Friction force)

Contact patch has no relevance. You could argue that AWD cars get phenomenal traction that can't simply be "imagined" but the normal force isn't actually consistent in that model. In a rwd car the only relevant normal force is that which is exerted on the rear two tires. A fraction of the car's weight is being supported by the front two tires which cannot support friction (in a RWD car) because they revolve freely.

In an AWD car 100% of the car's weight is contributing to the normal force in the aforementioned equation.

Now, feel free to correct me if you spot any inconsistencies. I'm only a first year engineering student with a limited knowledge of friction. But, if I'm right I'll be sticking with 245s next time.

DaddySS

Well I just typed a whole paragraph and it said invalid thread.

So...wider = more traction up to a point and looks cool!

ewhood

I am by no means an expert, but I believe the contact patch does matter. The more tire contacting the road surface provides more potential for traction (thus a greater frictional force coefficient).

mpe488

that would certainly be the conventional wisdom, but if you looked at the patch as a wider area supporting the same weight, then you could analyze a small fraction of the patch and solve for it's friction force by using the equation (like in solving an integral.) Now the friction force per square unit will be smaller because the same weight is being applied to a greater area, but obviously there will be more square units of contact patch because the tire is wider.

In practice you should find that the total friction force:

(friction force per square unit) * (square units of surface area)

should come out equal in both cases.

sawedoff

iTrader: (3)

True, very true. There's romantic reasons that are entered into it.


All things come into play. I mean everything man

[QUOTE=mpe488]Now, I know the actual equation for static friction force is too complicated to be used practically, but in the simplified model:

(coeff. of friction) * (Normal force) = (Friction force)[QUOTE/]Teams with mucho buckos have expensive and elaborate main frame computers that will do these things and none of it will see publication.

If you're coming at it from the angle that you are approaching it at, then no you don't need anymore tire than what's really needed or allowed. If there's a ceiling in the rules, then most go straight for the biggest limit. A wider tire will also dissipate heat better too, from what I've read in engineering info that I've come across. But you're right.

mpe488

Does dissipation of heat improve traction? I have yet to learn any equations that relate temperature to frictional force, but I'm sure that it's a factor.

sawedoff

iTrader: (3)

Sure it does. Teams set up a car to behave certain ways during the progression of the race as the race goes on. They have a spec tire that they are dealing with, and they usually have tire limits on the amount of tires that they can use during the race too.

Teams even use thermal imaging cameras to monitor the tires too. Their mounted near the tire and then the signal is then fed into the car's telemetry system and can then be downloaded at pitstops. Now they get so much info that they have to figure out what it means.

Manic Mechanic

iTrader: (3)

Tire width accounts for less than 10% of availiable traction. I pulled this from HOT ROD.com. I read it a while back and it's interesting, confirms my long held beliefs that wieght distribution and chassis setup are far more important before blowing big bucks on more power or narrowed rears.

"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."

Vernon

Manic Mechanic

iTrader: (3)

This from Car Craft.com:
"“Bigger isn’t necessarily better.” Although the Real Street class allows any size tire, Joel O’Donnell prefers to run a smaller 31x10.5 ET Drag for two reasons. Reason one: he gets a 50-pound weight break for using a 10.5 tire. Reason two: testing has shown that the smaller tire has actually netted him a 6-mph gain over a larger 33x16 slick. Tire pressure is left at 10 pounds, burnouts are kept to a minimum, and the slight hazing of the tires is enough to generate mind-boggling 1.19 short-times.

Name: Joel O’Donnell
Hometown: Palos Hills, Florida
Class: Drive Train Specialists Super Modified
Vehicle: ’73 Pontiac Ventura
Best e.t.: 8.14 at 166 mph
Best 60-foot: 1.19
Motor: 458ci big-block Chevy by Fast Times Motorworks
Intake: Dart 18-degree with Holley Dominator
Heads: Dart Big Chief
Power adder: N.O.S. Pro Race Fogger
Exhaust: Kooks 2-inch step to 41¼2-inch Flowmaster Outlaw mufflers
Transmission: Powerglide, 9-inch Mid West converter with 4,200-stall
Suspension: Stock with AFCO shocks, front; four-link with AFCO coilovers, rear
Wheels and tires: 15x3.5 Bogart Drag Star with Mickey Thompson ET Street runners, front; 15x10 Bogart Drag Star with 31x10.5 Mickey Thompson ET Streets, rear
Rearend: Fab 9 with 35-spline axles, spool, and 4.56:1 gears"



Here is my $0.02. Anyone who tells you that putting your battery in the truck of a drag car isn't as effective as putting a light wieght battery under hood has never done it. I have and I'll throw lead in the trunk if I can't hook. I sure as hell won't hang it in out past the front axle and ignore it. I don't care if it adds 20 lbs in cable and parts to the car, it will go from a stop to a quarter mile away in a shorter period of time.


Vernon

FieroZ34

iTrader: (1)

Mid engine ftw

We move our batteries to the FRONT, right on the front axle

wabmorgan

True in the straight. The wider and heavier tires rob hp.

However, it is in the curves where wider tire and wheels shine.

mpe488

Even in the curves I dont beleive the wider tires help, the shorter sidewall does. I was also referred here, by a different forum I posted this on:

http://www.physlink.com/Education/As...TOKEN=80000491

RevGTO

iTrader: (12)

Common sense says that the thinnest, lightest, tire that allows you to hook will get you down the track fastest.

RednGold86Z

There's more to it than friction and width. When hot enough, there's adhesion. Where the road isn't as smooth as glass, there's mechanical "gearing" mesh between the rubber and the road texture. The inflation pressure causes contact patch distortion and alters the size, which affects the other non-standard "friction" aspects of the "static friction" of a tire. The tire compound and treadwear are key also. Worn street tires get better dry road traction, as well.

billc5

iTrader: (1)

Most of the good info on traction comes from empirical testing, the simple force from friction model is a good gross indication, however the other factors are,

Tires are not exactly like a balloon so the contact patch is not a simple shape from Weight/Force. the balloon will have a round contact patch demonstrating an equal distribution of force. Tires have sidewalls which creates the elliptical patch.

This is important at the point of dynamic friction because now the strength of materials between the different surfaces become important. For example is the tire shearing off rubber or the Road losing aggregate? Is the tire losing more rubber off the side wall or off the center equally?

Under rotation does the volume of the tire change and how does that change effect the patch and traction?

It is the resistance to shear forces that makes the tire compound so important and in most cases even more important than the size of the tire.

So after all that what is the relevance of the contact patch??

With a smaller patch ( lets say you put 60psi in the tire (smaller patch area)) the ability to shear the rubber off is easier, ie F/A is larger compared to the tire with on 16psi.

But as usual I could be wrong.

87ciz

iTrader: (3)

not sure of the physics behind the whole thing but i tell you my 275's handle a whole hell of alot better than the 245's i currently have on the car. 245's even have a lower profile.

Marc 85Z28

iTrader: (8)

That tire is not being run soley because the rules stipulate it as does gearing and trap speed. If that car went to a identical compound and width tire with the sole change being a shorter tire and smaller diameter wheel (to keep sidewall height identical), the car would lose a significant amount of ET. Taller tires increase footprint and increase traction. The fact that he gained MPH has to do with the over 100lb weight difference. Why not test a 12.5" tire?

Go watch Forumula 1 racing...

Look, I'm no engineer or rocket scientist by any means. I come in this section for a laugh, because most here seem to over-analyze everything to death, coming up with all sorts of crazy conclusions. 10.5 cars go faster on 10.5W tires - know why? I've personally gained a VERY consistent .090 improvement in 60' times going from a Nitto 275 drag radial to a 315 drag radial, and the MPH improved too (there goes width vs additional rotating weight theory). Instead of doing some mathematical calculations, why not go out see what works in the real world. Wider tires to a point (reasonable cutoff being well over normal street tire size) offer better traction with all other variables being the same. That street tire width vs lateral acceleration experiment is laughable at best! - I'm sure ALL those cars had the same tire brand, model, compound, and suspension setups right?

Or maybe I'm completely wrong... I think I'll inform every seasoned drag racer I know that they need to go much smaller with their tires as it will improve traction.

mpe488

sigh...

I guess there always has to be a Marc85z. Firstly, the mathematics and physics are sound, and they do explain your universe better than you can. We're not talking about "theories" we're talking about proven equations.

I'm sorry, but your very casual observation about street tires and trap speed does not disprove Newton's "theory" F=ma, and if you think it does you don't belong in this forum, your car, or in this millenia.

Secondly, I can guarantee that if larger tires improve acceleration in anyway it can be, and probably has already been, explained by physics. I can't say that about all things, but something as simple as tire width vs. acceleration is very reasonably within the grasp of modern physics.

Thank you for contributing nothing.

Marc 85Z28

iTrader: (8)

I nor any others are breaking any laws of physics. But when real world occurances defy what you calculate on paper to be true, what then? 20 years ago, it was calculated by better minds than you or I that it was physically impossible for a Top Fuel Dragster to run within a half second of what they do now...

So, have you looked into the Formula 1 tires yet?

intheclouds1977

There's many factors. The wider the tire, the less pressure on any particular spot of the tire because the weight is distributed. However, the narrower the tire, the less contact patch and possibly the less traction. You have to find the happy medium for traction. GO for the widest tire possible without getting so wide that the weight is too distributed and you have wheel slip. You can't argue with simple physics. Take a bicycle tire and stand it up. You can drag it along the ground pretty easily. Take that same bicycle tire and lay it on it's side. It's the same amount of weight, but much more rubber is in contact with the ground and is much harder to drag, plus there's not so much contact patch that it's "floating" either.

You have to find a good balance. Yes, as a general rule wider is better. But if you get too wide and spread the weight out too much, you'll be slipping just as bad again. That would be pretty darn wide though on a 3k pound plus car! I wonder why dragsters don't run 275 wide rear tires? Duh!! Not enough traction!

Also, you only want to go just wide enough to prevent tire spin for the amount of power you have/want on your launches. Any more than that is just wasted weight in extra tire/wheel.

Another even more important factor is rubber compound. The widest tire in the world won't do you much good if it's made out of plastic.

BTW, if you're just taking Physics I for the first time, wait till the class is over before making threads like this. You're assuming that the decreased CoF from spread out weight is directly proportional to the increased traction from increased contact area. It is not...nor is it linear. As the tire gets wider and wider, the CoF is affected, but not in a 1:1 ratio resulting in final friction. Not only is your knowledge of proper application of Physics equations shaky, apparently so is your application of common sense.

If you still believe what you're saying, go put some 165 wide tires on your car and let us know how you do at the track.