So ... for a Vette ... wider is better? for all out traction ...

As far as I am concerned .. today ... I met a man with a Lingenfelter 500TT (#4) ... he runs his car in the summer and winter (we have snow) with stock sized tires ... he says that they break loose only when you STOMP it ... his take on this subject was ... it has a lot to do with your driving style ... some people could break loose 335's and other's cant ... if you drive with a heavy foot .... no amount of traction will help after 450RWHP ... but if you know how how to control the power ... you can hook up with the stock tires ....

I know 2 Supra owners who drive around with 600RWHP alll day ... and they can hook up with 275's (some use 295's) ... the main thing is that during 1st gear ... stay off the gas ... they cant/we all cant get low ET's with 275's or 295's becuase of the reasons above in other posts ... but for everyday use ... tires in Z06 sizes should be fine for anyone ...



[Modified by NewVetteLover, 6:57 PM 8/2/2002]

 

Mos tof the confusion comes from mixing apples and oranges here. First, the force that supports the car is derived not from the force of the ground pushing up against the wheel but from the opposing force of the air inside the tire pushing outward on the tire against the downward force of the weight of the car. Imagine four cube shaped beachballs, one at each corner of the car. When deflated, the car rests on the ground. As air is pumped into the beach ***** (beachcubes?), the upward force of the confined air conteracts the downward force of the car and when the two forces are equal the car rises. Pressure is force divided by area so the force nned to raise the car is 800 lbs at each corner. The total surface area of the inside of a tire is more than sufficient to allow for this force to be exerted. (It is unlikely that you could support a Corvette on four racing bicycle tires regardless of the amount of air you put in the tires, for example.) Going one step further, imagine that each pair of beachcubes, front and rear, has a metal rod stuck through them so that the rods run under the car at the front and rear. Now, it is clear to see that as air is pumped into the cubes, eventually they would inflate enough to raise the car on the rods similar to the way inflating tires raises the wheels and thus the car. As I am sure everyone is aware, it takes far less than 30 psi to raise the car off the ground so the 30 psi is a number chosen by the tire manufacturer that optimizes the downward forces against the lateral forces necessary to support the sidewalls. By the way, we have not begun to develop the issues involving the elasticity of the tire and the compressability of the air. The numbers Tom generated hold true in a very narrow case but begin the change as the car starts to move. Traction, on the other hand, is a function of the coefficients of friction existing between the tire and the road. Unfortunately, there is more than one kind of coefficient but in general it si in part a function fo the cantact area of the tire. The contact patch is only minimally dependant on the tire pressure since the tire is not all that elastic. So changes tire pressure only change the contact area to a relatively small degree. The contact patch of a Z06 tire is significantly larger than that of say a Geo regardless of the pressure in either tire so the coefficient of friction and therefore the traction of the Z06 is greater. Finally, the original discussion neglected the effect of centrifugal forces acting on the car and tire. Although the downward force on the tires at rest is 3200 lbs., the force on the contact patch of a Corvette tire going around a 90 degree turn at 140 mph is several tons. The relationship is a function of the mass of the car times its angular velocity squared. This obviously has little to do with the 30 psi of air pressure inside the tire. Now if I could just keep my radio antenna from coming unscrewed while I drive...

 

It looks to me that in your original post your are assuming that 30psi is recommended for the larger tire also and that's where your equation get lost.

 

Thanks for an informative look at the issue, however I still think that many of the things that people are worried about wash out when COMPARING two types of tires.

We don't have to define the exact formulas for every detail of tire performance to try and understand why a wider tire handles better than a skinny one in a slip situation.

Using my example above, admittedly contrived - but useful for this question, you have tires with the same aspect height, same diameter and 1/2 width on one tire.

The question then becomes, "why does the LONG, SKINNY (in relation to the slip direction) contact patch handle better than the more SQUARE, contact patch?

I think some of the links above have touched on that, particularly the tirerack quote.

To REALLY understand this question, I bet we could get a lot out of this link, but it's kind of expensive for a book! All the SAE stuff is though - but what I really hate about investing in it, is that SAE stuff also usually goes over my head in detail....
https://shop.sae.org/servlets/productDetail?PROD_CD=R-146&PROD_TYP=BOOK&COMMON_SUCCESS=TRUE

Anyone got this?

 

The basis of your assumption does not consider the dynamics of acceleration.
The guys at the dragstrip talk about getting hooked up, which mean transferring the force of the car to the rear wheels due to acceleration.

Consider that if we were on ice then it would not matter what size tire your had you get no acceleration and the weight of the car is of no consideration.

However the acceleration creates force and that force goes to the wheels and force distance per time is power.

My speculation is that good acceleration will compress the tire which increases the contact area and allows more power to get to the Road. Iterate this a few times and more acceleration more compression more traction more power to the Road.

Spinning the tires is easy because the acceleration is poor and the compress from it isn't there so we can Smoke the Tires when we apply more power than the traction will permit DYNAMICALLY.

Bigger Tires means more available traction, I would be embraced to try and argue otherwise.

 

It was mentioned that in the '50's it was calculated that no car would exceed a buck fifty in the quarter. This was derived from figuring that the grip of the tires couldn't exceed the 1 Gee translation of torque to acceleration. The tires could not hold the track once the sheer force exceeded the 1 Gee downforce. (Am I this old?!?)

Since I am revealing my age, the advance in tire technology that was the real advancement transition was rubber compounds. The reason that this 1 Gee barrier was broken is that more of the polymer chains' ends actually began to extend further into the the track surface. They also bond better chemically through electro-static forces. Tracks also employed better technologies for surface preparation. So the smoke from the rear tires is actually a combination of these expensive precisely designed chunks of rubber and torn off road surface. These effects are always demonstrated by tractor trailers pulling pebbles out of the road directly in front of you!

The flex wall design of drag tires, the distortion ripples on the side walls, promotes gripping ahead of normal patch. This allows better hook-up as the sticky is establishing grip prior to the weight and torque arriving. The rubber is being slapped into place before being tugged. This augments penetration through static contact and cling. Tire spinning in place is the opposing corollary. The tire is 'floating'. No hooking up is being established.
The other advantage of drag tires is that as speed increases, the tire 'grows'. The side wall flex diminishes. 'Effective' air pressure of the tire increases geometrically due to circumfrential inertia of the outer tire. The final drive ratio decreases and needs to be calculated for optimal final high speed trap gearing.
If I remember correctly (A potential senior moment here.) the dragsters only employed two gears. The lower gear and upper gear had the growth factor of the tires dialed in giving a 'variable speed gearing' effect. This allows for less shifting and more powering during those seven second and less runs.
Why don't they perfect this for me when I am doing a little freeway cruising? I'm sure my tires have grown larger on occasion!

A chemical engineer is really the guy who can define ultimate sheer forces and corresponding grip. I don't believe 'naked' physics' will define the parameters, particularly on varying surfaces. The CF is way too variable, especially factoring in track surfaces, temperatures of track and ambient, yada, ya.
Heck, at races they change tires like Hefner changed girl friends. What type do I need for this particular party? Tires for me are the only thing near affordable for varying my performance. Ask my wife. :blueangel:
:lol:

 

You are taking a scenario where the coefficient of friction approaches zero. We are looking at a situation with identical tire compounds, tire height, sidewall height and the only difference is WIDTH of the tire and asking, which tire will get better traction on a drag race start?

This will happen with a 6 inch wide tire OR a 12 inch wide tire. Without a doubt, the transfer of weight from front to back increases the amount of weight over the back tires. That increases the footprint, or contact patch. But the 12 inch wide tire will have the same amount of footprint (approximately) as the 6 inch wide tire.

The 12 inch wide tire will have a WIDE, SHORT (~12inches x 2 inches) contact patch during straight line acceleration while the skinnier (6 inch wide) tire will have a more SQUARE (~6 inches x 4 inches) shaped contact patch.

Bigger is a broad term. If you mean WIDER, then I am still trying to figure out why. I don't see that wider tires means any more tire touching the pavement...

 

Tom, where do you get the 2 inch (short) contact patch for the wider tire? A 12 inch wide tire is going to have more volume than a 6 inch tire. That changes your original contact patch fomula. 30 psi is not going to be recommended for both widths, all else being equal.

 

Let's not forget to factor in the circumference of the tire in this equation. The larger it is, the "longer" the contact patch.

Mike :flag

 

I further speculate that the behavior of the contact patch under acceleration is significantly different between wide and narrow tires because of dynamic forces.

Compression is created at the toe and released at the heel. Therefor a contact patch that is relatively long in the direction of travel will tend to buckle, and thus cause a spontaneous release of friction. In other words the narrow tire will tend to cup out in the contact patch putting the force onto the sidewalls. This should cause the tire to bounce when this occurs.

A wide tire where the same acceleration is applied will have less compression per length at the toe simply because it is wider. Less compression means less deformation and I would speculate less buckling. The effective area of contact patch, remains more stable and able to provide better traction for forward acceleration.

This would also explain why very low tire pressure will reduce performance by alowing the tire, at the contact patch, to buckle under load.

All this is direction dependant, the contact patch that is good for forward accelation may not perform equally laterally.

 

I'm assuming 30 psi for both tires. Might not be ideal for either, but it should be ok for trying to equalize the equation a bit...

Heck, lots of people go lower than that when drag racing, in order to increase contact patch and allow for more sidewall deformation.

30 psi might be a little low for the "6-inch wide tire" but it should still work. You should be able to easily get an ~4 inch front-to-back contact patch to the ground.

 

:cool:


[Modified by 66ImpalaLT1, 1:45 AM 8/5/2002]

 

This is where your reasoning goes wrong. We have 30 lbs per square inch of force from the compressed air pressing against the inside surface area of the wheel and tire. How many lbs per square inch pressure exerted onto the road would be the corner weight divided by the contact patch in square inches.

You cannot figure contact patch that easily from the PSI in the tire. If you could, your reasoning would mean that with 1psi in the tire we would have 800 sq/inches of contact patch, which is more than half the surface area of the tire (26.7 * pi * 12 = 1006.4).

What it all boils down to is that our theoretical skinny tire will have a 6 x 2 contact patch while our theoretical wide tire will have a 12 x 2 contact patch.

But honestly, its much more than 2" rolling that touches the road.

Eric


[Modified by 66ImpalaLT1, 1:56 AM 8/5/2002]

 

Without a doubt, the contact patch changes in size during cornering, the weight shifts over the outside tires (and comes off the insides) and that increases the patch size, as well as the lateral force deforming the tire somewhat...

BUT this happens to a skinny tire and a fat tire. To some degree this can be considered a constant since we are comparing tires in the same situation.

And we arrive once again at the unanswered questions:

1. Why is a wider tire better for handling;
2. Is a wider tire better for drag racing starts?

At 1psi the sidewalls would deform and collapse on a non runflat tire, thus the rims would be pressing on the rubber directly at a MUCH HIGHER PSI (and much lower surface area) than a properly inflated tire.

As you lower tire pressure, you lose your relatively even surface loading, and the sidewalls take up more and more of the load, increasing the PSI on the sidewalls dramatically.

So, yes you CAN use 30 psi (relatively) evenly distributed to come up with an approximate contact patch on a sitting car.

I don't think I understand why you are saying the 6 inch tire will only have a 2 inch forward length? It will have closer to ~4 inches.

And yes, those numbers will increase as the load is put on them, as I said above. But the underlying question still remains...

The fatter tire is going to have a LONG, SKINNY contact patch and the thinner (6-inch) tire is going to have a more SQUARE contact patch. Barring extreme circumstances with the sidewalls and such, they will both still have JUST ABOUT the same AREA of contact patch. So... why does the wider tire handle better?

The tirerack.com link above shows some pictures that I will try to link here, if I can figure it out ok, and their page answers SOME of this...

 

Why ..... :confused: no ones going to put 205's on if I can figure it out or not ;)

 

I agree that contact patch changes with the tires air pressure, but you cannot come up with one single formula that will give you contact patch for any tire.

In the picture you show off tirerack you're comparing tires with a different wheel diameter and sidewall height. Of course this will change the front to back contact length. But given the only difference is the tread width, the front to back contact length will be about the same.

1psi would significantly deform the tire and give it more contact patch, but more than half the surface area of the tire could never touch the ground, even with an infinately small wheel.

What it comes down to is that corner weight / air pressure inside the tire = contact patch cannot possibly be correct.

Eric


[Modified by 66ImpalaLT1, 1:06 PM 8/5/2002]


[Modified by 66ImpalaLT1, 1:18 PM 8/5/2002]

 

Any Consensus yet?

 

Formally stated, Newton's third law is: "For every action, there is an equal and opposite reaction."

The tire is pressing in every direction with 30 pounds per square inch. "By Law" there must be something pressing BACK with 30 pounds per square inch.

That means that the ground is pressing back with 30 pounds per square inch.

Since there are 800 lbs above the tire, then there must be 26.7 inches of contact patch (I shoulda made it 32 psi, woulda been easier numbers).

The only variables on a sitting car, are that the sidewalls will actually do more of the work than they are supposed to and the contact patch will actually be a little smaller. But that doesn't matter if you will look at the big picture. Because a 6 inch wide tire is gonna have just about the same contact patch as a 12 inch wide tire, if they have the same pressure.

I guess this is one of those really hard to believe statements because all our lives we have thought, "a wider tire means more rubber on the ground." According to science, that isn't really true.

I know that. They are for illustrative purposes. To show what the wide tire's contact patch looks like, and what the skinny tire's contact patch looks like.

But did you read what tire rack said?

"The shape of a tire's contact patch or "footprint" greatly influences its performance and is dependent on its profile or "aspect ratio". Low profile tires (most performance tires) have a short and wide contact patch that is effective in converting the driver's input into very responsive handling, cornering stability and traction...especially on dry roads.

High profile tires (light truck and most passenger tires) have a long and narrow contact patch which helps to provide predictable handling, a smooth ride and especially good traction in snow."

They agree with me. And they offer the vaguest of explanations to question one... It appears to be a vector-oriented thing. For reasons we haven't gotten into yet, a short, wide contact patch appears easier to turn and gives more stability.

Notice tirerack NEVER says that the wider tire puts more rubber on the pavement!

Once again, at 1psi you would deform the tire and the SIDEWALLS would see a huge increase in psi force that they were exerting. Note, psi IS NOT air pressure, just PLAIN PRESSURE. They RUBBER of the sidewall would be doing the work that the air HAD BEEN doing when the tire was properly inflated.

So, you would see spikes in the hundreds of psi along the sidewalls to make up for the lack of support from air pressure. This does NOT disprove Newton's Third Law, you just have to realize at extreme low AIR PRESSURES something else has to take up the slack: the sidewalls.

It IS reasonably correct for a correctly inflated tire. There is SOME extra load taken up by the sidewalls, even at correct inflation. But for all practical purposes, you are going to see ~ 2inch x 12inch contact patch for our theoretical tire at 30psi.

I have to go out this evening, but when I get back I'll see if I can find some more supporting information to further back this up.

I have already, while typing this, found some very interesting resources I want to share here.




[Modified by Tom Steele, 5:42 PM 8/5/2002]

 

True ... Tom ... I think that wider is better after all I have read ...

 

How do you arrive at 30 pounds per square inch, so far I've assumed that you are refering to tire inflation.

Just because your tire inflation pressure is 30psi, it doesnt mean the tire is pressing in every direction at 30 pounds per square inch. The opposite reaction to the inflation is that the tire is pressing against the air inside it at 30psi.

I'm going to state it clearly, YOUR SCIENCE IS FLAWED.

Tirerack does not agree with you. So I'll ask you, did YOU read what they said? They said that the contact patch size is dependent upon the aspect ratio of the tire. Your original argument did not mention aspect ratio, and only stated that a 6" wide tire would have the same contact patch as a 12" wide tire. I assume that you mean exactly this: A 265 35R18 tire will have the same contact patch as a 295 35R18 tire.

What I've been trying to say is that with the same aspect ratio and tire diameter the contact patch will change proportionally to its width. If I read you correctly, you are saying that it does not, it only shape changes.

Again, what is the figure you're quoting as 30psi?

good luck :cheers:

Eric


[Modified by 66ImpalaLT1, 8:56 PM 8/5/2002]


[Modified by 66ImpalaLT1, 9:09 PM 8/5/2002]