50 years different

[dad911]

Originally Posted by JackDidley

Would this car get traction on a little skinny tire of the same compound ? I dont think so.

It would until the tire failed.

https://physics.info/friction/

[island911]

Rather than talk about the nonlinear behavior of visco-elastic material under varying shear rates while energy inputs create nonlinear thermal gradients due aspect ratio of cord length relative to diameter ...

Let's talk bubble gum.

If you are making a bubble gum tire -a relatively small skinny tire- you would want to use old hard bubble gum - like the kind you find stuck to the underside of a table. Reason being that as that tire rolls it massages that big old contact patch -the flat patch much like can be seen on the dragster above. This means that before it starts making hard accelerations it creates heat. --just from rolling along. - at speed this is a big deal. So if you had made that tire from new soft bubble gum it would be really sticky, but it would all get hot fast and fall apart.

If you have a nice wide tire with the same contact patch (same amount of square inches on the pavement) you no longer have to worry about rolling along at speed driving the tire temp up (softening the bubble gum to the point where it slides off) This means that you can have a tire running softer stickier bubble gum. - Sticky for the work of accelerations (cornering, braking...)

Now let's for a moment imaging an old light 911 with that fat tire on the front. (take off the fenders for clearance.) That would be a performance disaster. It would be more like a ski than a tire. Well, at minimum it would be very difficult to bring the tier up to temp where it gets grippy. A fat tire like that needs some heavy loads to come to temp. But at least the aero would be horrible. And imagine driving in the rain, on a rutted highway. (water ski)

[hcoles]

Originally Posted by herr_oberst

What do they provide? Serious question.

Tires don't follow the classic laws of surface friction.

Here is a start from Wiki..
Tire load sensitivity describes the behaviour of tires under load. Conventional pneumatic tires do not behave as classical friction theory would suggest. The load sensitivity of most real tires in their typical operating range is such that the coefficient of friction decreases as the vertical load, Fz, increases. The maximum lateral force that can be developed does increase as the vertical load increases, but at a diminishing rate.

[Porsche-O-Phile]

My HS physics teacher made exactly that point and proved it to us (wider tires / wheels do not affect traction from a pure laboratory perspective). There are however other factors that aren’t considered (like cornering and sidewall flex and such) that favor the wider ones, aside from their looking cooler. 😎

No question modern tires have far, far better design and materials properties than those from just a few years ago. When you consider the abuse they take and conditions they’re subjected to - and how long they last, it’s almost magical.

[GH85Carrera]

Go get two identical tires, one a 285 width, one a 185 width. Same compound, which has more traction?

[JackDidley]

Originally Posted by dad911

It would until the tire failed.

https://physics.info/friction/

Pretty sure it would fail instantly. If I read the chart right, it is based on a constant velocity. The rail acceleration would be increasing, not constant.

[island911]

Originally Posted by GH85Carrera

Go get two identical tires, one a 285 width, one a 185 width. Same compound, which has more traction?

Same tire pressure too?

both cold?

Traction on what vector? Cornering or straight line? ..or parked (brakes on) on an increasing angle plate?

And since we are splitting hairs do the vehicles weigh the same?

And then we need to know the durometer.

[GH85Carrera]

Oh come on.

Yea, the very same car, both sets of tires are the exact same manufacturer and version of the exact same tire. Both have the exact same temp, and the proper tire pressure, and since it is the same car, the exact same suspension settings. The ONLY difference is one set of tires is wider.

Yea, I understand with tires, like many things there are an almost infinite set of variables.

I have been autocrossing since 1976. I remember vividly the best autocross tire was pretty much the Michelin XWX. I used up many set of them and a few XAS tires on my 1974 914 2.0. It was a very light weight (no ac) car and had great handling. I could beat a lot of the guys driving the 911s of the day.

The tires of today are clearly better in every measurable way, no doubt. But no tire advancement alone would be enough to put 205s in the rear of a modern 911. They have wider tires for more contact patch. Yea, the mass of the new 911 is way higher so it requires bigger tires to for many reasons.

Big wheels and tires are certainly driven by the style department, but the engineers want as much rubber on the ground as is feasible.

More than a few guys put super sticky race tires on a 914, and quickly found the weak points on the chassis. Porsche even sold a reinforcement kit to keep the suspension from ripping off with gummy race tires.

[island911]

"They have wider tires for more contact patch. "

How does a wider tire have more contact patch? ...given the same weight and inflation psi and rubber durometer.



per https://en.wikipedia.org/wiki/Contact_patch

[Sooner or later]

It will increase some but not as much as one would think.

A tire made out of concrete would not compress and when width is increased the contact area would also increase.

A tire made out of bubble gum would compress less as weight is increased so contact area would stay relatively the same.

I tire fits between the two examples. It will compress more than concrete but not as much as bubble gum.

[island911]

Originally Posted by Sooner or later

It will increase some but not as much as one would think.

A tire made out of concrete would not compress and when width is increased the contact area would also increase.....

The points of contact may spread but that is not the same as increasing contact area.

[Sooner or later]

Originally Posted by island911

The points of contact may spread but that is not the same as increasing contact area.

A tire is not bubble gum. It is not concrete.

Difference will be slight but up to a point the contact patch will increase by a small amount when only width is changed.

Do you agree that a concrete tire would have a larger contact patch as width is increased?

Btw, I worked for Goodyear and did work on tire design at one point in time.

[island911]

A tire contact patch is closer to bubble gum than it is to concrete. -nonlinearity from both strain rate and temp. Concrete OTOH is rather linear (strain rate) and temp independent.

Originally Posted by Sooner or later

..

Do you agree that a concrete tire would have a larger contact patch as width is increased?..

Because the weight would go up?

Let's say you keep the weigh the same you would need to decrease the circumferential beam thickness... do you have positive pressure lending support in this imaginary concrete tire?

Are you considering nonlinear compressive strain rates for concrete? If you are, that is one fine hair to split.

I'm trying to imagine how you could see a concrete tire having a larger contact patch as width is increased.

Clearly, a concrete tire displaces very little actual area, resting on a very few high points in the granularity of the pavement. This means also that the flatness of the road would dictate where on the cylinder the contact happens. -- a line of contact is not likely. Anyway, as I said before, a wider concrete tire may have a larger area were contact occurs, but that does not mean more area is in contact. --resting on a couple mm^2 high points 300mm apart does not make 600mm^2 of contact.

[RWebb]

They have wider tires for a wider contact patch.

This give you the ability to pull more G's in a corner.

[island911]

Originally Posted by RWebb

They have wider tires for a wider contact patch.

This give you the ability to pull more G's in a corner.

Yeah? ..that long line of contact does it?

Does that mean that they have less grip for braking and forward acceleration?

[unclebilly]

Well I know that on every race car (and kart) I have owned / driven, keeping all else equal, a 10cm wider tire has allowed us more speed through a corner and faster lap times. As mentioned, a bigger tire allows you to push harder before it steps out or ‘goes off’.

[GH85Carrera]

Find me a race team anywhere driving on dry pavement that wants to go to a skinnier tire to increase speed. Good luck.

[island911]

Originally Posted by GH85Carrera

Find me a race team anywhere driving on dry pavement that wants to go to a skinnier tire to increase speed. Good luck.

No one said that. c'mon.... reductio ad absurdum.

Although, OTOH, I promise you that race teams have gone to skinnier tires. A tire can be too wide. On a race team tire choice is an optimization problem.

I once had some 550 rims. Those were 16 x 3.25"! But hey, 1200lb car! Would that 998 tire give that car more grip?
On that thought, I knew an old autocrosser that had much fatter (than OE) tires on his Beck replica... until Porsche used it for a Boxster commercial and put on some OE approximations (tall and skinny) - the autocrosser was stunned by the performance improvement. (kept 'em)

Ask yourself, Why would a 930 have such skinny tires up front? --Clearly a wider version of the same wheel tire was available at the time. Was it because they wanted worse braking, worse cornering... slow the car down? OR was it to balance the front/rear grip? --I will note that car makers typically do like to set up ROAD cars with a bit of understeer, but that can be achieved with inflation pressure differences and/or alignment) So why would a light nosed, heavy assed 930 have relative steam-rollers only in the back?

Typically the width of a tire increases with the load. (have you noticed how much heavier a modern car is?) There are other factors of course, influencing tire shape, like road type expected, suspension kinematics and dynamics, scrub radius and aero concerns, but again, tires are engineered/optimized wider (or narrower) so that they can carry appropriately sticky compound and not easily exceed the optimal temp.

For any tire as the cord length (F/R of the patch) grows so too does the heat put into the tire from the rolling component. (drive on a flat and see how hot that gets) The more heat from rolling means less abuse available for accelerations - this is the fundamental. Make the tire wider and the cord shrinks (cooler, from both less dynamic deflection and from more available surface area(heat shedding) - again, thermal management)

The optimization of tire width is much more complicated than even that fundamental, and a looonnng ways away from the misguided tautology of "wider tires has more grip cuz wider is grippier." --that is simply not true.

hmmm... I suppose you could safely state, wider can provide more grip, in that it's a longer lasting grip. But that would be equivocating on the word "more" to mean longer lasting rather than the original usage of meaning more static friction.

[masraum]

Originally Posted by Sooner or later

A tire is not bubble gum. It is not concrete.

Difference will be slight but up to a point the contact patch will increase by a small amount when only width is changed.

Do you agree that a concrete tire would have a larger contact patch as width is increased?

Btw, I worked for Goodyear and did work on tire design at one point in time.

Contact patch doesn't increase as tire width increases. The contact patch just changes shape.

[masraum]

Originally Posted by GH85Carrera

Find me a race team anywhere driving on dry pavement that wants to go to a skinnier tire to increase speed. Good luck.

again, apples and oranges.

2 things, a wider contact patch allows for a wider (but same sized) contact patch which is better for cornering. Also as was stated earlier, a wider tire means more tread/mass to absorb heat which allows the tire to get up to the right temp for optimal grip vs a smaller tire with less tread getting up to a temp that's too high for optimal grip.


In theory, you could go to a skinnier tire that was also very tall (I believe Porsche did that with the 935), but then a larger diameter would mean more rotating mass and more unsprung weight.