Hercules Rockefeller

Yes friggin way. Reread what I wrote earlier. This isn't rocket science folks, it's not that bad.

Having a tread pattern decreases 'grip' for two reasons.
1) the tread blocks will deform and fold over under lateral force, this can't happen with a full racing slick.

2) putting grooves in the tire effectively reduces its width, which brings me back to what I said earlier. Narrower tires have the same contact patch area at the same time pressure, but you have to run higher pressures to prevent them deforming the sidewall and distorting the contact patch into a forward/rearward stretched ellipse.

In addition, a tread pattern, by virtue of having lower surface area, will wear out faster.

The only disadvantage to slicks is they cannot channel standing water, which will cause your car to hydroplane.

CivEngineer

Your buddy is correct, but only in the simplest form. Tire dynamics have a lot more to do with tire pressure, size.

This is why people lower their tire pressure when they drag race and why people who run slicks run a very low tire pressure. Tire PRESSURE and vehicle weight, vehicle downforce is what causes contact patch area (and sidewall stiffness). Tire width will give the tire it's contact patch geometry (wider tires will have a rectangle with the long side going from side to side, narrow tire will have a rectangle with long side going from front to back). The geometry effects the tire dynamics though, especially heat. If the rectangle has the long side front to back, then the sliver of tire that first contact the ground will touch the ground for more duration, creating heat. Think of a particular part of the tire touching the ground, it has to make its way from the front of the rectangle, all the way to the back.

There is a ton about tire dynamics that are beyond my knowledge, here is an article from an engineering forum with lots of good information.

http://www.eng-tips.com/viewthread.cfm?qid=102250

Johnjan

Oh, I forgot. You wrote it on an internet forum. That makes it true. My bad....

jschindler

Does the stiffness of the sidewall have an effect that could counteract this effect?

BTW, I fully get that lowering or raising pressure effects the patch, but that does not necessarily tell me that the tire width is not a factor.

Thanks for your thoughts. Have not had a chance to read the link yet. Pesky work is getting in my way today.

nitrojunky

super back of the envelope...

our 'vettes will put ~750 pounds of normal force on the tire. ignoring runflats (that's an interesting concept :think: ), the tire will probably push back with a force that is a nonlinear function in sidewall deflection; i'm going to speculate that at the deflections we see, the tire's sidewalls are pushing back with somewhere between 10 and 50 pounds of force. so probably not totally neglegible, but at the same time not a huge effect.

when i get home, i'll grab an extra old TA gforce i have for my hatch, let its air out and press down until it looks like it has a car on it... then report back. considering other factors, this will also be inaccurate, but better than nothing... (i.e., i'll find out if it'll hold my weight before collapsing)

jimman

Hi Jim missed ya when we came through Texas; hope I had the correct number. Anyway stirring up some thoughts on this one are you. It's a dynamic solution were both of you can be correct at some point or points. Fundamentally just start by thinking two wheels solid as in steel of the same diameter but one is one inch thick and the other is 10 inches thick. That example will lend itself to the one having 10X the contact. Now one can extrapolate that with all sorts of variables as in pressure, weight, tire compound, sidewall strength and anything else you can dream up and answers will be all over the map.

jschindler

Jim - yes, you had the correct number. I was out of town when you called, then I screwed up and deleted your message and number. My bad!

I respect your opinion - thanks for any thoughts on this. It's one of those topics that the more answers I get, the more questions I have. I have to be honest and tell you that based upon everything I've heard, I'm going to believe what I want to believe .

I'm not convinced that the physics of the issue are as black and white as some would like us to believe. That's not to say I'm not open minded to the responses - I am and I appreciate the responses.

Hercules Rockefeller

Simple physics makes it true. P=F/A. The internet's just here to teach you. But if free instruction on the internet doesn't work for you, you can come by my class and pay for it instead.

You don't have to take my word, or anyone else's word for it, you can look up what's been said here and verify for yourself if you like. At the end of the day, it's not that complicated. A larger tire can be run at lower pressure, and lower pressure increases the coefficient of friction, which increases grip. That's pretty much it. There are other secondary effects, including the adhesive nature of the tire at high temperatures, the sidewall stiffness, etc. but they are less important. Larger tire --> lower pressure = more grip.

jschindler

I get that if you lower the pressure the contact area increases. That has never been in doubt. The point I'm questioning is that if you take a 285 tire, and a 325 tire - both identical except for the width, they will have the same contact area at the SAME pressure. That is what I've been told.

Hercules Rockefeller

Unless pressures start getting really low, in which case the sidewall actually starts supporting a large part of the car's weight, this will be true. It has to be true, Pressure = Force/Area. If the force (weight) is the same, and the pressure is the same, the contact area will have to be the same. There's no way around it. The narrower tire will have to achieve this area by deforming more of the sidewall and providing a footprint that points in the same direction as the car, while the wider tire will achieve this with less deformation and have a footprint that points perpendicular to the direction of the car.

CivEngineer

Yup. But the narrower tire will have to deform more to achieve this (i.e. become more flat where it is touching the ground - becoming less round)

Here is a simple sketch to kind of show it (very simple sketch). You will also notice that an instant line across patch on the lefts tread surface will only touch the ground for half the time as the one on the right. The 100 lbs on the top is constant (cars weight and assuming car is not moving - when car accelerates, moment transfers more weight to back, less weight to front which would in effect change the 100 lbs to be more for that period of time). Again, this is extremely simplified. There is a lot more to it than this, especially when the car is moving.

hoefi

I didn't say that. I said your friend is correct that a wider tire has the same contact area as a norrower tire.

Some of you made two mistakes. One, you assume a wider tire gives you a larger contact patch. Two, you assume a larger contact patch is the reason why a tire has more mechanical grip. Both of those assumptions are wrong.

It is correct that wider tires (assuming all other parameters are the same) do provide better performance, but it is not via the size of the contact patch. The following explanation was given in a seminar by a friend of mine who is a tire engineer. He started his career with Goodyear and is currently with one of the import brands.

During the seminar (attended by mainly amateur racers), the question was asked if a wider tire is always faster. My friend's answer was "yes". His expalantion was that a wider tire provides an elongated contact patch shape compare to a less elongated shape from a narrower tire. Rubber, being a non-rigid material does not transfer lateral stress in a linear uniform fashion. The free edge (parameter of the contact patch) will experience higher stress than the inner area of the contact patch. As a result, mechanical grip generated by a unit area (say one square inch) of rubber at the free edge is lower than a square inch of rubber further behind the highly stressed free edge. Also, the parameter around the contact patch does not see the same stress, different sections of the parameter (free edge) see different level of stress depending what the vehicle is doing (cornering, braking etc). What an elongated contact patch does is that it will provide a larger percentage of "less stressed" rubber compare to a more square shaped contact patch, hence providing a higher level of mechanical grip, especially in lateral loading (mid corner)or a combination of lateral/acceleration (corner exit and corner entry) situation. Basically, what he said was that an elongated contact patch shape is a more optimized shape for high performance applications.

The reason why slick tires are faster (even if tire compound and carcass construction is the same) is it minimizes the free edges, hence provide a larger percentage of less stressed rubber, similar concept as above.

Some of you guys can believe whatever you want to believe. My recomendation is that next time you go to a MAJOR race event and you see one of those large transporters from a tire manufacturer, go up there and ask for the tire engineer. If you are lucky, he might just answer some of your questions.

jschindler

Thanks for the first reply that actually addresses the issue in a sense that explains why a wider tire provides better traction.......I think

track junkie

Thanks hoefi great explanation.

When you talk about the less stressed areas of the contact patch, for the wider tire. Would less stress mean lower tire tread temperatures for the wider tire with track use than the narrower tire?

CivEngineer

jimman

Interesting on how a question on how big a patch area vs. how wide a tire has morphed into all sorts of traction issues.

Johnjan

Agreed, that if this were simple physics it would be true. But this isn't simple physics. A spherical balloon is simple physics, a tire ain't so simple. This is a discussion about a very complex engineered product that is designed to transmit and absorb forces far beyond your simple formula.

So, to understand complicated stuff my simple practical mind likes to simplify things even more. There was mention earlier of a steel cylinder on a flat plate. By definition, the contact patch in that scenario is a line without width. The "275" cylinder would create a line about 10" long. The "325" cylinder would create a line about 12" long. Hmmm, wider tire, oops, I mean cylinder = longer line by 20%.

Let's now assume that the "275" and "325" cylinders are designed for similar purposes and manufactured by similar processes of similar materials. The load on each is also similar. And the smart engineers designed some flex into them so that they would spread out a bit and provide friction against a rough surface. I think it's safe to assume that they would flex in a similar fashion in response to the similar load. That said, the line would gain some width which equates to area, or contact patch. Assuming both lines expanded by a similar amount, I'm gonna go with the "325" line equating to a larger contact patch by virtue of it starting out as a longer line. So much for my back woods Texas math.

If this weren't the case, why (all else being equal), does a Z06 or GS with a 325 tire have better skid pad performance that a C6 with a 275 tire? Why would F1 reduce the width of the tire to reduce grip and make the series more competitive? Why does my pals GT1 car have front tires wider than my Z's rear tires, and rear tires that are wider still? Simple answer, more contact patch equals more grip and wider tires create more grip so there must be more rubber in contact with the road. Same reasons slicks provide more grip in the dry - more rubber in contact with the road.

Now, some smarty pants is going to chime in with the formula that says that sliding is independent of area because the coefficient of friction times normal force and all that. Yup, I took physics and I don't argue that at all - for uniform surfaces. Tires though, depend on mechanically locking the tire to the road by flexing in and out of irregularities in the road surface. Like gear teeth or a chain and sprocket. Here again, more gear teeth meshed together spreads out the load and increases the ability to transmit force. So, the larger the contact patch, the more teeth are meshed together, the more force can be transmitted.

That's why I like my 325's on my Z06.

jschindler

That's my story and I'm sticking to it

By the way, I'm in the electrical business - have been for 37 years. I'm not an engineer, but just from osmosis I've learned a little over the years. What I have learned is that you don't have to be able to see something move to know what the results are. Just saying....

JoesC5

hey Jim, maybe this will help.

http://www.boeing.com/commercial/air...ontactarea.pdf

Hercules Rockefeller

Just because it's simple, and the problem your trying to solve seems complicated, doesn't make a fundamental formula untrue. P = F/A is true in all situations, uniform or not, moving or not, whatever. That's one of the nice things about a good formula, it keeps us from making incorrect assumptions, especially in situations where our "gut instincts" lead us to incorrect conclusions. This is one of those situations.

The only reason wider tires give you more grip is because you can run them at lower pressure. A wide tire and a narrow tire, made of the same compound and run at the same temperature, will essentially provide the same contact patch size and the same grip (assuming no run flats so we can ignore what should be relatively insignificant effects such as sidewall compression and things like that).

Need an example? Look at my formula car. It runs very wide tires for its size, but at widths of 8" up front and 10" out back they are still smaller than the Z06 tire sizes. Nonetheless, because my car is so light I can run them at only 15 psi (hot), and that's why, even though the formula car has much smaller tires than the Z, it gets way more mechanical grip. WAY MORE.

Mechanical grip from tires is controlled by the following three things, in order of importance:

1. Compound
2. Tire pressure (lower pressure, more grip)
3. Tread pattern (lower profile/more widely spaced tread blocks, more grip)

Larger tires do not, in and of themselves, increase contact patch size or grip. What they do allow is for you to run lower pressures, and it's the lower tire pressures that increase grip.

If you run wider tires and do not lower the tire pressure, you will not see any grip advantage, but you will see a weight penalty to performance. Running larger tires at high pressures only provides advantages for tire wear and heat management.