JMatt

In theory, a 305 35R20 is a perfect diameter match for 245 45R20 on the front. It would ideally take a 1 1/2" widening of our rear wheels, which can be done for $500. Add a small spacer, and you're in business!

vvv90

Now THAT would be cool.

JMatt, what is your opinion on whether or not you'd get more grip with a 305 based on your physics equation posed to Hyperspec awhile back. Do you think the contact patch would be bigger?

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Hyprspec

How could it not be?!? The CP(contact patch) does not get smaller front to back just because you have a wider tire. The only way the CP gets smaller/bigger front to back wise is when you fiddle with air pressure. ie: dropping it for the drag strip.

Or raising it for your weekend "ghetto suspension" mod.

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tdola

The ghetto suspension mod seems better suited for a "PM" only since it is a little off the thread topic.

JMatt

Absolutely 100% wrong!!!!!!!

This is total internet misinformation. Please see prior posts on topic, including links to detailed discussions explaining why the above is WRONG.

JMatt

I've been thinking about this, and I don't "know" the answer, but here's what I "think."

For drag racing purposes, imagine how big the front-back contact measurement can get before you have problems. That length should be the same no matter how wide the tire is. Thus, with a wider tire you could lower your drag strip tire pressure more the wider the tire is.

Therefore, a 275 can use lower pressure (and a corresponding increased contact patch) compared to a 245. A 305 could use even lower pressure.

Also, imagine a tire with 1000 pounds of weight while sitting still, but transferring an additional 500 pounds at launch. With a narrow tire and low pressure, you end up with cupping, or at the limit theoretically bottoming out the tire. With a wider tire, you get a much bigger contact patch at the limits when combining low initial pressure with weight transfer.

That brings me to your earlier question: Why big tires on high performance cars? I think weight transfer is a big issue. When that Ferrari accelerates through a corner at high speed, or brakes heavily, a ton (literally) of weight is transferred to one or more tires. A narrow tire would quickly meet it's physical limitations and bottom out or roll sideways. Even though a wide and narrow tire may have a contact patch of 33 square inches while sitting still, the wide tire can safely create a 66 or 99 square inch contact patch under extreme conditions, where the narrow tire would pass it's maximum theoretical contact patch size and merely fail.

Any thoughts?

EDIT: I will be testing my hypothesis by running lower psi at the track with my 275's than I did with my 255's. I'll report back.

Hyprspec

I think we are both argueing the same point here, but the underlined portion of your post is what I was questioning. As I understood this, you are saying just because the tire is wider, its CP is smaller front the back. but wider so its even. I wanna know, from a physics stand point. How is a wider tire relate to a smaller front to back contact patch? That is where I think your mistake is

Ok I will admit my statement was not entirely correct.

I understand what your talking about. Completely. But here is the difference in our "misunderstanding". You are talking about the contact patch and the changes it endures under driving/racing conditions. I was merely speaking about the contact patch as it sits still.

Yes under accleration(+/-) weight is transfered to the rear/front of the car, pressing the tire into the ground. think of a plastic bag filled with air, and sealed shut, set it on the ground. the part that is touching the ground would be your "at rest or neutral acceleration contact patch(CP). Now push it into the ground and it flattens out eccentially making a bigger CP, As happens the rear wheels under accleration and to the front wheels under negative accleration.

So yes like I said my statement about the only way to change CP size is to lower/raise pressure is not completely correct.

Think of it like this. Lets keep a constant(32 psi). AT REST a 255 tire has the same depth(front to back) size as a 295 tire. The 295 tire IS wider than the 255 therefore the CP is bigger. Bigger CP = more traction(assuming the tire in made of the same rubber compound and stickyness, but let's not get into comparing a 295 "street tire" to a 255 "Drag radial or slick" that is whole nother can of worms)


Now take the 255 tire and lower the pressure(as some do for dragstrip racing) to allow for more compression of the tire under acceleration(+/-)


So with lower PSI the 255 tire is able to achieve a contact patch >= 295 tire(depending how low you go, but you can only go so low for safety).
So what happens when you put that same 15 PSI in the 295 tire ? well the same exact thing happens. It can be compressed more without causing a malfuction of the tire(explode), causing more surface area to touch the ground(CP)


So inconclusion(whew, takes a breath), at rest and in motion the a wider(295 as this thread is about) tire does provide more grip(CP) than a 245,255, or 275 tire(or anything smaller for that matter). All of us who travel the streets will more than likely us at least 32 PSI in our tires. Yes you can roll around on a near flat 245 tire achieving relatively the same grip as a proplerly inflated wider tire. BUT that is completely unsafe and NOT recommended by me. I hope this explains everything. It's all physics.

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JMatt

This is the part of your premise that is inarguably wrong. Sorry.


No. Wrong.

Tire A is 10" wide.

Tire B is 13 1/3" wide.

Both tires are inflated to 30 psi.

Weight (while standing still) on the tire is 1200 pounds (just keeping my math simple.

psi literally means "pounds per square inch."
1200 pounds divided by 30 "pounds per square inch" is equal to 40 square inches of contact patch.

It's a math equation - there are no other "interpretations" or opinions possible. If I say 2 plus 2 equals 4, do the math. You might think it's 6, but to debate the issue makes one look silly.

Tire A has a 40 inch contact patch, and that contact patch is 10" wide. Thus 40 divided by 10 equals a 4" long (front to back) contact patch.

Tire B has a 40 inch contact patch, and that contact patch is 13 1/3" wide.
Thus 40 divided by 13 1/3 equals a 3" long (front to back) contact patch.

This is all while sitting still.

PLEASE - review the link I submitted earlier. Don't believe me. Goggle it. Buy a math book. Buy a physics book. Read an article about tires.

I acknowledge, that most people don't realize this concept, and it never occurs to people that PSI literally means the Pounds of weight on the tire divided by the Square Inches of tire on the pavement. If you know any two variables, you can calculate the third. We know the Pounds. We know the PSI in the tire. Thus, we can contact the contact patch Square Inches.

Now: Length times Width equals Square Inches. We know how Wide. We know the Square Inches. Thus we can calculate the Length of the contact patch.

Nothing in this post is higher than 7th grade math. It's just a matter of realizing the concept, and applying it.

Hyprspec

Yes. You are right. I cannot deny the math. It has to be right. guys JMatt is correct here. and hyprspec is wrong. Completely wrong.

I sure did give it a good shot though!

Jmatt

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JMatt

Your sarcasm is dripping.

Look - I'm trying to be respectful. I know people just don't think about it, and the assumption is that wider tires put a wider, but still as long, contact patch on the pavement.

I'm sorry that they are wrong, but they are wrong. I invited you to google it and do internet or other research. No need to be condescending. I'm sincerely trying to explain a concept in writing as well as I can.

Heck - PM an SRT engineer.

You don't have to believe me. You don't have to rely on me.

But you also don't have to mock me. I did not mock you.