Contact Patch

[Bogie]

Don't ask me why. Sometimes we climb the mountain because it's there. Actually, there is a reason. I'm just trying to get a little smarter on tires, and I've found the world wide web isn't too friendly. So, I decided to strike out on my own. My assumptions aren't realistic, but I gotta start somewhere. And maybe someone else has some info that will help me refine the assumptions so I eventually converge on truth.

Assumption 1: Rectangular contact patch.
Assumption 2: Evenly distributed force across the patch.
Assumption 3: No contribution from sidewall.
Assumption 4: Tread width (contact) of 8.5 inches.
Assumption 5: Tread length (contact) of 3 inches.
Assumption 6: All four tires at 29 psi.

How much weight does the air balance?

4 tires X 8.5 X 3.0 X 29 psi = 2958 lbs [close enough to SKY curb weight]

So, a good upper bound on a contact patch for our stock tires is 8.5 X 3 per tire with an inflation of 29 psi. Since the sidewall will indeed provide some support, the actual patch will be smaller. On the other hand, the contact force falls off toward the side so assumptions 2 and 3 cancel somewhat. How much? I don't know.

Having started this discussion, does anyone here know the actual tread width and sidewall contribution at 29 psi?

[G8TR]

Man Bogie, there is no doubt you're an engineer.

[cerberus]

Ya might want so review this thread: Optimal tire size.

[lorennerol]

I can't speak to the accuracy of the data presented, but this page has some interesting info on motorcycle tires:

Pneumatici

This page shows that the contact patch is somewhat elliptical, at least when not subject to turn forces:

Tuner Sports Supercar Tires Q&A With Michelin - Motor Trend

And here is a book that you definitely need to snuggle up with while you are contemplating success metrics with your Corsican obsession:

Tire and Wheel Technology: SP-1439 - Google Book Search

[MidniteBlues]

Who cares?? and what fer? You need to get a life my friend, thinking about all these "silly" engineering crap will wear out your valuable brain cells over useless matter over time... You certainly are taking it to the extreme.. AAhh yes, to be young and inquisitive... Have fun, I will NOT contribute to your headache for this one..

[Bogie]

Originally Posted by MidniteBlues

AAhh yes, to be young and inquisitive...

I'm 45.



Oh, and thanks for the links folks. I do remember some, but some of the others are new for me.

[Elff]

This leads me to a question.
In my onstar reports, they say the optimum tire pressure is 28psi.
In my experience, the stock goodyears squeal like crazy at that psi.
I've increased mine to 33psi just to play around and they are much more quiet.

I am sure this relates to contact patch, so what psi do you think equals best performance?

[Elff]

How's this for contact area

[RedHeaded RedLine]

Originally Posted by Bogie

Don't ask me why. Having started this discussion,?

Assumption 1: Rectangular contact patch.
Kind of more ovoid than you might think. It tapers at the corners.
Assumption 2: Evenly distributed force across the patch.
Not really, with a high performance camber, the outboard of the tire sees less weight during straight driving.
Then, the outboard see much more when cornering. A pyrometer will tell you percentages.
Assumption 3: No contribution from sidewall.
Let all the air out of your tire and see how much the side wall carries…almost none; it really just keeps the air in place. NASCAR tells us that one psi of air (nitrogen) adds about 50# of spring rate to a 27/10/15 racing EAGLE. This translates probably none to what we are doing.
Assumption 4: Tread width (contact) of 8.5 inches.
I would get a measurement on that. That could take away one of your variables.
Assumption 5: Tread length (contact) of 3 inches.
Again a tape measure solves this conundrum.
Assumption 6: All four tires at 29 psi.
Your call…

[kudelt07]

Increased tire pressure does affect contact patch... more pressure = more force/unit area = less area to support the same weight. Your cornering stiffness will change with inflation pressure, generally directly proportional. The "best" performance will really depend on what you are trying to do. I assume you are using the car as it was intended to be used (throwing it around like a frisbee at a phish concert). In that case, I've found that 32 psi seems to work pretty well, and still gives a pretty good ride. If you are goin' racin' I would probably increase that to 35 psi.

Bogie, try taking four pieces of paper and setting them on each side of the tire so they are just touching the side of the contact patch. This won't give you a perfect picture, but it will get you pretty close. If you do that at each corner you should be able to fairly acuratly predict the weight of the car.

[Lil GTO]

Originally Posted by kudelt07

Increased tire pressure does affect contact patch... more pressure = more force/unit area = less area to support the same weight. Your cornering stiffness will change with inflation pressure, generally directly proportional.

Agreed that pressure on a tire not designed for off-road trucking will not have a huge impact on contact patch - but even a small difference can affect the tread wear - even a couple of percent difference can have an impact depending on the road conditions etc.


Tire Tech Information - Contact Patch

[ChgoBlues]

For just a moment it's this type of discussion I can get interested in even though I may not be able to contribute to. I hear what some were saying in Skip's thread but the forum is what we the population of members make it. ok back to the thread.

From my knowledge (limited) about tires, I know most contact patches are about the size of a cassette tape, not an 8-track. The stiffer the sidewall, the more support it provides to the tire in keeping the tread in contact with the pavement. A softer sidewall will allow more of the sidewall to contact on turns, causing more "push" or "slide" as I've heard it termed. As far as the engineering points Bogie is looking for, way out of my league but interested in finding out.

[RedHeaded RedLine]

Originally Posted by ChgoBlues

For just a moment it's this type of discussion I can get interested in even though I may not be able to contribute to. I hear what some were saying in Skip's thread but the forum is what we the population of members make it. ok back to the thread.

From my knowledge (limited) about tires, I know most contact patches are about the size of a cassette tape, not an 8-track. The stiffer the sidewall, the more support it provides to the tire in keeping the tread in contact with the pavement. A softer sidewall will allow more of the sidewall to contact on turns, causing more "push" or "slide" as I've heard it termed. As far as the engineering points Bogie is looking for, way out of my league but interested in finding out.

Glad you are trying but I think you may be a little off on the facts & terms.
Contact patch of modern tires are closer to a sneaker, or larger. If you look in Puhn’s book “How to make your car handle” you will see that the co-efficient of friction goes up gradually as you approach optimum grip, then falls off much mare rapidly with over inflation. All things being equal, it is better to be a few pounds sub-optimum than the same amount over inflated. This must be viewed that most people run much lower than optimum, so inflating, even more than manufactures recommendations, still does not provide optimum grip.
Slide isn't really a technical term, unless referring to an all wheel drift.
Push (Understeer) is when your car hits the wall with the front.
Loose (Oversteer) is when your car hits the wall with the rear.
The definition of Understeer is when the slip angle of the front tires exceeds the rear tires. -Joyce

[smithb]

The sidewall contribution is the hard part. I have never seen actual objective data on that. As an engineer, I would say it is very close to 0 as a percentage of load carried by a fully inflated tire. I would say this even for "run flat tires" (I believe the air would take almost all the load off the sidewall). If all of that is true, then the tread width and length should be easy. Just run over a piece of paper, and then you can measure it. If you have a concrete driveway like me, you probably have lots of good samples already out there. You can go to a truck stop and weigh your car with you in it for $5. It is 52/48 f/r weight distribution (assume 50/50 l/r), and divide by 2 to get closer for each tire. I'm sure you can figure out the rest, but if not... then divide those numbers by your tire pressure and you have the contact patch area. Divide out the width (from the measurement on the piece of paper) and you have length. Good luck, I hope I helped.

Late edit, that will give you contact patch. As for the shape of the contact patch, it is pretty close to rectangular. If our tires were not vertical, everybody and their brother would be crying on here about the uneven tire wear, and how crappy the design is.

[rufusmacblorf]

Originally Posted by RedHeaded RedLine

Glad you are trying but I think you may be a little off on the facts & terms.
Contact patch of modern tires are closer to a sneaker, or larger. If you look in Puhn’s book “How to make your car handle” you will see that the co-efficient of friction goes up gradually as you approach optimum grip, then falls off much mare rapidly with over inflation. All things being equal, it is better to be a few pounds sub-optimum than the same amount over inflated. This must be viewed that most people run much lower than optimum, so inflating, even more than manufactures recommendations, still does not provide optimum grip.
Slide isn't really a technical term, unless referring to an all wheel drift.
Push (Understeer) is when your car hits the wall with the front.
Loose (Oversteer) is when your car hits the wall with the rear.
The definition of Understeer is when the slip angle of the front tires exceeds the rear tires. -Joyce

Wow. How many of the geek guys on this forum have proposed marriage to you so far?

[ChgoBlues]

Originally Posted by RedHeaded RedLine

Glad you are trying but I think you may be a little off on the facts & terms.

Thanks..Don't beat me up too badly. Did a quick search on Puhns book. Interesting but based on 1973 technology. Tire and suspension technology has come a long way since then even though the basics are still a good foundation. It will make good reading though, thanks. What I've found so far too in what I've read, the actual contact patch is constantly changing but the "size" seems to be consistently stated as being smaller than we think. The Tire Rack comments that Shaq's sneaker has a larger contact patch than a car tire. One site states it's about the size of a hand.

Bogie, ran across this based on racing technology, not sure if it helps or hinders.

Tire Technology, excerpt no. 2 from The Racing & High-Performance Tire

HowStuffWorks "How do 30 pounds of air in your tires hold up 2 tons of car?"

The second talks about different psi and it's effects.

[RedHeaded RedLine]

Originally Posted by ChgoBlues

Thanks..Don't beat me up too badly. Did a quick search on Puhns book. Interesting but based on 1973 technology. Tire and suspension technology has come a long way since then even though the basics are still a good foundation. It will make good reading though, thanks. What I've found so far too in what I've read, the actual contact patch is constantly changing but the "size" seems to be consistently stated as being smaller than we think. The Tire Rack comments that Shaq's sneaker has a larger contact patch than a car tire. One site states it's about the size of a hand. Tire Technology, excerpt no. 2 from The Racing & High-Performance Tire
HowStuffWorks "How do 30 pounds of air in your tires hold up 2 tons of car?"
The second talks about different psi and it's effects.

While suspension technology has progressed in the last 35 years, ours has not. We surely benefited from CAD/CAM, but we don't have active suspensions, nor even computer controlled suspensions, like in my other car (‘06Volvo V70R). What we do have is an easily tunable, relatively (v.MacStruts) compact setup, that is not particularly heavy. I did curse him like a sailor when he had me under the car changing front swaybars. My mother says “that is not a ladies life”. He said “you like the plaque at the end of the year…you are going to earn it.
No one has proposed yet, thanx. I wouldn’t be surprised if he sold me off depending on the offer. -Joyce

[Bogie]

Thanks for the references everybody. I have a feeling a good bit of this stuff seems to be from folks talking about the way it is because they were taught the way it is, without anyone actually taking a step back and building the case from the ground up.

I run across this every once in a while looking at engineering stuff. Everyone says its ABC. I ask why is it ABC? And they say because that's the way it is...it's ABC. I start chasing sources, and then things go cold. Everyone is basing their knowledge on a foundation built by someone else to the point the ultimate foundation is wishy washy or outright wrong.

[Anecdotal aside: As an engineer, I use decibels (dB) a lot. One day I wondered....where did it come from and why. Bunch of talking heads in textbooks telling you what someone told them..... Even the web trails went cold. Try finding out where it really came from was madness. So I ran down to the Library of Congress and the National Archives (living near DC is great sometimes!) a few years ago and chased down the ORIGINAL source material from the early 1900s. Interesting stuff. I know of no other person alive today who knows the full story other than myself. Maybe I write a book some day].

[Anecdotal aside #2: Most science textbooks don't describe the action of the tides properly. If you are the product of today's education system, you probably think you know why...but you don't. Fortunately, there are a few places on the web that actually cover it properly, so you go have fun if you dare].

Back to tires. Reading all the stuff on the web is setting off the same bells and whistles that seem to be characteristic of of those authoritative presentations I've seen that eventually turn out to not properly tell the real story of what's going on.

It's almost like a Quest. The Quest to find dB. The Quest to find the tidal forces. The Quest to find Contact Patch. To ultimately understand something beyond the superficial web...to leave the castle walls....and gallop into the sunset....looking for dragons in the dark places few are bold enough to tread. Sigh....I bet you never thought engineering could be so fun? I even have time for the fair maiden in need of rescue from time to time...but you've already been introduced..so I'll stick to Contact Patch for now.

[uranium-238]

Please tell me I'm not the only one who has noticed this...

Has anyone else noticed that tires get wet they sometimes leave a black spot on the garage floor were they make contact after they're dried? I've seen this in my civic several times, makes for a (roughly, i'm guessing as I've never really measured this) 6"x4" rectangle with rounded corners. You can even see the tread pattern when this happens, so you could easily calculate the surface area.