http://www.g2ic.com/forums/showthrea...threadid=17183

I thought this may be of interest to some of the Solo II guys. Nothing really dramatic here or empirical, just interesting.

 

Yeah I did the same thing back in January with 205/50x15" Azenis, 215/45x16" Azenis, and 215/40x16" Kumho 712s

 

I must have missed it then. I check this board in cycles, and that must of been one of my down times . For anyone else who hasn't seen it, still kind of interesting.

 

Yeah I just measured my 712s and Azenis. For overall width they aren't that much different the Azenis are 8-3/8" wide and the 712s are 8-1/8" wide. However the real difference is in the contact patch hitting the ground.



The kumho 712s only give approx 6-3/4" of a contact patch due to the rolled sholder design.


Now look at the same size 205/50 Azenis contact patch!! 7-1/2" wide contact patch!! That's an extra 3/4" of rubber on the road for the same size tire.

This is why most of us with Azenis are also getting the rubbing on control arms or fender wells and also the illusion that the Azenis are really a 205 width tire.

 

Yup. My 215/40x16" Kumho 712s were almost the exact same size as my 205/50x15" Azenis.

My 215/45x16" Azenis:


My 205/50x15" Azenis:

 

You guys are a bit mixed up! A wider tire will not give you more rubber on the road or a larger contact patch. The contact patch area will remain the same only the shape will change.

 

so your telling me that a 175/65-13 tire will have the same contact patch amount as a 205/50-15 or a 205/45-16??? Then why the hell do they put wider tires on cars to improve traction???

 

you are the one mixed up. your theory and picture seem that they would only hold true if the wheels in question were not of the same size.

 

Actually this is true. The guys at the Panoz school show something very similar to the students in order to make them understand that just because they have a wider tire, it doesn’t mean that they have a larger contact patch and all around more grip. All it means is that you are transferring more longitudinal grip into lateral grip.

A wider tire by lets say 2 sizes will have the same contact patch area as a narrower tire by 2 sizes on the same car, given everything equal. The patch will have different shapes, just like in the illustration. And just as the picture suggests, the patch is wider on the wider tire, but also narrower. This means that a wider tire offers more lateral grip but less acceleration/braking grip as the slightly narrower tire does. But since most of us are after lateral grip, we go with the wider tire available. I haven't actually seen or felt physical proof of this, but this is what I heard the Panoz guys talk about and I also heard this somewhere else before, too. Of course you have to be reasonable and not compare a 335 tire with a 155 wide tire.

 

Hey Hracer,

Thank you, to many youngsters! Throw two bodies in the water equal weight different shapes, they will displace the same amount of H2O, Archimedes figured it out a long time ago!

 

lol, I was just going to make the same analogy. So weight distribution, and tire pressure can effect the area of the contact patch, but tire size can only effect the shape.

 

Actually this is true. The guys at the Panoz school show something very similar to the students in order to make them understand that just because they have a wider tire, it doesn’t mean that they have a larger contact patch and all around more grip. All it means is that you are transferring more longitudinal grip into lateral grip.

A wider tire by lets say 2 sizes will have the same contact patch area as a narrower tire by 2 sizes on the same car, given everything equal. The patch will have different shapes, just like in the illustration. And just as the picture suggests, the patch is wider on the wider tire, but also narrower.

The above is correct.

This means that a wider tire offers more lateral grip but less acceleration/braking grip as the slightly narrower tire does. But since most of us are after lateral grip, we go with the wider tire available.

This is incorrect; you don't exactly lose accel/braking traction because of a wider/shorter contact patch. More of the contact area is now along the flat axis of the tire as opposed to the circumferential one, and still contacts the ground.

It's as if you turned your shoe sideways so instead of rolling your foot through a stride you sidestepped left/right. Maybe if we're talking top fuel dragster there is some benefit of the longitudinal contact patch vs. lateral one on accel/braking, but in most cases there shouldn't be a disadvantage.


[Modified by 4WDrift, 2:48 AM 4/15/2002]

 

The picture isn't working right now but I saw it earlier. If I'm not mistaken, doesn't the picture just show the difference between contact patches of radial and bias-ply tires? If so, how does it still hold true when comparing a radial tire against another wider radial tire? You'll have to forgive me for not knowing much about Archimedes. But I do know that two objects should displace the same amount of water regardless of shape as long as they have the same volume.

 

Wouldn't the more narrow tire have a lower (unsafe?) tire pressure in order to have that much contact (given the same diameter wheel)? Contact patch is the same and all, but...

 

Actually, this is a great thread to post a couple pics I took yesterday. I wanted to get a good look at what the contact patch of the 205 Azenis looked like so I dusted a board with some baby powder and stuck it under one wheel. I lowered the car on it and came up with a perfect imprint of the true contact patch.



I measuered the patch and come up with approx. 7" of width. This was done at 30 psi on a 7.5" rim.



If anyone else wants to perform the same experiment with the same tire on a different width wheel, or at a different pressure, I'd be interested in seeing the results.


[Modified by Floyd, 7:58 AM 6/9/2002]

 

This would require tires of the same mass, though. Using your example of water displacement and considering weight in terms of mass (like a true physicist should! LOL...), object of different mass will displace different amounts of water. So in thew case of our tires, are we assuming the same overall diameter, with the width and aspect ratio of the sidewall to be changing (ex. 205/50 15 and 195/55 15 etc.)? In that case, then Floyd, Marc (DB1-R81), Hracer etc are correct.

Or am I missing something? If I am, please tell me because I would hate to miss something.

 

bibliography please??? or at least the description under 'figure 15'

D

 

If your tires are inflated to 32psi all the way around that means that each square inch of contact patch is supporting 32 lbs of the vehicle's weight. In the picture, you can see that the contact patch is approx. 7"wide, and in another picture I had, you could see the patch was approx 4" deep. That patch would be 7x4=28 square inches. Let's subract a few square inches for the fact that the patch is rounded in the corners and we will use 24 square inches of actual contact patch.

At 32 psi, that tire is supporting 768 lbs of the vehicles weight. Assume the other front tire is doing the same, then that means the front tires are supporting 1536 lbs.

If we assume the car weighs 2600 lbs, that leaves 1064 lbs on the rear wheels. Divide by two and that's 532 lbs per rear tire. Assuming 32 psi in the rear and the contact patch should be 16.6 square inches. Because the rear wheels are carying less weight, the contact patch won't be as large.

Now, let's up the tire pressures to 40 psi. Assuming the same weight distribution, the front tires would have a contact patch of 19.2 sq-in. That's quite a bit less rubber on the ground than at 32 psi.

If we wanted the front and rear of the car to grip evenly in the corners, we would want to match the contact patch of the front tires to those of the rear, and since the front tires are carying more weight, we would need to increase their pressures relative to the rears.

If you want the car to "rotate" in the corners, you lower the front pressures and raise the rears so thet there is a lot of contact in the front, and only a little in the rear.

I think I am making sense. I;m sort of putting all this together in my head while I write this. Did my calculations approximate the real F/R weight distribution of the ITR? 63/37?

 

hmm.. does a tire's load limit depend on the tire pressure? if so, how do you factor that in when deciding how much to inflate a tire?

 

good grief, a lot of things are being thrown around in this thread.. I'm not sure how bouyancy figures in with Force-Pressure relationships, but one thing to remember is that the "Load/Contact Patch = Tire Pressure" equation is a simplification of this situation. Sidewall stiffness also carries load. The trend of increased pressure = smaller contact patch still holds, just don't expect to be able to calculate actual contact patch area with that formula

TypeSH: tire pressure does not change the F/R weight distribution of your car (except if it results in a drastic change of ride height at one end).

DB1-R81: two bodies of the same volume if they sink, same weight if they float.

GSpeedR: even though bouyancy is not relevant (as I see it) to this discussion, bouyancy and water displacement is dependant on volume and weight (it's ok to speak in terms of weight because without some force field present -gravity- bouyancy is kinda meaningless).. and it's not so much the weight of the tire, but more the weight of the entire supported vehicle. I imagine full tank/empty tank would make a bigger weight difference than the difference in the weight of the tires.

Argh.. I'm starting to ramble.. summation:
*wider tire doesn't necessarily mean bigger contact area (as previously stated)
*you can't judge contact area solely from tire pressure
*and just to throw a new bit into the discussion I'd wager sidewall stiffness is probably as big a factor in traction as the small differences in contact patch that we're talking about.

blah..

 

That's a great breakdown. Sounds good to me.

 

I don't think that's true.. is it? 32 lbs/in^2 is a pressure rating of the air within the tire. I think you are confusing pressure with stress. Stress is force/area, so in our case, our units would be lbs/in^2, exactly the same unit, but a different physical meaning.


Using your assumption of 2600 lbs, the weight distribution you would get is 59/41 pretty comfortably numb close.


[Modified by carsaregood, 9:58 AM 6/11/2002]

 

I was thinking the same thing at first, and almost pointed it out too. But then it dawned on me, it is 32 all around the tire, but the only pressure being exerted is on the bottom of the wheel. If the car were heavier, and there was still only 32 psi in the same tires, the contact patch would be bigger, and still his formula works.

 

I could be wrong on my previous thoughts. I was just throwing them out there, trying to piece together common sense with what little high school physics I could remember.

Afterwards, I was thinking about it more, and trying to make sense of a balloon, or even a tire that is jacked up in the air. The pressure is still 32 psi, even when it isn't touching the ground.

Does anyone know for sure if my theory is correct?

 

This is a good discussion, and not to throw a wrench in it but to just share experience...

In my opinion the biggest difference between the two tires is the compound. Haven street driven and autocrossed on both, I can tell you that the Kumho behaves like a high performance street tire and the Azenis behaves like a mediocre R compound race tire (the Yoko 032R comes immediately to mind).

It would actually not at all surprise me to see the Azenis get banned from Street Touring in the near future. It is becoming too widely used, too dominant and is performing just like a race tire. Not really what the SCCA intended for that class.

Just my $.02.