JoesC5

My Z06 weighes right at 3200#, and has 325/30-19 tires at 26.7 " diameter,and with 30psi.

My 1964 coupe weighes right at 3200 # and has 205/70-15 tires at 26.3 " diameter, and with 30 psi.

I did a rough measurement on both. The contact area on the Z06 was 4.06" length X 10.38" width which equals ~42 sq in of contact area.

The 64 contact area was 5.06" length X 5.06" width which equals ~26 sq in of contact area.

Those meausements aren't exact but both tires/cars were measured in the same fashion so the relationship to each other should be close.

Now, who believes that if I installed the 205/70-15 wheels/tires (making believe the 15 " wheels would fit over the Z06 calpiers) that I would have the same traction as the 325/30-19 wheels/tires? And how did my contact patch area gain 73% in size since I have both the same weight and the same pressure in both size tires?

Remember, we have the same weight on each size tire and they are both at the same pressure and almost the same diameter and the tire pressure is what I actually drive both cars with.

haljensen

Just had to spoil all the longwinded nitpicking theory eggheads didn't you. The thread started about contact patch size, not traction.

Common sense and practical proof have no business on this thread.

'06 Quicksilver Z06

nitrojunky

my texas math disagrees with yours. excepting the force held by the tire's resistance to deformity, in a static situation the load is carried by the tire pressure. and for that F=p*A is a pretty good approximation. i don't know about you, but excepting runflats, i can pretty easily tell a full tire from a low tire from a flat tire.

the above says nothing about why a 325 is better, it only makes a statement regarding the area of the contact patch.

Hercules Rockefeller

In your first example, assuming you have about 800 lbs of load on the tire and a contact area of 42 in^2, the average pressure on the surface of that tire would have to be 800/42 = 19 psi. This is too low to make sense.

On your other car, you have 800 lbs of load distributed over an area of 26 in^2, so the average pressure on the contact surface of that tire will be 800/26 = 31 psi.

The second example is pretty close to what would be expected, but your first is not. If I had to take a guess, I would reexamine your measurement technique, it's pretty easy to incorrectly measure the "length" of a wide tire


Ah, anti-intellectualism at its finest. Just because you don't understand something doesn't mean you have to hate it. Why don't you just let the 'eggheads' do all the thinking, and when we're done we'll let you know. In the mean time, check out the flat earth society, they base all of their beliefs on "common sense" just like you, you'd love it.

jimman

Runflat

Hercules Rockefeller

Runflats, by virtue of a stiff sidewall, will *decrease* contact area, not increase it. That's why you can drive on a runflat when you have no tire pressure.

jimman

sure so how did you determine the comparative tire presure and mock him.

Hercules Rockefeller

What are you talking about? I calculated the average pressure at the contact patch of the tire directly from the weights and areas he provided. Pressure is defined as force over area. Then I said it works out with the tire *air* pressure he provided for one of his cars, but not the other (the contact patch pressure and tire air pressure should be equal in both cases). In the case where it doesn't work out (the Z06), he's either overestimated the area, or underestimated the inflation pressure in the tire, because the contact patch is too big. I suppose he could have grossly underestimated the weight of the car, which would also cause this to happen, but unless he's filled it with lead weights that's unlikely. You then responded with "runflats", which doesn't make sense, since runflats would cause the contact area of the Z06 tire to be *smaller* than expected, not larger than expected (which is what we are seeing here).

jimman

You calculated 19psi you could have that patcch with 0psi

RUBYREDVET

Enough already, my brain hurts.
One thing I know for sure is that the wider the tire, the cooler the look.

Hercules Rockefeller

There's no question that if his contact patch measurements are correct, and his weight estimate is correct, that the average pressure at the interface between the tire and the floor is 19 psi, it's defined that way. With a runflat, yeah, you might be able to get that patch size at "0" *gauge* pressure in the tire. That's because runflats, especially at lower gauge pressures, decrease the contact patch size. He stated that his gauge pressure was 30 psi, and I said he either underestimated the gauge pressure or overestimated the contact patch size. 0 gauge pressure would be an underestimate of 30 (albeit a crazy underestimate), and it could very well have caused the error we're seeing. But all things being equal, a runflat tire will have a smaller contact patch than a 'normal' tire, not a larger contact patch, which is what we are seeing here... therefore the error is not being caused by the tire being a runflat.

jimman

Your equation is not valid with a stiff sidewall, remove the 19pse term and replace it with zero and try and verifly the load weight with the so stated patch size. The patch size remains almost the same regardless of tire presure. ( I just did it) I feel like I'm talking to myself here.

hoefi

I doubt your 64 has the same fore/aft weight distribution as the new Z. Just because the vehicles weight the same, that doesn't mean the corner weights are the same. I don't even believe the left rear is the same as the right rear.

What you should do is to put the two front tires of the Z on the rear and then try to measure them for comparison. At least the tire construction is similar in this case. Don't be lazy and only put one front tire to the rear because it will put a wedge into the chassis and jack weight into the diagonal corners, hence distorting the weight on the tire you want to measure.

Have fun changing the four tires. If you want to argue, at least you have to sweat a bit and do some work.

Cheer.

jschindler

Okay, I'm going to say this AGAIN - but understand I'm still not sure which side to believe. If the answer is that you can run lower pressures, then why isn't the recommended pressure on the 325's on Grand Sports and Z06's lower than the 285's? Why do they put bigger tires on cars that have more power even though they don't recommend lower pressures?

JoesC5

C6 Z06 has 49.3% front/50.7% on the rear, weight distribution. That means each rear tire has a 811 # load.

1964 Coupe with auto transmission has 48% front/52% rear, weight distribution. That means each rear tire has a 832# load.

I measured the footprint on the rear tires of both cars and the measurements are very close as they were measured in the same fashion. Both are side by side in my garage. Where are the Z06 and the 64 coupe that you are measuring located? Or, are you saying my measurements are wrong, even though I have made actual measurements, or are you saying your measurements are correct even though you have not made any measurements?

Hercules Rockefeller

Well that's a good question. I don't know what guides the recommended tire pressures from the factory, but I think they are more concerned with safety and meeting fuel mileage standards than they are with performance. Whatever it is, there is no doubt that by using a lighter car and/or wider tires, you can run much lower tire pressures... that's the whole point. I know it's an extreme example, but the tire pressures (and resulting improved grip levels) of a formula car easily show this. Or you can look at my 245 width van tires, they are narrower than the vettes 325's AND they have to support a crazy heavy van, and that's why they run near 80 (!) psi. For tires with the same profile, the optimal pressure for a tire is determined by the weight it has to bear and the width of the tread. Lower weight, lower pressure. The lower the pressure, the higher the coefficient of friction, and therefore the greater the grip.

But don't take my word for it, here's an article written for road bike guys that spells out pretty much the same thing, the graph at the bottom of the first page is worth looking at:

http://www.bikequarterly.com/images/TireDrop.pdf

The graph shows how, for a given weight on the tire, you need less and less pressure as the tire gets wider and wider. For example, trace up from the 40kg line on the x-axis; as you go up, you see that for the 37mm wide tire you only need an inflation of about 40 psi, but with a 32mm wide tire you need to inflate to 50 psi, and when your tire is only 23mm wide you need to inflate to 90 psi.

Hercules Rockefeller

You might feel like you're talking to yourself because we're not talking about the same thing, I don't know exactly what you're getting at here. The equation P = F/A is always valid, it's the DEFINITION of pressure. The average pressure on the outer surface of the tire must, BY DEFINITION, be equal to the weight it is supporting divided by the contact patch area. Now the pressure between the tires outer surface is not necessarily equal to the gauge pressure inside the tire, especially in an extreme case such as a runflat at 0 gauge pressure. But pressure, regardless of where you get it from, always always always equals force/area. Since the sidewall can support some (or all) of the outer pressure on the tread of the tire, for a given gauge pressure inside the tire, you may very well see a *smaller* contact patch than you would expect, but not a *larger* contact patch like we were told. A runflat will NEVER cause the contact patch to increase in size; the whole point of a runflat is to decrease the contact patch (when the tire is flat) to a manageable size.

jimman

You made the accusation that he misread his pressure gauge, not sure how you drew that conclusion. This example can’t be explained with simple linear equation, yes the equation as it stands is correct but not for the example described. Most all the weight is distributed via the side wall and not the psi term you used. Also to further the explanation in describing grip with tire pressure is also a dynamic and not a simple three member equation. For example you have an inverse curvature which changes the cross section weight distribution and depending on tire composition it could have less grip. Again this has morphed to God only knows what, all Jim wanted is what happens to the patch size.

haljensen

Score;

JoesC5;
2 posts with actual measurements showing a larger contact patch with a wider tire.

HerculesRockefeller;
Multiple long doubletalk posts of theory and equations.

JoesC5 = WIN

HerculesRockefeller = Fail