Josh-

That's actually a cool idea. Of course it won't show the load distribution well and it will only be a static contact patch, so it'll most likely give more questions without answering anything conclusively, but it'll still be interesting to see and will at least give a better idea of how the static contact patch looks.

I'd be willing to try it too, although I have 255s as well.

Make sure you note your tire pressure. It might actually make sense to do at a couple pressures.

Also I was thinking it'd be good to drive forward a foot or so after putting the car down. So you have what'll look like a splotch from the tire deforming as it's loaded when it's lowered onto the paper, then a line of contact patch. Then you can measure the width of the line. Also I was thinking we should make sure you just put on barely enough paint to show up - too much paint and it'll drip and/or spread out.

 

26.4 lbs guys for the 255/40

 

I was rushing to work so I forgot pressures. We can do 36 and 38 since most people run in that range

 

Everything you said is right. Find someone willing to do the 245 and I'll do the 255 tomarrow

 

Variables, like camber and tire pressure, will cause issues when comparing the tires.

Josh, what's your rear camber? If we pick a tire pressure, we'll have to assume our tire gauges are similarly calibrated....which rarely happens.

 

2.5 in the rear for camber, I have the $180 longacre or how ever you spell it gauge. As long as the other person uses a good gauge then they Gould be really close.

 

If we can get a couple people to do it then that'll help take tire pressure gauge differences out of the equation. Camber should be noted. I'll do it right after I get the car aligned later this week so I'll be 100% on the alignment.

My front and rear camber should be pretty different - I'll probably be a little under 2 in the rear and around 3 in the front, so it might be interesting to do both.

I have a moroso tire gauge, it's pretty decent. It was like $50 from amazon so it's not bargain basement.

Also, as another idea....I was thinking you could do this with a small sheet of fairly hard durometer foam rubber. Drive over the end and you could potentially see the width that's loaded significantly vs. the part that's got a very slight load on it in the period before the foam rubber returns to it's original shape. Also you wouldn't need paint. Might be interesting to see it both ways.

 

What ever you all decide I'll do. Well just have to all do the exact same thing.

 

I've never seen a light wheels vs heavy wheels test that showed that large a difference in times -- and I don't remember it being a big difference in weight anyway. Maybe 2 pounds per wheel? Where was that test published, maybe Sports Car?

 

Prediction, all things being equal, which they aren't, the contact patch will show less for the car with more camber, higher pressures, and lighter corners.

The only way this has any chance of meaningfull data is if you test it on the same corner of the same car.

In reality, if you know your corner weights, you can try to plug the numbers in to the following forumula:
area (contact patch)=load/pressure

The formula shows that as pressure increases, contact patch decreases, which is a common tuning method used to chang the balance of our cars.

There is one exception to this formula though, the side wall. This formula only works when the air pressure is the only force acting to hold the tire up. To account for the side wall you have to add a correction factor.

Now, this goes back to my original statememt. All things being equal (load, camber & PSI) the tires will have the same contact patch area, but the shapes will differ. A drag tire and a road race tire, with the same PSI and load, will have the same contact patch , but the drag tire's contact patch will be longer.

Ref: Rowley 3rd ed. Pg 12-13