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How much does the contact patch of a tire change from a static unloaded tire on a level surface to a tire loaded with the static car weight, applicable alignment, and dynamic loading? Taking my tires off the trailer when I got home from Greenville, I noticed that I could see a good deal of light between the support bar of the trailer and the bottom of the tire. So I put the tire on a level flat surface at around eyeball height with a light behind it. I was shocked by how little of the tire actually touched the surface (I can't believe I hadn't noticed this before!). On my front Hoosiers (225/50/16) only about 3 1/2 inches of the almost 9 inch tire width actually touches in the unloaded condition. The rears (245/45/16) are similar, only touch in about 3 3/4 inches. This does not seem to be a wear issue because I tired the same thing with my recently new Kumho 710s. Does the loading of the car weight bring most of the width of the tire into contact?

Ignoring the mechanical strength of the tire (which is negligible when talking about fully loaded):

Assuming a tire at 30 psi, 3000 lb car, 4 tires...

in^2 = 3000 lb / (30 lb/in^2) (4) = 25 in^2

Or about 5x5", sounds close to right.

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Mike Whitney
whit32@gmail.com, 919-454-5445
V10, V8, V8t, I6, I6, V6, F4t, I4, I4, I4, I4, I2, 1, 1

Easy way to find out: get a piece of masonite or other hard smooth surface, carbon paper (remember that stuff?) and a sheet of paper. Jack up car with tire in question mounted and aired, place hard surface, carbon paper messy side up and paper under tire. Lower car. Jack car back up, remove paper and see image of contact patch pattern on underside. Remove carbon paper before re-lowering car to avoid dotted line on driveway.

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SPIN or WIN!
there's no glory for going slow.

Based on experimental work I did years ago, if you want to go slower, lower air pressure to get a larger contact patch.

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Dick Rasmussen

FS 50 2018 Mustang GT

Dick-
Please explain in more detail.

Rick,

From my experience (which was years ago), assuming the tire pressure was in the "known good range" for the tire, lowering the pressure just to get a bigger contact patch usually made for slower times.

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Dick Rasmussen

FS 50 2018 Mustang GT

I'm curious if that is still true today Dick. Many of the street tires used today for autocross have sidewalls stiffer than the r-comps you used years back.

Maybe the older tires with lower pressure rolled too much and would impact your cornering force. Todays Azenis, MX, F1 SC, or entry level r-comps like the R2 or RA1 have really stiff sidewalls. So a little less air in the rear on a RWD drive car might get it to get better traction without killing cornering speed.

For autox I ran 2-5lbs less in the rear to get grip. On track I'm at least 2lbs lower in the rear.

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http://www.greywinds.com

The thing that surprised me about this observation was that although I expected there to be a small contact length along the longitudinal axis, I was surprised that so little of the width direction actually touched the surface.

Graham,

You may be right. Even though the tires I used to run at 50 psi had STEEL in the sidewalls (Michelin XWX's) they also were 70's series on a 5.5 inch rim. Other tires I used were also relatively high profile compared to the 45 to 35 series modern tires on wider rims. A lot of the benefit of high pressures was grip (as measured on skid pads) but for a stock class car the improved steering response tended to compensate for the soft suspension.

FYI I was running 45 psi fronts and 35 psi rears on "normal" street tires in F Stock (245/45X17 Firestone Wide Ovals on 8 inch rims). I chose the high front pressure to prevent roll over on my daily driver full tread tires and, I hoped, to improve the response and maybe the grip. The balance was slight understeer (to be expected with a Mustang) but since I had no test time and knew there was more time in the driver than could be found with tire pressures I kept the car the same for each run. A look at the results will show that the driver learned a little on each run. Final PAX (compared to stock TIR times) indicates grip was at least decent.

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Dick Rasmussen

FS 50 2018 Mustang GT

Wow, that's a substantial difference from the 29.5 front 28 rear on Kumho V710 215/50 13s on an 8 inch rim.

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Has no responsibility whatsoever.

I could actually feel a difference between 33 and 35 psi in my V710's. The car turned in better and had more grip at 33psi. It also ate teh tires in a hurry (this was in Sanford). I choose to run 34-35 psi to keep them on the car for more than one event.

However, I do agree with Dick. I experimented a lot with the Azenis 215 tires we ran a few years ago. I could lower them excessively (20 psi) or raise them excessively (55-60 psi) in the rear, and both conditions would result in more oversteer.

I honestly forget which felt better, I think it was the low pressures, it was more progressive (roll onto the sidewall vs no contact patch at all).

Wes,

How did you decide on that 1/2 pound? You must be one heck of an engineer/test driver.

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Dick Rasmussen

FS 50 2018 Mustang GT

More seriously (than my comment to Wes) I suggest when fine tuning a tire pressure choice that testing under controlled conditions will result in the best data. This usually means a timed skid pad for grip and a timed repeatable transient course segment for transition feel. This also would require a consistent surface and consistent tire and pavement temperatures.

It all boils down to you cannot have more precision in the "answer" than in the experimental variables. For most of us in autocrossing competition conditions the biggest variable is driver "learning curve" followed closely by "driver error".

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Dick Rasmussen

FS 50 2018 Mustang GT