drag tire contact patch discussion

[lindertw]

I realize we have a tire/wheel sub forum, but as this discussion will be specifically for drag tires I hope it can stay here...

As a continuation to my post in the dumb question thread, I am considering going to a from a 26" rear tire, to a smaller one to change my effective gear ratio and get me through the traps at a higher rpm. Conventional wisdom says that a smaller diameter tire yields a smaller contact patch.

my current rear tire specs
M/T ET street (p/n 3743) 26x11.5-17LT
measuring rim: 11"
section width: 11.1"
tread width: 9.5"
overall dia: 26.5"
circumference: 83"

I slid sheets of paper front/back/sides until they touched the tire and taped them down - presumably what's left is the contact patch.

representative pic showing my OEM pirelli

on a 17x8" OEM wheel, with the M/T ET street bias ply tire
18psi cold
8.25" wide
6.25" long
52 sq-in contact patch

I found a guy on another forum running a 16" hoosier drag radial (24.9" diameter) on a 16x7 OEM V6 wheel on his 2007 Shelby GT, and asked him to measure the contact patch as I did above, and let me know what the measurements (and psi he had in the tire). Conventional wisdom says the contact patch should be less than my 26" tire:

Hoosier DOT drag radial (p/n 17320) P225/50R16
measuring rim: 7"
section width: 9.3"
tread width: 8.8"
overall dia: 24.9"
circumference: 78.3"

on a 16x7" OEM wheel
24psi cold
8.5" wide
6.75" long
57 sq-in contact patch

15psi cold
9" wide
8" long
72 sq-in contact patch

His smaller diameter tire had a greater contact patch!

There are two 24.5" slicks for 15x8" wheels that I'm considering, M/T ET Drag p/n 3015, and M&H drag slick p/n MHR-25. Either of these should give me a larger contact patch than what I'm running with my current setup.

I'd like to see how the numbers shake out with your differing setups. If you could, please provide the following:

tire brand, part number (and link to specs)
measuring rim:
section width:
tread width:
overall dia:
circumference:

wheel size (diam x width)
tire pressure
measured width
measured length
contact patch size (sq-in)

 

[86GT351]

Smaller tire will lower your RPM not raise it.

 

[skwerl]

Smaller tire will lower your RPM not raise it.

Please clarify this statement.

 

[lindertw]

Smaller tire will lower your RPM not raise it.

rpm = (mph x final gear ratio x 336)/tire diameter

smaller diameter tire = higher rpm

 

[86GT351]

You guys are correct. Was thinking backwards!

 

[dadoctah]

mythbusters-lindertw

that's a neat way to measure contact patch. If you yourself had measured both tires, With the same air pressure and the SAME air gage, your results would be different. for the most part, static contact patch size is a mathematical function of vehicle weight and tire pressure-picture a "flat" tire. Most tire pressure gages are just a little more accurate than kicking the tire with your boot. Both tires should have same size contact patch. Length and width would vary but L x W = same. Conventional wisdom is that longer contact patch(higher diameter) is more efficient for drag racing due to how a drag tire deforms(wrinkle walls) and mechanically interacts with the road.
dadoctah