How
do we determine our real Crawl Speed? Thanks to Chet Szymecki for some of the original
formula.
We have heard how to figure
out Crawl Ratio. That's easy Transmission*T-case*differential, but
how fast does the Jeep really travel. Do larger tires make a difference
when you really want to go slow.
A = 1st gear ratio of transmission
B = transfer case low range
C = axle gearing
D = loaded tire radius *see note below
E = engine RPM .
FPM = feet-per-minute
FPM = E/(A*B*C)*(D*2*PI/12)
76.6 FPM = 750/(4.01*4.0*4.88)*(15.25*0.524)
The 0.524 is 2*PI/12.
Loaded tire radius. This measurement is taken from the center of
your hub straight down to the ground. If I were to air down for the trail
this number would likely decrease and would have a net effect on the rolling
diameter and hence, the true feet-per-minute number.
Here are a few examples:
Stock '97 with 3.08 gears and 215/75R15 tires. 160.8 FPM = 750/(3.83*2.72*3.08)*(13.15*0.524)
Stock Rubicon with NV3550, 4.10 gears and 245/75R16 tires. 78.6 FPM=750/(4.01*4.00*4.10)*(15.25*0.524)
Average mod TJ with 4.56 gears and 33s (aired down to 18psi). 110.2 FPM = 750/(4.01*2.72*4.56)*(14.00*0.524)
Mod TJ with NV4500, Atlas, 4.88s, and 37s (aired down to 12psi). 45.8 FPM = 750/(6.34*4.3*4.88)*(15.50*0.524)
How
does a 32" tire have a loaded radius of only 15 1/4"?
Here
is the simple solution to the changing tire size.
Basic Assumptions: (loaded/ unloaded doesn't apply)
1. Assume tire perfectly round has a contact patch of a straight line.
2. As you let the air out the contact patch gets bigger.
3. We know the circumference of the tire to be pi*2 radius.
4. You can only use the distance from hub center to ground since this
is the only part of the tire under load.
The Test: (Zero Contact patch) (30 inch tire)
1. Mark the tire straight down from the center to the ground.
2. Roll tire until Mark returns to straight down.
3. Distance rolled will equal pi*2 radius. (94.25 inches)
The Test: (6 inch contact patch) (30 inch tire)
1. Mark the tire straight down from the center to the ground. Notice it
will bisect the contact patch leaving 3 inches in front and 3 behind.
2. Roll tire until Mark returns to straight down.
3. Measure distance rolled. This will equal pi*2 radius-3 inches. (91.25
or a 29 inch tire)
So the above 32's (265/75R16) even though the tread circumference is 100.2
inches unloaded, the loaded diameter is 30.5 inches/ circumference 95.8.
His contact patch at the widest distance should be 8.8 inches.
I verified this on mine. Tire patch just over 6 inches wide, distance
from hub to ground 14 7/8 inches. That makes my loaded diameter 29 3/4
inch. This on a 245/75R16.
