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Tires & Wheels/Size and Pressure Test:
Date sent: Fri, 16 May 1997 16:41:34 MST
Subject: HML: Tire Size/Pressure Test
OK people, I did the test and here are the numbers...
Test Description:
| Measure the Height between the center of the wheel and the
ground. Measure the distance traveled in 10 revolutions of the
tire Perform above measurements at two different inflation
pressures. |
Test Data:
| Tire Pressure | Height | Distance |
| 35# | 17" | 92' 1" |
| 10# * | 15-3/8" | 91' 4" |
| (difference) | 1-5/8" | 9" |
| (% difference) | 9.6% | 0.8% |
* Tires were bulging noticably (about as expected for 10#).
Test Result Analysis:
| Reducing the vehicle axle height by about 10% did not reduce the
distance traveled by anywhere near 10%. Actual reduction was
less than 1%.
Since the distance traveled was changed by only about 1%, the
effective torque was also increased by only about 1%. It should
be noted that the rolling resistance of a low-pressure tire is
also higher, so the net difference on torque is probably near
zero, if not negative.
Speedo difference should also be about 1%. |
Effective Tire Size (Calculated from distance traveled):
Tire
Pressure | Effective
Circumference | Effective
Radius |
| 35# | 110.5" | 17.59" |
| 10# | 109.6" | 17.45" |
Bottom Line:
| Do not lower your tires for increased torque.
As mentioned in a previous posting, reducing tire pressure does
greatly increase traction and floatation. These, in addition to
smoother ride, are excellent reasons for reducing tire
pressure. |
Notes for Nit-Pickers:
| The test was performed only once at each tire pressure. The
truck speed during the 10-tire-revolution test was very slow -
about 1 mph. The axle-height to ground was measured at the end
of the test. The tire was turned about 1 revolution at low
pressure before the test was started to allow for tire
deformation while deflating. Test was performed on the rear
tires of a '95 Diesel wagon with very little cargo inside.
Front tires were not deflated, transfer case in H (not HL).
Tires are Goodyear MT tires with about 50% of tread remaining.
Altitude: about 5500'. Ambient Temp: 82 deg. F. Surface used
for testing: graveled asphalt. Grade: between 1% and 2% down. |
Dave Breggin
'95 Diesel Wagon
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