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A popular rumor or misconception is a car that has poor traction has a higher
1/4 mile MPH. The false idea is the spinning tires and slower ET allow for more
time to apply power. This rumor is as bad as the “engine overheats because water
flows too fast without a thermostat or restrictor” myth.
MPH or velocity is a result of the time and the “push” or torque from the the
tires to the ground, provided time is a preset value. The formula is:
a = ∆V / ∆ t
a is the acceleration
∆V is the change in
velocity
∆ t is the change in time
For distance, the formula is:
Distance
| distance |
∆d = V * t
|
| time |
t =∆d / V
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| velocity at given distance |
∆V = d / a
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| average velocity |
V = ∆d / t
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The third formula above ∆V = d / a
is another way of saying difference in start and finish speed is simply
the distance times the acceleration.
People claiming spinning tires accelerate a car to a faster trap speed have
ignored the basics of acceleration. Final speed difference from starting speed
is always average force applied to the
ground over a fixed distance. If tires are slipping and spinning, and if
distance is the same and average engine horsepower is the same, we always will
have less speed. There is no way around this.
Fortunately, you do not need to know the math. You can use online calculators
like this
Velocity Calculator or you can do a test.
One test would be to accelerate your car to the fastest speed possible when
the track is dry, and then repeat the test in winter with a snowy track. Another
would be to replace slicks or drag radials with small street tires, and watch
the MPH change.
Summary
Some of the confusion behind this might be elapsed time. When the clock is
running, we want to capitalize on maximum possible acceleration from zero MPH.
Since speed is zero at the start, we travel no virtually no distance at all at
the start in a one second period. Traveling sixty feet, the standard distance
for the first time clock reading, takes about 2 seconds in a relatively fast
street car.
1 foot per second = 0.68182 MPH
Our car’s average speed in MPH over any distance in feet is MPHavg
= .68182 * (d/t)
| 60′ time |
formula |
Avg MPH |
| 2.5 |
.68182 * 60 / 2.5 |
16.35 |
| 2 |
.68182 * 60 / 2 |
20.45 |
| 1.5 |
.68182 * 60 / 1.5 |
27.27 |
| 1 |
.68182 * 60 / 1 |
40.91 |
The ultimate MPH at 60 ft depends on how the car accelerates at every point
along the 60 feet, and cars are never perfect. The exact formula is Vf 2d
/ t = at + 2Vi
| 60′ time |
typical MPH @ 60 ft |
| 2.5 |
29 |
| 2 |
35 |
| 1.5 |
45 |
| 1 |
62.2 |
The more tires spin, the faster MPH has to be for the same ET. This is
not the same as saying more tire spins results in higher MPH. Actually,
the exact opposite is true. The more tires spin, with power and distance
constant, the slower the final speed will be. ET slows right along with MPH when
traction is reduced over a fixed distance, although traction generally affects
ET much more than MPH.
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