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Tested: Can sticky tires make your straight-line speeds slower? | Articles

Tested: Can sticky tires make your straight-line speeds slower? | Articles

[Editor’s Note: This article appeared in the December 2013 issue. Some information may be different today.]

Friction is what makes tires fun. The more they generate and the more grip they have, the more smiles they put on our faces. But that same friction that helps us cling to the road when clipping apexes also slows us down when we’re on our …

The Formula:

We’re trying to figure out the force of drag (F). Since we know how much the car weighs (which gets us m) and we have the deceleration (a)–or at least the data to calculate it–we can solve the simple equation: F=ma. 

Since we’re talking about testing the car on a relatively flat surface–or, in this case, averaging the data in opposite directions on the same surface–we can assume that the mass of the car acting against the drag is the weight of the car converted to mass.

To convert the car weight to mass, we need to divide the weight of the car, in pounds, by the gravitational acceleration. The gravitational acceleration for our purposes can be approximated at 32.2 ft./sec.2. [Ed. note: It varies with altitude; ask your local astronaut.]

This converts the weight of the car in pounds to the mass of the car in slugs. [Ed. note: Really, who comes up with these unit names?]

The acceleration is the difference in speed divided by the time between the two speeds.

The speeds from the start and the end of the deceleration need to be in ft./sec., not mph, so we’ll multiply our speed in mph by 1.466667.

The deceleration time needs to be in seconds and as accurate as you can get. Here’s the formula for deceleration: (starting speed–ending speed)/deceleration time.)

The deceleration is effectively a negative acceleration, so throw a negative sign in front of it, and the units are now ft./sec.2. For a relative number, you could divide the deceleration by 32.2 to get the g-load.

Now we have everything we need to calculate the drag force.

The output of F=ma will give a force in pounds.

This force is the average force on the car from your starting speed to your ending speed. Put another way, it’s the drag at the average speed.

This drag force is going to include everything from aero and tire resistance to bearing drag and any slopes in the road (which were eliminated by doing multiple passes in both directions).

Now we can convert that drag force in pounds to an equivalent…

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