Weight and Acceleration

During Conversion

As you remove the internal combustion engine parts, it’s likely you’ll discover additional parts that you hadn’t seen or thought of taking out before. Parts snuggled up against the firewall or mounted low on the fenders are sometimes nearly invisible in a crowded and/or dirty engine compartment. Get rid of all unnecessary weight, but do exercise logic and common sense in your weight-reduction quest. Substituting a lighter-weight cosmetic body part is a great idea; drilling holes in a load-bearing structural frame member is not.

After Conversion

Break your nasty internal combustion engine vehicle habits. Toss out all extras that you might have continued to carry, including spare tire and tools.

After all your work, give yourself a pat on the back. You’ve probably removed from 400 to 800 lbs. or more from a freshly cleaned-up former internal combustion engine vehicle chassis that’s soon to become a lean and mean EV machine. The reason for all your work is simple—weight affects every aspect of an EV’s performance: acceleration, climbing, speed, and range.

Weight and Acceleration

Let’s see exactly how weight affects acceleration. When Sir Issac Newton was bonked on the head with an apple, he was allegedly pondering one of the basic relationships of nature—his Second Law: F 5 Ma; or force (F) equals mass (M) times acceleration (a). For EV purposes, it can be rewritten as

Fa

5 CiWa where F a is acceleration force in pounds, W is vehicle mass in pounds, a is acceleration in mph/second, and Ci is a units conversion factor that also accounts for the added inertia of the vehicle’s rotating parts. The force required to get the vehicle going varies directly with the vehicle’s weight; twice the weight means twice as much force is required.

Ci, the mass factor that represents the inertia of the vehicle’s rotating masses (wheels, drivetrain, flywheel, clutch, motor armature, and other rotating parts), is given by

Ci 5 I 1 0.04 1 0.0025(Nc)2 where N c represents the combined ratio of the transmission and final drive. The mass factor depends upon the gear in which you are operating. For internal combustion engine vehicles, the mass factor is typically: high gear 5 1.1; 3rd gear 5 1.2; 2nd gear 5 1.5; and 1st gear 5 2.4. For EVs, where a portion of the drivetrain and weight has typically been removed or lightened, it is typically 1.06 to 1.2.

Table 5-1 shows the acceleration force F a, for three different values of Ci, for ten different values of acceleration a, and for a vehicle weight of 1,000 lbs. The factor a is the acceleration expressed in ft/sec2, rather than in mph/second 5 21.95 5 32.2 3 (3600/5280)—used only in the formula (because acceleration expressed in mph/second is a much more convenient and familiar figure to work with). Notice that an acceleration of 10 mph/sec, an amount that takes you from zero to 60 mph in 6 seconds nominally requires extra force of 500 lbs.; 5 mph/sec, moving from zero to 50 mph in 10 seconds, requires 250 pounds.