I don't follow the logic of the rolling start performance figures:
a. as a good comparison. Who goes to 5 mph and then does max acceleration?
b. We all stop at red lights and proceed when they turn green. If someone is going to accelerate at maximum, they are going to do it there. There are other test we see in car mags. The 50 mph to 80 mph for example. This is to show how a car cruising at highway speeds can pass the car in front when the road becomes clear. A useful statistic. I can't fathom anyone driving along at 5 mph and then having the need to accelerate at max to 60 mph.
c. As a comparison figure, how do you get good data with the rolling start? Some cars will have shifted to 2nd gear by then, some not. How will you ensure each car starts at exactly 5 mph?
Not knocking the excellent article, just trying to understand.
Thanks for the questions, Jim.
The primary benefit of the 5-60 rolling start test is that the results represent how a car is normally driven around town, or to put it another way, "real-world responsiveness". Like you said, nobody actually does a max acceleration from 5 mph, but the test gives us easily repeatable data on how quickly a car can accelerate at speeds up to 60 mph.
Even during aggressive road driving, such as accelerating vigorously from a stoplight, the rolling start test better represents the results. That is unless you activate launch control, or torque brake with an automatic or rev and drop the clutch in a manual; but who really does this routinely and on the street? If you do, you're at risk of getting famous on YouTube for the stupid results that can ensue.
Other tests are done to represent passing times. Car & Driver does pulls from 30-50 mph and 50-70 mph in top gear. Motor Trend does a pull from 45-65 mph. The results are a function of gearing, engine torque and transmission responsiveness. I rarely pay attention to these test results.
The rolling start test is easy. First realize that 5 mph is really very slow. The car will be in 1st gear and lugging along at idle. Five mph is about as slow as a car will go in gear at idle before the engine quits. Because there's no power spike, like you'd get from dropping a clutch at a dragstrip, the car's tires and the pavement surface play less of a role in the outcome. Also since the car's gearing defines the engine speed, there's no ability for the driver to rev the engine or otherwise use techniques to maximize the results. Hence, being more independent from driver skill, launch control software, pavement stickiness, tire stickiness, etc the test is much more repeatable and therefore a better metric to use as a comparison of responsiveness.
If you're racing, then 0-60 and quarter mile times are definitely a better but clearly different test. They show the maximum the car can do under those circumstances, with all the tricks available depending on whatever technology is in use.
To characterize what a car will feel like during normal driving, the rolling start test is much better.