Thread: S65 vs. S62
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      01-25-2013, 04:25 PM   #96
bruce.augenstein@comcast.
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Quote:
Originally Posted by urBan_dK View Post
A flat torque curve (at a given throttle position, for simplicity's sake) means constant acceleration. The jerk (derivative of acceleration over time, da/dt) is 0.

A peaky torque curve will have non-zero jerk. Due to hysteris of the tire carcass, it can only tolerate so much jerk before the static coefficient of friction is overcome and the tires spin. While tires do have an outright torque maximum for given conditions, inducing enough jerk would be enough to spin tires that otherwise wouldn't from torque alone.

Example: If I apply 100% throttle in my supercharged S2000 from 2000 RPM to redline in 3rd gear, I won't break traction. If I punch it at 6k, I will. The amount of torque applied at 6k is no different. The jerk is.
Good post.

But listen, an engine with a building torque curve, even a steeply building torque curve, will create next to zero actual "jerk", as you say. I am not familiar with any engine on the planet that will gain significant torque within, say 25 or 50 rpm. Even a car gaining an outrageous 347 pound feet of torque over 1500 rpm (in the example given by pbonsalb) cannot be called a "jerk" in second and third gear.

Your example of punching it from cruise in third gear at 6000 rpm is an example of jerk from the takeup of driveline lash when going from light-loading cruise to full power.

Look, I'm not saying that the jerk factor is immaterial. What I am saying is that it's of little consequence in terms of a building torque curve.

Of course, saying it's of little consequence is not the same as saying it's of zero consequence, so I guess, point made, at least to some extent.

Bruce

PS - Just got back from the auto show in Harrisburg. It ain't a big deal like Detroit, L.A. and new York, but I'm still a happy guy.