Seven degrees of slip
37signals.com
37signals.com
I believe the author is slightly incorrect. First off, there is no "optimal" slip angle for all situations. Putting on your physics hat for a second, think about a race car tire at the Indy 500. As the car enters turn 1 at 200 mph, the driver turns the wheel. The tire, due to rotational and centrifugal forces, will slightly change shape under these pressures. As a result, the contact patch of the tire can be pointing in a slightly different direction than the tire itself during the turn. The anglular measurement of that difference is "slip angle." A small amount of slip angle can be good, as it can increase grip up to a certain point. However, beyond that point, grip is no longer improved and you can transition from a slip angle to a slide angle, where the tire has lost grip. Depending on whether you want the back tires lose grip first (oversteer, where the back end comes around) or understeer (where the front tires lose grip first and you lose some steering, think driving in snow) you can use the slip angles to tailor the car's handling to the driver and the racecourse. The optimal slip angle is that angle which maximizes grip without inducing momentum-sapping slide angles or creating too much drag. Optimal slip angle depends on all sorts of things: chassis set up, tires, racetrack temperature, etc. While the author may know more about this than I do, I've never heard of seven degrees of slip being optimal for all situations.
Also, I think the author's metaphor is inelegant: driving beyond the limit means you are not allowing the full power of your engine to be transmitted to the tires. The fastest drivers are those who drive at the limit of available grip. If you drive beyond the limit of available grip, the power of your engine will not be transmitted efficiently to the road because the tires are not gripping it fully.
If you go to a rally, you can really tell by sound which drivers are doing the best job by the tire squealing.
Yes, sliding too much is bad. Sliding just enough is necessary if you want to win.
Amusingly as relates the author's point, the latter link seems to indicate that optimal slip angle isn't fixed at 7 degrees but varies based on type of tire - as it says, "For race and high performance tires this optimum slip angle is around 6 to 10 degrees while this number is a little lower for street tires."
This is due to two things:
Improving yourself in the most efficient way needs you to push your envelope right behind the limit. Example: do as much push-ups as you can, rest a few seconds, then do one more. You will improve much faster than by simply doing "as much as you can" (or so you thought) because you will learn to truly feel what your real limit is and work out accordingly without trashing yourself.
Knowing where your limits lie allow you to quickly go from any point to the limit without overshooting your target, which would then imply overcorrection. The inability to correctly evaluate how much to correct is extremely dangerous and can lead to unstable systems (like one who tries to control a sliding car and by doing so only makes it worse). In a feedback system, the initial ramp-up has to be reasonable else you will overshoot your target, requiring costly corrections later on. Equally undershooting produces undesirably slow results. See [0] for a visualization. Experience of the limits allow you to directly aim for the optimal action.
[0] http://en.wikipedia.org/wiki/File:Step_response_for_two-pole...