Monday, April 16, 2007

Static Friction

What about Static Friction, you say? I thought you'd never ask....

Static Friction is the measure of friction of an object against a surface when the object is, well, static, not moving. The coefficient of friction for a moving object (slipping against the surface) is less than the coefficient of static friction; that's what Anti-lock Breaking Systems (ABS) are all about.

It turns out, that the point of maximum friction is at the crisis point between static friction and kinetic friction. Here's a lovely little picture:

So, decades ago, a friend, investor, co-worker and mentor who happened to really love trains, explained that locomotives were actually the first beneficiaries of early Anti-lock Breaking System technology. Control in turns is obviously not a factor. Instead, the goal with trains is to break at the point of maximum friction: the crisis point between static friction (wheels not slipping) and kinetic friction (wheels slipping). This is, of course, important to locomotives which tend to pull large numbers of incredibly heavy cars loaded with even heavier materials and goods.

Googling for "train locomotive breaking" yielded a variety of results, but no specific detail on the rail-wheel interface, or the systems responsible for adjusting breaking to optimize stopping power....

However, there's another, more novel application in widespread (relatively) use today... wait for it.

Yes! You guessed it, motorcycle racing. Specifically, MotoGP!

In MotoGP, although they do have advanced breaking systems, the novel use of static friction is in the firing sequence used to accelerate the bikes.

It turns out that under hard acceleration, the rear tire is unable to generate maximum traction, because the high speed at the interface between the tire and ground is always slipping -- it's never firmly planted. In 2006 and even more so this year, the MotoGP gurus have realized that by delaying the firing order of each piston, they can tune the application of force at the road to manage optimum traction under acceleration.

Instead of a sequence of fire-rest-fire-rest-fire-rest-fire-rest, which never gives the tire a chance to gain maximum traction, they now have tested a variety of schemes, with one of the more successful being fire-fire-fire-fire-rest-rest-rest-rest. It has got to be hard on the motor, but it helps with traction on the tire, and enables better acceleration, for some....

SpeedTV writes: "Ducati has abandoned the ‘Twin Pulse’ or ‘big bang’ firing order and gone back to their original and conventional 360 degree V4 with symmetrical firing order...’screamer’ style."

So, who says that physics can't be cool!?

No comments: