03 nov 2009
Lift-off oversteer, gravity defeated
This is a summarized version of one of the post I made some time ago on the other dark side of the internet, where cookies have four wheels and generally no less than 4 cylinders. You can read the full version by clicking here.
The theme of the day is how to survive yourself, while braking hard specifically.
Long story made short, we all know that while we are braking, a relative rotation between the vehicle and the front tire tries to lift the tail and to sink the nose.
That's an inertial effect engaged by the braking. As the center of gravity is positioned higher than the front axle, that rotation generates always these effect, regardless of where you are looking from.
This applies to everything, from bikes to supercar.
What's happening now is that the tail is either slightly suspended in the air or less pushed against the road. That's because the initial relative rotation, generated by a momentum, can be seen as a force pushing the rear end of the vehicle toward the sky, as opposed to the force of gravity which is pushing it toward the ground.
This is very important as a lot of people wrongly assume the rear end is now “lighter”, which is not. The correct version of the story says the rear end still has its own mass (which isn't half of the overall mass), only the force applied to it changed.
The mass is the volume you occupy, multiplied by your density (no offense for those beer-belly equipped out there), while the weight is that mass multiplied by the gravitational acceleration. Hence, the weight is a force. When this force is contrasted by another one, it doesn't mean your volume or density changed, right?
So now the tail is up in the air as the middle of a rope while two persons are pulling it. It is there, it isn't moving vertically, but still has its own mass.
What's the problem then?
The problem is that now your still subject to movement toward the transversal direction, mainly due to the contact with the road, wind or even your own movements/steering, which means a small movement and its intrinsic acceleration will still match with the original mass of the tail, while the inferior or absent contact with the ground won't provided the grip you're used to, if any at all.
So every movement in those moments has to be slow, soft, and well thought.
That's also why rally drivers use the handbrake for tight corners. Not only the tail is lift because of the braking (but less than in a road car due to the harder suspensions an less movement allowed), but blocking the rear tires the friction coefficient is even lower, allowing to move the tail much more easily.
It is a brilliant way to get you out of tight corners or even out of troubles in certain moments, but it's also a good way to crash.
Now that you know how everything starts (somehow), you should also know how to avoid to waste something precious as your race suit, your bike, or even that pink aging thing you can't replace, which is called skin.