Re: O/T why does a skidding tractor trailer jackni

Posted by MarkB_MI on October 14, 2006 at 04:42:27 from (64.31.11.113):

In Reply to: O/T why does a skidding tractor trailer jacknife?? posted by baffling question on October 13, 2006 at 14:41:16:

Not being a truck driver, I haven't thought much about the dynamics of a jacknife. But jmixigo got it right that a sliding tire has no traction in any direction.

The thing to understand is that the friction between two surfaces is different depending on whether the surfaces are sliding or stationary. The static (stationary) coefficient of friction is usually much greater than the dynamic (sliding) coefficient. For example, if you are trying to push a heavy box across the floor, you will find that it takes much more force to get it moving than it does to keep it moving. When you push on the box, you must first overcome the static coefficient of friction; once it starts to move, the friction drops significantly.

This explains why you loose the ability to steer a car when you lock up the front brakes: although the tires are sliding, they loose traction in all directions, not just the direction in which they're sliding. It also explains why a vehicle with a limited slip differential has a much worse tendency to fishtail than one without limited slip: the non-spinning tire on the car without limited slip keeps the vehicle from sliding sideways.

OK, getting back to the jacknife, here's what I think happens: On a semi, the tires most likely to loose traction are those on the driving axles. That's because they are subject to torque any time the clutch is engaged and the transmission is in gear. (You have to step on the brakes to get the wheels on the other axles to break traction.) Once the driving axles lose traction, the rear of the tractor will tend to slide sideways, since there is no longer any lateral resistance. When the rear of the tractor slips, it causes the trailer to rotate out of alignment with the rig's direction of travel. Once the trailer is a few degrees out of alignment, the entire rig becomes unstable: As long as the trailer tires have traction, the trailer will tend to force the driving axles to slide, causing the trailer to rotate further and pick up rotational inertia. At a certain point, the trailer tires will break traction, and the trailer's rotational inertia will cause it to "pass" the tractor.

A similar condition can occur on a motorcycle. I took a motorcycle safety class a while back, and one thing they stressed is that if you ever lock up the rear brake, you must keep it locked up until the bike comes to a full stop. Why? Because once the rear tire breaks traction, it will tend to slide out of alignment with the cycle's direction of travel. If you release the rear brake, the tire will instantly "grab" and try to turn the bike in the direction the tire is pointing. The result is inevitably a "high side" crash, where the driver is on the forward side of the sliding motorcycle. (This is on dry pavement, on dirt it's no big deal to lock up the rear brake and release it, since the tires have much less traction.)

Getting back to your point about drag racers doing burnouts: The power-to-weight ratio of a dragster is so great that wheel spin is inevitable. What they are doing is to modify the properties of the tires by heating them up, basically changing the tire from hard rubber to something like rubber cement. Even in this heated state, the static coefficient of friction is higher than the dynamic coeffficient, but the dynamic coefficient of the hot, sticky tire is much greater than it is for a cold tire. All of this has litle relevance to the jacknifed semi.

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