Big Rig Dynamics 101

This is true. I explained it the original dead thread. It was one of those freak deals; I'd been on wet roads for an hour or so, no sign of freezing on vegetation or mirrors, temperature was climbing. It could have been oil or fuel, although I didn't see any of those tell-tale signs. I always figured that road had likely been shaded just a bit more in that spot the previous day, or maybe a touch of the frost hollow scenario, as it is slightly bowled terrain even though it wasn't in a deep bottom. Like I said, a freak deal; and an unwelcome one after having spent the graveyard shift skating down the height of Kansas in freezing rain.

But this story is more about the reaction, as Numb put it; "The whys of the do's and don'ts..." Because understanding the reactions of your truck to certain inputs might just save someone's bacon down the road.

 

Let's title this next section, Big Rig Dynamics 102


Condo nailed the gist of it, just enough throttle to keep the kingpin load from the tractor to the trailer. Let's look a little deeper at why. This is the straight road scenario, cornering forces change things a bit.

When the trailer starts to rotate after the trailer tires lose lateral grip, it wants to do so around the center of gravity, not the kingpin. What this means is that when the trailer starts to swing, it also pushes sideways on the tractor through the king pin (Which is why you feel the trailer break loose in the seat of your pants.) So here we are with the trailer tires sliding, adding little to braking effort, and therefore allowing the trailer to push forward against the kingpin. Combine this with the sideways push, and they tend push the drives the opposite direction as the trailer swing, rotating the nose of the tractor the same way the trailer is moving. So now the drives are much closer to losing grip as they have to deal with that side load. If they do start to slide, you have to react very quickly or a jackknife is imminent!

Think of the forces involved: the steers are now centered to the inside of the center of gravity (cg) of the truck, and are still producing braking force, wanting to force the tractor to rotate further towards the trailer. The drives are sliding, being pushed forward and out by the trailer, also tending to rotate the tractor towards the trailer as they are centered outside of the cg's path. And the further the tractor rotates, more of these forces are directed to further rotating the tractor because their force centers are moving further away from the cg's path. Picture two pencils; one pointed at nine o'clock, the other at eleven o'clock. If you thump the erasers of each in the twelve o'clock direction, you can visualize how more of the force is converting into rotation with the nine o'clock pencil. It's why jackknifes tend to snap around, the rotation speed increasing as the truck and trailer scissor together. And realize the whole time this is happening up front, the truck is also pushing the nose of the trailer back, increasing it's yaw rate as well.

So back to the point Condo made; by keeping a very slight forward tension on the kingpin by the tractor, the trailer can now induce a small side slip in the drives, but the momentum of the trailer is no longer driving the kingpin forward into the truck, and the drives have a much lighter force from the trailer to deal with. And while the trailer may off-track a bit, the friction of the tires and any small force on the kingpin from the truck tend to push and pull the trailer back into line with the travel direction. Picture a trailer yawed to the right; the tires pushing the rear of the trailer left, and the tractor gently tugs the nose to the right. This is where the "pull it into line with the johnny bar" thinkingncomes from, the only problem is that it is entirely too easy to lock the trailer tires because there is no feedback through the trailer brake control, defeating the benefit of the increased tug on the nose. Also, in a big, high angle slide, even with ABS, it can take several seconds to get the trailer tires rotating once they have locked, and those may be several seconds you wish you could rewind and get back.

All that said, if you watch the jackknife videos on youtube, you'll notice that most of them don't involve large angle slides of the trailer. The forces are all the same, you just don't see the trailer move much because the truck is already knifed before much movement takes place. I don't have the stats, but studies have shown many, if not most, are the result of unbalanced braking, specifically that the tractor brakes are out performing the trailer brakes. This can be from improper equipment matching which has the tractor brakes engaging first because of incompatible crack pressures, improper shoe selection on truck vs. trailer, or just flat out poorly maintained or defective trailer brakes. Any of those end up with the trailer pushing the tractor, generating the forces we talked about above; and that can be squirrely on dry pavement during a hard stop, and outright hazardous in slick conditions. And if I were an O/O pulling someone else's trailers, it's something I'd think long and hard about, even to the point of chatting with the shop about their brake shoe choices, just to make sure there's be no issues. Of course, way too many people in our industry slap the cheapest shoe they can find on trailers, and that might not always be the same shoe over time.

Just some thoughts for y'all to ponder...

 

Rockstar, you're misunderstanding the gyroscope effect and the forces involved. Those forces don't straighten the front wheels, that's the caster effect. The gyroscope forces actually try to prevent the tires from turning about the vertical (kingpin) axis in that case. Even in the case of a tire spinning perpendicular to the direction of travel, the gyroscopic forces resist rotation about any axis other than the axis of spin.

Even on motorcycles, the gyroscope forces only play a minimal role in moving the bike around during lean angle changes. Yes they provide stability, but the precession forces are minor compared those generated by moving the front footprint relative to the cg.

The centering forces you're referencing are driven by traction. Rolling in a direction perpendicular to rotation axis is the lowest friction movement, so rolling tires seek to roll with zero slip angle.