Big Rig Dynamics 101

I see a ton of misinformation and misunderstandings about dealing with slick conditions. I'm pulling this section from an old thread to avoid the dreaded "Dead Thread Resurrection," but the topic addressed is important in that it will help you understand the whys of how a truck reacts in a skid or slide.

The scenario is a truck at speed, cruise engaged, in a corner that was unexpectedly slick and caused the trailer to step substantially out of line. Your choices follow.

One of these is the right choice, the others will have various unpleasant outcomes. So, do you:

A) lightly apply the brake pedal to slow the truck.

B) use the Johnny bar (trailer brakes) to pull the trailer back in line.

C) disengage cruise to allow truck to slow down

D) disengage cruise, maintain neutral throttle (an in-gear coast)

E) accelerate to pull the trailer back in line.

F) touch clutch pedal to disengage cruise and allow the Jake brake to engage.

Hhmmmmmmm..............

'A' and 'B' would have similar outcomes. Your trailer tires are already sliding, teetering on the edge of adhesion. Any brake application will almost surely lock them up, reducing their traction (a rolling tire has more traction than a sliding or spinning tire, even if it's rolling with a high slip angle.) and, combined with the slowing tractor, send the trailer swinging to the right. Well, swinging until it smacks your cab in a jackknife, anyway.

'C' is an iffy choice. As the tractor slows, it's pushing back against the kingpin. Which would be fine if your trailer was inline. But it's not, and as you push back, the momentum of the trailer pushes the back-end forward. Not a good idea! You might get lucky and have enough traction at the trailer to keep it from coming all the way around. But you might not! Do you feel lucky?

'E'. Well, you know how when you come up on a wreck and the truck and trailer are on their right side, across the road, with the top of the rig facing his former direction of travel. And you wonder, "how'd he do that?" This is how you do that! Nuff said?

'F' This is the option you use when you wish to find out how it feels to have the hands of God fold your rig in half! If it is slick enough that the trailer starts to slide in a fairly low g corner, the Jake will most likely send your already slipping drives (the trailer will be tugging your drives into higher than normal slip angle. ) into a full skid. The result of that? Well, if you're lucky, a long slide that hopefully scrubs off enough speed before the tires catch on something substantial enough to flop you over on your side. If you're not lucky? Well I've heard that a hard jackknife will rattle your brain a little, but I've never been there!

Which leaves us with 'D'. The idea being to minimize the traction you're requesting from your tires. Some people prefer the 'push the clutch in and coast' method, which is basically equivalent to this. Just be careful reengaging that you match RPM or else you might cause another skid, with the tractor this time. I've always had a good feel with the throttle and prefer that way. There's still no guarantee that you won't further lose control with this technique, but it gives you the best chance of a safe recovery.



What you need to understand is that a tire only has a given amount of traction to offer. If you use all the available traction to brake or accelerate, the tire can not produce any resistance to lateral movement. And the converse is also true: a tire generating maximum lateral traction can not offer any force to accelerate or brake. On dry roads, this doesn't really matter because you have far more available traction than you are ever likely to use. However, on an slick road that is offering only tiny fraction of the dry traction, it matters a great deal. It is really the whole key to safe slick road driving; if you understand the principle, you can adjust your driving to account to the limited traction, never asking more from your tires than they can provide. It's very important to keep the wheels turning, providing the lateral traction necessary for you to control your rig.