Big Rig Dynamics 101

Discussion in 'Experienced Truckers' Advice' started by Hammer166, Feb 1, 2013.

  1. Hammer166

    Hammer166 Crusty Information Officer

    Aug 18, 2007
    ~8600+' and loving it!
    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.


    '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. :sad:

    '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.
    Rocks, Terry270, TruckerNerd and 17 others Thank this.
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  3. Saddletramp1200

    Saddletramp1200 Road Train Member

    Sep 4, 2011
    Houston Texas,USA
    Why are you bothering us with this, it ain't even funny.
    Canned Spam Thanks this.
  4. Numb

    Numb Crusty Curmudgeon

    Jan 30, 2012
    Charlotte, N.Carolina
    because the new ones need to know and it explains the why to the "do's and don'ts".

    some good info there.
  5. dptrucker

    dptrucker Road Train Member

    May 14, 2012
    not suppose to be funny. but to be used as a learning tool. thanx to the op for sharing
    Canned Spam Thanks this.
  6. Tonythetruckerdude

    Tonythetruckerdude Crusty Deer Slayer

    Dec 8, 2012
    hunting...../ retired
    Nice to see some constructive info based in real facts....not opinions from wannabes , and newbies that already know waaay too much.
  7. Njnoob

    Njnoob Light Load Member

    Oct 26, 2012
    Northcentral NJ
    When's the next chapter???
    S M D Thanks this.
  8. Blind Driver

    Blind Driver Road Train Member

    Aug 7, 2006
    New Albany, IN
    There are many variables, but I think the OP meant on a flat road surface.

    I had my trailer trying to pass me on a secondary ice covered road. I played with the trailer brake handle making it work kind of like anti-lock brakes while droving near the edge of the other lane. It worked, but I took the next hill slower. Trailer still tried a little, so I accelerated a bit and brought the trailer back in line. Good thing the hills weren't more then 1/2 mile long.

    Me and the truck driver in front of me eventually found a place to stop for a few hours while we waited for the salt truck. Salt truck passed us and we went on our way 30 minutes later only to catch the salt truck because it slid into the ditch.

    I started to wonder why salt trucks don't unload from the front :biggrin_2556:
    Rocks Thanks this.
  9. EZ Money

    EZ Money Road Train Member

    Every situation is different too...
    After 30 years my gut feeling and reflexes without thinking have saved me many times. I have never wrecked in my career and have had many close calls.
    It is something that comes with time as your body and mind become part of the truck.

    Good info though as a starting point for new drivers.
    Rocks Thanks this.
  10. Hardlyevr

    Hardlyevr Road Train Member

    Jul 30, 2009
    Mapleton Depot,PA
    If you are in conditions where the trailer could slide like this, you should NOT have the cruise control on to start with!
    KMac, S M D, Lonesome and 3 others Thank this.
  11. Blind Driver

    Blind Driver Road Train Member

    Aug 7, 2006
    New Albany, IN
    I've had my drives spin while going uphill. That a weird feeling especially when the truck slows down, but the trailer tries to maintain speed. It even worse on the small back roads with small steep hills.

    I kept giving the driver behind me grease reports. :biggrin_2559:
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