Are there any " Warning signs" for A Jack knife?

Actually did that one once.... and yes it worked (gently hit the throttle at the same time). Not sure if it was right just did it on instinct, but everyone walked away and no damage to anything so it was a good day.

 

To understand the laws of physics as it pertains to how semi trucks go into a jack knife, remember that the tractor and trailer are two vehicles connected by the 5th wheel and trailer's king pin. When you apply the brakes, the trailer slows down by both, the tandem brakes on the trailer and your 5th wheel. As you slow down, you feel the forward surge as the trailer pushes against the 5th wheel. Any vehicle (or object) in motion is kinetic energy. Catch a tennis ball at 40_mph versus a baseball at the same speed, and you'll feel the impact force is greater on the solid baseball than the hollow tennis ball. Thus, weight and speed is proportional to kinetic energy in motion. This is why a 2,000 pound compact car will not produce as much damage on impact as a fully loaded semi truck.
Your brake system converts kinetic energy into heat energy. As the brake shoes presses against the disk wheel (front brakes) or drum brakes (drive axle or trailer tandem axle), it heats up immediately as it converts kinetic energy into heat energy. An analogy to understanding this is like solar energy; solar panels converts the energy from the sun into electrical energy. As your disc or drum brakes heats up, the air current passing through as you're in motion cools the brakes. However, when you're going down a steep 5% (or more) grade, the heat you generate can exceed the heat dissipated by the air current, which is why your brakes begin to smoke and the disc or drum brakes turns glowing red.
When you apply your brakes in slippery conditions, or brake hard in an emergency condition, the tractor is slowing down faster than the trailer. Thus, the trailer has more kinetic energy than the tractor; the tractor is converting kinetic energy into heat energy faster than the trailer because the cargo weight and trailer has a longer braking distance than the tractor. To alleviate this, you equalize the energy conversion by either letting go of your brakes and using the trailer brake handle, or let go of the brakes completely and steer into the emergency shoulder (or next lane over if its open) to increase your braking distance. The latter is called "looking for an escape" to buy yourself more braking distance.

Or at least that's what my physics teacher said when I asked how semi trucks jack knife. I'm assuming she's not telling the entire class a lie.

 

Lets don't forget on ramps off ramps. If you enter an on ramp off ramp to fast when its slippery you are asking for a jack knife.A fuel truck could have just came through leaking leaking fuel on the ramp or maybe a truck is leaking a little fuel from his tanks,You know he could have left his cap off when he fueled and when he turned the curve fuel splashes all over the ramp.Here you come along and not realizing the speed limit for the ramp is 25 but your only going 35. In the past you have made this curve easy.This time you are not aware of the lurking killer which is the spilled fuel on the ramp so here you go and you feel the tractor sliding, so your first response is to hit the foot brake. Huh Oh! Now the trailer is trying to pass you. Your last mistake,you hit the foot brake even harder which causes your trailer to collide with your truck.The first thing you could have done is obeyed the posted speed limit. When the speed limit is 25. I usually go 15 or 20 while the road is slick to make sure I can maintain control.Also you could have used you hand brake to keep your trailer behind you.Note this does not always work because if your truck is sliding then both truck and trailer could wind up in the ditch. Also some trucks are not equipped with hand brake. Hope this is helpful.

 

Yes you can use the Trolley Valve or Trailer Brake but you would have to know what your doing. I have used mine in situations before to straighten her out behind me but like i state you have to know how much to pull on that handle. i would not recomend it to anyone new out here ever. If you want to learn about it if your at a terminal and there is snow on the ground and you have enough room give it a try and see what the reaction is. Main problem ive seen in all my years are speed, weather conditions and slamming on those brakes do it just once and you will remember it for sure.

 

All sounds good.. except all the trucks I have ever done a brake job on had drums and shoes on the steer axle as well.. Never have heard of disks on a big truck..

 

There are two kinds of jack knives and both involve loss of traction and weight pushing you. Most happen on wet or slippery roads but can happen on any road surface. ABS brakes have made huge improvements.

Either your tractor drives can break traction or your trailer tandems can break traction. Multiply that with the weight of your load pushing and you have problems. The key is prevention and to prevent losing traction in the first place.

Older trucks it was easier because you could lock the brakes up and smoke them. It's not so easy with ABS brakes. Most jack knives today are weather related. Ten years ago there was problems with the converting over to ABS brakes. Companies would keep tractors 4 years on average and trailers 10 years on average. Many times drivers had a tractor with ABS and a trailer with no ABS. A very dangerous situation on slippery surfaces. The non-ABS trailers are about gone now. But when you see that little yellow light on the back of your trailer means your trailer ABS is disabled from a fault. Many drivers don't think that's a biggie but it can be. No way will I pull a trailer in need of ABS repairs especially in winter.

To a certain point jack knives are recoverable if you can regain traction and take away the weight pushing you factor. That might be letting off the brakes, counter steering if necessary and accelerating. There is a point of no return where regaining traction isn't possible or traffic doesn't permit it.

Every driver should understand how a jack knife happens in order to prevent it.

 

Watch your speed, keep your eyes on the mirrors.

 

Yes, as a matter of fact. If you're driving, and you suddenly notice your trailer is perpendicular to your tractor... you're about to jackknife. If you suddenly see other cars slamming on their brakes and throwing it in reverse, you could potentially be having a jack knife situation! LOL! I'm sorry, I couldn't help myself

 

When you work in fleet operations and order trucks from the factory to rotate your fleet (get rid of older trucks and replace with newer trucks), the trucks are cheaper if you spec drum brakes on all 6 brakes instead of owner-operator standard package of disc brakes in the front. If drum brakes on the front is $300 cheaper per truck, an order of 100 trucks specd with all drum brakes is a savings of $30,000. Fleet management pockets this as a salary bonus, telling top corporate management "...I saved you $30k because of my college education in business administration." Drum brakes have longer braking distance than disc brakes, so they're being rewarded with $$ bonus for endangering truckers' lives by increasing their brake distance, and increasing the likelihood of a jack knife. When I was an owner operator, my International COE had disc brakes in the front. Next time you visit a trucking show, look at the front brakes on owner operator truck package; or look at the front brakes of owner operators hauling electronics and show exhibits. They're pulling household goods' trailers. Disc brakes dissipate heat faster than drum brakes, and not as likely to overheat on steep downgrades.

Incompetent driver trainers produce incompetent newbie drivers. The number of drivers taking the curves and turns too fast is a reflection of how many trainers out there have no business training and certifying newbie drivers. Trucks have higher center of gravity, which makes it easier to loose control and end up laying on their side. This is further compounded when the trailer has a payload. You can take a turn or curve faster with an empty trailer than a loaded trailer. IMO, when a driver with less than 1-year driving experience puts a truck on its side for taking the curve or turn too fast, they need to identify who trained and certified him/her, and put a federal restriction on that trainer from training newbie drivers.

When the tractor stays in control (remains parallel to pavement) and you see the trailer going into a jack knife, the trailer has lost traction with the pavement. The trailer wheels are turning slower than the trailer's speed of travel. In slippery condition, there's an increased likelihood that both tractor wheels and trailer wheels will loose traction as ALL THE WHEELS are turning slower than the vehicle's decelerating speed. When you feel the tractor wheels going into a slide, LET GO OF THE BRAKES. Releasing the brakes will cause the wheels to re-establish traction as the wheels catches up with the vehicles actual speed of travel. Look for an escape to increase your braking distance, which is the shoulder lane, the left lane (if open), or median lane. Keeping the truck upright, yet stuck in mud is the lesser evil of letting the truck and/or trailer laying on its side. You can easily get a tow truck to hook up a winch cable to pull you out of the mud, then send you on your way to make final delivery. If the equipment is laying on it's side, they'll need to dispatch another truck, trailer, driver, and workers to unload the trailer and reload the cargo. If they only need to pay the towing fee to pull you out of the mud, they'll forgive you if you say "This 4-wheeler cut me off without warning, so I used evasive steering to keep myself from rear-ending him. Maybe I should have just rear ended him." Versus "I don't understand how the tractor/trailer ended up on its side, it just happened so fast." With the latter excuse, you're definitely unemployed.