2 questions regarding traction and accidents

As noted a few times before in this thread it's hardest to stop a bobtail. Here's why. A typical bobtail will have about 12,000 lbs on the steer tires, or 6,000 lbs per tire. It will have about 9,000 lbs on the drives, or 1,125 lbs per tire.

It's the lack of down pressure on the drives that creates the problem. The only time I have ever smoke any tire was stopping at a fast light change coming to a complete stop from 35 mph. Throw in a little rain or snow and the problem is magnified.

The OP seems to have two questions regarding how to handle accidents you aren't involved in, the first question is MUST you stop to lend assistance? That's a judgement call. If I WITNESSED the accident AND it looks like injuries are likely AND I can safely stop where I am not creating a hazard I will stop. Otherwise and especially if I will be blocking traffic or creating additional hazards it's best to keep going and call 9-11.

The second question seemed to be regarding getting in the backup behind an accident on multi lane freeway, then finding out your lane is blocked by the accident and now you have to merge across two or more lanes.

If I am in a backup like that the first thing is to NEVER try to be so close to the vehicle in front of you that you are constantly coming to a complete stop, as if you are trying to block folks from merging in front of you. Try to maintain as constant a speed as you can, shifting to alter your speed as needed, getting to the stopped vehicle in front of you just as it starts moving again.

Second, if you have a CB turn it on. You COULD have avoided the backup in the first place. But now that you are in the backup at least you can find out well in advance that you need to merge over WELL in advance of the problem.

If you DON'T have a CB, as you are leaving some room ahead of you, keep a sharp eye out for turn signals and trucks merging ahead of you. As soon as you see that truck merging five cars ahead of you TURN ON YOUR TURN SIGNAL and pace the lane next to you. Don't worry that there's now a big gap forming ahead of you, just pace the lane you need to merge into. Eventually folks will let you over.

When I am running at the speed limit and see stopped traffic ahead I turn on my four ways as I get on the brakes. Once I see that traffic behind me has slowed safely I TURN OFF THE FOUR WAYS!!!! There's no reason to run for miles with your four ways in a backup. What's your intention? Now the four wheelers BESIDE you have NO IDEA you want to merge after they've been beside you thirty minutes with your four ways going. Turn OFF the ###### four ways!

 

Either I am still asleep or you have weird weighted bobtails.

 

It depends on the tractor. My example was a T680 I drove several years ago.

 

Nothing typical about the weights you stated. 12,000 on a bobtail’s steers? Come now! Not typical at all!

 

I'm puzzled why you state that. The engine block doesn't move. That's the primary weight on the steers. How would lack of weight on the drives shift weight away from the steers?

FWIW I had to have a bobtail CAT scale ticket in my cab binder. This was to help dispatch get the right tractor hooked to heavy loads. You can't have a load that maxed gross weight with a bobtail get pulled with tractor with a sleeper berth. Yes indeed the bobtail steers were right around 12,000 with a T680 with a condo sleeper.

 

They teach you how to pass the test. And it was very much more a reality in the days of spring ride and bias ply tires, as empty trucks had a hard time keeping their wheels on the ground.

But to answer the why: traction, and thus braking force, is fairly proportional to weight. So as long as your brake is strong enough, an empty and loaded truck can produce roughly the same deceleration. At least at the beginning of the stop, by the end of the stop, the heavy truck will be experiencing brake fade and lose some braking force.

That last sentence is why I wish the books would be updated. Because while there are situations where the loaded truck stops better, there more situations where the empty would, because of that fade.

 

Well, I'm going to disagree. For all practical purposes, inertia is used most often when discussing rotating bodies. When thinking about inertia, think about the effects of a light weight flywheel versus a heavy flywheel.

Velocity can make a bigger difference in KE but velocity isn't changing in the question asked so it can be assumed to be constant or at least the speed at which the brakes are applied. So this would be higway speeds. 60 or 70 mph.

I think in the OPs original question, weight is still the main issue and a fully loaded tractor trailer has significant momentum. We could get crazy discussing kinetic energy and velocity.

But all of this is spitballin' on my part as I didn't sleep in a Holiday Inn last night.

 

Y'all got me curious...

in·er·tia
Dictionary result for inertia
/iˈnərSHə/
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noun

1. 1.
   a tendency to do nothing or to remain unchanged.
   "the bureaucratic inertia of government"
   synonyms: inactivity, inaction, inactiveness, inertness, passivity, apathy, accidie, malaise, stagnation, dullness, enervation, sluggishness, lethargy, languor, languidness, listlessness, torpor, torpidity, idleness, indolence, laziness, sloth, slothfulness;More
   
   
2. 2.
   PHYSICS
   a property of matter by which it continues in its existing state of rest or uniform motion in a straight line, unless that state is changed by an external force.

 

Mass is dependent on an object's inertia.

Momentum is dependent on mass and velocity. It's useful for analyzing collisions, as momentum is conserved.

Kinetic energy is dependent on mass and the square of velocity.

When we're talking about braking , KE is what we're dealing with, as it determines how much energy the brakes must dissipate to slow a vehicle. Twice the weight, twice the energy. Twice the speed, four times the energy.