Engine Torque-Induced Chassis/Frame Twisting (BIG RIGS) 2

[nik_ace]

Hello all,

I am trying to analyze the behavior of the chassis/frame of a big rig truck during initial launch. I want to understand how much force the engine torque would cause the front of the truck to twist, lifting the driver side, similar to the attached photo. This twisting most-likely occurs when the truck is hauling heavy cargo, keeping the rear wheels on to the ground. Is there any way to calculate the reaction forces with given big rig specifications such as engine torque and power? Ultimately, this would help me understand how this twisting effect of the chassis would affect parts and accessories attached to the truck such as headache racks, custom built cabinets, etc.

Thanks!

 

[BrianE22]

It's just drive shaft torque going to the rear wheels of the cab causing unequal left/right vertical loads at those rear tires.

 

[BrianPetersen]

Dead simple. Engine output torque x gear ratio inside the transmission is the amount of torque that the engine mounts have to react against, and thus, that's the amount of torque reaction that has to pass through the frame between those engine mounts and be countered at the suspension attachment points.

 

[enginesrus]

Also shows how weak and flexible those frames are. Just my opinion, those class 8 trucks are dangerous, the rules do not allow them to be built correct and strong enough. I've spent time around them in years past.
Air brakes? Horrible idea, just have an airline break on an icy road.

 

[BrianPetersen]

Can you cite a single case of an accident being caused by a truck frame twisting and breaking (and which was not related to overloading, or decades of corrosion together with completely-absent safety inspections)?

And, there is sound reason behind the use of air brakes in those applications. Yes, they have failure modes not present with hydraulic brakes, but hydraulic brakes have failure modes not present with air brakes. How would you propose to make trailer brakes work properly otherwise, and in a way not subject to additional failure modes?

FMEA is real. Do it.

 

[CWB1]

Federal motor vehicle safety standards don't allow frames to be built safely? Ladder frame design is driven largely by the need to accommodate bodybuilders with modular design and many lengths and wheelbases, FMVSS has nothing to do with it.

As for air brakes, you clearly dont have much experience driving or working on vehicles. An air leak causes the brakes to be automatically applied without any loss of braking force, hence why they're required. A leaking hydraulic line causes a major loss in braking force, hence why they're not allowed on semis.

 

[enginesrus]

^^^
Airbrakes correct. Wouldn't that be fun automatically applied brakes on an icy road at almost any speed. Or the same in a sharp turn at a decent speed. I never like the idea of having the brakes suddenly apply at highway speeds. There are many work arounds for hydraulic brakes with a leaking line. The main reason for air is like in rail, they need a good way to have trailer brakes.

 

[TugboatEng]

In the marine industry we have both air and hydraulic brakes on our towing winches. From an operations standpoint air is far superior. When leaks do develop they announce themselves. There is an infinite reserve of make-up fluid available. They have more options for failure modes. For a towing winch you want it to fail in free spool and this is not an option with hydraulic brakes as they are all spring set.

 

[LionelHutz]

You'd have to be an idiot to drive a truck with an air leak until the brakes all locked up due to a loss of air pressure. You get lots of warning that there is a leak. A line won't spring a leak in a corner on an icy road and cause the brakes to immediately all lock up. That's just a completely stupid statement...

Also, there are separate service brakes and parking brakes. The leak would have to be on the parking brake system at a wheel to possible lock the brakes up on that wheel. Well, unless again you ignored the blaring low air warning buzzer and kept driving until the air pressure dropped enough to apply all the parking brakes.

 

[CWB1]

Even if a driver were to ignore a low-air warning on ice nothing particularly spectacular is likely to happen, most trucks are long enough and DOT inspections frequent and rigorous enough that you have to actively work to fishtail them. In the military we regularly pulled the parking brake knob to dump air on dirt and ice, both to test the slack adjusters and to amuse ourselves by terrifying new drivers.