why does the truck lift up

[zach103]

when a pickup truck or tractor trailer starts go from a slow speed or dead stop why does only the left front end lift up why not the right side

 

[TilliamWe]

Re: why does the truck lift up

Rotation of the engine.

 

[mla2ofus]

Re: why does the truck lift up

Rotation of the engine.

What he means is the torque of the engine is literally twisting the truck frame which makes the left front corner of the frame lift. If the engine rotation was opposite w/ the driveline set up to accomodate it, the right front corner would lift.
Mike

 

[Bifflefan]

Re: why does the truck lift up

What he means is the torque of the engine is literally twisting the truck frame which makes the left front corner of the frame lift. If the engine rotation was opposite w/ the driveline set up to accomodate it, the right front corner would lift.
Mike

Excelent Explaination.

 

[zach103]

Re: why does the truck lift up

oooo i see now thank you... so because cars and most suvs don't have as much torque as trucks the engine doesnt twist the frame enough to notice?

 

[TilliamWe]

Re: why does the truck lift up

I notice it on SUVS, which usually have the same HP as their pickup truck counterparts. But maybe their "one piece" bodies make it harder to tell? Where a pickup's cab seems to move independently of the bed?

Now, "most" cars are front wheel drive, and the motors are "sideways" in the engine compartment. When they accelerate their torque actually lifts the front of the car. Surely you have noticed this? If you haven't, pop the hood of almost any car, rev the engine, and watch it rock back towards the firewall.

 

[jonesg]

Re: why does the truck lift up

Whilst the torque is generated by the engine, its applied by the rear wheels.
The frame isn't twisting that much, its more the suspension relaxing.

If you see a truck spinning it wheels on ice, only one wheel will be turning.
In a rear wheel drive, touching the brakes will cause both wheels to engage and increase traction.
Learned that trick pushing a snow plow with a pickup.

 

[Tim Frank]

Re: why does the truck lift up

Whilst the torque is generated by the engine, its applied by the rear wheels.
The frame isn't twisting that much, its more the suspension relaxing.

The torque is actually applied through the drive shaft and Newton's Third Law aplies.
If you are backing up, you might get the front right lifting.

 

[dlindeblad]

Re: why does the truck lift up

I thought it was because the right tire was heavier therefor keeping contact with the pavement while the left tire lifted. Either that or helium in left tires vs. right ones get you better MPG.

 

[CN Spots]

Re: why does the truck lift up

That's some seriously relaxed suspension.

 

[Bob_VT]

Re: why does the truck lift up

Just to confuse the issue....... wait until you drive a High Performance front wheel drive and it torque steers into another lane!!

 

[jonesg]

Re: why does the truck lift up

look at the right rear tire, thats where the torque is being applied.
Engine generates it, driveshaft transmits it and it gets applied by the tires.
Most direct engine torque goes into motor mounts and dissipates as heat.
Its just physics.

 

[CN Spots]

Re: why does the truck lift up

Agreed, but, THAT frame is twisting.

 

[roscoe]

Re: why does the truck lift up

Its just physics.

understatement of a lifetime.

everything is "just physics", but 80% of the population doesn't understand it.

 

[justchange]

Re: why does the truck lift up

when a pickup truck or tractor trailer starts go from a slow speed or dead stop why does only the left front end lift up why not the right side

It's not the frame twisting on a semi. It's the torque of the engine making the front suspension raise. It tends to show more in the left front from engine rotation direction. Frankly, If you see a semi truck doing this, they're probably using far to much throttle and not shifting when they should.

 

[CN Spots]

Re: why does the truck lift up

This one bugged me enough that I called the Peterbilt service department here in Memphis this morning. I asked them if truck frames flexed under acceleration. His official response was "He11 yes they flex, they'd break in half if they didn't! That's why they're made from C-channel and not box beams"

Torque applied to the rear wheels that is not transferred to forward momentum or wheelspin remains in the drivetrain and has to go somewhere.

 

[dlngr]

Re: why does the truck lift up

[QUOTE Frankly, If you see a semi truck doing this, they're probably using far to much throttle and not shifting when they should.[/QUOTE]
Apparently,you've never driven an 18 wheeler.

 

[QC]

Re: why does the truck lift up

When you take 1750 lb ft. and apply it through a trans reduction of 12:1 the theoretical result is 1750 x 12 = 21000 lb ft. to the driveshaft and that is before rear end reduction of 3 - 4:1 . . . Yes, the frame flexs as that torque has to go somewhere and it tries to go everywhere it is allowed to, tires, motor mounts, frame flex etc. and hopefully moving the load if it doesn't start flopping around like a broken helicopter As the driver goes through the gears the torque applied to the driveline drops on each upshift until OD where it even drops below Fylwheel torque i.e. 1750 x .83 = 1450 lb/ft. All #s assuming full load (power) at peak torque RPM with an engine rated at 1750 peak torque . . .

 

[45Auto]

Re: why does the truck lift up

Most direct engine torque goes into motor mounts and dissipates as heat.
Its just physics.

Now that I would like to see!!! Those suckers would be glowing white hot! That's not physics, that would be magic!

It's not the frame twisting on a semi. It's the torque of the engine making the front suspension raise.

I can see how this could be. You could saw off all the motor mounts, then put a board across the frame to support the front of the engine. Then you run a chain from the lower front suspension arm to the top of the engine. Then when the engine torques around, it wraps the chain around itself and lifts the suspension! This way it never twists the frame until the suspension hits the upper stop.

In the real world, Newton's 3rd Law applies:

''To every action there is always an equal and opposite reaction: or the forces of two bodies on each other are always equal and are directed in opposite directions''

This was first published on July 5, 1687, so it's only been around about 332 years.

If a torque is exerted on the driveshaft of 21,000 lb ft, using QC's example, then the engine is trying to rotate in the opposite direction with a torque of 21,000 lb ft. Since the engine is bolted to the frame, the frame has to resist this 21,000 lb ft. If the frame rails are 6 feet apart, this is the same as pushing down on one frame rail with 3,500 lbs while simultaneously lifting up on the opposite frame rail with 3,500 lbs. So the frame twists, raising the suspension on one side. This is the same reason the frame is twisted on the picture of the dragster doing a wheelie, and also the reason the right rear tire is planted harder than the left rear tire. There's a big down-load on the right frame rail, and an up-load on the left frame rail.

 

[rbh]

Re: why does the truck lift up

yup.