Turning with a 4x4

[KRSmitty]

Has been close to 30 years since I had my last 4x4 truck. I sort of remember with my old 4x4 that while driving when I turned the wheels in 4x4 mode I would get this loping sensation (lack of a better term). I notice it also does it in my 2017 Ram. I am assuming that this is normal on 4x4's? What causes it?

Thanks

 

[Wiredawg]

Yes sir, the same technology...front CV joints, drive shafts are binding due to nothing to slip, usually a slippery road. That's why manufacturers say don't operate 4x4 on hard dry surfaces.

Cheers, Ron

 

[KRSmitty]

Thanks Ron...I do notice that it still does it even in dirt/mud etc... Guess it will always do it?

6 years USAF...Thanks for your service.

 

[Wiredawg]

Thank you too for your service... had a great time!

Cheers Ron

 

[KRSmitty]

Same here...still kick myself in the butt for not stating in for the full 20. But, money was to good on the outside for my AFSC.

 

[RVTRKN]

I thought that the reason for not running 4X4 on dry paved roads, is that the front axle has a slightly different rotation than the rear, and if ran to long that way it will cause extreme binding in the drive lines. As for the pulsation when in 4X4, I thought it is the u-joints in the axles between the axle and spindle (knuckles) binding when steering hard left or right under the torque of the axles.

 

[Wiredawg]

RVTRKN,
You said it better than I did.. a knuckle is like a CV joint.. the driveshafts include the u-joints, and me leaving out the axles was the most important omission.

Ron

 

[petersonj]

Yes sir, the same technology...front CV joints, drive shafts are binding due to nothing to slip,

I could be wrong here, but to my knowledge the front axles on newer trucks still do not use CV joints. Constant velocity u-joints (a double u-joint with a ball in the middle) do not bind when steering because the input and output ends of the u-joint are rotating at the same velocity. No wheel hop.

I believe that even the newest heavy duty 4x4 trucks are still using a single u-joint on each steering axle shaft. With a single u-joint the input velocity is constant, but the output velocity is not constant as the output of the u-joint accelerates and decelerates twice in one revolution when the u-joint is angled as in steering left or right.

The large diameter of the wheel and tire magnifies the two-times-per-revolution velocity change. Add the fact that each steering tire is rotating at a different speed during a turn, then you can understand where wheel hop comes from, especially when traction is excellent.

As far as driving on pavement in four wheel drive, binding will occur because there are no differential gears in the transfer case to accommodate the difference of front and rear axle speeds. For example, if an all-wheel-drive vehicle has three differentials and CV joints on the steering axles, there will be no binding between front and rear axles nor any wheel hopping under any steering conditions.

- John

 

[Wiredawg]

I'm glad there are folks on here who know what they are talking about. I learned the correct terms for our trucks drive components.

Thanks,

Ron

 

[petersonj]

You're welcome...

- John

 

[Ozymandias]

CV Joints have also downsides, at a steep angle with torque added they can burn up, even explode within seconds.

All to me known HD Vehicles use U-Joints in the axles - but better ones have double u-joints that do not jump.

 

[KATOOM]

CV joints are "constant velocity" joints. U-joints are not... Therefore u-joints require two exact opposing u-joints to cancel out each others harmonics. Watch the short video below to understand what I mean...

On a solid front axle 4wd truck u-joints are used at the axle ends where the tires are. The front driveshaft uses u-joints too. A single u-joint at the differential end and a double cardan joint at the transfer case end. The double cardan (DC joint) houses two u-joints and a ball/spring in between them. The DC joint cancels out its own harmonics allowing that third single u-joint to be run at 0*, or close to for no harmonics and optimal driveline angles.

CV joints are used on independent suspension 4wd front axles because they produce no harmonics and allow more angle for the shorter axle shaft. U-joints are still on the front driveshaft though...

That said, I understand that the new 4wd RAM's are implementing CV joints on the front driveshaft (not axles). Not completely sure why either other than they claim better reliability. I'm sure one reason it to eliminate the need for a driveshaft DC joint.

As for why a 4wd truck "crabs" while turning on solid surfaces... The most simplistic way to think about it is knowing that on a 4wd system (not an all wheel drive system) the front and rear axles are spinning at the same speeds. Driving straight everything is running smoothly but when the front axle is turning then the front tires are no longer traveling in the same path as the rear tires, which means that both axles are now trying to spin at different speeds. This fighting of which axle gets to spin at which speed it wants causes the tires to slip/hop as the front and rear axles arent spinning the same speed. On loose/slick surfaces that axle fight goes unnoticed but on solid dry surfaces the tires jumping around is very noticeable.

Also...its VERY hard on the drivetrain to drive a 4wd truck in 4wd on solid dry ground. The binding taking place will eventually eat up u-joints and stretch the transfer case chain. Do NOT run in 4wd on dry cement roads.

 

[Wiredawg]

KATOOM,
That was outstanding. Thanks for the education. Lots of science and engineering in making our trucks smooth and vibration free.

Cheers, Ron

 

[mfurrh55]

Year before last i had to run in 4 wheel drive for over 400 miles to help keep the dreaded death wobble under control, didn't hurt a thing, and i was on dry highway the whole time, ive put nearly 30,000 miles on it since

 

[petersonj]

Driving straight everything is running smoothly but when the front axle is turning then the front tires are no longer traveling in the same path as the rear tires, which means that both axles are now trying to spin at different speeds.

I agree with everything you are saying and the video clearly demonstrates the velocity changes that occur. However, I believe that the majority of the wheel hop is caused from the sharp angle of each front axle u-joint when the steering wheel is turned sharply because each tire will independently accelerate and decelerate twice for each axle revolution. The fellow in the video does not mention this, but you can hear the acceleration and deceleration twice per revolution in the cards, especially on the middle sprocket which could represent the front tire on a truck. If that sprocket was a 31 inch diameter tire with good traction, you could easily imagine the wheel hop that it could generate.

Anyway, I like your post and you picked an excellent video for demonstration.

- John

 

[Ozymandias]

Year before last i had to run in 4 wheel drive for over 400 miles to help keep the dreaded death wobble under control, didn't hurt a thing, and i was on dry highway the whole time, ive put nearly 30,000 miles on it since

As long as you're driving in a (almost) straight Highway there isn't a problem, the trouble starts doing narrow curves or worse turning on a parking lot.
On a straight road one can drive 100000M in 4x4 without any problem.

 

[mfurrh55]

As long as you're driving in a (almost) straight Highway there isn't a problem, the trouble starts doing narrow curves or worse turning on a parking lot.
On a straight road one can drive 100000M in 4x4 without any problem.

yeah i know Ozzy, especially when you get that wheel hop on them short turns, i found myself having to use it in 4 WD just to try to keep the wobble under control till i got home

 

[Ozymandias]

yeah i know Ozzy, especially when you get that wheel hop on them short turns, i found myself having to use it in 4 WD just to try to keep the wobble under control till i got home

I hate that wheel hope and wish for permanent 4x4 with Center Differential.
That would be a dream on narrow mountain tracks.

 

[GrantP]

yeah i know Ozzy, especially when you get that wheel hop on them short turns, i found myself having to use it in 4 WD just to try to keep the wobble under control till i got home

OK, I'll bite. How in the world does running in 4wd prevent death wobble? I'm struggling with the logic on that one.

 

[petersonj]

I hate that wheel hope and wish for permanent 4x4 with Center Differential.

A center differential will not make wheel hop go away. CV joints will. I had an old 4 wheel drive Subaru that could be manually shifted in and out of 4 wheel drive. There was no center differential and you could feel the strain on the drivetrain in sharp turns when traction was good, but since it had CV joints on all drive axles there was no wheel hop. A center differential would have made the strain on the drivetrain go away.

I would like to see CV joints on our front axles, but I don't think it will happen soon. They would have to be very heavy duty (which means costly) to handle the torque of the diesel compounded with the torque multiplication of a low range transfer case.

- John