Why turn 4wd off?

[scrappy isb67]

Unless your truck has a custom diff or it came with lockers as an option it's unlikely to have a locking diff.

The Dodge/Ram Powerwagon has electrically locking diffs front and rear from the factory.

That's really interesting. I knew that the tires hop when you're in 4wd, and I knew that it was much worse when doing tight turns. I always assumed it was a locked diff between the left/right wheel on the axle. I never thought that the axles didn't have locking diffs and it was just the center diff between the front and rear axles that was locked.

unless your truck is full time or AWD there is no center diff. that is why they are not supposed to be driven in 4wd on pavement.

 

[joshuabardwell]

unless your truck is full time or AWD there is no center diff. that is why they are not supposed to be driven in 4wd on pavement.

Right. Because if there was a diff, the wheels wouldn't skip. It's the lack of a diff that makes them skip.

Maybe I just need to go back to 4wd 101 and remind myself how the whole thing works :laughing:

 

[MHarryE]

Many axles use a limited slip diff to limit torque that can be sent to one wheel. For example if you have one tire on ice and the other on dry concrete, you could potentially send 100% of available torque to the tire with traction. This might be good for you, but not for the axle which is designed to send 50% to each tire. Gleason Gear Works developed the Torsen differential, a combination of TORque SENse, that will limit the amount of torque to about 70 - 80% of available torque before it stops transmitting torque to the wheel with traction. Detroit also developed their Trutrac differential for customers who were using Detroit Lockers (100% locking) and breaking axle shafts, gears, etc. I am sure there are others but these are 2 with which I am familiar. The product for which I was responsible used a Detroit Locker in its New Holland axle and we started blowing planetary reducers as we asked more from the axle. We ended up changing to the Trutrac until we could get a stronger axle for the next generation. All of the machines I have driven with a diff lock have taken a good deal of effort to keep them locked because the axle will not last long with the potential of one wheel transmitting all the torque.

If you are doing something like pulling a plow at 5 mph, the transmission will see the same load whether you are in 2wd or 4wd. The rear axle, however, will love you for being in 4wd because part of the torque to the ground is transmitted by the front axle so although the transmission sees the same load, the axle load is reduced. In addition your ground speed will generally increased due to reduced tire slippage. I can really notice this on my cousin's 150 HP JD which has a 3 position rocker switch - 4wd off, on, or auto (senses wheel slip and automatically engages the fronts as needed). Pulling a 30 foot cultivator, I can notice it speed up when I switch from 2wd to 4wd and I assume it is mostly due to reduced slippage. However, I also see a slight bump up in engine speed like it is easier for the tractor to pull the load. My first thought would have been that I would see more power transmission loss due to transmitting power through additional gears. The auto position on that tractor is nice for people who forget to turn 4wd off when the hit the asphalt, but I found manual 4wd is better all the time in the field except for baling where traction isn't so important.

 

[tungularafishcamp]

The only pavement my tractor sees is the 1 1/2 foot strip on the threshold of the gear shed so I always leave it in 4 wheel drive EXCEPT

when ripping up the tundra with the bucket to clear a pad with the front bucket I bent both tierods because of front wheels spinning and then engauging as I lifted the load. Didnt realize how I did it to the first tie rod, thought I hit a rock:ashamed:but saw it happen the second time when I broke the tierod on the other side:ashamed::ashamed: Now when I am digging hard I put it in 2wd while digging into the tundra, dont know if I would have had the problem if I didnt have duals up front.
Rick

 

[namesray]

Many axles use a limited slip diff to limit torque that can be sent to one wheel. For example if you have one tire on ice and the other on dry concrete, you could potentially send 100% of available torque to the tire with traction. This might be good for you, but not for the axle which is designed to send 50% to each tire. Gleason Gear Works developed the Torsen differential, a combination of TORque SENse, that will limit the amount of torque to about 70 - 80% of available torque before it stops transmitting torque to the wheel with traction. Detroit also developed their Trutrac differential for customers who were using Detroit Lockers (100% locking) and breaking axle shafts, gears, etc. I am sure there are others but these are 2 with which I am familiar. The product for which I was responsible used a Detroit Locker in its New Holland axle and we started blowing planetary reducers as we asked more from the axle. We ended up changing to the Trutrac until we could get a stronger axle for the next generation. All of the machines I have driven with a diff lock have taken a good deal of effort to keep them locked because the axle will not last long with the potential of one wheel transmitting all the torque.

If you are doing something like pulling a plow at 5 mph, the transmission will see the same load whether you are in 2wd or 4wd. The rear axle, however, will love you for being in 4wd because part of the torque to the ground is transmitted by the front axle so although the transmission sees the same load, the axle load is reduced. In addition your ground speed will generally increased due to reduced tire slippage. I can really notice this on my cousin's 150 HP JD which has a 3 position rocker switch - 4wd off, on, or auto (senses wheel slip and automatically engages the fronts as needed). Pulling a 30 foot cultivator, I can notice it speed up when I switch from 2wd to 4wd and I assume it is mostly due to reduced slippage. However, I also see a slight bump up in engine speed like it is easier for the tractor to pull the load. My first thought would have been that I would see more power transmission loss due to transmitting power through additional gears. The auto position on that tractor is nice for people who forget to turn 4wd off when the hit the asphalt, but I found manual 4wd is better all the time in the field except for baling where traction isn't so important.

thank you. that mostly answers my one question that i posted earlier in this thread. explained well.

 

[SPYDERLK]

I also see a slight bump up in engine speed like it is easier for the tractor to pull the load.[/B] My first thought would have been that I would see more power transmission loss due to transmitting power through additional gears.

Reasonable assumption, but more than made up for ... It is easier to pull the load in 4wd on soft ground or, correspondingly, over obstacles. This is because the fronts "climb thru" rather than push.
larry