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Yeah, 4WD should never be used on dry pavement. It to be used on slippery surface only. All four wheels are rotating at same RPMs so when turning on dry, one side starts trying to skid to compensate. Kills transfer case. On slippery surface wheels simply slip on the turn outer side.
This is very true, but so as to not panic the OP, the GM Auto mode is separate from the 4WD mode, and can be used on dry pavement, as in normal operation in still powers the rear wheels only. Isn't as good as the full time 4wd-HI mode in bad conditions if that is what is needed, because there is a slight delay until the traction loss is detected in the rear and the fronts engage. It is a decent "set and forget mode" if that's what his sister wants to use and only has that little fuel mileage penalty.
And then there's AWD, which you often cannot turn off...
Quote:
Originally Posted by ukrkoz
Yeah, 4WD should never be used on dry pavement. It to be used on slippery surface only. All four wheels are rotating at same RPMs so when turning on dry, one side starts trying to skid to compensate. Kills transfer case. On slippery surface wheels simply slip on the turn outer side.
There are two different kinds of "four wheel drive" systems, for example Jeep's NP242 transfer case has settings inclueding "4HI" (locked differentials, for slippery surfaces only) and 4 FT (full-time) which is fine on dry surfaces.
Yeah, 4WD should never be used on dry pavement. It to be used on slippery surface only. All four wheels are rotating at same RPMs so when turning on dry, one side starts trying to skid to compensate. Kills transfer case. On slippery surface wheels simply slip on the turn outer side.
That hasn't been true for 20 years. There is a viscous coupling in the front drive that allows limited slip between front and rear. Plus, of course, the differential allows right and left wheels to rotate at different RPMs. Electronic traction control will apply breaking to any wheel that starts to slip, just like brakes on a tractor. This transfers more power to the wheels with traction. Some e-trac systems also adjust the suspension to transfer weight to the slipping wheel.
That hasn't been true for 20 years. There is a viscous coupling in the front drive that allows limited slip between front and rear. Plus, of course, the differential allows right and left wheels to rotate at different RPMs. Electronic traction control will apply breaking to any wheel that starts to slip, just like brakes on a tractor. This transfers more power to the wheels with traction. Some e-trac systems also adjust the suspension to transfer weight to the slipping wheel.
It is still true depending on the type of 4WD system being used. Unfortunately people interchange AWD and 4WD all the time, including car manufacturers. What you said is true for AWD and "full time" 4WD systems like mentioned in the OP.
But true 4WD (4-Hi or 4-Lo) is still not meant to be driven on dry pavement.
I think she is better off in 2wd and if she gets nervous say in rain and sleet go ahead and put it into the Auto4wd and if the vehicle senses slippage the power will be applied where needed. When the snow starts to pile up and she is still far from home put it into 4wd.
The Envoy never got good mileage due to its heavy truck base but hers seems to really suck the petrol. I was hoping the auto 4wd would be more of an reason for the poor mileage.
I have owned 4x4 for years and I remember having to get out and lock the hubs. today the systems are so easy, they are either automatic which is fine for 95 % of drivers since they never go off road or they are as easy as pushing a button or turning a knob.
My Ford truck has a knob to turn. My JeeP wrangler still has a lever to the transfer case.
4wd on dry pavement and even wet will chatter so don't turn too hard.
Who actually uses 4 low unless they are off road rock climbing.
I just thought of something 4x4 is like having a gun when you need it you really need it and are happy you have it.
I'm not too familiar with these newer AWD systems as pretty much everything I've ever owned has been the traditional 4 wheel drives, with the exception of my first car, a 76 Jeep Cherokee with quadrotrac full time and an "emergency drive" dial that would lock the transfer case like a standard 4by.
My current RAM and my previous Wrangler has/had auto hubs which would engage when put in 4hi. My previous brooncos had old school locking hubs. When driving around in those vehicles with my hubs locked my MPG's dropped off pretty good - and they weren't much to begin with. Even though everything was free from the transfer case, the front axle and shafts were still rotatating, causing drag.
Perhaps that is what is happening on your vehicle. I'm sure some components would be connected and causing drag if in auto mode so the front is at the ready to connect when needed. It doesn't take much extra drag to cause a drop in MPG's and when your MPG's aren't high to begin with 10 or 20% can be a lot.
Yea, standard, traditional 4bys do chatter on hard pavement because the rear is pushing the front as the front is pulling the rear (rear pushing straight and front pulling to left or right) It doesn't take much looseness in terrain to allow a tad bit of slip for all to be good. Full time systems suc as the old quadratrac have transfer cases that disconnect on turns (think of a limited slip differential basically). And I guess the AWD systems may sense slippage anywhere to give power to a non slipping wheel, good for rain and snow and it seems the fancier systems are for on street performance as well.
Low is something good to have but for the majority is seldom used. It iwas great on a trooper I had in Costa Rica because it let me go down steep, muddy grades with still having control as the truck only went a couple MPH. Likewise when climbing I could have the engine in it's powerband without spinning a tire as it just crawled along.
Low range also works good for pulling boats out of the water so it has it's purposes! Even used it to pull my brother out of a ditch he was stuck in pretty bad (high centered and all) My truck on the street in low hooked up to his, I never touched the gas as it just eased. I'd never own a 4x4 without it.
4L is great for off roading, but most people who drive 4x4s never get off of pavement, or at the worst a gravel road. For real cross country driving, you rarely go faster than 5 mph. If you can go faster than that, you didn't need 4wd in the first place.
With free hubs, the difference in mileage between a 2wd and a 4wd being operated in 2wd is mostly due to the several hundreds of pounds that a transfer case, front drive shaft, differential and axles add to the vehicle.
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And then there's AWD, which you often cannot turn off...
