four wheels good, two wheels bad?

November 20, 2013 10:42 PM

What's wrong with running my 4WD with the front wheels always engaged?

Like many 4WDs, I can engage all four wheels by choice, or I can have the front two disengaged, via a button on the dash. Why should I have it set in 2WD, what damage could be caused by running the vehicle in 4WD all the time? At highway speeds, 100 kph.

posted by wilful to Travel & Transportation (8 answers total)

Best answer: If there's no center differential in the transfer case, the four wheels will not be able to turn at different speeds. You'll notice why this is a problem if you attempt to turn a tight circle on dry pavement with 4wd engaged. The problems are increased wear and fuel consumption, and decreased safety.

This is not a problem for AWD vehicles.
posted by klanawa at 10:51 PM on November 20, 2013

Best answer: To expand on klanawa's correct comment, the problem is that if you make a turn, the front wheels make a larger circle than the back wheels. (You can see this easily if you look drive through a puddle or in the sand and look at the tracks.) If you are in 2wd, no big deal -- the back wheels are connected to the engine, and the front wheels can free spin. But in 4wd (with no center differential, which is the case with pretty much every part-time 4wd system) the front wheels are forced to turn at the same speed as the back wheels. So in a curve on pavement, the front wheels want to turn faster and the back wheels want to turn slower, and all that stress is not good. Either something slips (usually a tire), or something breaks (usually an expensive part).
posted by Dip Flash at 11:45 PM on November 20, 2013

Best answer: Even if there is a diff between front and back I believe you may notice that you get poorer fuel economy in 4WD.

If you don't have a centre diff you aren't going to enjoy the experience at all, as outlined above.

What is the car model?
posted by deadwax at 12:58 AM on November 21, 2013

Response by poster: why worse fuel economy? parasitic load? it's a ssangyong actyon 2008.
posted by wilful at 1:00 AM on November 21, 2013

Best answer: Apart form the extra scrub on the tyres from forcing all of them to rotate at the same speed, if you put load through a transmission it sucks power. This is why 'at the wheels' power figures are different from 'at the flywheel'. So the fewer transmissions/drive systems you run power through the less power loss there is in the system.

So yes, parasitic losses is one way of thinking about it. It's significant, too, as a rear wheel drive typically loses 20-25% of its power by the time it is transmitted to the wheels.
posted by Brockles at 5:09 AM on November 21, 2013

To answer the highway speeds question - most transfer cases hold less oil than a transmission and don't have any additional cooling. At highway speeds, the transfer case will get warm enough to cook the oil.

And really, if conditions are good enough to go highway speeds, you don't need 4WD for traction.
posted by Pogo_Fuzzybutt at 6:28 AM on November 21, 2013

To answer the highway speeds question - most transfer cases hold less oil than a transmission and don't have any additional cooling. At highway speeds, the transfer case will get warm enough to cook the oil.

This is going to vary by vehicle. Modern switch-on-the-fly 4wd systems allow operation up to 60mph (100kph) or more, though there may be exceptions. And I'm pretty sure that the transfer case is still spinning in 2wd, though of course the front driveshaft is disconnected -- the transfer case is part of the drivetrain regardless of whether you have 2wd, 4wd, or 4wd-low selected.

And really, if conditions are good enough to go highway speeds, you don't need 4WD for traction.

I agree, though I've always thought of it the other way around: If conditions are bad enough to need 4wd, you shouldn't be driving that fast.
posted by Dip Flash at 7:17 AM on November 21, 2013

Response by poster: thanks everyone.
posted by wilful at 1:58 PM on November 21, 2013