FWD vs RWD for snow and ice

[Isaac Zachary]

Originally Posted by cRiPpLe_rOoStEr

Makes me wonder to which extent a traditional 4WD truck would behave in the snow with the high-range 4WD engaged according to the load as the weight bias would be shifting.






I'd take with a grain of salt the viability of repurposing a transfer case into some sort of power-split device.




I'd rather not rely on a clutch-operated transfer case. Viscous coupling is more reliable.

To clear up how the Astro and other GM transfer cases work from that era, it's what you could think of as a viscous clutch. There are several friction plates in the transfer case that can be squeezed together at various levels by what I do believe is a type of stepper motor.The plates are in the case in a special viscous lubricant along with the gearing and chain drive. When in 2WD or 4Lo/4Hi modes the clutch pack is either completely sqeezed or not. When in Auto mode (this is the only mode for the Astro) when rear wheel slip is detected the stepper motor sqeezes the viscous clutch pack just enough to virtually eliminate the wheel spin. That way the front axle isn't completely locked to the rear axle.

Anyhow, as far as what locked front to rear axle 4WD does on snow and ice goes: it can make understeer and oversteer even worse in some situations.

One time my brother in his GMC Sonoma needed to turn left. But there was a berm of snow in the middle of the street the snow plows had made. Where he was going lots of other cars had already worn down the berm so it was passable. However he thought it still would be a good idea to put it in 4WD and get a run at it. That made the truck understeer and he ended up stuck on the high part of the berm with all four wheels of the ground.

I've also noticed when decelerating that 4WD can cause oversteer as the weight comes up off the rear wheels and onto the front wheels while turning. I noticed the Dodge Durango I had would oversteer more at times with 4WD engaged. Basically it would understeer as I pulled out onto the highway, for an example. And so then I'd let off the accelerator and that would make it snap into an oversteer. It would react much the opposite in 2WD, oversteering when accelerating and then understeer when letting off of the accelerator. But it was the one vehicle I didn't get snow tires for, so that was a large part of it.

For the most part 4WD (or even AWD) is only helpful when driving slow. That being said, carrying around a snowplow on the front of a pickup with nearly no weight in the back makes pickups oversteer from wheel torque quiet terribly. So it becomes imperative to put it in 4WD even when not plowing snow.

[JSH]

Originally Posted by Isaac Zachary

I'd be interested in how that would work. The Astro's "AWD" transfer case is basically the same clutch operated transfer case that the Chevy/GM trucks use only without low range and manual 4x4 mode. It's just "stuck" in auto mode.

When I had my 2001 Chevy Astro I dreamed of installing an S-10's or Sierra's transfer case and hooking up the controls so it could work as a fully controllable low range 4x4 transfer case. Seems like this has been done quite a few times in the Astro/Safari community.

It would work the same as a belt mounted hybrid motor but instead of feeding power into the front of the crankshaft through the accessory drive pulley the motor would feed power into the transmission. Controls would be the same.

Downside vs belt mounted is that the motor couldn't be used as a starter.
Positive is that you could put the transmission in neutral and drive at low speed in EV mode without spinning the engine.

Yes, some people convert the Astro to 4wd using S-10 parts. I haven't really been interested in 4wd but a selective rear locker like an air locker would be nice.

Realistically I have never been stuck in the Astro even with the one wheel drive open rear differential. I've never even needed to chain up although we don't do winter camping with it so no snowy passes.

[cRiPpLe_rOoStEr]

Originally Posted by Isaac Zachary

For the most part 4WD (or even AWD) is only helpful when driving slow.

So, considering there would be no heavy off-roading involved, would you rather do it the old-school way and resort to a small amount of ballast weight closer to the rear-end of a RWD truck?

[Isaac Zachary]

Originally Posted by cRiPpLe_rOoStEr

So, considering there would be no heavy off-roading involved, would you rather do it the old-school way and resort to a small amount of ballast weight closer to the rear-end of a RWD truck?

AWD/4WD makes sense if you're hauling a trailer, you plow snow or live where snow removal is non-existent. Or like you said, it's good for true off-roading.. Other than that it's usually overkill.

RWD with more weight in the rear has advantages, but mostly for acceleration and very well thought out cornering.

FWD is better than RWD when going very slow or stuck. It also doesn't require as much thinking in an emergency.

FWD is much more predictable. Hit a corner too fast a and you go straight regardless of the road condition.

In rear-biased vehicles it's much different to hit a corner too fast that's wet, than one that's loose gravel, than one that's snow packed, than one that's covered in ice.

[cRiPpLe_rOoStEr]

Originally Posted by Isaac Zachary

Or like you said, it's good for true off-roading.. Other than that it's usually overkill.

Considering what either a Beetle or a Citroën 2CV and all those Jeep-like utility vehicles derived from them could do, 4WD is most likely overkill to an overwhelming amount of ordinary drivers even through some harsh terrain conditions.

FWD is better than RWD when going very slow or stuck.

No wonder beauties like these became so widely used even among rural users, from small farmers to tractor maintenance providers, even though they're only FWD.

[Isaac Zachary]

There were the VW trucks based off of the Golf that were FWD too.

[Autobahnschleicher]

Over-/understeer is not just about the weight ballance of the vehicle.
If it was, my car would oversteer all the time, yet it normaly understeers slightly.
And no, FWD isn't safe from oversteer either, many FWD cars are prone to liftoff-oversteer.
Especialy ones with twist-beam rear suspension.

[freebeard]

I gave it my best shot at Permalink #36.

[cRiPpLe_rOoStEr]

Originally Posted by Isaac Zachary

There were the VW trucks based off of the Golf that were FWD too.

There is still a similar product in Brazil, yet it's not Golf-based. It's maybe the 2nd trucklet I see more often being used for maintenance of telecom structures.

[freebeard]

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