Two wheel drive vs 4 wheel drive (Torque, traction?) 1

[TrustButVerify]

Hello folks,

So, just imagen you have two commercial trucks, one with 2-wheel-drive and another with 4-wheel-drive, and their WEIGTH is the same.

Great, they are being manufactured to be able to be driven on a flat surface, both of them.. Like driving on the road.

Now, this is the question... Imagine you start scrapping both trucks the same way, I mean, you start to take weight off the truck, like reducing their self-weight...

Can you find the situation that the 4-wheel-drive truck will NOT go forward due to lack of weight on each axle (and thus less friction), but to start SLIDING due to much torque?

The 2-wheel drive truck will be able to go forward since the two wheels need to overcome their own resistance, plus the resistance of the other two wheels?

Where I can read about this?

Have a nice weekend.

 

[IRstuff]

Trust -

Pictures of a toy model is not useful. A video of the actual slipping would be useful; anything short of that just leaves too much room for speculation

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[3DDave]

Guy calls a mechanic. Says the car stopped working. Mechanic asks if anything happened. Guy say yes. Mechanic, now irritated, asks what. Guy says he hit a small animal. Mechanic says that's odd, asks, did it do any damage? Guy says, didn't leave a mark, but car definitely doesn't run. Mechanic asks if the guy can have it towed. Guy says maybe. Mechanic, getting angry, asks why. Guy says park police are involved. Mechanic asks if the animal was endangered and protected. Guy says he thinks it wasn't but he doesn't know much about fish.

 

[IRstuff]

Are ALL the wheels spinning, or only some? If the latter, which ones?

Another possibility is that the transport bed/frame is warped and not all the tires are at the same vertical position, so the ones that are higher might slip because they're not actually loaded at all

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[3DDave]

"Another possibility is that the transport bed/frame is warped and not all the tires are at the same vertical position, so the ones that are higher might slip because they're not actually loaded at all"

This cannot normally happen on an actual Scheuerle system as the height is managed by groups of mutually plumbed hydraulic cylinders. This is a major feature to guarantee the ground contact is uniform. On our unit the only way to stop it is by using chain to hold the single-acting suspension cylinder in a collapsed position. There is no reason to expect any Scheuerle units would be different.

 

[MintJulep]

I saw a pig fly once. Here's proof.

 

[BrianPetersen]

I figured out 3DDave's puzzle. (Nice.) I haven't figured out the original poster's puzzle. The rest of us are missing some critical piece of information.

It is well known in this neck of the woods that in the first winter snow, the first vehicle spotted in the ditch will be a 4x4 of some sort. Happens every year. (having all wheels driven may keep it going better, but doesn't help it stop or turn any better, and doesn't make up for not having winter tires)

4 wheel drive is a good way to get 100 metres further into trouble. (usually down a snowed-in path that you have no business driving into)

 

[3DDave]

At the risk the OP is describing some other thing, the typical power pack weighs 7.6 metric tons and has a length from the main pins of 4.6 meters. This cantilevers off the end of the platform. See

https://youtu.be/2NScGEWVDEs?t=57

The 8 axle (4 axle "lines", though it can be just 2 axle lines when going directly sideways) unit is 5.6 meters long, overall, and has an empty weight of 17.3 metric tons.

https://youtu.be/nxqO726h-DI?t=68

Recall the typical suspension links pairs of axles on th long side so the effective reaction is halfway between. This makes the wheel base less than expected; the spacing is just 1.4 m between suspension units.

Calculation of where the CG ends up when empty will be an exercise.

 

[IRstuff]

This is a major feature to guarantee the ground contact is uniform. On our unit the only way to stop it is by using chain to hold the single-acting suspension cylinder in a collapsed position. There is no reason to expect any Scheuerle units would be different.

Given what the OP has been posting, I would suspect that pigs indeed are flying and one or more of the suspension cylinders are bone dry and stuck in an up position, and the extra weight of the cargo is squashing tires until all tires are in contact with the ground.

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[TrustButVerify]

IRstuff.. Unfortunately, I do not have a video of it.

 

[cibachrome]

4wd always wins because it doesn't have to fight the brake friction and tire rolling resistance of the un-driven wheels. All wheels driven means all available tractive force goes to propulsion. Same with boats with multiple propellers, aircraft with feathered props, and centipedes with a few inop legs. Audi & Subaru claimed better fuel economy with 4wd in spite of the transmission losses.

I have a John Deere tractor with hydraulic Traction Assist. This allows you to run substantially higher slip with modulation as necessary. I don't leave home without it.

And 4w BRAKES are better than 2w brakes (Just ask my golf carts: 1 does, the other does not). Tractor agrees, too ..

 

[3DDave]

It's not a truck. It is a motorized pallet with 4 wheels/2 axles near the CG and a power pack hanging off one side to move the CG over just 2 of the drive wheels; it also is apparently plumbed to have an effective open differential in 4WD, so the unloaded wheels spin.

Not sure why it is a mystery that a not-a-truck doesn't behave like a truck, but it is a mystery that so many false clues were used to describe the situation.

 

[gruntguru]

4wd always wins . . .

Except 4WD with full differential action where loss of traction on one wheel = loss of drive to all wheels. The same vehicle with 2WD and a non-driven wheel in the mud hole will have greater drive to its two wheels.

je suis charlie

 

[TugboatEng]

A differential shares torque evenly between the wheels. When a wheel slips that wheel becomes the torque limit. The slipping wheel gets 100% of the power because while the one wheel is slipping the others are stationary and force x 0 speed = 0 power.

 

[GregLocock]

And that's what happens with a hydraulic drive if the designer just feeds all 4 wheels from the same reservoir. All the flow goes to the spinning wheel. Which is why nobody who has done it before designs a system like that.

Cheers

Greg Locock


New here? Try reading these, they might help FAQ731-376

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[TrustButVerify]

cibachrome, 4WD will always win? Even when the surface has a pretty low friction coefficient?

 

[TugboatEng]

Your tractive effort is proportional weight on the driven wheels.

Example: 1000 lb 4 wheeled vehicle with even weight distribution between all wheels. That's 250lbs per wheel.

If 2 wheels are driven your traction effort is 2 x 250 x CoF 500*CoF

If 4 wheels driven your traction effort is 4 x 250 x CoF = 1000*CoF

If the wheels are driving and steering the traction effort will be shared between the two actions. If the steering wheels are driving at near 100% effort they will not be able to steer the vehicle and it will continue to be pushed forward.

 

[LittleInch]

Tug, you need to correct your sums - 2 x 250 = 500, not 100?

Remember - More details = better answers
Also: If you get a response it's polite to respond to it.

 

[3DDave]

As described, the original problem isn't a uniformly loaded 4WD exposed to low friction - it is caused by placing the CG over only two drive wheels, leaving two wheels with no load on them, but not changing the flow of fluid to match the load case, until it is moved to only supply flow to the wheels with weight on them. If the flow was diverted to 2WD mode with the other two wheels it would also not move.

 

[TugboatEng]

Fixed.

3D, you are correct. Let's bias the load 90/10. That would give 900*CoF at the rear and 100*CoF at the front. If the front and rear are driven through a differential or by hydraulic motors in parallel the tractive force must be equal between front and rear. The lowest force is the limit. So both axles will have 100*CoF each or 200*CoF total. A 2wd vehicle would have 900*CoF. In this case, the 2wd would be superior.

For an automotive comparison there are 3 basic types of 4wd. There is all wheel drive, full time 4wd, and part-time 4wd.

All wheel drive has an open differential coupling the axles. It does not work well off-road for the reason above. Some AWD vehicles have viscous clutch in the differential to transmit some power to the other axle if a wheel starts slipping.

Full time 4wd is the same as AWD except thee differential is lockable. This ensures an even power split between axles.

The part-time 4wd has no differential, only a dog clutch to engage the other axle. This system also splits lower evenly but causes some problems at high speeds on surfaces with high CoF.

 

[cibachrome]

All the TEST DATA from traction trailers I have collected thruout the years indicates that surface Mu goes DOWN with increasing Fz, except for 1 type of surface: Dry concrete.

In God we trust, all others bring data....