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]

Wheels start to spin (not moving forward or backward)

Seems pretty clear to me; the controls are either not sufficiently fine-grained, or the operators are all lead-footed, or tires are just bald

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[LittleInch]

Trust...

Your situation is a bit odd with for individually driven wheels with no known interaction between them or apparant anti slip control.

In the vast majority of cases, four driven wheels will be better in terms of grip and traction than 2 as torque / force on each wheel is halved if all are connected as a single entity (locked differentials all round)

You seem to have found a peculiar situation which is poorly described. There may be some sort of interaction between each wheel when powered individually. Two wheel drive might be limited in torque compared to 4. Who knows? We certainly don't and it doesn't look like you do either I'm afraid

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[gruntguru]

"Vast majority of cases" = all cases.

je suis charlie

 

[TrustButVerify]

Hey pals,

Attaching here a real picture of the equipment (maybe makes it easier for me to explain myself?).

Wheels surrounded by a yellow circle are driven wheels, they have inside them a driven engine, the gears are driven by oil being pumped at a certain pressure from an oil reservoir built-in.

This equipment has such amount of driven engines to transport massive load above their platform (15 times their self weight) when a lot of traction is needed to overcome steep slopes.

In flat surfaces, with low friction coeffcient, and the equipment being empty loaded we have seen you need to turn off half of the driven engines in order for the equipment to be able to move (linear motion, steering, whatever).. if you don't turn off half of them, wheels will start to spin in the same point, and the equipment is not able to move in linear motion.

Hope this is a better explanation.

 

[LittleInch]

It might be if anything was actually attached....

Or use the image button to stop everyone downloading it. Please

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Also: If you get a response it's polite to respond to it.

 

[LittleInch]

My only guess her is that the wheels, being independent, are simply not acting in unison. So when fully loaded, this difference in speed or oil flow gets transferred to torque and with the higher load doesn't slip.

But when very light, any small difference in oil flow or the speed will cause the tyres to spin when 4 are working.

When only 2 are working the impact is eliminated and can be counteracted by steering or reducing speed on one wheel. Trying to do that with four wheels is clearly beyond the capabilities of the control system or driver.

So nothing to do with 2 vs 4 wheel drive and everything to do with this very strange system and its control system which has not been described. IMHO.

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Also: If you get a response it's polite to respond to it.

 

[TrustButVerify]

Don;t know what is not attached to previous reply. Here is second try...

 

[LittleInch]

That looks like a picture of a toy version, but this is the proper thing - an SPMT.
Anyone else just look up mammoet spmt

The control on those things are just incredible.

The videos show no slipping even when not loaded. e.g.

https://www.youtube.com/watch?v=I3paAUAcLVE

So the issue is really about how the control system works when the unit is not loaded. Clearly its anti slip system wasn't working on the one you saw.

Nothing to do with 4WD vs 2WD.



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

 

[TrustButVerify]

That's why I didn't want to share the real thing, because you will google up and get the wrong assumption.

Of course, you cannot see slipping when not loaded, because you are not looking at the specific situation I'm describing.

The very situation I'm speaking of happens when you have a single 4 axle spmt (with double engine driven). It does not happen with trailers with 5, 6 or 7 axles, or any configuration from 4, 5, 6 o 7 axles together.

It just happens when you a single 4 axle SPMT (with double, than standed, engine driven).

 

[TrustButVerify]

Here you can see the difference from 4 axle with standard driven engine, and 4 axle with double driven engine.

 

[3DDave]

Edit because my post was a bit behind - seems like that happens when incomplete posts happen so rapidly.

You are having trouble with a cheap Chinese knock-off of the original German design which my company paid to license? No kidding.

Same as before - figuring out the reason requires a copy of the hydraulic schematic.

---

I expected it to be larger.

I was on the design team for this (not the planes):

On the OP vehicle, if the model matches, the suspension puts equal loads on connected cylinders - probably the OP vehicle ties each corner pair for stability. It means the load on the drive wheels is half the total load, acting effectively between the pairs. Since the load is distributed, the 4WD configuration sees only 1/2 the download and 2WD sees only 1/4th om the driving wheels.

Our vehicle beat the competition largely because a snowfall on the airbase used for testing had only one transport vehicle able to move on the slippery surface - ours - with 4 of 10 axles being powered. I don't recall the plumbing but I doubt they were simply in parallel, allowing one wheel to slip and stop it from moving would have spoiled it.

 

[GregLocock]

There may be a clue in the hydraulic drive. If all the motors see the same pressure then it is analogous to open diffs on a 4wd ie equal torque to each wheel. But I doubt the controller is as unsophisticated as that.

Cheers

Greg Locock


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[LittleInch]

But I doubt the controller is as unsophisticated as that

We don't know, Trust doesn't know, We don't even know how many wheels are powered on how many axles.

This thread is a waste of time.

I'm checking out.

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

 

[CWB1]

Seems pretty clear to me; the controls are either not sufficiently fine-grained, or the operators are all lead-footed, or tires are just bald

Not necessarily. Engine torque at idle can be enough to spin the tires on a slippery surface without additional throttle input.

Winter roads and parking lots in truly northern climates like Alaska, Canada, Russia, etc are almost-frictionless glare-ice every winter. Extreme temps prevent snow-removal via salt/chemicals, and the heat of vehicle traffic causes the top layer to melt and quickly refreeze. When I lived in AK I saw many folks get stuck at a red-light or in a flat parking lot, often drivers would hop out with the tires still spinning/idling to push.

 

[MintJulep]

I'd expect a vehicle with a 15:1 loaded:empty ratio to have an automatic system to limit torque in proportion to load.

Maybe yours are broken.

 

[TrustButVerify]

3DDave. No, not chinesse. The picture was just for reference. The equipment is german (Scheuerle/Nicolas/Kamag).

I will try to find the hydraulic schematics, but does it really make a difference to explain this situation? I mean, oil is pump through a pipe below the deck platform, and that pipe feeds all the driven engines at the same time.

How can you turn off one driven engine? Just closing a valve, so not more oil can be feed to that wheel. Each leg/bogie has their own set of valves. So by turning off one driven engine, you are not turning off the other ones.

As I said before, I've seen this situation several times, in different locations.. It's not related to something specific as somebody thought before when asking too many questions.

It is just like that.. this equipment is design with standard amount of driven engines, and double than standard driven engines. Why double? Sometimes you need more traction to transport heavier loads in steep slopes.. What is the downside? Well the situation I'm describing with just this single equipment.. When the 4 axles is empty loaded in a low coefficient friction surface, the driven wheels start to spin instead of pushing the trailer in linear motion, that means, the trailer does not move at all, in any direction.

So, is this a issue of TOO much torque, and few weight (thus friction), causing the wheels to spin instead of pushing forward (not enough traction)?

 

[TrustButVerify]

I'm just trying to find here the reasoning behind a situation we have seen several times, in several locations, with several trailers (same design).

The trailer/equipment is not broken.

 

[MintJulep]

Draw a FBD of a wheel.

It's a dynamics problem, so you need to include linear and angular inertia forces.

 

[3DDave]

DOES IT HAVE TO BE WRONG INFORMATION AT EVERY SINGLE POST?

Don't bother posting more until you have a video of your equipment doing exactly what you claim.

Yes - the schematic absolutely does as the fact you dont mention the location of the CG or the power pack. Don't tell - post a video of the problem.

Also, why are you not asking Scheuerle?

 

[IRstuff]

Engine torque at idle can be enough to spin the tires on a slippery surface without additional throttle input.

Not relevant; OP claims there's a difference between 2-wheel and 4-wheel, so surface is not the issue.



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