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

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

(OP)
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?

Have a nice weekend.
Replies continue below

### RE: Two wheel drive vs 4 wheel drive (Torque, traction?)

Yes.
No.
Any dynamics textbook.

### RE: Two wheel drive vs 4 wheel drive (Torque, traction?)

(OP)
Are you sure? In reality, I've seen the opposite.

I want to understand the science behind.. but for sure. I've seen the opposite.

Something I would like to add (just in case it is relevant): the mechanism driving the tires is not mechanic, but hydraulic. There is a driven engine (planetarium), inside each driven tire.

So two trocks like this:

One truck, 4 wheels, two of them has driven engine inside the wheel.

One truck, 4 wheels, four of them has driven engine inside the wheel.

### RE: Two wheel drive vs 4 wheel drive (Torque, traction?)

Are you sure?

If the two trucks are equal and hence need the same force to accelerate or move, then the force for the 4WD on each wheel will be half what it is for the 2WD version.

too many unknowns here to give you a good answer. Formulate your question better.

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

### RE: Two wheel drive vs 4 wheel drive (Torque, traction?)

(OP)
But I just noticed you changed your answer, first it was No, No.

The trucks are the same.

My question is:

If you start reducing the same weight in each truck, will you reach the situation when the 4 wheel driven wheel will start sliding due to lack of friction force to start moving, AND 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? Surface for friction remains the same in both trucks. Four wheels always in contact with the road.

### RE: Two wheel drive vs 4 wheel drive (Torque, traction?)

It will always be possible for enough torque to spin the driven wheels.

Any weight.

Any number of driven wheels.

### RE: Two wheel drive vs 4 wheel drive (Torque, traction?)

(OP)
To spin, to slide, or to go forward? I think there is a difference.

The torque will make the tyre spin, but can it make the tyre to go forward, instead of sliding?

Again, I've seen this in reality, but I'm trying to understand the science behind.

I used commercial truck as an example, in reality I saw a heavy-lift equipment being employed to transport heavy-load cargo above them. It is design to carry 1000% more weight than their self weight. Thats why sometimes the four wheels driven engine is needed, but when the equipment is empty, four wheel driven engine start to slide.

### RE: Two wheel drive vs 4 wheel drive (Torque, traction?)

I am puzzled by your question. You talk about trucks, then mention planetarium.

But if we are talking about smooth, untextured surfaces like a steel plate, then the answer will be different.

So I think you need to give us a better description of what you are trying to understand. Be as specific as possible.

### RE: Two wheel drive vs 4 wheel drive (Torque, traction?)

Draw a FBD of a wheel.

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

### RE: Two wheel drive vs 4 wheel drive (Torque, traction?)

#### Quote (CapriRacer)

But if we are talking about smooth, untextured surfaces like a steel plate, then the answer will be different.

Umm. No.

The physics is exactly the same. The equations are exactly the same.

### RE: Two wheel drive vs 4 wheel drive (Torque, traction?)

(OP)
I will try to look for a picture, and the share that later today. I will think more about my wording as well.

### RE: Two wheel drive vs 4 wheel drive (Torque, traction?)

Thinking about this a bit further, the issue is the amount to torque being supplied to wheels. If the torque is capable of being throttled down to zero, then even at zero load, a tire on a road surface will have enough grip not to spin the wheels=- regardless of whether the truck is 2WD or 4WD.

But if the torque is not capable of being throttled down to zero, there will obviously be a load below which the tires will slip - regardless of whether the vehicle is 2WD or 4WD.

### RE: Two wheel drive vs 4 wheel drive (Torque, traction?)

Spinning = rotation without linear motion.

Sliding = linear motion without rotation.

Of course torque can make a vehicle move forward without spinning the wheels. See most any ordinary car on most any ordinary day.

### RE: Two wheel drive vs 4 wheel drive (Torque, traction?)

#### Quote ((MintJulep))

Umm. No.

The physics is exactly the same. The equations are exactly the same.

Unfortunately tires on a road surface don't follow conventional friction theory, so the physics is different.

### RE: Two wheel drive vs 4 wheel drive (Torque, traction?)

This appears to be a misrepresentation of some occurrence.

Can we play the 20 questions game in order to finally discover why this seems to be a reasonable situation?

Are the trucks, tires, CG location, operating slope, driver habit, operating surface, temperature of tires, temperature of surface, contamination of surface - all verified to be identical? If so, list the measurements made to confirm these are true.

11 questions to go.

### RE: Two wheel drive vs 4 wheel drive (Torque, traction?)

Practically, tractive effort for a 4x4 vehicle on dry pavement is equal to the weight of the vehicle. For a 2 wheel drive vehicle the tractive effort would be equal to the axle load of the driving axle.

(OP)
All identical

### RE: Two wheel drive vs 4 wheel drive (Torque, traction?)

No data = not identical.

### RE: Two wheel drive vs 4 wheel drive (Torque, traction?)

(OP)
It's not a misrepresentation of some ocurrence, this is something that has happened many times in several locations with the facts provided in the paper. You don't need to answer the question you are formulation to come up with a conclusion.

### RE: Two wheel drive vs 4 wheel drive (Torque, traction?)

(OP)
To make it easier... you turn off the driven engines of two wheels, and then the truck is able to move in linear motion (no spinning of the wheels). How?

### RE: Two wheel drive vs 4 wheel drive (Torque, traction?)

The facts are clearly incomplete.

What were the trucks being used for when you observed this many times in several locations?

### RE: Two wheel drive vs 4 wheel drive (Torque, traction?)

(OP)
Ok. As I said previously, it is easier than that..

Just imagine that you have a truck with four driven wheels, and when you try to move it in linear motion, you cannot, the wheels just spin... then, you turn off the engines in two wheels, ending up in four wheels touching the ground, two with driven engine, and two in neutral (you turn off their driven engines), the truck start moving in linear motion.

How?

### RE: Two wheel drive vs 4 wheel drive (Torque, traction?)

(OP)
I mean... what is the science behind?

### RE: Two wheel drive vs 4 wheel drive (Torque, traction?)

I think I'm starting to understand your question. In 4wd your vehicle doesn't track straight, in 2wd it does. The answer to this is that steering and driving both require traction. If you have 100 units of traction available and you are using 90 units for driving then you have only 10 units left for steering. If it is not a driving wheel then all 100 units are available for steering. Available traction is the weigh on the wheel x coefficient of friction.

### RE: Two wheel drive vs 4 wheel drive (Torque, traction?)

Open differentials are not providing equal torque, and someone built a truck with a continuous rear axle.

Did I solve the riddle?

I'm going to make a one-wheel drive truck that will be twice as good as two wheel drive. And I'll make it with 1000 non-driven wheels. Sort of a homeopathic medicine approach to undercut the 4WD community offerings.

### RE: Two wheel drive vs 4 wheel drive (Torque, traction?)

I believe OP's case has 4 independently motor driven wheels. It sounds like a mining truck of some sort.

Your 1 wheel drive system sounds like a Detroit Locker. One wheel is directly connected to ring gear and opposite wheel is driven through a torque sensing clutch.

### RE: Two wheel drive vs 4 wheel drive (Torque, traction?)

(OP)
They are 4 independently motor driven wheels.

### RE: Two wheel drive vs 4 wheel drive (Torque, traction?)

Isn't a tuk-tuk a commercial truck? Sure, only 3 wheels, but then I'm back to:

What were the trucks being used for when you observed this many times in several locations?

### RE: Two wheel drive vs 4 wheel drive (Torque, traction?)

#### Quote:

They are 4 independently motor driven wheels.

Hydraulic motors or electric motors?

### RE: Two wheel drive vs 4 wheel drive (Torque, traction?)

#### Quote:

If you start reducing the same weight in each truck, will you reach the situation when the 4 wheel driven wheel will start sliding due to lack of friction force to start moving,

This does not compute. The self-weight of the wheel, suspension, frame, and engine alone is sufficient; you have control of the torque being generated. The fact that mechanics roll wheels and tires around in a tire shop by hand says the premise is bogus.

TTFN (ta ta for now)
I can do absolutely anything. I'm an expert! https://www.youtube.com/watch?v=BKorP55Aqvg
FAQ731-376: Eng-Tips.com Forum Policies forum1529: Translation Assistance for Engineers Entire Forum list http://www.eng-tips.com/forumlist.cfm

### RE: Two wheel drive vs 4 wheel drive (Torque, traction?)

Your hypothetical is backwards. 4wd vehicles have higher driveline losses than 2wd vehicles so torque measured at the wheel will be lower than an identical 2wd. IOW, the 2wd will begin spinning before the 4wd in your hypothetical weight-reduction scenario.

### RE: Two wheel drive vs 4 wheel drive (Torque, traction?)

There is no drive line on OP's vehicle. It has independent drive for each wheel. I think OP is describing a mining truck. His descriptions are convoluted and misleading. It sounds like someone describing their perpetual motion machine but maybe there is a translation issue?

### RE: Two wheel drive vs 4 wheel drive (Torque, traction?)

Traction is still torque/wheel. If vehicle torque is equal between two and four-motor vehicles, two-motor has twice the torque/wheel and will spin before four motor. OTOH, if each motor's torque is equal then torque/wheel is the same between two and four-motor vehicles and they'll spin the tires simultaneously, even tho the four-motor vehicle will have twice the total torque.

### RE: Two wheel drive vs 4 wheel drive (Torque, traction?)

I think OP is complaining about difficulty steering while in 4wd operation. They are describing it in a very unusual way.

### RE: Two wheel drive vs 4 wheel drive (Torque, traction?)

There cannot be torque without resistance to motion and the OP has not said where that resistance comes from.

Q remaining 11: What were the trucks being used for when you observed this many times in several locations?
Q remaining 10: Hydraulic motors or electric motors? I missed the previous answer.
Q remaining 10: Show the hydraulic schematic of the two different trucks.
Q remaining 9: How many wheels spin when all are driven?
Q remaining 8: Was power to two wheels actually removed or is this a guess?
Q remaining 7: Was power to the slipping wheel(s) removed?

### RE: Two wheel drive vs 4 wheel drive (Torque, traction?)

OP's problem seems to be that the wheel is sliding sideways. Torque will not correct this problem.

### RE: Two wheel drive vs 4 wheel drive (Torque, traction?)

Not seeing sideways motion in the diagram. This leads me to believe the 4WD has all motors tied in parallel with no flow balancing, so if one spins there is insufficient backpressure to move the other motors. Whoever designed the 2WD did use a flow balancing circuit so it applies flow regardless of torque requirements. The advantage to not having flow balancing is to avoid Ackermann steering speed problems.

### RE: Two wheel drive vs 4 wheel drive (Torque, traction?)

Hydraulic motors in parallel behave exactly the same as an open differential. Equal torque to all wheels.

The line "not moving in linear motion" leads me to believe his is a steering problem, not a propulsion problem. Both are traction problems.

I have a new coworker with good knowledge but limited English. I'm starting to get use to mis-descriptions like this.

### RE: Two wheel drive vs 4 wheel drive (Torque, traction?)

Nowhere near enough information. Question not clearly defined.

je suis charlie

### RE: Two wheel drive vs 4 wheel drive (Torque, traction?)

#### Quote:

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

TTFN (ta ta for now)
I can do absolutely anything. I'm an expert! https://www.youtube.com/watch?v=BKorP55Aqvg
FAQ731-376: Eng-Tips.com Forum Policies forum1529: Translation Assistance for Engineers Entire Forum list http://www.eng-tips.com/forumlist.cfm

### RE: Two wheel drive vs 4 wheel drive (Torque, traction?)

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

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

### RE: Two wheel drive vs 4 wheel drive (Torque, traction?)

"Vast majority of cases" = all cases.

je suis charlie

### RE: Two wheel drive vs 4 wheel drive (Torque, traction?)

(OP)
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.

### RE: Two wheel drive vs 4 wheel drive (Torque, traction?)

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

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

### RE: Two wheel drive vs 4 wheel drive (Torque, traction?)

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.

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

### RE: Two wheel drive vs 4 wheel drive (Torque, traction?)

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.

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.

### RE: Two wheel drive vs 4 wheel drive (Torque, traction?)

(OP)
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).

### RE: Two wheel drive vs 4 wheel drive (Torque, traction?)

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.

### RE: Two wheel drive vs 4 wheel drive (Torque, traction?)

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

New here? Try reading these, they might help FAQ731-376: Eng-Tips.com Forum Policies http://eng-tips.com/market.cfm?

### RE: Two wheel drive vs 4 wheel drive (Torque, traction?)

#### Quote (Greg )

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.

### RE: Two wheel drive vs 4 wheel drive (Torque, traction?)

#### Quote:

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.

### RE: Two wheel drive vs 4 wheel drive (Torque, traction?)

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.

### RE: Two wheel drive vs 4 wheel drive (Torque, traction?)

(OP)
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)?

### RE: Two wheel drive vs 4 wheel drive (Torque, traction?)

(OP)
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.

### RE: Two wheel drive vs 4 wheel drive (Torque, traction?)

Draw a FBD of a wheel.

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

### RE: Two wheel drive vs 4 wheel drive (Torque, traction?)

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?

### RE: Two wheel drive vs 4 wheel drive (Torque, traction?)

#### Quote:

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.

TTFN (ta ta for now)
I can do absolutely anything. I'm an expert! https://www.youtube.com/watch?v=BKorP55Aqvg
FAQ731-376: Eng-Tips.com Forum Policies forum1529: Translation Assistance for Engineers Entire Forum list http://www.eng-tips.com/forumlist.cfm

### RE: Two wheel drive vs 4 wheel drive (Torque, traction?)

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

TTFN (ta ta for now)
I can do absolutely anything. I'm an expert! https://www.youtube.com/watch?v=BKorP55Aqvg
FAQ731-376: Eng-Tips.com Forum Policies forum1529: Translation Assistance for Engineers Entire Forum list http://www.eng-tips.com/forumlist.cfm

### RE: Two wheel drive vs 4 wheel drive (Torque, traction?)

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.

### RE: Two wheel drive vs 4 wheel drive (Torque, traction?)

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

TTFN (ta ta for now)
I can do absolutely anything. I'm an expert! https://www.youtube.com/watch?v=BKorP55Aqvg
FAQ731-376: Eng-Tips.com Forum Policies forum1529: Translation Assistance for Engineers Entire Forum list http://www.eng-tips.com/forumlist.cfm

### RE: Two wheel drive vs 4 wheel drive (Torque, traction?)

"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.

### RE: Two wheel drive vs 4 wheel drive (Torque, traction?)

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

### RE: Two wheel drive vs 4 wheel drive (Torque, traction?)

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)

### RE: Two wheel drive vs 4 wheel drive (Torque, traction?)

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.

### RE: Two wheel drive vs 4 wheel drive (Torque, traction?)

#### Quote:

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.

TTFN (ta ta for now)
I can do absolutely anything. I'm an expert! https://www.youtube.com/watch?v=BKorP55Aqvg
FAQ731-376: Eng-Tips.com Forum Policies forum1529: Translation Assistance for Engineers Entire Forum list http://www.eng-tips.com/forumlist.cfm

### RE: Two wheel drive vs 4 wheel drive (Torque, traction?)

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

### RE: Two wheel drive vs 4 wheel drive (Torque, traction?)

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 ..

### RE: Two wheel drive vs 4 wheel drive (Torque, traction?)

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.

### RE: Two wheel drive vs 4 wheel drive (Torque, traction?)

#### Quote (cibachrome)

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

### RE: Two wheel drive vs 4 wheel drive (Torque, traction?)

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.

### RE: Two wheel drive vs 4 wheel drive (Torque, traction?)

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: Eng-Tips.com Forum Policies http://eng-tips.com/market.cfm?

### RE: Two wheel drive vs 4 wheel drive (Torque, traction?)

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

### RE: Two wheel drive vs 4 wheel drive (Torque, traction?)

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.

### RE: Two wheel drive vs 4 wheel drive (Torque, traction?)

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.

### RE: Two wheel drive vs 4 wheel drive (Torque, traction?)

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.

### RE: Two wheel drive vs 4 wheel drive (Torque, traction?)

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.

### RE: Two wheel drive vs 4 wheel drive (Torque, traction?)

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....

### RE: Two wheel drive vs 4 wheel drive (Torque, traction?)

A solution to the problem: https://imgur.com/gallery/HHquGbB

Drop a section of tree trunk in the gap. Instant traction.

### RE: Two wheel drive vs 4 wheel drive (Torque, traction?)

(OP)
Thanks for the video. First time I see it.

By any means, I was trying to find the reason behind the problem.

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