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Lego Differentials vs. Real Life Counterparts

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I do want to start this topic off by saying I understand what a differential is, how it works and why it is necessary for standard 2wd setup. What I am curious about is the function of a differential when placed in a drive train between two or more driven axles or 4x4 setup. Is a center differential necessary? Are there performance increases or decreases? What type of results have you had when running PF on a setup with and without a center differential?

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In principle, a center differential is not necessary if the front and rear axle are spinning at the same average RPM. This assumes that all tires have the same rolling radii (inflation pressure vs. load per axle). This assumption however does not hold in real life and that is why a center differential is preferred. There are many reasons why (in real life) the average rpmss on the real and front axles could be different ... I hope this answers your question, else ask again.

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Well the 8110 unimog has a central diff and it was pretty good for off-roading but it does have it's limitations.

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A differentials allows the wheels to be powered even if they are spinning at different speeds which they do when going around a corner.

This video may help you understand it better:

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A differentials allows the wheels to be powered even if they are spinning at different speeds which they do when going around a corner.

This video may help you understand it better...

This video is great, thanks, now I understand the lego 9398 working principle :sweet: Edited by bjorkan

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None of the posts seem to have completely answered his question, which is about the need for diffs in four wheel drive situations. JB is right, and the principle of two tires moving at different speeds due to difference in distance travelled in a curve, not only applies to right and left, but between front and rear as well. This is the most important reason.

The front of a 4wd front steered car always has the greater radius, thus needs a little more speed on the wheels in comparison to the rear wheels. The front on a 4wd all wheel steered car travels the same distance as the rear axle, because the imaginary prolongued lines drawn from both the axles to the side of the car will meet in the middle of the car. This configuration is almost never seen in real life though: even on crawlers the rear axle often steers less degrees than on the front. A center diff will be needed either way, because wheels independently slip, brake, etc.

The 9398 doesn't need a center diff because the axles are powered separately and can't interfer with eachother.

This picture helps a little. Look a the front wheels, both of them driving on an imaginary circle which is greater (54 and 50) than the circle the inner rear wheel is driving on (48):

turn.ft96.jpg

Edited by PhyBuilder

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a central diff is required because when turning, more so in sharp turns, the front wheel travels in a sligtly larger radius/arc to the trailing rear wheels so not only do the wheels on each axle have to rotate at different speeds, both axles have to rotate at different speeds also, not by much but without it one there is extra drag/resistence in the drive train.

EDIT:: and while i'm typing a picture is posted that shows what i meant. :blush:

Edited by locoworks

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A differentials allows the wheels to be powered even if they are spinning at different speeds which they do when going around a corner.

This video may help you understand it better:

...

Heh, that video is just epic! Discovery channel was way better in 1930ies ;-) Where did you get it???

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fpr central diff imagine a car where front wheels steer 90 degrees. The front wheels make cricles, but the rear wheels just spin in the spot. Thats why you need a central differential. When steered front wheels always travel a longer path.

Edited by Zblj

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The 9398 doesn't need a center diff because the axles are powered separately and can't interfer with eachother.

Precisely!

I must mention that the 9398 setup is NOT typically implemented in real life as it could lead to (lack of) stability. You should never power front and rear axles separately, unless controlled by some electronics/software. There are however multiple vehicles with dual 'motors', such as construction vehicles (dozer) with hydro-static transmission. In such case there are separate motors for the left and right tracks ... and since they're 'separate', the vehicle has a tendency to steer 'erratically, especially is the soil (left/right) is different. This is of course less problematic than the 9398/stability. Also, such tracked machines require some fancy electronics/software for 'straight tracking' i.e., to keep the vehicle going on a straight line, especially on rough terrain (non-paved roads).

Edited by DrJB

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in reality, a centre differential is only required for smoothness and reduced tyre scrubbing when driving all wheels on hard surfaces. On soft surfaces, there has never been a need for a centre differential and 'real' off road vehicles with a centre diff will lock that diff (ie stop it acting like one) so you don't get all the available torque going through one wheel.

That's the mechanical solution - these days with traction control using arbitrary application of any brake, you can do the same thing that way - you just get lots of brake wear.

Of note for previous post about never drive axles independantly, there are a few short wheelbase race cars that did exactly that. Suzuki made one for the pikes peak hill climb I remember.

Edited by bonox

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A lot of older 4x4 vehicles don't have a center diff, and the extra resistance on the drivetrain is very noticeable. My 2000 Toyota 4Runner is a part-time 4x4 and has no center differential. The lack of a center differential makes it more difficult to steer on hard surfaces, but it is a simpler system and the front and rear axles stay locked together at all times. 4Runners with the Limited trim package (mine is a SR5 with Sport package) have full-time AWD with a lockable center differential. In 2001, all V6 4Runners gained a lockable center diff, regardless if it was full or part-time 4x4. When bombing across washboard roads, I find that throwing it in 4-hi offers quite a bit more stability.

Trucks with more then one drive axle in the rear also have an inter-axle differential. It's for the same reason as previously mentioned above. When a truck turns, the drive axles rotate at different speeds and the inter-axle differential compensates for this. The Volvo and Mack trucks I've driven had locking inter-axle differentials, which are only supposed to be activated when driving in a straight line.

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There is one exception to needing a center diff on 4x4 vehicles, and that is if they have synchronized 4 wheel steering. If they have, the front and rear wheels are following the same radius in turns, and thus there is no need for a center diff. This is the case in 9398, but it's not comparable as it has individually powered axles.

-ED-

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Not lego related story:

I was about 20 years old, and I took my fathers Lada Niva for a spin, on our hills outside of the city, without telling him (he was at the weekend cottage with other car). As you know, Niva has AWD, with three differentials, with reduction unit, and lockable center differential. We had that Niva almost for a decade, and it never failed us, never got stuck in snow or mud, and we towed other cars uphill during winters etc...

What I did not know is that my father was working on a Niva, and left system for locking center differential unfinished.

I took the NIva, drive it with center diff unlocked to the muddy, grassy hills, and as soon as I feel tires slipping, I locked the center diff. Car behaved much better, and I had a lots of fun on mud and grass and hills.

Time to go home- I drove it to the hard-earth road that lead to the main road, and then I tried to unlock the center diff. It could not be set to unlocked! Father did not finish work on it and it could not be set back to unlocked.

What I also did not know is that one front tire was smaller than rest of the tires. When I was on a mud, it all was good, but as soon as I got Niva on the asphalt, troubles started. It was like driving a car with handbrake on. After just 200 meters, reduction-box between the seats was so hot, that I could feel the heat.

I had no other way than to call my buddy wih tow truck and we went to him and he somehow disingage the locked center diff. I was the car and take it back. At the end I told my dad what happened.

From that day on, I have a great respect for center differentials :)

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Not lego related story:

I was about 20 years old, and I took my fathers Lada Niva for a spin, on our hills outside of the city, without telling him (he was at the weekend cottage with other car). As you know, Niva has AWD, with three differentials, with reduction unit, and lockable center differential. We had that Niva almost for a decade, and it never failed us, never got stuck in snow or mud, and we towed other cars uphill during winters etc...

What I did not know is that my father was working on a Niva, and left system for locking center differential unfinished.

I took the NIva, drive it with center diff unlocked to the muddy, grassy hills, and as soon as I feel tires slipping, I locked the center diff. Car behaved much better, and I had a lots of fun on mud and grass and hills.

Time to go home- I drove it to the hard-earth road that lead to the main road, and then I tried to unlock the center diff. It could not be set to unlocked! Father did not finish work on it and it could not be set back to unlocked.

What I also did not know is that one front tire was smaller than rest of the tires. When I was on a mud, it all was good, but as soon as I got Niva on the asphalt, troubles started. It was like driving a car with handbrake on. After just 200 meters, reduction-box between the seats was so hot, that I could feel the heat.

I had no other way than to call my buddy wih tow truck and we went to him and he somehow disingage the locked center diff. I was the car and take it back. At the end I told my dad what happened.

From that day on, I have a great respect for center differentials :)

Typical system wind-up :classic: A simple lift of a wheel with a jack would probably help the system unwind.

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I think that a central diff is needed for those vehicles that run on smooth surface with good grip. Without it, it would generate significant stress to the transmission and the tyre, probably causing loss of grip of one or more wheels and difficulty in steering and that's not the result sought.

For surface with poor grip, there is no needed, I work on a vehicle with 8 wheels but only one differential, it's called "H" transmission, but its use is intended primarily off road. If you drive on asphalt road it will eat the tyres as if there were no tomorrow.

Edited by Lucio Switch

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Typical system wind-up :classic: A simple lift of a wheel with a jack would probably help the system unwind.

The same would happen when I drove our old Fiat Panda 4x4, which it did not have a centre differential. You could only connect/couple both axles with a lever. In low traction surfaces it worked miracles, especially combined with its low weight. It was really awful when driving on normal tarmac and the system would wind up. In those conditions centre differential is necessary.

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In fact, driving on a smooth (with good traction) surface with a locked center differential is a very bad idea. It can not only damage the vehicle. but may also cause stability issues as part of the tractive force (in the footprint tire/road interface) is used not to control the vehicle, but to counteract the other tires ... In principle, this situation could be alleviated with a 'smart' TCS (Traction Control System)... but that's another discussion.

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In all transmissions there is a little bit of slack (or maybe a lot if its an old Land Rover!), plus a little bit of spring. When one axle runs faster than the other it is winding up the transmission between the axles like a clock spring, but because the transmission is much stiffer than a clock spring it takes only a very few turns (or fraction of a turn) to become fully wound. When fully wound there is a very high load going through it and something has to give, usually a tire slips, and then the transmision winds up again until the wheel slips again. If a car is driven in this condition you will get much increased tire wear due to the tyre slip, and a short life from the overloaded transmission, and eventually it will break like an overwound clock spring.

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wow, I didn't know you could lower the drive gear on the differential gear like that and it still works. Thats just so cool. We could use something like that in LEGO.

Nothing new for me. Technical names for this differential setup are hypoid gearing and amboid gearing.

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Strictly speaking, it's hypoid gear set, and that has nothing to do with how the differential works. It is just a way to push the drive-shaft lower than the car's floor. It was other advantages though and yet, hypoid gears are a cross between regular and worm gears where the meshing is accompanied by friction/sliding as well ...

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