Conceptual differential layout for an 8x8

[tripletschiee]

I created a conceptual differential layout for an 8x8, just to see if it could work.

It does it so far, even there is obviously still some minor tension happening between the axles.

However it is better than not to have any center diffs at all.

Any opinions?

[Pluto_MkII]

In my opinion this setup will only work if the stiffness of the whole chassis is low. Otherwise if one wheel is off the ground your vehicle will stop and transfer all the power to one wheel.

[Zerobricks]

The rear two axles unless steered dont need a central differential between them. Depending on the wheelbase a central differential may be needed in the front two axles, depending on the difference of steering angles between them, which as stated in the start is dependent on the wheelbase between the front and rear axles.

Put simply a truck with a long distance between front and rear pair of axles may not need a front central differential...

I usualy dont bother with central differential between SAME axle pairs, but a central differential between front and rear is good for steering... Or in the case of my latest 8x8 truck the front 4 and rear 4 wheels were not mechanically coupled and were driven individualy.

Another solution that I did use in older models was to seperate left and right wheel drive, but keeping the front and rear side wheels mechanicaly coupled...a real life example of this is ALVIS 6x6.

Edited January 4, 2016 by Zblj

[DrJB]

Rather interesting kinematics that you have developed. With 4 axles, I would 'expect' a total of 7 differentials, 4 for left/right, and of course, 3 between the axles. You managed to use only 2 between the axles. While this appears to be working, I'm sure there are situations where the contraption could get in trouble. Nonetheless, fitting so much 'kinematics' and gears in there is no trivial feat. Thanks for sharing.

Edit: agree with zblj, differentials are NOT needed for steered axles, only for driving (motorized). But, your title says 8x8, so I'm assuming you'll be powering all 4 axles.

Now, with so many gears, friction (and losses) is bound to be a problem. I can't help but wonder how, if you were to 'test' this, you'd be able to separate friction from diffs not working ...

Edited January 4, 2016 by DrJB

[tripletschiee]

@Pluto_MkII:

It is basically two 4x4 gears hooked together at the center diffs. One is vitually axle 1 and 3 and the other is axle 2 and 4.

The advantage in that setup is that there will always be two wheels spinning, one in each 4x4 setup.

As @Zblj said, the rear two axles can be consideres as one, as there is no steering included.

So in a "perfect" setup, where no tension/torsion during curves can happen in the drive train at all, there will be 6 differentials needed:

• 1 per axle
  
• 1 between the two front axles (as they will have different turning angles)
  
• 1 between the center diff between the front acles and the two rear axles.
  

That setup however will lead to the situation (one wheel spnning) that Pluto_MkII described in his post above.

[DrJB]

I agree with the above point that there should be no need for a diff between the teo rear axles, but this argument holds only for THIS lego model. The reason being that there is no difference due to steering. In real-life however, differentials are used to 'compensate' for many different reasons:

1. Travel distance as during steering

2. Road surface condition/variation

3. Different inflation pressures of the tires

4. different tread wear/depth on the tires

5. Different traction/friction of the tires, because of e.g. different compounds ...

[Rockbrick]

if you use a tatra layout are the axle diffs needed?

surely the two centre ones (longitudonal ones) would be enough - they you could have front left steering side paired with rear right and vice-versa

[tripletschiee]

@Rockbrick:

yes, a Tatra has diffs for each axle. But they are mounted lengthwise instead of crosswise.

I just build an optial layout for the diffs for an 8x8, considering the two backwise axle as one as they are not steered.

That means one differential between the two steered front axles, as they have different turning angles.

And one between the two backwise axles and the front center diff.

But without any limited slip differentials or locks it would not make a lot of sense as this setup could lead to the situation Pluto_MkII described above: one wheel will spin (rather fast).