Saturday, July 9, 2011

8110 Unimog U400 - The Panhard Rod misalignment mistery explained

As you certainly have noticed, with the recent presentation and reviews of the LEGO Technic 8110 Unimog U400, some questions have raised regarding the small offset on its front suspension.

Here we have the official explanation on how and why it was done this way.

"We got a couple of questions on the geometry of the front suspension of the new LEGO Technic U400 Unimog. Some of you is wondering why the front suspension is offset by half a module.

There is a geometrical reason for this, the front axle is stabilized by a Panhard rod, the panhard rod is there to prevent the entire front axle from moving from side to side. The shock absorbers would simply be too flexible to support the front axle on their own.
When the suspension is fully compressed, the front axle needs to be centered right underneath the vehicle in order to prevent the wheels from hitting the chassis and wheels arches, when turning.
That means when the suspension is fully extended the panhard rod will make a “circular” moment that pushes the front axle slightly to the right of the vehicle.
In the real world, these geometries are balanced out by fine tuning the length the panhard rod.

So if your front axle is offset by half a module, you have built your new LEGO Technic U400 Unimog the right way.

Have a nice summer and stay creative.
Ricco"



In fact I've noticed the Panhard rod explanation from Wikipedia which says,

"The advantage of the Panhard rod is its simplicity. Its major disadvantage is that the axle must necessarily move in an arc relative to the body, with the radius equal to the length of the Panhard rod. If the rod is too short, there will be excessive sideways movement between the axle and the body at the ends of the spring travel. Therefore, the Panhard rod is less desirable on smaller cars than larger ones. A suspension design that is similar but dramatically reduces the sideways component of the axle's vertical travel is Watt's linkage.
Some vehicles including Land Rovers with live axle suspensions use a Panhard rod as a component of the front suspension where Watt's linkage is not an option."

The Panhard rod has the advantages of lighter weight and less complexity. A Panhard should be as long as possible to minimize lateral displacement as the bar swings through its arc. The bar should also be level for the same reason.



What was not realized was the fact that the alignment is more important when the suspension gets fully compressed, to prevent the wheels from hitting the chassis and wheel arcs (specially when steered), than when it is fully extended. Thus we observe the misalignment most of the time, when the model stays at rest.

On the video below you will see the lateral displacement of the chassis, relative to the front axle.
Pay attention to the dark tan studs or the black tow balls. When the suspension is fully compressed it becomes perfectly centered. It displaces again up to one half module, as the suspension gets fully extended.



The equivalent situation is not as relevant for the rear axle, because it does not steer. So by design, it keeps aligned with the chassis when the suspension stays on its normal position (fully extended).



Thanks Ricco, for your enlightening explanation!

11 comments:

Junkstyle Gio said...

And so it seems that the designers did their work quite nicely!

The explanation was very clear and understandable.

Thanks!

Felix Schiffler said...

And again, great job TLG!

spamsac said...

For those interested, this problem is why the Watt's linkage is often preferred to a Panhard rod set up http://en.wikipedia.org/wiki/Watt's_linkage

Allanp said...

It's nice of TLG to explain this stuff. Ther didn,t have to but they did and even with a nice illustration. Thanx guys and gals!

Junkstyle Gio said...

The above mentioned article as given by Spamsac says Watt's linkage is prefered on rear axles. But does it work on front axles as well?

And is it possible to build this Watt's linkage with Lego-parts and get it into the Unimog?

Gerhard said...

@ Junkstyle Gio:

I was wandering also about that problem. And for your question about implementing this into the Unimog: I just opend a new thread on EB (http://www.eurobricks.com/forum/index.php?showtopic=57543) that should show all the MOCs, changes, improvments, new equipments and attachments people will build for the Unimog. So maybe someone will also include a solution for the Watt's linkage.

Regards,
Gerhard

Conchas said...

Remember the Panhard rod was used, because it is also used on the real Unimog, unlike the Watt's linkage.

A Watt's linkage design for 8110 would be great though! :)

Gerhard said...

Thanks for the interessting explanation and the perfect illustration!

But in fact it is only a theoretical reason. At least for me I never managed to get the suspension fully compressed, when haveing an heavy equiment attached at the front. I did not try the snowplow, but still I believe it will not force the suspension to compress fully. And If I compress the suspension manually, I can hardly turn the steering wheel, because of the massiv friction between the tires and the ground. At least the whole steering train twists so much, that I am afraid of breaking it.

So for me I prefere haveing a centered front axle in the uncomressed situation.

Regards,
Gerhard

Marin said...

Well, that's all really nice but my Unimog doesn't have interference when compressed while steering with the modified front panhard rod so I'm not changing it back to "the right way". Sorry, LEGO, but as Gerhard said, it's only theory. In practice it doesn't work. I still think it's a great set. :)

Vlad said...

I own the set and in my opinion they did that in order to compensate the weight of the battery box. If you will try to center the two axles, you will find out that the whole body of the will lean on the battery box side. On the right side of the vehicle there is nothing heavy enough to compensate the weight of the battery box on the left. By offsetting the axles slightly to the right of the vehicle the whole body will rotate compensating the roll caused by the weight of the battery box.

Jetro said...

That's an interesting observation, and it makes a lot of sense. LEGO usually does things for a good reason and this may very well be it.

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