[HELP] Live Axle Geometry

[piterx]

hey guys!

Even if i've already made live axles i still don't get the exact geometry...

where should i place the links? and should i use 2 cardanic joints? because otherwise, if i use the ball joint the wheels tend to rotate around that point...wich is a thing that i don't want....i just want the axle to go straight up

thank you :)

[DrJB]

somehow the picture does not show ...

[Lipko]

Um, it's not very clear what you have can you post an image?

Without that info I think I can only give some trivial answers.

For a live axle suspension, you'll need some other links too. 4 links for example (with ball joints at every ends), making two trapezoids that are orineted in the opposite direction, and that are not in one plane.

Or a Panhard rod (that's almost parallel with the axle) with a ball joint on the chassis end and a hinge at the chassis.

Or you can use an arm with a ball joint at the axle and a hinge at the chassis. It's very similar to the 4 link setup, if you move the ends of the links very close to each other.

Edited February 26, 2014 by Lipko

[Alasdair Ryan]

somehow the picture does not show ...

That is probably because he did not link one.

Edited February 26, 2014 by Alasdair Ryan

[Lipko]

Oh, I guess the emphasis is on the WHERE.

For a four link setup, you'll need to have the attachment points on planes (chassis and axle) that is approximately perpendicular to the planes defined by the links.

Link

The upper two arms could be substituted with a single arm with a hinge at the chassis end and a ball joint at the axle end.

EDIT: I'm not sure about the Panhard rod.

Edited February 26, 2014 by Lipko

[piterx]

eheh i haven't linked any image!

I just wanted to know the theory of the live axle...for example, if the rotation point of the link should be parallel to the one of the cardanic joint

bsically i'd like to make something like this for my pajero...

but i guess that if i use the 4 link it gets easier because the cardanic joint would be in the center and the links are one on top and the other below...am i right?

Edited February 26, 2014 by piterx

[Lipko]

Yes, you are right about the 4 axle setup. But note, that the driving (input) axle will change it's length if you articulate the suspension, even if the centre of the U joint lies on the plane defined by the 4 attachment points.

The Panhard rod: I think (I'm not sure at all), that you need to have a hinge at the chassis, and a ball joint at the axle. And the 3 attaching points on the axle (for the 2 trailing rods and the Panhard rod) have to define a triangle, that is parallel to the suspension travel (perpendicular to the trailing arms). So the trailing arms should be below the axle, and the Panhard attachment point should be above the axle.

So the Panhard rod is just like the upper triangle in the four link setup, but rotated horizontally by 90°.

note, that these are truth for Legos, an in theory. In real life, I think every joint is the same with hard rubber bearings.

Edited February 26, 2014 by Lipko

[piterx]

thank you really much lipko :) it's clear now

now i just need to find a way to dismount the spring from the shock absorber XD

[Lipko]

Actually, I think it doesn't matter which end of the Panhard rod is the ball joint or the hinge. All that matters is that one and has to be a ball joint, and the other a hinge.

So long story short: Go for the Panhard rod, especially because there's that new suspension part. With small suspension travel, you should be fine with a Panhard rod with two hinges (so a plain liftarm).

My first MOC had Panhard rod suspension. It wasn't RC, but the suspension seemed to be stable and well defined without wobbliness.

Edited February 26, 2014 by Lipko

[Paul Boratko]

I recently did a new car with a live rear axle set-up and coincidently just posted a video of it a few minutes ago... I timestamped the part with the live rear axle... Maybe you can get some ideas from it... My main goal was to make certain that none of the parts were being flexed(Shocks, connectors, etc..) It works pretty well...

[dhc6twinotter]

If you are going to use a panhard bar, you will need ball joints at both ends of the panhard bar. As the axle moves up/down, the panhard mount on the axle will be moving in an arc relative to the other links (if that makes sense--basically the axle moves forward and backward a bit). The movement isn't much, but ball joints at both ends would be the proper way to build it, similar to the Unimog or Paul's Mustang in the above post. In some cases while off-roading real vehicles, the link geometry on the rear axle can give a rear steer effect when one side of the axle is stuffed or drooped.

eheh i haven't linked any image!

I just wanted to know the theory of the live axle...for example, if the rotation point of the link should be parallel to the one of the cardanic joint

bsically i'd like to make something like this for my pajero...

but i guess that if i use the 4 link it gets easier because the cardanic joint would be in the center and the links are one on top and the other below...am i right?

The picture above is a suspension with radius arms. There are no upper links. The two lower links each have two mounts on the axle housing. This seems to be a fairly popular setup on real SUVs with live axles on the front (80, 105, and newer 70 series Toyota Land Cruisers, and I believe the Nissan Patrol and older Range Rovers as well). I don't think I would go this route with LEGO though. There are some funny movements that go on with a suspension setup like this, and real cars seem to compensate fine with rubber bushings, but I'm not sure how you could do that with LEGO.

Many cars with 4 link suspensions, like my Toyota 4Runner, have upper and lower links that are different lengths. My upper links are much shorter than the lowers. Real vehicles have bushings and slip joints in the dirveshaft to compensate for this. However, when building with LEGO, I think it would be easier to use equal length links on both the upper and bottoms. If you are using a 4 link with equal length links, the u-joints don't have to line up with the swivel points of the links, as long as the links and distance between the u-joints are the same length. Since you would be using a panhard bar also, I would recommend using a CV joint on one end of the driveshaft. This will compensate for any slight driveshaft extension caused by the axle housing moving side to side as the suspension compresses.

Another option is to use triangulated links on either the top, bottom, or both sides of the axle. If you go with a triangulated link on at least the top or bottom, you will not need a panhard bar. As Lipko mentioned above, you could use a single link on top with just a hinge point at one end (like the LEGO a-arm piece). This will act like a triangulated link and keep the axle centered, but due to stresses on the hinge end, this will probably only be good for smaller scale mocs. You could also just angle some 6 or 9L links to form a triangle. I believe ZBLJ (sp?) has done some triangulated suspensions.

Anyways, sorry for being long-winded here. Just my $.02.

Edited February 26, 2014 by dhc6twinotter

[AndyCW]

• The amount of triangulation in the links also contributes to the centering effect the linkage has on the axle.
  
• With a four link setup, the point of articulation is between the upper and lower links.
  
• Increasing the distance between the upper and lower links will increase the axles resistance to caster and pinion angle changes with application of torque.
  

v/r

Andy

[Doc_Brown]

I'm keen to learn about this too. In the video below features leaf springs from a 71 mustang and Ford transit van!!!! LOL.

[DrJB]

The last time I worked on car suspensions ... I recall that the leaf-spring is a thing of the past ... inherited from horse carriages. True, Ford had them on their Explorer Sport-Track up until 10 years ago (Colleagues and I were pondering what was Ford thinking ...). But then again, ABS did not become a standard feature until 10-15 years ago in the US. Also, when you stagger multiple leafs of various lengths, you get a non-linear/progressive stiffness (which is desirable in many instances), but that was oh-so-last-century, with an understanding of vehicle dynamics I'd qualify of rather primitive ... We have come a long way since those.

[dhc6twinotter]

Yeah, thankfully we've moved away from leaf springs, but there are still a fair amount of vehicles that have them. Most of the pick-up trucks still have leaf springs in the rear, with the only exceptions that I can think of being the Dodge Ram 1500 and Honda Ridgeline (if you consider that a pick-up). The Dodge Ram 1500 has a 4 link coil sprung live axle in the rear, and the Ridgeline has independent rear suspension. Plenty of vans still have leaf sprung rear suspension, including all the full-size vans, and I think the Dodge and Chrysler mini-vans still have leaf sprung rear ends. I was pretty happy Toyota went with a 4-link rear-end on my '90 and '00 Toyota 4Runners. So much better!

The vehicle that really surprises me is the Chevy Corvette. It has been using leaf sprung suspension for years, and even the new C7 Corvette uses leaf sprung suspension front and rear. As far as I know, it's the only automobile sold in the U.S. that still uses a leaf sprung front suspension. Pretty amazing what they can do with that car.

And then there are torsion bar suspensions….

[Lipko]

@Dhc6: I don't think that's truth about the Panhard rod. If both ends are ball joints, and the trailing arms have ball joints too, the axle whole axle could roll. The Lego Unimog could use rods with two ball joints, because the other degrees of freedom were locked by the big ball joint.

[piterx]

hey guys thanks for your help!

at last i've done the 4 link system....did i make it right?

it still need the panhard rod...it seems to work fine...the axle goes up straight

if you notice something wrong please warn me :D

[dhc6twinotter]

@Dhc6: I don't think that's truth about the Panhard rod. If both ends are ball joints, and the trailing arms have ball joints too, the axle whole axle could roll. The Lego Unimog could use rods with two ball joints, because the other degrees of freedom were locked by the big ball joint.

The only purpose of the panhard bar is to keep the axle centered beneath the vehicle. Without it, the axle would have too much side-to-side movement and would be very unstable. A properly built 3 or 4 link suspension uses the links to keep the axle from rolling. That is why the lower links usually connect somewhere under the axle housing, and the upper links connect somewhere above the housing. Many triangulated suspensions have the upper links mounted to the top of the pumpkin (part of the axle housing that holds the differential).

A panhard bar is not needed with triangulated links because the triangulation keeps the axle centered. Panhard bars have more than 2 directions of movement at both ends, and most real vehicles just use bushings to compensate, but since we are talking LEGO here, ball joints will be needed at both ends.

The closer the upper and lower links are, the more axle roll (axle wrap) will occur. The further apart they are, the less axle wrap will occur. This is what Andy was mentioning a few posts above. A properly designed 4 link does not need a panhard bar to keep the axle from wrapping, but just uses the panhard to keep the axle centered.

The image below shows how the upper and lower links are mounted below and above the axle. Also, the upper links are triangulated, so no panhard bar is needed.

Suspensions that use radius arms instead of a 4-link have very little axle roll. Each radius arm is mounted to the axle housing in two places, so there are a total of 4 axle mounts to the two radius arms. The entire axle and radius arms pivot around the point at which the radius arms connect to the vehicle frame.

Most of the discussion so far has centered around panhard bars and link triangulation. There is another method used to keep the axle centered, and it's called a Watts link:

http://en.wikipedia....t's_linkage

The beauty of the Watts Link is that it eliminates much of the side-side axle movement that a panhard bar causes.

hey guys thanks for your help!

at last i've done the 4 link system....did i make it right?

it still need the panhard rod...it seems to work fine...the axle goes up straight

if you notice something wrong please warn me :D

Looks great! Nice job so far.

I do have some suggestions though:

-Make the driveshaft 2 studs longer. Replace the red 2L axle with a 4L axle. The middle part of the driveshaft should be as long as the links.

-Add another direction of movement to both the upper and lower springs mounts. You will need both side-to-side and front/back movement in both the upper and lower spring mounts. Check out the Unimog suspension. It allows the springs to move in all directions.

-You may want to add more distance between the upper and lower links. You may have some axle wrap in your current setup, especially if you add another degree of movement in the springs. This may take some experimentation though, and your current setup may be just fine in this regard.

Otherwise, it looks good! I'm looking forward to how this project turns out!

Edited February 27, 2014 by dhc6twinotter

[Zerobricks]

hey guys thanks for your help!

at last i've done the 4 link system....did i make it right?

it still need the panhard rod...it seems to work fine...the axle goes up straight

if you notice something wrong please warn me :D

Also since you use hard springs you may want to reinforce the axle itself so it doesnt bend in the middle.

[Lipko]

I was talking about a 2 trailing arms + Panhard rod setup, where the trailing arms have ball joints at both ends. And I have to correct myself: it's better to have a hinge at the chassis side of the Panhard rod.

With a properly triangulated 4 link setup, there is no need for a Panhard rod at all, because there can't be sideways movement (think about triangulation to an extent that the lower and upper trapezoids become two triangles. Many Lego builders already used that technique with a single A-arm suspension part for the upper triangle).

With a Panhard rod with two ball joints, Piterx' setup is okay. Again, my comment about he hinge+ball joint applies for a two trailer arm setup.

I'm not at my PC for some time, so I can't make a video or an LDD animation to prove I'm right, so I'm backing out from the discussion.

Edited February 27, 2014 by Lipko

[Meatman]

I've built this model and it has a nice live axle design.

http://www.brickshelf.com/cgi-bin/gallery.cgi?i=5387480

[dhc6twinotter]

I was talking about a 2 trailing arms + Panhard rod setup, where the trailing arms have ball joints at both ends. And I have to correct myself: it's better to have a hinge at the chassis side of the Panhard rod.

I can see how that would work with LEGO parts. I'm not aware of any real vehicles with this setup, but I'm not an expert in suspension designs. Your suggestions seems very doable with LEGO parts.

With a properly triangulated 4 link setup, there is no need for a Panhard rod at all, because there can't be sideways movement (think about triangulation to an extent that the lower and upper trapezoids become two triangles. Many Lego builders already used that technique with a single A-arm suspension part for the upper triangle).

With a Panhard rod with two ball joints, Piterx' setup is okay. Again, my comment about he hinge+ball joint applies for a two trailer arm setup.

I agree with all that. Using the A-arm piece was a suggestion I made in an earlier post as well. I think we are on the same page here.

[AndyCW]

To be the best of my knowledge, equal length parallel 4 links are actually fairly rare in production vehicles. They do not transfer the torque to the chassis very well. This setup I great for massive travel, but not for traction or articulation. Air bagged pickup trucks use these layouts to get the door sills or frame on the ground, but they never really get the tires to hook up.

Depending on the torque applied to the axle and the weight of the finished vehicle, you may need to separate the planes between the upper and lower links.

What link arrangement does your real Pajero use?

v/r

Andy

[Lipko]

Okay, I may have misread your post too.

Anyway, I do believe that the two trailing arm + Panhard rod setup is common, you can see a drawing of it in your Piterx quote on this page. But As I said earlier, real cars don't bother with ball joins and hinges, pretty much every joint is a rubber bushing because it works almost like a ball joint but for only a very limited angle, so it is also a bit like a hinge joint. This angle may be comparable to the softness and backlash of Lego parts, so maybe you could use simple liftarms as trailing arms and other rods, but that's too much for my purist ego.

EDIT bwah. I should stop posting in the morning. My reading is very selective and I missed all the info Dhc already wrote. Of course he's right and already said everything I wanted.

The setup I push here like crazy is very similar looking to that radius arm setup, and it works quite well with Lego parts.

Edited February 27, 2014 by Lipko

[Doc_Brown]

Most of the discussion so far has centered around panhard bars and link triangulation. There is another method used to keep the axle centered, and it's called a Watts link:

http://en.wikipedia....t's_linkage

The beauty of the Watts Link is that it eliminates much of the side-side axle movement that a panhard bar causes.

WOW, I was wondering if anything like this was possible. Has anyone made a technic version?