[WIP] Rate my axle setup

[damian_kane_iv]

I'm trying to design a realistic 5-link axle for an upcoming crawler idea. I wanted to make a genuine 5-link with as much travel as possible and a differential locker. I think I have the geometry 99% there. I also plan to add pneumatic shocks to complement/dampen the coil springs. Lego's new springs were appealing and compact, but the 9.5L springs have much more travel needed for high axle articulation.

Let me know what you think/how it can improve:

Here you can see the coils, the shock and most of the suspension links.

 

This is a close-up with the pneumatic cylinder removed.

 

The axle is designed for ~1:9-1:11 sized mocs and can be fitted with either a portal axle mounts (pictured here), or traditional ungeared mounts. Here you can see the lower control arms, the panhard rod, and diff locker.

Here's a picture of the underside. I based this design off of Madoca's Dacoma axle. This was originally designed to accommodate a power ball joint for a more trophy truck-like suspension, but I have converted it into a 5-link axle design and scaled it up for 1:10 models.

Finally, the underside of the chassis contains the upper control arms, mounting for panhard (bottom), and the driveshaft As you can see, this was formerly a powerjoint setup and can be converted back if desired. 

[gyenesvi]

Cool that you are doing a genuine five-link setup instead of using the ball joint (cheating).

As for the link geometry, I'd say your upper link is too short, and not mounted in very good positions. Your lower link is 9L (I'd try using the existing 9L lego links, it's a bit harder to mount because it's one sided, but much slimmer for tighter spaces), so the upper link could also be a 9L. In case of a 5 link setup, the 4 main links could be roughly parallel, since the panhard rod will take care of fixing the axle sideways. The upper link on the axle end should be mounted higher and more to the back. The role of the upper links is to prevent the whole axle from rotating forwards, and in the current configuration it would be weak in serving that purpose. A nice test for that would be to take off the springs and see how much play the axle has in terms of forward rotation (now I think it's mainly the spring that prevents the axle from rotating forwards).

The mounting of the spring on the axle end is also a bit problematic, as it cannot follow the angle of the axle, which is already tilted on your image, and may change during suspension travel. You may think it's not a big deal, and it has some play anyway, but even slight sideways loading of the spring can effect the smoothness of the spring travel, and ultimately, the axle may not articulate as much as it could. So that end may also need two degrees of freedom for the spring mount. However, if you ensure by 4 parallel links of equal length that the axle never tilts forward/backward, and also your links are close to horizontal, so there's not much movement of the axle forwards/backwards moving away from grid positions, then straight springs mounted with one degree of freedom can also work smoothly. But your current setup does not satisfy that.

Also, for the driveshaft on the chassis end, I'd try using another strong CV joint instead of the U-joint, though that may only be the relic of the conversion you mention. I know that's also harder to mount, but it's stronger.

Keep experimenting!

[damian_kane_iv]

1 hour ago, gyenesvi said:

Cool that you are doing a genuine five-link setup instead of using the ball joint (cheating).

As for the link geometry, I'd say your upper link is too short, and not mounted in very good positions. Your lower link is 9L (I'd try using the existing 9L lego links, it's a bit harder to mount because it's one sided, but much slimmer for tighter spaces), so the upper link could also be a 9L. In case of a 5 link setup, the 4 main links could be roughly parallel, since the panhard rod will take care of fixing the axle sideways. The upper link on the axle end should be mounted higher and more to the back. The role of the upper links is to prevent the whole axle from rotating forwards, and in the current configuration it would be weak in serving that purpose. A nice test for that would be to take off the springs and see how much play the axle has in terms of forward rotation (now I think it's mainly the spring that prevents the axle from rotating forwards).

The mounting of the spring on the axle end is also a bit problematic, as it cannot follow the angle of the axle, which is already tilted on your image, and may change during suspension travel. You may think it's not a big deal, and it has some play anyway, but even slight sideways loading of the spring can effect the smoothness of the spring travel, and ultimately, the axle may not articulate as much as it could. So that end may also need two degrees of freedom for the spring mount. However, if you ensure by 4 parallel links of equal length that the axle never tilts forward/backward, and also your links are close to horizontal, so there's not much movement of the axle forwards/backwards moving away from grid positions, then straight springs mounted with one degree of freedom can also work smoothly. But your current setup does not satisfy that.

Also, for the driveshaft on the chassis end, I'd try using another strong CV joint instead of the U-joint, though that may only be the relic of the conversion you mention. I know that's also harder to mount, but it's stronger.

Keep experimenting!

Yeah its tough finding the right mounting points for each component. The problem I've been having is incorporating a differential locker into the axle. 

Both my upper and lower control arms should be as close to the differential as possible, but they're so far forward that this is affecting how the axle rotates through the spring travel, but changing the upper control arms should help too. Interestingly, upper control arms tend to be shorter than the lowers: https://www.autoblog.com/2020/01/23/toyota-4runner-suspension-deep-dive/ , so I dont know how that factors into the axis rotation equation.

When I compress the spring all the way, the angle of the axle is actually better/fixed

I agree that the upper arms are suboptimally mounted. Im going to need to find some way to mount them so that they dont get in the way of the differential locker. Maybe a longer set mounted further back will help

As for the spring mounts, I have a plan for an axle mount that allows some rotation so its not fixed, but ideally they would be mounted on top like the real thing.