MOC 4x4 Rock Bouncer

[sacm93]

G'day from Australia everyone!

This is my first topic created on Eurobricks and my first serious MOC since the end of my "dark age". I'm seriously super glad to be here and keen to share my Technic creations with you all from now onward. With this in mind, if there's any way I can improve how/what I post, by all means let me know so I can keep everyone (including staff) content. I am committed to giving this site the full respect it deserves. :)

As a few of you have suggested, it's a good idea when coming out of your dark age to develop designing/building skill by drawing on other people's MOCs for inspiration and guidance for different building techniques...

Partially inspired by SevenStuds' recreation of Tim Cameron's rock bouncer "Showtime", I present my own 4x4 Rock Bouncer.

Features..

Drive: 2 PF XL motors (1 per axle). Final ratio is 3:1.

Steering: 1 PF Servo motor with rack and pinion

Suspension/axles: Full-time locked solid live axles with portal hubs (geared 3:1), suspended by 9.5L shock absorber (soft) (2 per axle) and stabilized by a double triangulated four-link setup.

Tires: Third party scale RC tires similar to the "Rock Crusher" by RC4WD.

Battery: 1 x PF AA battery box

Receiver: 1 x PF V2

I initially began designing some kind of rock crawler which was to include PF XL motors, third party tires of some description and (after quite a while researching suspension design) double triangulated four-link suspension. This kind of suspension is ideal as it provides maximum articulation and strength of the axle while eliminating the requirement for a Panhard Bar or Watts Linkage because the triangular positioning of the upper and lower control arms oppose each other, eliminating sway and allowing all desirable movement.

By far the most difficult part of designing was the requirement for a steering shaft which moved harmoniously with the suspension cycle of the front axle. Because the upper control arms are shorter than the lower ones, the angle of the axle relative to the chassis changes through it's cycle and this means that when positioning the steering shaft, it must be such that the radius of it's motion doesn't change (due to angle change) as TLG doesn't offer any part which works purposely as a slip joint to negotiate the effect of plunge. After many, many, many... many attempts, a sweet spot was discovered which offered a negligible discrepancy. (This was a happy moment).

It's biggest performance drawback would have to be that when the angle of climb and drive torque applied is too great, the rear lower control arms buckle and the rear axle begins to walk under the chassis. Trust me, it's cringe-worthy. With 15L beams instead of 16L links and consequently different suspension geometry, however, this could be resolved. The turning circle also suffers due to the wheel base.

Overall, I am reasonably happy with the final product as it is capable of most of the things I intended it to be and in my opinion, the body could look worse. ;)

I unfortunately don't have video footage, but I do have photos (see below).

Enjoy! All comments welcome. :D

[JJ2]

Nice crawler, the axle design looks really rigid and I like the way you made the steering with 5L wishbones.

Does it have enough torque with only 1 XL geared 3:1 per axle?

Did you try having the steering servo mounted to the axle? It saves space and gets rid of the backlash that Lego U-joins have.

[Lox Lego]

Crazy car for your first MOC! Keep it up, nice work ?

[Sylvian]

at first i thought it was something jj2 made,. very cool machine!

[sacm93]

13 minutes ago, JJ2 said:

Nice crawler, the axle design looks really rigid and I like the way you made the steering with 5L wishbones.

Does it have enough torque with only 1 XL geared 3:1 per axle?

Did you try having the steering servo mounted to the axle? It saves space and gets rid of the backlash that Lego U-joins have.

Thanks JJ2, but I'm afraid that's one thing SevenStuds shared with me before he published his crawler. We actually corresponded together for a while regarding various building techniques, he was a great help to me.

It actually has quite a lot of torque. The AA battery box can supply up to 9 volts rather than the 7.4 volts supplied by the Li-Po battery box which enhances motor performance while the added weight of the AA box assists traction. The major limiting factor is really just the flexibility of the lower arms. Knob wheels were used to transmit torque from motors to along the axles, so teeth skipping was not an issue.

I did. It looked ugly in my opinion and added certain other suspension-related limitations. I really wanted to set and achieve the goal of having some kind of shaft working properly alongside the suspension links. Backlash is moderate but actually surprisingly workable. I found having drive on the front axle helps the wheels the "claw" their way over obstacles instead of being "pushed aside" (if that makes sense).

[Leonardo da Bricki]

That is really cool! The suspension is great, though I would have liked to see a video... Good work!

[Blakbird]

I see that you are already an expert at the most important part of this hobby: having fun!  Great first MOC.