42099 B model 'Rocky'

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Rocky - 4WD RC Rock crawler buggy

I would like to present to you 'Rocky', a rock crawler buggy with a body tilting angle that averages the angles made by the front and rear axles. My shot at a 42099 B-model.

Change log
2019/11/01 Added the option to build it with locked differentials. Parts list covers both the open differentials option and the locked differentials option.
2019/10/24 Added wider images showing the orientation of the model better (p162-p189).
2019/10/23 Updated front page render.



When I saw the first images of 42099, I noticed that the amount the body tilts sideways, is mostly defined by the rear axles angle, because that axle's suspension is the hardest - it carries the battery/control unit - and it's not pendular. That got me thinking; wouldn't it be nice to make a setup in which the body tilting angle averages the angles made by the front and rear axles? Just like how a Mars Rover averages it's body angle between it's rocker bogies - with a differential - but now sideways, not lengthwise. That way it should be possible to mimic the character of 4-link suspension, which is often seen in rock crawlers. So that was my objective with this B-model and the nice thing is that this model contains exactly the parts needed to build something like that.

Axle articulation
Here is the setup that interconnects front and rear axles. Like in rocker bogie suspension, you should regard the body as the differential house. The body tilting angle is defined by the two axles that point sideways. I used 4 gears in the differential itself to minimize slack in the system. There is some rotational slack of course, but this is even further reduced by 1:3 given the 20:60 gear ratio with the turntables.



Center of gravity
Besides the differential, the center module also houses the battery/control unit, because that unit includes the tilting sensor and I wanted the tilting sensor to show the tilting angle of the body. I also wanted to keep the center of gravity low and centered. However, putting the unit in this central spot did cause issues later on... The battery/control unit - not depicted here - plays an essential role in form-locking the whole center module. The battery/control unit can be slided out sideways after removing a few pins and parts.



Spring suspension
Besides axle articulation, I of course also wanted to include actual spring suspension, so I attached two main suspension arms to the turntables, one for the front axles and one for the rear axles. I suspended the main suspension arms with springs placed between the turntables and suspension arms. The springs are mounted differently to the front and rear suspension arms, giving the car a little more lift in the back, which adds to a nice inclination, or rake angle, of the whole model. The whole model nicely sinks into the spring suspension under its own weight up to about 40% percent of the overall spring travel.


In RacingBrick's video, you may notice that it's hard to demonstrate the spring suspension while the model is standing on the ground. This is because when you compress the suspension by pushing down the cabin, the wheels rotate relative to their frames, which causes them to drive the motors. This is also why the model doesn't easily return up after pushing it down. When the model drives around, it easily returns to its ideal spring compression level, which is about 40% of the overall spring travel. To demonstrate or get a feel of the spring suspension, the best option is to lift the model with your hands underneath the front and rear differentials and let it bounce a little.


Drivetrain
I wanted to have the most simple drive train possible, so the motors are directly attached to the frames holding the differentials. This is a crawler and with the new portal hubs, there is no need for any up or down gearing. The motors add to the stiffness of the main suspension arms. I also wanted to have a track width that is two studs wider than the stock 42099 build. After some playing around I found out I could use the new CV-joints the other way around to make that possible.


The instructions come with two drivetrain options: Option 1 with open differentials and option 2 with locked differentials.


Steering
For steering I wanted minimal slack and double sided steering rods like in the stock 42099. I limited the steering angle to make sure the maximum angle the CV-joints make, does not cause any damage. I noticed the CV-joints start wobbling when the angle they make is too big. The steering rack assembly - as well as its back side counterpart - use a trick to minimize unintended movement (slack): The assemblies are 3 studs deep and incorporate 3L axles with end-stop. The end-stops are sticking out of the assemblies and make them slightly deeper than 3 studs. For this to work the end-stops need to slide along a smooth surface. This trick makes for a very nice fit with little play and still allows the assemblies to move very smoothly.



Ground clearance
To increase ground clearance I used a double wishbone setup, not suspended, to take advantage of the extra lift provided by the inclined wishbones. The rear wishbones are inclined more than the front wishbones, because there the CV-joints don't need to deal with the steering angle. At this stage I also added a set of minimalistic fenders ;-).



Bodywork
Finally, bodywork. This was the most challenging part for me. It needed to be removable, to provide access to the battery/control unit and I wanted it to live up to my foolproof standards. The whole model can be lifted by the roof or by the A(?)-pillars. At this stage I practically used all the pins that came with the set, so I had to do a lot of backtracking to get some pins available. I ended up using all pins, including the ones that came as spare parts.



Interior
RC don't have interior ;-).

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MOC-30630

Name 42099 B model 'Rocky'
Designer Didumos
Designed 2019
Inventory 837
Theme Technic > Model > Off-Road
Alternate Build of 42099-1 4x4 X-treme Off-Roader
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PDF - Photo Sequence
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Help Guide - BI File Types
Submitted by vladoniki Oct. 27, 2019
Submitted by vladoniki Oct. 27, 2019
Submitted by vladoniki Oct. 27, 2019