42099 B model 'Rocky'




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.

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.

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 ;-).

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.

RC don't have interior ;-).

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Name 42099 B model 'Rocky'
Designer Didumos
Designed 2019
Inventory 837 parts
Theme Technic > Model > Off-Road
Alternate Build of 42099-1 4x4 X-treme Off-Roader
Old Trafford


  • 6 days, 8 hours ago jchobbes Level 3
    I will build this MOC after I do efferman's ultimate MOC. Found some different tires, wrote a post in the forums.

    • 4 days, 23 hours ago csy2020 Level 7
      如果有,请发给[email protected]的邮箱或留言上,我将感激不尽!
  • 2 weeks ago csy2020 Level 7
  • 2 weeks, 2 days ago Interstellar_1 Level 13 MOC Designer
    What's the difference between Option 1: Open diffs and Option 2: Locked diffs?
    • 2 weeks, 2 days ago Didumos Level 13 Designed this MOC DESIGNER
      Open differentials means differentials that work normally and locked differentials means differentials that force both attached wheels to have the same speed. See also the second video, the one from RacingBrick.
      • 2 weeks, 2 days ago Interstellar_1 Level 13 MOC Designer
        • 2 weeks, 1 day ago A_morti Level 5
          If you build the version with locked differential, you will have the parts left at the end to choose either. If you build with open differential, you'll only have that option (without digging into your own spares anyway)
          • 2 weeks, 1 day ago Interstellar_1 Level 13 MOC Designer
            But wouldn't you have to take it apart either way?
            • 2 weeks, 1 day ago A_morti Level 5
              You can change the differential from open to closed by dismantling a relatively small amount, it's easy enough to do but only if you built the locked diff version, as then you'll have parts for both open or locked available. If you build open diffs, you use the little 2L plates which are needed for locking the diff in other places
    • 2 weeks, 1 day ago Interstellar_1 Level 13 MOC Designer
  • 1 month ago A_morti Level 5
    I've built it, and it's a cool model. Few suggestions though. Front suspension popped open first time I drove it outside, so I haven't played much. Where the front shocks fit I've used some plain 3 long beams from another set, and a 3 long axle with a bush in the middle to maintain a distance between those beams. Similarly on the back, a 3 long beam maintains a distance there. Appreciate this may not have been possible without extra parts, I've not been keeping it separate from spare parts so I'm not sure.

    You did a great job getting freeplay out of the steering. Way better than the A Model.

    The starting button and battery replacement procedures are awkward. However I think it's just how it has to be on this model.

    My main problem though, is the turntables. They're not happy being used to support such a big weight on a cantilever, and they do not move nicely at all. I'll try some silicon spray tomorrow.
    • 1 month ago Didumos Level 13 Designed this MOC DESIGNER
      I'm sorry to hear you ran into issues. You're the first to report on 'suspension popping open' or turntables that 'do not move nicely at all'. I have also not experienced any of these myself. About the battery box placement being 'ridiculous'; the battery - alike the motors - play an essential role in form-locking the whole structure. That's why it was placed like this. Not ideal, but reaching the on/off button is still fairly easy, takes 30 second, and is well instructed.
      • 1 month ago A_morti Level 5
        Well, I got some time to put it back together and have a little play. It really is a cool model, and it'll climb up and over some impressive terrain!

        If anyone else got frustrated on the power button, try using the decorative tube from the A Model's A-pillar. It's a bit easier than an axle.
        If you pick up the model by the middle and try to rotate either end, the turntables don't like it. However, in use it seems fine. I still think there will be wear occurring here, but within the limits of Lego where ball bearings don't exist, actually it's as good as it could be.
        The suspension popped out when it rolled forwards off a drop. Bench testing it shows that as the spring over extends, it can tend to flex things where the spring joins, so I guess that's how it happened. In this case adding something between there can help prevent it. Or maybe  I should remember it's only Lego (!)
        Now I played with it, I need to change the batteries :D I understand why the box is buried, and agree it's useful that such a large part of the model needs to be structural. I guess it's a fairer frustration at Lego for not putting a lithium battery and USB socket on there. I'm looking at it now and thinking it'll be easier to take the bottom off the chassis to access the batteries than unbuild the body off the top.
        • 1 month ago A_morti Level 5
          Under the battery box are 2 black L pieces, held by two black pins and two blue 1.5 pins in to a long grey beam going up. If those are swapped for those 3-long pin/bushes, then the whole bottom piece becomes a hinged cover for the battery box :)
          There's none spare at the end, but there are 4 inside the door panels that could be swapped for normal pins. 
        • 1 month ago Didumos Level 13 Designed this MOC DESIGNER
          If you pick up the model by the middle and try to rotate either end, the turntables don't like it.

          It's not so much the turntables that don't like it, but the gearing. It works well when the front and rear modules are used to drive the body, not when the rear module needs to drive the front module or vice versa.  The latter is also not what's happening in use. When you want to test what happens in use you should lift the model with two hands and rotate the front and rear modules independently and see how that determines the body's angle.
  • 1 month, 1 week ago dody Level 8
    Hello . I have a hard time using the compare sets feature on Rebrickable . Can anyone confirm or not ..are extra parts needed to build this alternate build or the original set will suffice ?
    • 1 month, 1 week ago raystafarian Level 14 MOC Designer
      This is an "alternative" build so you shouldn't need anything other than the original set. If it's a "modification" then you might.
      • 1 month, 1 week ago dody Level 8
        thank you , didn`t noticed the modification/alternative thing .
    • 1 month, 1 week ago Didumos Level 13 Designed this MOC DESIGNER
      No extra parts needed.
  • 1 month, 2 weeks ago prototyp Level 15 MOC Designer PRO
    I dig RC cars and obviously dig Lego, but have dismissed Lego for RC as its likely going to be too slow and heavy to really be engaging. I'm not particularly keen on the main 42099 4x4 truck, but this rock crawler has changed all that. It's so cool, with interesting engineering and design, and I think the pace and challenge of rock-crawling is ideally suited to the combination of Lego together with RC. The 42099 set and these instructions are solidly on my to-get list now. Nicely done!
  • 1 month, 4 weeks ago qasweder Level 3
    This 42099 was a christmas surprise. But this moc is much better than the original. THX. In this MOC i have some problems with the steering motor:
    1. in the calibration of the control+ app the steering moves to both mechanical limits. after the calibration the steering is way of the center and does nor reach the mechanical limits. is there a way to set end points and center point manually?
    2. I use eneloop accu in the controller, could this be the reason for calibration problem?
    3. Is there an alternative app for controlling this central unit?
    Edit: This request is moved to: https://www.eurobricks.com/forum/index.php?/forums/topic/173733-moc-42099-b-model-rocky/&page=3
    • 1 month, 4 weeks ago Didumos Level 13 Designed this MOC DESIGNER
      I don't know about controlling 42099 with BuWizz app. To successfully calibrate the steering with the Control+ app, you may consider dropping the parts that form-lock the upper and lower suspension arms. Without these parts the max steering angle is exactly the same as for the A model and calibrating should behave the same too. This B model can actually do without these parts:
      • 1 month ago A_morti Level 5
        I took those off. They seem to have the function to stop the ball joints popping open, but I don't think it can really happen anyway, I would rather have a little more steer angle.
  • 2 months, 2 weeks ago R.goff1 Level 3
    I just bought these instructions (it’s a beautiful design by the way!), but I just wondered if I could ask 2 things?
    1. Could you tell me which parts I should keep spare, in case of possible stress breakages?
    2. Did you manage to link a Buwizz unit to the Powered Up motors? As I thought it might not be compatible.
    Many thanks for your time.
    • 2 months, 2 weeks ago Didumos Level 13 Designed this MOC DESIGNER
      1. I did not experience any stress breakages so far.
      2. I did not link a Buwizz to the Control+ motors.
  • 3 months, 2 weeks ago ftedd Level 3 PRO
    Can’t wait to build it!
    Built it.  Great kit, excellent instructions very easy to follow.  Made it with front diff open and rear diff fixed.  Thanks for a great MOC.
  • 4 months ago psmyth Level 16
    What a terrific design and a great reason to buy this LEGO kit!! Thanks
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