[MOC] Rocky 42099 B model

[Didumos69]

Rocky - 4WD 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. Instructions are available on Rebrickable. 

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.

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.

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 .

When I wanted to test drive with a first bodywork attempt, I found out the hard way that I could not reach the on/off button of the battery/control unit. I had not taken that into account. Eventually I found a solution in making the roof openable, as if it were a hood, just by releasing two pins. The red 10L axle in the back can then be used to turn the controller on. After opening the roof, it can be removed easily, after which the sides of the body can be removed separately to access the battery/control unit.

All together this has been a great experience. Especially the limited and pre-defined set of parts made it a real challenge. It forced me to revisit all constructions over and over again, and leave in only what is essential, without making concessions to my self-imposed building standards. I ended up using 828 of the 958 parts.

Edited November 11, 2021 by Didumos69

[suffocation]

Love it

And it's great to see you building again!

[Attika]

Glad to see you back in business. 

I've seen the video earlier and I knew there must be a trick behind the curtains. That chassis is a very clever approach, especially being limited to the set's inventory. Also the shape is pretty authentic.

Something got me thinking though.  I've noticed some pinholes without pins. What happened? 

[Kio Liex]

Great alternative model, I know my son would love to build this. He is already complaining about the lack of a B-model from LEGO. Will you be sharing you digital build or can we be so lucky that you are making instructions?

[kbalage]

Another great build from you and a nice challenge to build a B model :) I'll definitely build it if you decide to share the file or create instructions!

[Permo]

It works pretty well,... great engineering !

[Didumos69]

12 hours ago, suffocation said:

Love it

Thanks!

12 hours ago, suffocation said:

And it's great to see you building again!

11 hours ago, Attika said:

Glad to see you back in business. 

I don't spend as much time as I did before, but I still enjoy building.

11 hours ago, Attika said:

Something got me thinking though.  I've noticed some pinholes without pins. What happened? 

Yeah, that's new for me too. And I even covered some empty holes with redundant connectors 

1 hour ago, Kio Liex said:

Great alternative model, I know my son would love to build this. He is already complaining about the lack of a B-model from LEGO. Will you be sharing you digital build or can we be so lucky that you are making instructions?

1 hour ago, kbalage said:

Another great build from you and a nice challenge to build a B model :) I'll definitely build it if you decide to share the file or create instructions!

Yes, I will be making instructions. Won't share the digital file, because some building steps can not be deduced from the digital file alone.

40 minutes ago, Permo said:

It works pretty well,... great engineering !

Thanks!

[PetSild]

Totally original sophisticated construction and visually nice model.

[agrof]

I really dig the shape, and the usual high level engineering from You is the top of the cake on this B model. I wonder, if AFOLs like Didumos can create such a capable and viable alternative model in their spare time within couple of weeks (let's say 1 week netto work), why is TLG not capable/interested to put same effort (khm... creativity, capacity) for a product like this?

Whatever, awesome B-model, love it! 
 

[kbalage]

1 hour ago, agrof said:

I wonder, if AFOLs like Didumos can create such a capable and viable alternative model in their spare time within couple of weeks (let's say 1 week netto work), why is TLG not capable/interested to put same effort (khm... creativity, capacity) for a product like this?
 

Designing the B model is the easiest part, do you know how much effort is needed to add a control interface to the app with a different design?! 

[agrof]

11 minutes ago, kbalage said:

Designing the B model is the easiest part, do you know how much effort is needed to add a control interface to the app with a different design?! 

Here: forward-backward-steering? I guess, like zero effort?  (Sorry if I didn't get the sarcasm...)

Edited October 11, 2019 by agrof

[Didumos69]

21 hours ago, PetSild said:

Totally original sophisticated construction and visually nice model.

2 hours ago, agrof said:

Whatever, awesome B-model, love it! 

Thanks!

1 hour ago, kbalage said:

Designing the B model is the easiest part, do you know how much effort is needed to add a control interface to the app with a different design?! 

Luckily this model works with the control interface of the A-model, even without flipping the device upside down. I now even fall in love with the tilt-indicator, as it really follows the angle of the body, which is the interesting aspect of this B-model.

Edited October 11, 2019 by Didumos69

[kbalage]

2 hours ago, agrof said:

Here: forward-backward-steering? I guess, like zero effort?  (Sorry if I didn't get the sarcasm...)

Ohh sorry I forgot my sarcasm sign...

[Didumos69]

Kids like to challenge it 

 

[Permo]

"Couchcrawler"

 

I'd love to have those new geared down hubs...

[Didumos69]

On 10/12/2019 at 7:00 PM, Permo said:

"Couchcrawler"

I'd love to have those new geared down hubs...

They are really fun. From couch to outdoor..

Take 1:

Take 2:

 

Edited October 13, 2019 by Didumos69

[PetSild]

Excellent buggy, very please lxf file.

[Teo LEGO Technic]

Very cool! You really can build some solid buggies.

[Didumos69]

On 10/13/2019 at 7:26 PM, PetSild said:

Excellent buggy, very please lxf file.

I won't share the digital files. I will share instructions later this month.

On 10/13/2019 at 9:52 PM, Teo LEGO Technic said:

Very cool! You really can build some solid buggies.

Thanks!

To give an impression of the size; it's wheelbase is one stud longer than the Greyhound. Track width is 2 studs narrower.

[PetSild]

The instructions I'm looking forward, thank you for the answer.

[Didumos69]

Here is the final video (turn on your speakers ):

Edited October 16, 2019 by Didumos69

[Didumos69]

Sorry to bother you again guys, but this model is just so much fun. Also the low center of gravity pays off...

[Didumos69]

Here's a first impression of the instructions. This is in fact the first and most delicate stage. It uses auxiliary parts just to get the gear alignment right:

https://photos.app.goo.gl/nApcjaJFDebufCrx8

Edited October 20, 2019 by Didumos69

[Jim]

This is a very impressive build. Looking forward to the instructions.

[PetSild]

3 hours ago, Didumos69 said:

Here's a first impression of the instructions. This is in fact the first and most delicate stage. It uses auxiliary parts just to get the gear alignment right:

https://photos.app.goo.gl/nApcjaJFDebufCrx8

Awesome  big thanks for instructions.