[MOC] Make 42070 Rock! - Azure Rocker alternate

[gyenesvi]

Hi Eurobrickers!

I'd like to share with you a build that took me quite a long time to finish. Around the beginning of last summer, when the Zetros was rumored, I started thinking of building a trial truck, or some other capable off-roader out of 42070. As it is codenamed All Terrain Tow Truck, I wondered if it is possible to build something out of it that actually performs reasonably well off-road. The biggest obstacle obviously seemed to be the lack of proper suspension parts, so I started experimenting with a suspension concept inspired by the Rocky alternate of @Didumos69; linked pendular suspension where the body tilt angle is the average of the axle tilt angles.

However, the implementation of the concept had to be different from Rocky, where the averaging is achieved by gearing together the two large turntables. In Rocky, the two turntables are relatively close to each other, and the PU hub makes up the center of the chassis, with the drive motors placed in the two axles. However, set 42070 has only a single drive motor, so a drivetrain must connect the two axles, so it seemed straightforward that the motor and the drivetrain would be in the center, which would have been in the way for any geared linkage between the two axles. Also, the distance between the turntables used to mount the axles is much bigger, and at this distance, if the front and rear axles were connected through gearing, the thin lego axles connecting the front and rear axles would twist so much that the linkage would be ineffective (gooey). So I decided to mesh gears to the turntables sideways, which transmits the rotation to the horizontal direction, and that way the two axles can be connected by a long link built from liftarms, which is solid enough. This method also leaves room for the drivetrain in the middle. I had to fiddle some time though to frame this in a solid way. I must note here, that I recently discovered a build of @Thirdwigg that seems to use a similar implementation, though I only found some photos of it.

After building a proof of concept for this part, I started building the axles themselves. The rear axle was simple, but the front proved to be hard given the parts and my requirements. The difficulty was the steering. The set has an M motor for steering, so it has to be geared down significantly to be useful, and some clutch mechanism must also be added for protection. I tried both on and off axle motor placement. The off-axle version required a ton of gears to route the drive to the steering coming in from the front of the axle through a small turntable, it ended up awfully complex. So I opted for on-axle motor, but that resulted in really small space (I did not want to build a large front axle, because that would have caused problems with the room for articulation, would have required a lot of space around the axle in the front). I tried both with a clutch gear/worm gear combo and a small linear actuator based mechanism but could not get it to fit and work properly. After giving it a few more tries, I set the project aside for a while.

(In the meantime, the Technic contest of 2021 came, and I used the same suspension concept in my small scale 6-wheeled moon buggy).

Recently I started fiddling again with the steering concept, and finally was able to make a compact front axle using small linear actuator based steering. Though I had to give up one detail of perfect geometry: as the tie rod moves to the sides, it also moves backwards towards the axle center, and the linear actuator cannot pivot in that direction, so it can only follow that movement with a slight bending (the axle that it's driven by, and the actuator piston itself), but I think it's within the tolerance of lego parts, and I have seen this technique used in other MOCs as well (I don't really think there would be a good way to avoid it, since there's no chance of flipping the actuator by 90 degrees due to the tie rod being right under it, no matter how I build it). Apart from this detail, the other difficulty was mounting the M motor in a solid way, but I think that part came out fine. Furthermore, I had to attach the end of the actuator on a half stud offset to make use of its full range of 3 studs movement (1.5 studs in each direction), resulting in quite good max steering angle (also supported by the U-joints going into the portal hubs, as opposed to CV joints for other types of hubs). Here is the whole axle:

So after solving this bit, the project got new momentum and I started working on completion of the model. I built a rolling chassis, made a test drive on some rocks and it performed surprisingly well, so I started to work on a gearbox and bodywork.

For the gearbox, I thought I'd use a 24T differential available in the set to make a 3x gear ratio, keeping the same ratio as the A model for high gear, and 3x slower for low gear to support climbing. However, I encountered an interesting failed scenario here. The differential is easiest to use in a setup that results in a 3x speed-up (rather than a down-gearing, and the same would be true for a 24T clutch gear if existed), because as the drivetrain goes from the motor higher up in the chassis downwards to the driveshaft, the gear stick is easiest and most natural to place above the clutch gears (including the differential with its 24T gear), so the drivetrain below has to receive the drive through the 8T gear, resulting in up-gearing. So in order to have down-gearing in the system, I had to place some down-gearing before the gearbox, which is cancelled by the up-gearing on one path, resulting in 1:1 ratio, and not cancelled in the other path, resulting in a 1:3 ratio (another option could have been raising the whole gearbox and placing a down-gearing below it, but I don't like that scenario as there would be a big gear sticking out at the bottom). Easier shown than described:

However, while this looks good on paper, it failed in an interesting way. In low gear it worked quite okay, without significant clicking around the clutch gear, but in high gear it started to click as soon as the model was not going straight on a flat surface, but something more demanding, even while turning :) I figured that the problem was that the gearbox in this setup is under high torque, so less stress is enough to make it crack. I had to redesign the whole thing where there is no down-up gearing in the system. (I also have to mention that the clutch piece can go less deep into the differential than into the 16T gear, and that may have also contributed to the fact that that route clicks more easily). So the other way to use the differential as a clutch gear would be to drive it with an 8T gear from the top, resulting in a down-gearing, but that presents a fundamental problem: where to put the gear stick, as it's just in the way for an axle above the gearbox (alternately, the clutch gears could be driven from the side as well, or the switch could also be sideways, but the model does not have space side-ways because of the suspension linkage running exactly there). So I thought about an unorthodox solution: could the gear stick piece be moved one stud to the side, out of the way of the drive axle? At first I thought that would be weak and disengage too easily, but luckily the set has enough of those pieces, so I could try two of them, one on both sides. So I did, and it works in a surprisingly solid way (if the tilt of the stick itself is limited)! Here is the final gearbox design, in which only the gearbox itself performs down-gearing on one path, and the clutch gears are not under so high torque, so it does not click under normal usage (of course, it can be pushed so hard that it becomes the weak spot of the drivetrain, but that's inevitable).

And here are all the functions color coded (red is the suspension itself):

As for the shape and bodywork, first I thought I'd build some kind of a truck, for example a trial Unimog (that would have been a good match for the portal axles). However, the available azure parts were not so good for the shape, there are mainly large flat pieces and few triangular ones, so I quickly moved on to another idea. For a while I have been admiring Flatgekko RC crawlers (check out some awesome videos on YouTube), so I thought I'd try building something in that style. It gave me an interesting idea to test: putting the center of gravity into the front of the model. The motors have already been well placed for that, so I thought I'd try placing the battery above the front axle. By the way, that's also the lowest point where I could place it; as said above, I could not place it low in the middle as Rocky does, since the drivetrain and suspension linkage would be in the way (and I definitely wanted to avoid placing it high at the back). Although this hub is hard to use as a structural element, placing it right above the front axle was easier than I first thought. Here is the complete chassis, including a roll-bar to hold the roof, and the IR receiver in the back of the cab.

The long rounded panels available in dark azure were good to build a Dodge Power Wagon style cab roof and nose, and the few wing-shaped panels were okay to make the sides. I finished the back with a tail bed, that can hold a spare wheel fixed by chains. Here are some renders of the finished model.

And some photos in its natural habitat :)

More images are available on my Bricksafe, and building instructions are available on Rebrickable.

Finally, I want to note that this has only been my second build with the PF system :) and I was pleasantly surprised here and there in comparison with PU. First, the PF XL motor is just so powerful in comparison to the PU version. I don't know if the actual motor itself is that different, I know the gearing is, but I also suspect that the PF one is not limited by control software, and that can matter a lot. I was surprised that a single motor can move such a big model so powerfully. If I push it really hard upwards, it can seriously bend some axles and break some gears (or at least crack them), but it definitely won't stall.. The other thing that I really liked is the physical remote (I built a steering mechanism onto it). It makes such a difference wrt the phone control of PU, it was much more fun to play, and much easier to film and control the model at the same time. A steering system with the M motor and the linear actuator is an interesting technique, and while it works better than I expected, it is still more cumbersome to drive around and somewhat weaker than a PU servo based solution (the clutch inside the LA may click if the wheels are blocked hard), so I have to give that to the PU system.

Let me know how you like the model!

Cheers,

Viktor

[2GodBDGlory]

Great writeup as usual! Placing the battery that far forward is a really interesting solution that I may have to try someday! The suspension is also pretty unique. One detail I quite liked was the way you secured the yellow crankshaft part that the steering actuator connects to. The moment I saw the render, I worried that that part would pop off immediately thanks to their very low clutch power, but then I saw you had placed the 1x2 Technic brick overtop of it to prevent that. Nice solution there!

[msk6003]

This is quite interesting model. I also made some kind of rock crawler out of this model but I can't think use linear actuator in that time.

 

[Thirdwigg]

Great job @gyenesvi on another great alternative build. It's always interested to see what solutions you are able to come up with using the limited parts available in various sets.

I am happy I was able to give you a little inspiration in this build. I hope it worked well for this application.

The build turned out great, and all the functions you were able to add is impressive. The stance and proportions look great as well. I like the long hood, and the cabin placement that give a long bed on the rear. Again, great job with this.

[thekoRngear]

Great! Always like the description. BUT, this time Stunning photos as well! 

I should also add, with the continuation from RC Jeep Bruiser MOC @gyenesviis making some serious Lego RC Crawlers.

Edited May 2, 2022 by thekoRngear

[1gor]

Victor,

How does it perform on flat surface? I have always issues when using XL motors without reduction hubs and with central differential, only when upgearing XL motors 36 to 12 gear and then with using hubs from 42099 set MOC has continuous power output...

Edited May 2, 2022 by 1gor

[gyenesvi]

Thanks guys, glad you like it!

20 hours ago, 2GodBDGlory said:

One detail I quite liked was the way you secured the yellow crankshaft part that the steering actuator connects to. The moment I saw the render, I worried that that part would pop off immediately thanks to their very low clutch power, but then I saw you had placed the 1x2 Technic brick overtop of it to prevent that. Nice solution there!

Nice that you spotted that :) The crankshaft part seemed like a neat solution to make that half stud offset, but I had the same worries, and started testing it moving it back and forth 50 times, and of course slowly it would detach itself. But then I stumbled upon this simple fix with the 1x2 brick, which fits into the tight space and it holds it surprisingly well, it never came off during driving.

17 hours ago, msk6003 said:

I also made some kind of rock crawler out of this model but I can't think use linear actuator in that time.

I also learned it from others' builds :) And works better than I expected!

4 hours ago, Thirdwigg said:

I am happy I was able to give you a little inspiration in this build. I hope it worked well for this application.

Actually I only saw some photos of your model somewhere and suspect that it has a similar suspension technique, but could not find details about it. Do you have some description of that model somewhere? I'd be curious how similarly we implemented the same concept.

3 hours ago, thekoRngear said:

Great! Always like the description. BUT, this time Stunning photos as well! 

I should also add, with the continuation from RC Jeep Bruiser MOC @gyenesviis making some serious Lego RC Crawlers.

Thanks, glad you see this as a continuation of the Jeep. And yes, a better camera can make a difference for the photos, and also the videos :)

3 hours ago, 1gor said:

How does it perform on flat surface? I have always issues when using XL motors without reduction hubs and with central differential, only when upgearing XL motors 36 to 12 gear and then with using hubs from 42099 set MOC has continuous power output...

I think I get what you mean about the continuity of the output, I noticed this too and wanted to ask you guys whether this is a feature of the PF system. I noticed in other people's videos and with this model too, that it feels like the output is not completely continuous but a bit periodic. This is something I never experienced with the PU motors, those seem to have quite stable continuous output (though I always used them with planetary hubs). First I thought it might be because of the gearbox or the usage of U-joints in the front axle (instead of CV joints), but it also happens when it's running straight, which should not be effected by the U-joints. Furthermore it's still noticeable when I remove the gearbox and simplify it to fixed gearing. So is this something others have also experienced with PF motors? Or is it specific to the XL? When I motorized the Creator Mustang with 2 M motors, I did not notice such a thing, neither when I built Madoca's lightning buggy with 2 L motors (although I was running them from a BuWizz, not sure if that could make a difference).

[Thirdwigg]

2 minutes ago, gyenesvi said:

Actually I only saw some photos of your model somewhere and suspect that it has a similar suspension technique, but could not find details about it. Do you have some description of that model somewhere? I'd be curious how similarly we implemented the same concept.

You bet! Which model was it that you saw? I have used this type of suspension a lot, though never with the transversally mounted bevel gears rotating the walking beam links. I first used a linked walking beam suspension on a U500 from about 12 years ago, and have used it a couple of times since as it works quite well. I used gears to the turntables parallel to the driveline (longitudinally) like you referenced on the MAN LE and on the GMC truck as well. This does has more suspension slop than the linked walking beams, but still works well enough to use.

I'm happy to dig up any other photos if you need more. Let me know how I can help.

[1gor]

@gyenesvi I have managed to solve that issue with help from @Zerobricks.

My solution is like this:

From 2 coupled PF XL motors I use 36 tooth gear to central axle with 12 tooth gear. Instead of using axle that could twist I use axle connectors like this one https://www.bricklink.com/v2/catalog/catalogitem.page?P=6538c#T=C 

Then I use new differential from Zetros with 12 tooth gear combination and at the end I use reduction hubs from 42099 set.

I hope that this images could help you:

I'm sorry if I have derailed your topic to much. Unfortunately I wasn't able to make the same thing with unequal tires on smaller scale. Work in progress posted above is for 1:15 model and 1:18 model is extremely challenging for me...

 

 

[gyenesvi]

1 hour ago, Thirdwigg said:

You bet! Which model was it that you saw?

That's my problem, I don't remember, and can't find it again, it just came up during some unrelated search :) But thanks for the links, now that I went through your models in there, it may have been this MackMagma. However, I think I mistook the large turntables for small ones, and the suspension works differently than I thought. I remember seeing those red triangular links connected by the long beams that move back and forth as the axles tilts (that part is similar to mine), but in your model, they are connected to the axles through vertical 6L links, not by gearing from the turntable. Now it's clear how yours work, so it's different from mine, but in the end it did help me understand your technique, thanks!

57 minutes ago, 1gor said:

From 2 coupled PF XL motors I use 36 tooth gear to central axle with 12 tooth gear. Instead of using axle that could twist I use axle connectors

I like that massive up-gearing, that's interesting in combination with planetary hubs. Yes, I also tried to use more connectors and less long axles that could twist, because my main driveshaft is pretty long. And one other thing I did to minimize twisting is that the driveshaft gets drive in two places closer to the axles instead of just in the middle.

59 minutes ago, 1gor said:

Then I use new differential from Zetros

That would have been my other guess, that those newer diffs in the Zetros may be more solid in transmitting the drive than the regular ones that are in this set. Maybe it's just all of these factors added together that result in a less solid drivetrain in case of this older set.

1 hour ago, 1gor said:

I'm sorry if I have derailed your topic to much.

No problem, I like when there's more technical discussion.

[1gor]

I think it is combination of factors; when I used gear 40 to 24 tooth reduction and then combination of 12 tooth with new differential but without planetary hubs it didn't work. Problem with planetary hubs is that they fit to arms with ball joints and there is no 4 studs long suspension arm; smallest is 5 studs long...

[gyenesvi]

11 hours ago, 1gor said:

I think it is combination of factors; when I used gear 40 to 24 tooth reduction and then combination of 12 tooth with new differential but without planetary hubs it didn't work.

Interesting phenomenon, but I still don't quite understand why it works better with a massive up-and-down gearing. It sounds a bit counterintuitive. I mean this model has down-gearing in the portal hubs in the end, but it is still not smooth.

Quote

Problem with planetary hubs is that they fit to arms with ball joints and there is no 4 studs long suspension arm; smallest is 5 studs long...

I agree that's a problem, I'd even welcome shorter connectors with ball socket to make that hub more usable..

Edited May 3, 2022 by gyenesvi

[1gor]

It seems that torque from XL motors is to high for Lego material so with direct downgearing resistance is not big enough to prevent axle twisting (I think axle twisting us what causes this rubber band phenomenon)

[Sipus3747F]

Hello, maybe a stupid question, but I would like to ask how big a difference is there in speed on the flat compared to the A model? Or is the speed the same?

Thank you

Michal

 

[gyenesvi]

18 hours ago, Sipus3747F said:

Hello, maybe a stupid question, but I would like to ask how big a difference is there in speed on the flat compared to the A model? Or is the speed the same?

In high gear it is about the same speed as the A model (maybe slightly faster as it is lighter).