gyenesvi

[MOC] Trial Unimog - Zetros (42129) alternate

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Hi All,

Although I have posted a few pictures previously, I wanted to make a proper writeup of my Trial Unimog alternate of the Zetros set (42129), because I think sharing the design process is valuable for people around here. Also, I am going to share my thoughts about the set itself as a parts pack. But for the curious ones, here's an action video of the end result and a short summary of the features:

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Features:

- good actual outdoor performance :)

- permanent AWD, no differentials

- 2-speed gearbox, faster gearing than Zetros

- relatively large increase in ground clearance

- 4-link suspension on the back, large articulation, soft suspension

- 3-link suspension with Panhard rod on the front, slightly larger and softer articulation than the Zetros

- opening (and lockable) doors and trunk bed

 

When building alternate models, I always try to build something different from the A model, both functionally and in its looks. So when considering what to build out of the Zetros, keeping the Mercedes brand but going for a different form factor, my first idea was a G-Wagon - quickly taken by Grohl himself. Then I though, let's fall back to a Jeep - also taken quickly by Tim.. Unimog was on my list of potential alternates, but I wasn't sure about it yet. But one thing was sure: I wanted to improve the actual off-road performance over the Zetros.

So putting the form factor aside a bit, I started experimenting with chassis and suspension designs. One option I had in mind was to keep the live axle setup, but improve it to make it actually useful for off-roading: increase suspension travel, ground clearance and responsiveness. Furthermore, I wanted to see how lightweight the axles could be built, because the Zetros's axles are pretty bulky. I was okay with doing away with the diff-lock, since I wanted to use the M motor for a 2-speed gearbox anyway (just like others).

During experimentation with axle designs and suspension travel, I have realized that the 3-link live axle setup of the Zetros is not only limited by the springs' short travel, but also by the 3rd link on the top. That link cannot rotate sideways on the chassis side and is mounted very high on the axles, and as the axle tilts, its top can move sideways significantly, and as the link cannot follow it sideways, it limits the axle's tilting movement; hence very limited articulation. The setup only works for a short range of movement as on the Zetros, which is limited anyway by the springs, but cannot work for a setup that aims for more articulation. One way out of this is to allow the 3rd link to rotate and follow the side movement of the tilting axle, but then the axle needs fixing sideways. There are two ways to fix that. Either a 4-link triangulated setup, or a 3-link setup with a Panhard rod. Although the Zetros does use a Panhard rod at the front, it is unrealistically mounted and too short, that would cause kind of bump-'steer' for larger axle articulation (not exactly bump steer, but more like shifting the whole axle sideways).

So after taking the limited number of suspension parts into account (only 6 towball sockets left out of 10 since 4 must be used for mounting the front wheel hubs, and only 1 6L link available in the set), I opted for the following design: on the rear axle, I used 4 towball sockets for a 4-link triangulated setup with long links, allowing the upper links to rotate sideways and follow the movement of the axle. The springs are attached to the middle of the lower links. This allows for long upwards movement of the axle at the end of the links, and makes the suspension much softer even with the hard springs. Pretty solid and allows for large articulation, just what I wanted. Furthermore, the axle itself is quite slim and rigid, and has good ground clearance, especially in the middle where it's most needed (as proven by my off-road tests).

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The front axle was more challenging. First off, increasing the ground clearance is only possible with one trick: the towball socket liftarms must be built at an angle, going upwards in the middle, and that limits possibilities. Second, the steering motor and its mounting takes space on the axle, and is in the way for the suspension mounting points. I have experimented with two other options: steering through a driveshaft coming from the chassis, but the placement of the motor in the chassis was problematic do to space required by all other motors and the battery, along with the largely articulated suspension design; second I tried to put the steering motor above the front axle, actuating steering through a linkage system. Although, this worked quite well mechanically (verified with the Powered Up app), unfortunately, the Control+ app itself killed this direction: the app limits the steering motor angle to about a 25 degrees less than the calibrated range, which is normally 90 degrees or motor rotation, resulting in about 65 degrees in case of the Zetros (that's why it's steering is so bad by the way). Now in my alternative steering design, the max calibrated angle would have been 45 degrees, and the 25 degrees minus by the app resulted in about 20 degrees of movement, resulting in almost no actual steering at the axle. Pretty sad that you can build something mechanically sound and then not able to control it with the app :(

So I fell back to mounting the motor on the axle, and used the remaining 2 towball sockets for two lower links, which again, I built longer to allow for a bit more articulation. Luckily, I was able to mount the motor in a way that it allowed to use the single 6L link as the 3rd link, allowing free sideways tilting movement of the axle, as it is attached to a towball pin on the chassis end as well. I also experimented with the spring mounting technique of the rear, but it was weak and wobbly on the front without the 4-link setup, so I went back to mounting the springs on the axle. However, I managed to move the springs closer to the center of the axle, resulting in both slightly larger articulation and softer suspension. Finally, the axle is fixed sideways by a long Panhard rod going from one end of the chassis to the other end of the axle, eliminating bump 'steer' quite well even at larger articulation.

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As a side note about parts, it would be really nice if the set had another 6L link somewhere, for example as a Panhard rod on the rear axle. It could be used to build another 4-link suspension to the front, and also as a steering link in an independent setup (which is problematic to build for other reasons as well).

So after sorting out the two axles and the suspension, I though this could be used in a Unimog chassis. Though not too different in form from the Zetros, but at least it keeps the brand. Furthermore, I decided to make it different in its functions: the first step was focusing on a proper suspension. The second was the drivetrain. And that also caused some difficulties.

Unimogs are short. You'd think that's not a big problem at this scale, as many RC Unimogs have been built before, but it is with the latest drivetrain parts. The new CV joint with a sliding axle hole is a whopping 8 studs long!! (Compare that to a 3L U-joint; sure, the CV joint has the axle built in on one side, but this construction limits builds quite a bit. Not sure why the new sliding variant needed to be 2 studs longer, I thought it would be just 1 longer, enough for the sliding, which is typically about half a stud, but 1 stud max). So two of those, plus, two non-sliding ones at 6 studs length, and your driveshaft is already 28 studs long. Add to that the amount of space required by the differentials (2 studs on each end until the axle center), and your axle distance cannot be less than 32 studs. Add one more to get a nice odd length middle section, and we are at 33 studs from axle to axle. It is quite long for a Unimog. (As a side note, I tried to split the driveshaft in two halves, shifted sideways in the opposite direction to let them overlap in length and hence make things shorter, but it did not work out due to the axle length limitation of the CV joint part and also space limitations from the motors).

So because of this, and other considerations I decided to do away with the differentials altogether. It saves 2 studs in length. It allows for a slightly faster 12T / 20T gearing; the Zetros is just too slow. Furthermore, it allows for larger ground clearance, as the differential does not stick out from the axle. It even allows for a slimmer axle design, as there is more space in all directions, such as for mounting the steering motor. And last but not least: no differential - no need to lock them :) With unlocked differentials the off-road performance would be bad anyway.

Once I settled for the length and the drivetrain, I needed to place the remaining motors and the gearbox. I was able to sandwich the gearbox between the drive motors, and put the gearbox's driveshaft motor on top, the flip-flop beams and the frames proved to be very useful for building a strong but lightweight chassis. The final challenge was the driveshaft from the gearbox motor. I am not sure why TLG used this motor here as it is super cumbersome to work with. It needs substantial down-gearing (and the right amount for your actual application, which is 180 degrees final turn for a gearbox, but 90 degrees for a diff-lock), a clutch mechanism for safety and physical end stops for calibration. These all take up space as they need to be routed somewhere. Things would have been much easier with another L motor.. (Maybe to make the set slightly cheaper? Or TLG is trying to get rid of these motors on stock? They don't seem to fit well into the PU system without positional control.) Anyway, finally the chassis of the Unimog was complete. I used the remaining CV joints to connect the drive to the spinning fan in the front :)

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The rest was just bodywork and placing that big hub somewhere. I opted for roughly the same position in the cab as the Zetros, but with the cables inside, to allow easy access to the batteries for replacement. For the design and styling, I took some inspiration from online sources like this one because of its color scheme, but I wanted to minimize the cab length as there weren't enough panels for a 4-door version.

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I built the cab with no opening hood to make it solid :) And also made the doors lockable to avoid them opening automatically when driving around.. Finally, the black panels were just the right size and amount to cleanly finish the bed, both the sides and the floor. The bed floor opens (but lockable) to see the gearbox, and to give room for accessing the batteries. I have also added fenders and the usual decoration like the rollbars, exhaust pipe, mirrors, ladders. Here are some renders of the whole build. As you can see, I did not use any green parts, as they would have only added clutter to the otherwise clean bodywork I think.

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Some more notes about the parts of the set apart from the ones mentioned above. The set has a good amount of paneling and beams and connectors, although the green parts are a bit less useful since they are limited in number and size. Another shortcoming of the set is the available gears. It has all sizes, but their number is lacking in some cases and hence high gearing ratios are hard to achieve. For example if you try to deviate from the default drivetrain, those gears will be missing for example for a simple down-gearing for the gearbox motor..

More images can be found on Bricksafe.

Building instructions are available on Rebrickable.

I did not know too much about Unimogs in the beginning, but during the research and designed process I actually came to like Unimogs quite a bit. I like that they are compact but seemingly efficient machines, and the trial versions actually look pretty cool. And I am actually quite happy with the end result.

Let me know how you like it and your opinion on the set's parts!

Cheers,

Viktor

Edited by gyenesvi

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Really enjoyed reading this - thank you for the detailed look at your design process! Sounds like there are an unusually large number of constraints, even for a B-model. The idea for a combo diff-lock/gearbox seems pretty doomed, haha. 
 

The performance that you’ve achieved working within all these constraints is really exceptional - great job!

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This is really cool. I love crawlers, and your thought processes in designing this were good to read through.

What is the final gear ratio at the wheels?

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What a great B-Model! Looks and worksbetter than the A-model :D

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Thanks for sharing and describing the design proces Viktor. Very valuable for me and a nice read! 

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This 404 Mog is a legend :thumbup: and your extraordinary work just raised the barr

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The appearance is gorgeous! I just don't really like the inverted Ackermann geometry in your steering, I think it spoils the handling of the "Premium" mос. This is despite the fact that the front axle also doesn't even have a differential.

It's a pity not many people read notes like this, for example:

 

Edited by Igor1

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I'm happy someone turned the Zetros into a Mog: it had to be done. @gyenesvi you did a great job with the design, and the functions works very well with the parts you had available. Thanks for the writeup as well, it was a joy to read your process.

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An absolutely wonderful MOC! it's one of these cases where being a B-model benefices the MOC and elevates it to a new level. Special thanks for the write-up. This is what I dig on that forum. It's entertaining and educational, and kind of give you a sense that you are not alone in your struggles. To me the process is equally important to the end result, so it is nice to see that presented properly too.

The model itself is really good. It seems to be a capable off-roader, the solutions for the axles are very clever and the frame looks clean. And most importantly it's a MOG!!! ;)

P.S. Can you share a picture of the underside of the front axle? I am interested in how these angled towball socket liftarms are mounted. Thanks!

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On 10/12/2021 at 5:56 PM, Pattspatt said:

The performance that you’ve achieved working within all these constraints is really exceptional - great job!

On 10/12/2021 at 6:47 PM, Ullum Zurt said:

This is really cool. I love crawlers, and your thought processes in designing this were good to read through

On 10/12/2021 at 7:00 PM, Jundis said:

What a great B-Model! Looks and worksbetter than the A-model :D

18 hours ago, 1gor said:

This 404 Mog is a legend :thumbup: and your extraordinary work just raised the barr

4 hours ago, proran said:

An absolutely wonderful MOC! it's one of these cases where being a B-model benefices the MOC and elevates it to a new level. Special thanks for the write-up.

14 hours ago, Igor1 said:

The appearance is gorgeous!

Thanks for the kind words guys, I really appreciate that!

On 10/12/2021 at 5:56 PM, Pattspatt said:

The idea for a combo diff-lock/gearbox seems pretty doomed, haha. 

Well, I wouldn't say it's not possible to do out of these parts, but it would definitely take more space and hence would require a different form factor, for example something with a less space-hungry suspension.

On 10/12/2021 at 6:47 PM, Ullum Zurt said:

What is the final gear ratio at the wheels?

So in low gear the gearbox is 1:1 and instead of the diff there's a 12:20 gear mesh, and there's the 1:5.4 ratio of the hub, that's 1:9 in total. In high gear, there's a 20:12 up-gearing, so that's just the 1:5.4 altogether.

4 hours ago, proran said:

P.S. Can you share a picture of the underside of the front axle? I am interested in how these angled towball socket liftarms are mounted.

I have added one more render from the front (I forgot that previously), where the mounting is well visible, does that do it for you?

14 hours ago, Igor1 said:

It's a pity not many people read notes like this, for example:

@Igor1, thanks a lot for bringing my attention to this thread, I missed this completely, although very interesting topic (I was actually on holiday that week, and the discussion only lasted two days, probably went out of view pretty quickly). I only saw the video from @kbalage about the resolution. Anyways, the whole explanation in there is really useful.

14 hours ago, Igor1 said:

I just don't really like the inverted Ackermann geometry in your steering

And thanks for noting this! I'm not even sure why, but I never really took the time to think this inversion effect through properly. If I understand correctly, you are referring to the fact that if the steering rod is in front of the axle, then the Ackermann geometry reverses. I have been using this geometry because I saw it in many builds, from builders that I respect, for example in @keymaker's Hornet or even @Didumos69's Rocky uses it (although that one is a bit more complicated in this respect, because it uses two steering rods, one behind the axle with Ackermann geometry, and one in front with anti-Ackermann geometry (actually steered here), so not sure what the total result of those is, but I think it works in lego because of all the tolerances of the loose links). Not trying to point at people here, just quoted them because I am curious what they have to say about this matter.

So this problem made me think for a while about how to resolve it. It's not enough to fix it by mounting the steering link further out on the hub as @Attika did (I adopted his solution to this axle, see the image below), because then the steering rack will be 1 stud in front of the mounting point, and that also creates a minimal anti-Ackermann effect in my case (in the case of @Attika, it creates a slight Ackermann effect, which is just what is needed, so that's really nice when behind the axle). So to resolve that, the steering rack would also have to be brought backwards, which then would collide with many other things (not just the motor, but the springs and some support as well). The thing is that routing the steering rack only two studs away from the center of the axle is pretty hopeless for a driven axle with a servo on it if you don't want it to be huge (for example, the solution of the Zetros would not fit into the short nose of the Unimog), as the towball liftarms need quite some support that's always going to be in the way. And a differential would just make things even worse (actually, this axle could house the differential, it's just that the driveshaft on the back would have to be moved back by 1 stud). However, using a 13L steering rack could elegantly solve the problem, because more link mounting options would be available there, but that's out of the question for this build as there is no such a part in the set.

So while the solution below solves most of the anti-Ackermann problem, it raises another issue. Now the steering angle is more aggressive, because the mounting point on the wheel hub moved one stud closer to the axle. At that steering angle, the wheel starts to bump into the chassis.. So I had to insert a half pin to the steering link to limit the angle a bit (then the wheel does not seem to touch anything). However, even at this limited angle, I can hear that the angle sounds a bit too much for the CV joint. It starts to move slightly jerky at max angle; it is especially noticeable when reversing, or when moving slowly, the sound of the drivetrain is not uniform but bumpy. I wonder if that happens to the version of @Attika, did you test reversing or slow driving, observing the sound and see if it gets jerky?

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So at this point, I am not sure if I should update the model to this version. The steering performance is definitely improved, but I am worried a bit about complaints about the jerkiness and damaging the CV joint. You know, when selling instructions, it may be more important to avoid damages and complaints than to have the perfect geometry (obviously, if it was just for myself, I'd go for the better geometry).

So let me know what you think about this possible fix!

Edited by gyenesvi

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Amazing build, hats off, really, regardless of any steering questions. :-)

I've never took the challenge to build alternate models due to the limitation on parts. There is enough compromise to make in a building process, could not handle a self inflicted one. :-D 

Considering that, I feel like the way you've designed the (1st) steering mechanism is ok.

Although @Igor1 is making a valid theoretical point on the geometry (and hereby I'm thankful for him, carrying this flag in my absence) I'm more apologetic in this case for a few reasons:

- Limited part usage (as mentioned above)

-On the video it did not come accross as bad as I've expected. In fact I hardly even noticed the problem. (I've read the topic first, then went for the video) Might be due to the freeplay of the frictionless pins are counteracting and the steering lock is also modest.

-Without differentials.... :-) a small anomaly in the geometry doesn't even tickle my nerves (don't get it wrong, I totally agree with the no diff policy, you've applied)

- and finally, probably you did something here what is sort of common in similar designs. I've came across some rc rock crawlers explicitly using the same, inverted ackermann setting, yet I can't tell if it is some clever stuff or budget cut on the engineers desk, so I leave it for someone who has a grasp on this question.

I'm closing with a few words on the jerkiness of the new cv's in the 2nd design. My bet is on the lack of the differential. In this setup the rotation difference between the two front wheels is high enough to make one of them skid in the corner, this could manifest as a periodic hop of the given wheel. I think that can be confused with the cv's limit, but I doubt the latter. If you can test it by making the 2 wheels independent from each other for an experiment... I didn't have this problem in models with diffs.  Anyhow, it is up to personal taste I suppose, when it comes to smooth rollin', but I can assure you, it takes way more to damage the cv or the hub. As a reference, I offer my video, titled "wannabe tatra" to your attention, with the sidenote, that those hubs and cv's are still fully operational. As a rule of tumb, 45 degrees doesn't hurt them. 1200 rpm in the other hand does ;-) if lube not applied

Great job again

Edited by Attika

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5 hours ago, gyenesvi said:

So I had to insert a half pin to the steering link to limit the angle a bit (then the wheel does not seem to touch anything). However, even at this limited angle, I can hear that the angle sounds a bit too much for the CV joint. It starts to move slightly jerky at max angle; it is especially noticeable when reversing, or when moving slowly, the sound of the drivetrain is not uniform but bumpy. 

Assuming this is new behavior that wasn’t a problem with the old steering solution - are there any 1x1 round tiles, or 1x1 round plates, or 3/4 pins that you could add/swap, and see if that sorts it? Or a non-round 1x1 plate could work too, with the stud pressed into the pinhole.

Edited by Pattspatt

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@gyenesvi In general, the geometry of Ackermann is changed to negative not by the fact of the location of the steering rod in front of the axle. But by the place of the articulated fastening of this steering rod to the wheel hubs, relative to the pivot axis of the hubs themselves. Thus, for any location of the steering rod relative to the axle - positive, negative or zero Ackermann angles can be obtained. Of course, this also depends on the fastening of the pivot axis of the wheel hub itself, which in this particular case you cannot change.

Given the fact that in lego the axis attachment points go in one stud increments, it's most likely simply impossible to get the correct Ackermann angles on such a scale. Therefore, many are inclined to make the Ackermann angle zero, since with the backlash of the parts that we have, this isn't critical at all (on this scale!).

So, initially you had a negative Ackermann angle, which led to the fact that the inner wheel (which was located closer to the center of the turn), turned at a smaller angle relative to the outer wheel. Perhaps it even compensated for the speed of rotation of this wheel somehow, since you don't have a differential on the front axle. I just initially thought that the geometry of the wheels when turning was not ideal and that this would lead to slipping of the inner wheel when turning. However, if you were to say that the negative Ackermann angle was deliberately made to compensate for the lack of a differential, I would gladly take note of that. But it looks like you were guided by other considerations. Indeed, the MOC rides very well in your off-road video.

I don't think that now you should urgently change something in the front axle, since any change can be critical and lead to even greater problems. Such things need to be thought out initially, because when you change the fastening of the steering rod, the lever and the maximum angle of rotation of the wheels change. As I understand it, you are very limited in the current size of the MOC, so you can't put a differential in the front axle either. And it's simply necessary for the correct steering geometry, and without which wheel slippage and even jerks can occur when turning. Also when adding a differential an additional question will arise with the location of the servo motor, most likely they will interfere with each other.

Nevertheless, I think your solution is quite balanced. Given that you are limited in the choice of parts, the design was most likely very difficult! I would never do that myself. But in fact, someone also makes from Lego sets and alternative models with the addition of a small number of critical parts.

I made my initial remark more for discussing technical solutions that are not often encountered when someone presents their MOC. Yes, Lego sets are being poured with mud, but there is practically no technical criticism of designer's MOCs, or very little of it. This leads to the fact that the authors don't immediately receive more feedback on the correctness of their technical solutions. Which they may simply not know or not notice during the design. As a rule, everyone only praises the appearance of the MOC or is simply silent, which of course is good form here. I just wish the technical discussions of the MOCs were a little more varied as well. Thank you @Attika for joining us on this topic. I'm always pleased to read your thoughts on any occasion. :thumbup:

Edited by Igor1

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15 hours ago, gyenesvi said:

So in low gear the gearbox is 1:1 and instead of the diff there's a 12:20 gear mesh, and there's the 1:5.4 ratio of the hub, that's 1:9 in total. In high gear, there's a 20:12 up-gearing, so that's just the 1:5.4 altogether.

That's very good crawling performance even in high gear!

I really need to learn how to build a 2-speed gearbox for my crawler.

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21 hours ago, Attika said:

Amazing build, hats off, really, regardless of any steering questions. :-)

I've never took the challenge to build alternate models due to the limitation on parts. There is enough compromise to make in a building process, could not handle a self inflicted one. :-D 

Considering that, I feel like the way you've designed the (1st) steering mechanism is ok.

Thanks for taking the time to chime in! I actually enjoy the parts limitation sometimes, as it restricts the options and gives you some finite limits to build with, instead of the unlimited possibilities you would have otherwise, and also forces you to be creative with what you have available. But I understand you, nowadays I also want to push the limits and design build with all possibilities that lego allows, which is also somewhat limited.

21 hours ago, Attika said:

I've came across some rc rock crawlers explicitly using the same, inverted ackermann setting, yet I can't tell if it is some clever stuff or budget cut on the engineers desk, so I leave it for someone who has a grasp on this question.

I've only read that anti-Ackermann geometry is used intentionally in race cars, that it has some benefits when cornering with high speeds. I guess that does not apply to crawlers though..

21 hours ago, Attika said:

I'm closing with a few words on the jerkiness of the new cv's in the 2nd design. My bet is on the lack of the differential. In this setup the rotation difference between the two front wheels is high enough to make one of them skid in the corner, this could manifest as a periodic hop of the given wheel.

That's a good idea to test just to understand the situation, I'll give that a try, because the axle actually has space for the differential, so it should not be too difficult to test, I will only have to modify the driveshaft to use an older CV joint which is shorter.

22 hours ago, Attika said:

As a reference, I offer my video, titled "wannabe tatra" to your attention, with the sidenote, that those hubs and cv's are still fully operational. As a rule of tumb, 45 degrees doesn't hurt them.

Thanks, haven't seen that video before, nice one, and good to know.

22 hours ago, Attika said:

1200 rpm in the other hand does ;-) if lube not applied

Do you mean driving them with buggy motors? :) I just want to get some BuWizz motors, but a bit worried about putting wear on some parts with it. Does lubrication eliminate the problem? What kind of lubrication is safe to use with Lego?

18 hours ago, Pattspatt said:

Assuming this is new behavior that wasn’t a problem with the old steering solution - are there any 1x1 round tiles, or 1x1 round plates, or 3/4 pins that you could add/swap, and see if that sorts it? Or a non-round 1x1 plate could work too, with the stud pressed into the pinhole.

Yes, this is new behaviour, I have tested it with the original setup and it wasn't a problem there. I did think about those 1x1 tiles or plates, haven't tried it yet, but my guess is that it will limit the steering angle too much. Anyways, will give it a try.

11 hours ago, Igor1 said:

However, if you were to say that the negative Ackermann angle was deliberately made to compensate for the lack of a differential, I would gladly take note of that. But it looks like you were guided by other considerations.

No it, was not deliberate :) Not sure if that could work as a compensation either, but there might be some explanation, just like in the case of fast cornering race cars in my comment above..

11 hours ago, Igor1 said:

I don't think that now you should urgently change something in the front axle, since any change can be critical and lead to even greater problems. Such things need to be thought out initially, because when you change the fastening of the steering rod, the lever and the maximum angle of rotation of the wheels change.

Exactly, agreed.

11 hours ago, Igor1 said:

I made my initial remark more for discussing technical solutions that are not often encountered when someone presents their MOC. Yes, Lego sets are being poured with mud, but there is practically no technical criticism of designer's MOCs, or very little of it. This leads to the fact that the authors don't immediately receive more feedback on the correctness of their technical solutions. Which they may simply not know or not notice during the design. As a rule, everyone only praises the appearance of the MOC or is simply silent, which of course is good form here. I just wish the technical discussions of the MOCs were a little more varied as well.

I have also observed such a focus on the appearance by many comments, which is good as encouragement and appreciation, but as you say, the technical aspect should also be viewed and occasionally 'criticised' constructively. I'm totally glad that you did bring up this technical aspect, as I have already learned from it. And it's not just for this MOC, but also for others in the future, as I know I will use similar axle constructions in my future builds. So the reason I want to see possible solutions for this is because I want to understand how well it is possible to build such an axle. Unfortunately, as I see, it's really hard to build a proper steered and driven axle with a differential using any of the currently available joints and wheel hubs in an RC car in medium scale, even at large scale. You will have to compromise something, something will either be bulky, or some geometry will always be just approximate..

8 hours ago, Ullum Zurt said:

I really need to learn how to build a 2-speed gearbox for my crawler.

That's not too difficult as long as you are okay with a modest gear ratio, just check out how the hi/lo gearbox is in the Defender for example. And then remote controlling it is also simple with a PU L motor, for that you could check out the Volvo hauler's gearbox.

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9 hours ago, gyenesvi said:

I just want to get some BuWizz motors, but a bit worried about putting wear on some parts with it. 

It is plastic only on the end of the day, so you may rightly expect some wear. with safe design, it canbe reduced though. Appears as withe powder around axles rotating in the holes of beams or frames. 

9 hours ago, gyenesvi said:

Does lubrication eliminate the problem? What kind of lubrication is safe to use with Lego?

I go with silicone based lubricant. Namely WD-40 has a silicone spray that  used to use. Didn't experience any negative consequence yet, although I'd use the verb "reduce" instead of "eliminate". Got to accept that every magic comes with a price. :-) It is sand you wanna keep those cv's away from. No lubricant can help against that.

Edited by Attika

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Quite good silicone lubricant LIQUI MOLY Silicon-Fett (7655), which I use occasionally. Unlike sprays, it is thicker.

If you choose from sprays, LIQUI MOLY LM-40 (8048) probably works well. But the spray is difficult to apply locally only to the desired friction pair. It's very strongly sprayed and spreads over all parts. As a result, the entire MOC may end up in grease.

Edited by Igor1

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On 10/14/2021 at 1:58 AM, Attika said:

on the jerkiness of the new cv's in the 2nd design. My bet is on the lack of the differential

@Attika, you were right! I have popped in differentials to both axles (had to use older CV joints to make space in the driveshaft), and the jerkiness disappeared at high steering angle. Thanks, another lesson learned! I would have thought that these tires would just silently slip without me noticing it..

25 minutes ago, Attika said:

Appears as withe powder around axles rotating in the holes of beams or frames.

I have experienced that already with PU motors when geared up or when pushed hard off-road..

10 minutes ago, Igor1 said:

Quite good silicone lubricant LIQUI MOLY Silicon-Fett

Thanks for the tips from both of you!

On 10/14/2021 at 5:09 AM, Pattspatt said:

are there any 1x1 round tiles

I have also tested this, put 1x1 tiles on the half pins in the second steering link design, and it worked better than I thought; the resulting steering lock is not very bad, quite similar to the original design, and at that angle still there's no jerkiness (without diffs).

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Did you think that only pathetic weaklings use the differential? :laugh:

Actually, I was faced with the fact that even on the rear axle it's not worth doing a differential lock without extreme need, since this also leads to increased loads on the transmission, so that the gears may start to crackle somewhere. You can only get away with all this if the MOC is really very light. On heavy MOCs it is better to use differentials.

Edited by Igor1

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21 minutes ago, Igor1 said:

Did you think that only pathetic weaklings use the differential?

Of course not, haha :wink: But I thought the lack of diff would manifest itself differently, just slipping, since these tires slip quite easily (my test was done on a flat floor).

23 minutes ago, Igor1 said:

In fact, I was faced with the fact that even on the rear axle it's not worth doing a differential lock without extreme need, since this also leads to increased loads on the transmission, so that the gears may start to crackle somewhere.

So what counts as extreme need for you? On the terrain I tested this (and the Zetros), I found that it gets stuck quite easily with open diffs, that's why I decided to remove them. But I think it depends heavily on the capabilities of the suspension as well; with a suspension that's highly articulated, it gets by much better with open diffs as well; for example this build of mine has a more articulated suspension, and I found it still climbs still quite well with open differentials too. But it's good to know that the closed system may stress the gears a bit too much..

 

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57 minutes ago, gyenesvi said:

@Attika, you were right! I have popped in differentials to both axles (had to use older CV joints to make space in the driveshaft), and the jerkiness disappeared at high steering angle. Thanks, another lesson learned! I would have thought that these tires would just silently slip without me noticing it..

:pir-huzzah2:

 

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41 minutes ago, gyenesvi said:

So what counts as extreme need for you?

I'm probably exaggerating a little, since in the case of using planetary hubs, the load on the transmission is really low and you can put the differential lock without any special consequences. But if you don't use planetary hubs or portal hubs with large reduction, this can be a real problem.

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On 10/13/2021 at 5:29 PM, gyenesvi said:

does that do it for you?

Thanks a lot. I can't quite figure, it is not a pythagorean triangle, is it? I know it doesn't matter much, just curious.

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1 hour ago, proran said:

I can't quite figure, it is not a pythagorean triangle, is it?

No it's not, but it's really simple: the lower A-arm is simply rotated downwards as it goes out to the hub, its outer end is 1 stud lower than the inner one. As a consequence, it becomes shorter, and the hub itself is tilted inwards a bit at the bottom to reach the A-arm, so the wheel is not vertical, but this whole effect is so small that it's hardly recognisable (and part of it is kind of offset by the freeplay in the wheel mount). Unfortunately, this is the only way to mount these hubs with more ground clearance in the middle. I did not invent this trick, but learned it from others here.

An unfortunate side-effect is that the vertically middle point of the hub also gets closer to the center. It's not a problem for the driveshaft (again because of some tolerances), but there's a very slight toe-out in the steering. But it's still a good compromise I think.

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3 hours ago, gyenesvi said:

No it's not, but it's really simple: the lower A-arm is simply rotated downwards as it goes out to the hub, its outer end is 1 stud lower than the inner one. As a consequence, it becomes shorter, and the hub itself is tilted inwards a bit at the bottom to reach the A-arm, so the wheel is not vertical, but this whole effect is so small that it's hardly recognisable (and part of it is kind of offset by the freeplay in the wheel mount). Unfortunately, this is the only way to mount these hubs with more ground clearance in the middle. I did not invent this trick, but learned it from others here.

An unfortunate side-effect is that the vertically middle point of the hub also gets closer to the center. It's not a problem for the driveshaft (again because of some tolerances), but there's a very slight toe-out in the steering. But it's still a good compromise I think.

Also what matters is where you attach the steering arm to the hub. If you attach it at the same level as the upper arm, than the slight tilt of the hub will have a zero affect. If you attach all the way on the height of the lower arm, than it will matter a lot.

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