LEGO 42129 MB Zetros Foundry Sand Trials: Is this set playable or not?

[Kamil]

Some say Lego Zetros Trial Truck can only be played at home by using books as obstacles. To prove the opposite, a foundry sand track was built for Lego Zetros Trial Truck with bigger Lego wheels mounted on it. The truck completed the track. This set is playable on a sand track with variable road conditions.

Plus Sides:

Truck has enough power to be played with bigger wheels. 

The steerability of the truck is good on sand surfaces with bigger wheels. 

The speed of MB Zetros has increased by using bigger Lego tires. 

Rear differential-lock is useful for climbing slopes. 

Minus Sides:

Turning radius of MB Zetros is insufficient for narrow tracks.

Light rear end of MB Zetros causes problems whenever rear traction is needed. 

Front differential can not pull or push the vehicle under hard conditions. 

 

The trial video is here on Youtube:

 

I think, this set is still playable.

Thanks for watching.

 

[gyenesvi]

Well, I'm not sure I agree that this is what playability means. Sure, you can drive it on sand. But taking 12 minutes to complete such a track would get me really bored. And the fact that it is not really turning would get me irritated sooner.

Furthermore, on the capabilities of the set. The problem is that most of the technicalities in the set are useless gimmicks when it comes to trial and play. Take for instance the diff locks. In any practical RC trial truck there would be no diffs, all would be locked, saving a lot of gears and space, allowing for better ground clearance for example. Second, the suspension, it is practically not articulating too much. So, my guess would be that the truck would perform almost equally if you just ditched all the diffs and locks, and maybe even the whole suspension and just made it rigid, or one of the axles pendular. It would be interesting to see a comparison with those modifications in place.

[Jundis]

Interesting experiment.

But why did you change the wheels? This tire profile seems way worse than the tractor tires of the original. Only the increased diameter enhances traction.

Really not an expert in building trial trucks, but you need better steering angle and, as @gyenesvi said, better suspension.

[Kamil]

On 10/23/2025 at 12:22 AM, Jundis said:

Interesting experiment.

But why did you change the wheels? This tire profile seems way worse than the tractor tires of the original. Only the increased diameter enhances traction.

Really not an expert in building trial trucks, but you need better steering angle and, as @gyenesvi said, better suspension.

@jundis That is a good question. When I first seen this set, I immidiately thought something is wrong with it. I realized that the wheels are out of scale. The proof is here:

In addition to looking closer to scale, these bigger tires are handsome, increase speed of the truck, give better ground clearence, and their specs differ on different terrain types. For example, you are right for loose sand conditions. However, it does not matter on wetted stable sand. I have videos on preparation (part 2, part 3 etc.) so the answers may remain in them. I will try to add them here in my new posts. Thanks in advance.   

This is my second video on Lego Zetros foundry sand trials. Performances of three different Lego wheels were compared in it. Dry and wetted stable sand surfaces were obtained. 

 

Edited October 26, 2025 by Kamil
İmage file sizes

[keymaker]

Hmm here I think is more informative video and also comparison with trucks, which were actually built as trial trucks.

 

[Kamil]

On 10/27/2025 at 11:16 AM, keymaker said:

Hmm here I think is more informative video and also comparison with trucks, which were actually built as trial trucks.

 

Yes, I have seen this video previously, thank you for posting it here. I am not sure this comparison is right, Zetros and other vehicles may be different classes. Zetros is a safety concerned commercial set (no buwizz motors etc.) for 12+ customers. It has more finished exterior shell than others, so it is heavier and slower than others too. I wish it had two manual diff. locks, and a high gear, but not available here. Maybe they always leave incomplete functions in a commercial set, so we have to go and buy new ones to complete it.  I would compare Zetros with some of these more finished MOC's below:

Sariel's pug proof MAZ-535 8x8 truck (having more finished exterior):

 

  The "Zetros Killer":

Or, maybe this Unimog:

Or a similar Zetros:

 

To me, Lego Zetros is more a Unimog than a Zetros. It is more a rock crawler than a trial truck. Here, I want to stay on the "playability" scale. To avoid misunderstanding, I give my Lego off-road playability scale here:

12 points:

10 Points:

7 Points:

The three trucks on Sariel's comparison video above

3 Points:

Lego Zetros original

3.2 Points:

Lego Zetros with bigger tires

My new aim here is to upgrade Lego Zetros into rank 4 by adding few modifications to increase playability of the vehicle. 

Thank you

Edited October 31, 2025 by Kamil

[gyenesvi]

3 hours ago, Kamil said:

Here, I want to stay on the "playability" scale. To avoid misunderstanding, I give my Lego off-road playability scale here:

That all makes sense :)

3 hours ago, Kamil said:

To me, Lego Zetros is more a Unimog than a Zetros.

Then maybe you could include this one in the comparison, a Unimog built from the Zetros, specifically to improve capabilities (locked diffs, higher speed, better steering, softer suspension, lighter weight) :D

 

[Thirdwigg]

I love when I see a Thirdwigg Unimog in someone else's video and it holds up to testing! 

[Kamil]

On 10/22/2025 at 10:37 PM, gyenesvi said:

Second, the suspension, it is practically not articulating too much. 

gyenesvi thank you for the suggestion. For sure the red springs are too stiff for the rear axle of the vehicle. I have replaced them with softer grey springs. Now suspension became softer and gives more articulation. However, the real Unimogs and Zetroses also have a flexible chassis. The front cabin and the rear bed articulates in opposite directions while off-roading. This vehicle is far from this ability. The articulation I have added may seem insufficient to you (it wont be like the one used in the x-treme off-roader), but I think it is better compared to the original (see video Part 3 below). I have also added 107x44R tractor tires to the back axle. 

On 10/22/2025 at 10:37 PM, gyenesvi said:

The problem is that most of the technicalities in the set are useless gimmicks when it comes to trial and play. Take for instance the diff locks. 

 I recorded more videos for making the Lego Zetros, the ultimate beast (:-D) outdoor trials. On that video, I have discovered a hidden function of the diff. lock while trying to climb. The diff. lock transfers more power to rear axle or front axle. If the rear is stuck, it is possible to transfer traction to front axle. I have planned to merge these videos as Part 4, I need days to edit all. 

LEGO 42129 MB Zetros, with bigger wheels, foundry sand trials (Part 3): Is this set playable or not?

Cheers  

 

Edited October 31, 2025 by Kamil

[Kamil]

17 hours ago, gyenesvi said:

Then maybe you could include this one in the comparison, a Unimog built from the Zetros, specifically to improve capabilities (locked diffs, higher speed, better steering, softer suspension, lighter weight) :D

@gyenesvi Thank you for introducing me your cute little kitty. I really missed it within my searches. I wish I could build it for learning more. For sure, I will try to 

PS: The original tires matches the overall scale this time.

Edited November 1, 2025 by Kamil

[Kamil]

My last video on being the angel of Lego 42129 Zetros. The beast mode (Thanks to Viktor Gyenes for his suggestion for adding more articulation):

Video includes:

LEGO 42129 MB Zetros, with bigger wheels, outdoor trials:

1. The beast did not like sand trials

2. Outdoor climbing = Good performance

3. Outdoor tree root trials : Good performance

4. Outdoor rooftop ceramic tiles trials = Good performance (forward only)

5. Outdoor dirt trials = Acceptable performance

6. Outdoor dirt downhill trials = Good performance

6. Night driving, diff. lock on/off traction transfer trials = Good performance

 

Cheers

 

Edited November 4, 2025 by Kamil

[gyenesvi]

Glad you find my suggestions/link useful.

On 10/31/2025 at 7:35 PM, Kamil said:

I have discovered a hidden function of the diff. lock while trying to climb. The diff. lock transfers more power to rear axle or front axle. If the rear is stuck, it is possible to transfer traction to front axle.

I am not sure what you are referring to here. Is it with open or with closed diff? I am guessing open diff, because otherwise there would be no power transfer from one axle to the other when one is stuck. But that's actually a disadvantage of the open diff, not an advantage. Power always gets transferred towards the least traction, away from wheels that have traction, which seems like the car is trying hard, but trying at the wrong spot. So in the end it's not useful. Or do I misunderstand something?

Also, I guess sand does not work well even with tractor tires because once the sand gets into the cracks of the tires, it behaves like a big slick tire, which is not great.. Wet sand may require tire with special tread pattern / material that does not let the sand fill up the tread.

[Kamil]

3 hours ago, gyenesvi said:

Glad you find my suggestions/link useful.

I am not sure what you are referring to here. Is it with open or with closed diff? I am guessing open diff, because otherwise there would be no power transfer from one axle to the other when one is stuck. But that's actually a disadvantage of the open diff, not an advantage. Power always gets transferred towards the least traction, away from wheels that have traction, which seems like the car is trying hard, but trying at the wrong spot. So in the end it's not useful. Or do I misunderstand something?

Also, I guess sand does not work well even with tractor tires because once the sand gets into the cracks of the tires, it behaves like a big slick tire, which is not great.. Wet sand may require tire with special tread pattern / material that does not let the sand fill up the tread.

@gyenesvi Okay, thank you, good to talk about this.

Mine is an observation, not based on technical knowledge (or maybe not knowing like a mechanical engineer/mechanic etc.). In my video, diff. lock = on means engaged, on the phone control app. the button is at up position. “Off” means diff. lock is disengaged, and the button is at 0 position.

Let me explain the operation step by step, while giving the exact moments in one of the video scenes of the last video.    

-          At 18:33, I started the truck while diff. lock = on mode (I do so in all of my trials).

-          At 18:42, wheels start slipping, truck stops while at maximum throttle (truck is stuck). I release throttle.

-          At 18:48, I disengage the diff. lock (deep click is heard), bring it to off (zero position).

-          At 18:50, at full throttle, the front wheel starts turning faster, and the truck has moved few cm.

-          At 18:59, front wheel spins fast, and truck stops again. I engage diff. lock, deep click is heard.

-          At 19:00 the truck starts moving again.

 

I would be grateful for a technical explanation for this.

 

Coming to tires, okay, I see now. 

Thanks in advance

Edited November 4, 2025 by Kamil

[gyenesvi]

33 minutes ago, Kamil said:

I would be grateful for a technical explanation for this.

Well, I think you kind of got lucky there :) Sometimes just releasing and trying again, or moving the steering around can help move past a stuck position. Also, often (with proper RC cars) giving a burst of throttle can help move out of those situations, but lego motors are not powerful enough to allow for such bursts (since they were already running at full speed when the model was crawling slowly).

What happens is when the lock is on, the wheels are forced to spin together. If the motors cannot move the truck, it gets stalled. If you unlock it, the wheels that are less blocked will start to spin (almost freely), but typically, that's not enough to move the vehicle once it got stuck like that because those wheels have little traction. You might have had a bit of luck there though, as that allows at least the front wheel to spin up, so that might have helped a bit, but not for long. In general, the locked position has more chance of moving the vehicle.

[Kamil]

On 11/4/2025 at 10:12 PM, gyenesvi said:

so that might have helped a bit, but not for long. In general, the locked position has more chance of moving the vehicle.

@gyenesvi Absolutely, I leave it to the viewers to decide.

 Summary:

To be honest, I never liked this set before buying. I would prefer to buy Lego technic G 500 set instead. There are issues that do not fit the overall scale of the truck.

- The wheels are smaller than original,

- The cabin roof is higher and wider than original,

- The nose (engine cover) is shorter and wider than original,

- The shape of the truck's nose doesn't seem very realistic either.

I found this set for half a prize, it had unopened packaging, and I needed some parts of it for my new project. 

I built and played with this to satisfy my curiosity. 

My findings are:

- This truck is too heavy (around 3.3 kg) and battery consumption is high. I have used three sets of 6x AA battery packs during my trials,

- Some of its accesories come lose easily (like side mirror plates, fire extinguisher nozzles, snorkel and exhaust pipe) so precautions must be taken before using it outdoors or on sand,

- Instead of using an app. controlled motorised diff. lock, two manual diff. lock controls (controlled by sticks from interior cabin if possible), and one app. controlled high-gear selector would be a better option,

- It would be better to release this set as an expedition truck, instead of a trial truck, together with a rear cabin on the rear bed. Then its off-road capabilities would be reasonable on this basis. Here 94.3x38R Lego tires with black rims would be a better choice for this purpose,

- As the truck is too heavy and slow, momentum can not be used as a driving force to overcome obstacles while climbing. Only full power and traction combination is helpful here,

- Creating a flexible chassis, and gain more articulation via this chassis (cabin and rear bed can articulate in opposite directions as seen in off-road tests of these vehicles) would be fun to watch on Lego based Mercedes Zetros and Unimog vehicles,

- This set is a good part source for app. controlled  MOC's,

- If I were the official Lego builder, I would create this truck chassis as a set (Mercedes-Benz Zetros 3643-S 6x4 Tractor Head). While having 6x4 drivetrain, I would build a front steering system with higher turning angle to match original Mercedes specs. I would leave the buyer free for what to build on the rear bed. 

Zetros 6x4

- Playing with the original set was challenging, it was barely playable. I think modifying this vehicle to the beast mode in video 4 was more fun to play with. Plus, the beast mode is 4-4.5 on the playability scale above. What do you think?

 

Thank you

Kamil

 

 

Edited November 7, 2025 by Kamil

[Jundis]

1 hour ago, Kamil said:

- If I were the official Lego builder, I would create this truck chassis as a set (Mercedes-Benz Zetros 3643-S 6x4 Tractor Head). While having 6x4 drivetrain, I would build a front steering system with higher turning angle to match original Mercedes specs. I would leave the buyer free for what to build on the rear bed. 

The first leaked pics in 2021 were in 6x6 configuration, looked way better imho ;-)

 

[Kamil]

"The first leaked pics in 2021 were in 6x6 configuration, looked way better imho ;-)"

@jundis I am not sure the leaked Zetros is fake or real (it also has some similarities with the red Zetros 6x6, released four yrs before 42129, which is posted above).  I feel like I saw two XL engines in the leaked one. Okay, did the Lego CEO's told the builder "remove one axle, shorten the bed, change the two XL motors to L motors, then decrease speed of the truck?" I havent witnessed such a torture, from design to production, before. Something like designing a G63 6x6, and Multipla is coming out of the production line  

Edited November 7, 2025 by Kamil

[Jundis]

Would have been strange that this leak was faked since the medium tractor tires weren't around that time.

There were many speculation in the forum while the third axle has been left out. Could be just cutting costs, but maybe had more to do with traction, since Lego advertised the vehicle with it's climbing ability (at least on wood or other flat surfaces, not on dirt). Also the Zetros got a facelift at that time and the outer shape of the grill changed. 

[Kamil]

26 minutes ago, Jundis said:

Would have been strange that this leak was faked since the medium tractor tires weren't around that time.

There were many speculation in the forum while the third axle has been left out. Could be just cutting costs, but maybe had more to do with traction, since Lego advertised the vehicle with it's climbing ability (at least on wood or other flat surfaces, not on dirt).

@jundis thank you for info. 

27 minutes ago, Jundis said:

Also the Zetros got a facelift at that time and the outer shape of the grill changed. 

@jundis Yes, I learned this by chance. I was building Zetros from computer screen, and realised my MB emblem holder pieces were different from the pdf instructions. That was weird being the same set having two different instruction files. 

[Kamil]

On 11/4/2025 at 7:23 PM, gyenesvi said:

Glad you find my suggestions/link useful.

@gyenesvi For sure, it's great that you have created at least three different vehicles while staying within the 42129 parts limit 

[gyenesvi]

13 hours ago, Jundis said:

Would have been strange that this leak was faked since the medium tractor tires weren't around that time.

I think the one on the leaked image had large tractor tires, not medium. Count length of the green beam above the front tire..

[Jundis]

1 hour ago, gyenesvi said:

I think the one on the leaked image had large tractor tires, not medium. Count length of the green beam above the front tire..

Hmm pretty sure in the leaked pic it's this 43.2 rim, also counting the chevrons: 18 on medium, 20 on big tractor tire.

But the proportions to the liftarm above is indeed strange. Maybe perspective?

Edited November 8, 2025 by Jundis

[Kamil]

3 hours ago, Jundis said:

Hmm pretty sure in the leaked pic it's this 43.2 rim, also counting the chevrons: 18 on medium, 20 on big tractor tire.

But the proportions to the liftarm above is indeed strange. Maybe perspective?

Another issue on this leaked pic is written by another user in another platform (not sure to mention these names here). There says no suspension clearence exists between the front tire and the mentioned liftarm. So, the front axle must be a fixed one. I think, even it is fixed, there should be a visible clearence between them to appear normal to the eye.  

Edited November 8, 2025 by Kamil

[Kamil]

Hi, I disassembled the Zetros, found some wear and dirt on the parts due to these trials (everything was brand new before trials). Oiling is needed to prevent this wearing issue, maybe graphite grease... 

Edited November 12, 2025 by Kamil