Review - 42129 - 4x4 Mercedes-Benz Zetros Trial Truck




This is the second set of the 2H wave of 2021 I will be reviewing. To be honest; this wasn't a set I was really looking forward to building. We've seen a few preliminary pictures of this model and it has changed quite a bit since its first appearance. This model started as a 6x6, but ended up as a 4x4. Due to the smaller sized wheels, it got dubbed "sausage dog truck". While I absolutely agreed and had a laugh about it, I do think the final design looks better than I expected. The smaller diameter of the wheels provide more torque, so I do expect outstanding performance. I am looking forward to seeing this bad boy climb hills like there's no tomorrow :wink:

This set is designed by Milan Reindl who has been working at TLG for nine years now. Time flies. It seems like yesterday, he was churning out B-models here on Eurobricks hehe. In those nine years he has really made a name for himself and he has delivered an impressive list of cool models. Let's find out if this one can be added to that list.

Made for extreme conditions, the 4x4 Mercedes-Benz Zetros Trial Truck has the power to tackle the roughest terrain. Now you can build your own LEGO® Technic™ version of this mighty machine. Enjoy a challenging build then use the CONTROL+ app to steer the truck and operate the differential locking. Set up obstacles and use the flags to create a trial course. Can anything stop this mighty truck?

Pictures can be clicked to view hi-res versions. My Flickr album contains all the photos taken for this review.

The CEE Team of TLG has provided this set. It's not my goal to promote this set. It's my goal to give you an honest opinion about it. Therefore, the opinion in this review is my own and is in no way linked to TLG.


Number: 42129
Title: 4x4 Mercedes-Benz Zetros Trial Truck
Theme: Technic
Released: 2021
Part Count: 2108
Box Dimensions: 57,5 cm x 47,2 x 8,4 cm
Weight: 3210 gram
Set Price (RRP): 299.99 EUR / 274.99 GBP / 499.99 AUD / 299.99 USD / 399.99 CAD
Price per Part: 0.142 EUR / 0.130 GBP / 0.237 AUD / 0.142 USD / 0.190 CAD
Links: Brickset, Bricklink


The front of the box shows the trial truck in action. Apparently, it's perfectly fit to deal with rugged terrain. Looks like a ton of action. Makes you wonder how aggressively this model will climb steep hills. 

The back of the box is used to emphasize the Control+ functions. The picture at the bottom right show a pretty steep incline. That definitely needs to be verified in real life :wink: 


The box contains

  • 1x Instruction manual
  • 1x Sticker sheet
  • 1x Inner box
    • 5x PowerUp elements
    • 5x Bag for steps 1 and 2
  • 4x Tire
  • 11x Bag for steps 3 to 6

The book and sticker sheet were a bit wrinkled. I sure hope we will get the cartboard envelope packaging for all bigger sets soon.

I have tried to straighten the book a bit.

The sticker sheet before recovery.

The sticker sheet after having it pressed tight between the pages of the instruction manual.

Four new tires.

There is an inner box inside the regular box. This inner box contains all of the PoweredUp elements.


And the bags for steps 1 and 2.





81 x 35 TIRES
The most obvious new part is the new 81 x 35 tire. It's the smaller version of the "Claas tire". This tire will make a lot of tractor builders very happy. 

This set contains four motors. Three PoweredUp L motors and a WeDo 2.0 motor. I would have prefered a new PoweredUp M motor, but it does show that other motors can be used in Technic models as well.

This set contains two 15L and nine 11L perpendicular liftarms. 


This part already appeared in the Ferrari and two other 2021 sets. Actually, I have taken the wrong picture, because this set also contains these parts in dark bluish grey, making its first appearance.

New-ish clutch parts and a new longer CV joint part.

This part already appeared in the 4x4 off-roader. It's great to see it make it into another set. The name on Bricklink has a nice ring to it:
Technic, Steering Axle with 2 Pin Holes, 2 Axle Holes and 2 Tow Ball Arms with Dark Bluish Gray Wheel Hub (46490 / 92909)

This part isn't new either, but I do think it's worth mentioning. It consists of a gear and housing.

In case you are in dire need of a couple of 5L liftarms in olive green, this is the set for you :wink:


The complete parts list consisting of 2108 parts.


Upon opening the bags for the first step, I found a selection of various black panels and frames. These are used to build the center section of the chassis. The manual shows we will be using three of the four motors, which are probably used for the drivetrain. Besides two L motors, the WeDo motor will be used. I wonder how that motor is incorporated in the chassis and which function it operates. Let's find out!

As with most trucks, we start by building some sort of gearbox, placed in the center of the chassis. Two motors are connected to the center differential and the WeDo motor is connected to the gearbox. While I am not really sure, it looks like the WeDo motor is used to lock the center differential. It is linked to the gearbox which incorporates the two clutches.

Same picture from a different angle.

Having a gearbox makes a build more complex and interesting. While this step wasn't very complicated, it was less straightforward than the tow truck build. I almost missed an axle connector, which could have been hard to add at the end of this step. I strongly recommend hooking up the hub and testing the build with the Control+ app. At the time of building, I didn't have the update Control+ app to test the build. Of course, I could have used other apps to test my build, but I took the plunge. Who needs testing anyway?! :wink: 

Here's the entire chassis at the end of the first step.

The second step is all about adding the rear suspension to the chassis. The bags for this step reveal another modest selection of black panels and frames. The parts also contain the new differential housing and as expected the recently releases planetary hubs, commonly referred to as Technic Steering Axle with 2 Pin Holes, 2 Axle Holes and 2 Tow Ball Arms with Dark Bluish Gray Wheel Hub :tongue:

The rear differential will be locked simultaneously with the center differential.

The video below demonstrates locking the rear differential.

The picture below shows the rear suspension added to the chassis. While this build is totally different from the tow truck, I am equally enjoying it. It's been a while since I built Technic, so it's great to be building a gearbox and suspension again. The mechanism to lock the differentials is genuinely intriguing. I love the number of gears packed in such a small space in the rear suspension. I am looking forward to building the next step, which is the front suspension with steering mechanism.

Bottom view of the rear suspension, where you can see part of the locking mechanism.

The last motor is used for the rack-and-pinion steering.

The front suspension assembly also uses the new differential housing.

When I first saw this new-ish part, I wondered why it wasn't just a 3x2 liftarm. The picture below demonstrates how doubles as a "panel extension".

Bottom view of the front suspension added to the chassis.

Here we are at the end of the third step. I noticed my building speed is significantly lower than during build of the tow truck. Not that this is a difficult set to build, but it definitely is more advanced than the tow truck. So far it's a solid build. I'm not an experienced trial truck builder, but I reckon the experts will be quite happy with the chassis.

The fourth step is the one with the most parts. We will be adding the cabin and hooking up the wires to the hub. In my review of the tow truck I mentioned the huge number of pages. This manual isn't too shabby either, counting 634 steps for 2108 parts. Again, a nice selection of panels, but this time they are dark bluish grey. A nice overall selection of dark bluish grey parts in this step (and this model entirely). We also get the smooth axle pin connector toggle joint in dark bluish grey. This set is turning out to be an incredible parts pack, in case you don't like the main model.

The cabin packs some nice details, like the windscreen wipers, exhaust and floodlights. Quite a few left-over pieces, even some duplicates. Made me wonder whether I missed a few. But since they all are the usual suspects, I'm probably cool. 

The doors feel a bit flimsy, but that might improve when we attach the hood and secure the cabin assembly. This will be done in the next step. Looking at the bags, this step won't take long.

The hood is attached to the chassis/cabin and it has indeed been reinforced with two liftarms. The doors (and entire assembly attached to it) still feel somewhat flimsy. Nothing to really worry about, but this being a trial truck, I'd expect it to be firm to stand a chance to the rugged terrain it will be traversing. But like I said, no biggie. 

Like the tow truck I have built this model without stickers and really think it looks awesome at this point. One can argue that it looks a bit boring with the grey and black, but I actually like it. Time to open up bags numbered #6 and finish the model. This step will add some color to the model, so it won't end up in boring grey :wink:

Pay attention when you attach the rims to the tires, because it's easy to mess up. Like the Claas and other models, there will be a lot of "you attached the wheels incorrectly" comments on social media.


Here is a selection of pictures of the completed model. I apologize for the cat hairs in some pictures. We used to have a we have two...and it shows :tongue:


This being a trial truck is only fair to judge the set by its capacity to handle rought terrain. I have composed a video to show the truck in action. I'm sorry that one of the cats is photobombing the video, but he has severe FOMO, so I can't do anything without him checking it out. He is sorry for the hairs in some of the foto's :wink:

As you can see in the video, the truck is basically worthless if you keep the front bumper attached. If you want to climb a steep hill, you really need to lose the front bumber. The ground clearance is also a limiting factor. Not as much as the front bumber, but the wheel base is rather long, so it's easy to get stuck on a bump or small hill top. 

The rear lights are prone to fall off when you are bumping into something. This is a minor gripe, but I would expect everything to be rock solid.

If we ignore these limitations, the performance is outstanding. Its capability to climb steep hills is very impressive. Without the differential lock, the front wheels aren't up to the task, but with the differential lock engaged, the sky is the limit.

More torque means less speed, so it's no surprise that this set doesn't move that fast. Its top speed is rather low. I guess you can't have your cake and eat it too. Mods will start pooring in quickly and I am looking forward to seeing them. Slightly shorter wheel base, bigger wheels, higher ground glearance, stuff like that. That being said, I am very pleased with the out-of-the-box performance.


This set has really surprised me, in a positive way. The build was way more enjoyable than I expected and I do think the finished model looks great. It's definitely not the sausage dog we feared hehe. Is it a proper trial truck? Since I am no expert, I am not the right person to judge. I do think it has two obvious limitations, which could have been avoided. One possible solution would be to lose the license and the need for this set to resemble a Zetros. Its capability to climb hills is impressive, so the picture on the back of the box is justified.

This is an outstanding set, with lots of potential. It will generate lots of interest, especially in the modding scene. Somehow, this set reminds me of the Unimog, which was immensely popular back in the days and still has a decent following. Personally, I do like this set better than the Unimog. So in my book, this is another win for Milan! Job well done.


  • Outstanding climbing performance (high torque)
  • Gearbox with intricate differential lock mechanism
  • Enjoyable build
  • New tires
  • Great selection of drivetrain parts
  • Lots of black and dark grey panels (and frames)
  • Great parts pack overall
  • Looks great without stickers.


  • Front bumper needs to be removed for proper climbing
  • Ground clearance can be limiting
  • It's a bit slow (is only logical given the high torque)
  • Rear lights fall off easily


How do I rate this set?


I do like how this set turned out.


Build was better than I expected.

It does what you would expect.

Okay, but the front bumper and ride height are kinda limiting.

This set packs a lot of interesting parts.

Price is a bit steep but you do get a lot of interesting parts.



Thanks you for reading this review. All pictures can be found here.