42129 Mercedes-Benz Zetros rear axle issue and fix

[kbalage]

As it was already mentioned by multiple reviewers, the rear axle of the Zetros seems to have a potential problem. Under constant heavy load pushing the truck beyond its limits, a 5L beam that holds the gear driving the rear differential gets pushed out a little bit, and as a result the gears start to slip, crack and might get damaged on a longer term. I tried to run several tests to see what might be the cause, can it be a friction variation of the axle pins or is it something else. We also tried to come up with a quick fix with @Attika that eliminates the issue under "normal" circumstances without completely rebuilding the rear axle. You can see the details in this video:

 

 

[Sariel]

Great fix, thank you. I'm honestly shocked by how this rear axle design got approved in the first place, since it's so very easy to make the issue happen and since it's quite obvious when you look at it, that the whole axle depends on a beam that is not properly supported. The 5x7 frames were introduced specifically to put and drive differentials inside them and in my experience it's extremely difficult to use something else without risking a damage like the one here. And any load-bearing beam should always be secured at both ends. How this was overlooked here is impossible to fathom. Driving a truck into a wall is a standard drivetrain test that our local Truck Trial builders have been using for years, and it appears that they are actually more thorough with their testing that LEGO is with theirs.

[suffocation]

I wonder if the original design actually had a similar reinforcement which was then dropped because the 7l half-beams fell off during regular testing. There was something similar on 42069, if I'm not mistaken.

[kbalage]

21 minutes ago, suffocation said:

I wonder if the original design actually had a similar reinforcement which was then dropped because the 7l half-beams fell off during regular testing.

Those beams are definitely more difficult to rub off then to make the gears crack without them :)  

[nerdsforprez]

2 hours ago, Sariel said:

And any load-bearing beam should always be secured at both ends.

This is one of the first lessons I learned when reading your books.  

I agree with @Sariel's statements.  THough driving a model straight into a wall, or at such high of an angle is not normal driving for Lego, they DID market the truck as a trial truck.  They also, marketed, quite aggressively, the purported steep slope in which the truck can climb.  

Seems like there are many, many examples of sets being released with fundamental design flaws. Whether it be a gearbox (42056- too much friction and other issues), suspension not returning (42083), other gearbox issues (Landrover) and being fixed by AFOLs in a week or so.  

We often hear of TLG being a business and really being concerned with running like a business.  Well, if that is the case here is some advice.  Like all successful businesses, CONSULT.  Have a few highly regarded AFOLs, not full-time designers or employees, but AFOLs  on retainer or something to adequately test these models before release.  Not only could they adequately test them, but time and experience has also shown they can correct problems, and FAST!  

I think this has to be an embarrassment for TLC.  Seems like this keeps happening over and over  and over again.... 

[emielroumen]

That is a great idea @nerdsforprez, kind of closed user group of AFOLs with a track record of designing well built MOCs in terms of suspension, gear train, etc. In the end, we would most likely get sets that at time of release more resemble the pimped up versions that enthusiasts like members of the forum co-create together. I vote for this approach!

[efferman]

I have a fix which is going more deep. In fact it is a complete new Axle. I hope the mounting points are in the same place, because i dont own the instruction.

https://bricksafe.com/files/efferman/vehicle-axles/42129 rearaxle Fix.io

[Attika]

Hello there good people of Eurobricks,

haven't been around for a while, as I've moved country and turned my life upside down. Anyhow, thanks to @kbalage I had the pleasure to peek into this upcoming model and share some workshop hours on the forementioned problem. Upon the quick fix I've felt the urge to make a deep fix, just as my noble colleague @efferman did. Here is my take on it:

Made some sloppy building instructions too, please find in the link below:

https://drive.google.com/file/d/1O_F8eo6dFYrfoVrZ9gtxTZivxtvLxiU2/view?usp=sharing

I've the pleasure to report that it's been tested IRL and have passed it. (yes, straight against the wall, that's the way it is  )

Link for stud.io file:

https://drive.google.com/file/d/1dkpPFU93LUcXu_0qT2FqZnrjFijmKwVZ/view?usp=sharing

Edited July 29, 2021 by Attika
aging

[nerdsforprez]

This is exactly what I mean.  Well done you two....

58 minutes ago, emielroumen said:

That is a great idea @nerdsforprez, kind of closed user group of AFOLs with a track record of designing well built MOCs in terms of suspension, gear train, etc. In the end, we would most likely get sets that at time of release more resemble the pimped up versions that enthusiasts like members of the forum co-create together. I vote for this approach!

 

[Johnny1360]

Great job guys, I have the feeling this will become a very popular thread, lol.

   I will likely do one of the more complete overhauls straight off and not even bother with the original setup, when I get this set. Thanks for looking into this and offering a better option.

Edited July 29, 2021 by Johnny1360

[Bartybum]

Great work @Attika and @efferman. Any chance one of you would redesign the front axle to have tighter steering?

[efferman]

6 hours ago, Bartybum said:

Any chance one of you would redesign the front axle to have tighter steering?

when i have the set, maybe.

[Ngoc Nguyen]

Attika's fix makes me wonder why the designer didn't go for the 7x11 frame from the beginning.

[Attika]

On 7/30/2021 at 12:32 AM, Bartybum said:

Great work @Attika and @efferman. Any chance one of you would redesign the front axle to have tighter steering?

So, I had a go on this problem and in advance I have to say it is a compound problem, hence it requires some dissection.

1. The designer states in his related video that the steering lock is limited to about 30 degrees to protect the cv joints. 

I -respectfully- tend to disagree. Personally I've found the best point of these joints at their debut is the max working angle is around 40 (+) degrees. Since then I've built them in many application and never came across any problem in this regard.

2. In the same video the ackermann geometry is mentioned to be used to improve the steering lock. 

Sadly it does not do the job, here is why: 

     - The angles are not correct. If the wheelbase was the half of what it is, the designed angles would be moreless accurate, but at the current distance between the axles the given angles are too high. Having this kind of excessive ackermann geometry is even worse for the the steering lock as having no ackermann built in the design. Why is it worse?

     - If the maximum angle for the cv is given, it means -with ackermann steering- that the inner wheel takes that maximum angle. The angle of the outer wheel is going to be lower by defenition. So we've got 30 degrees inside and 26(ish) outside. Due to the reason that these angles don't line up with the wheelbase, it means, one of the front wheels going to skid sideways when cornering. Usually it is the inside wheel with the higher angle that is skidding, so our practical steering angle is down to 26(ish) what is represented by the outside wheel.

The rest of the problem is in the code. Using the PU profile (first time I did btw) there is a callibration process to go through. At this the motor takes both endpoints and settles at the middle point. When it comes to playtime however the maximum angle is lower,  I assume it is to protect the components. Nevertheless it also decreases the steering angle.

And now to the good news:

To be honest I forgot to measure the original turning circle but I had two sources to get data. The designer states it is 195cm and @Sariel's gadget displays something over 2 metres (would have been useful if you show us the method instead of playing with the cat)

Anyhow, I've rebuilt the front axle taking out the ackermann design from the equasion and connecting the steering rods one stud closer to the hub pivots (basically where it supposed to be by default).

This gave me about 20-30 cm decrase on the diameter of the turning circle. A bigger chunk has been chopped of from this number by using the BrickController2 app made by @imurvai  (God bless him). This method of control doesn't apply safety measures, it just does what you want. The end result is 90cm diameter between the inside wheels (130cm between the outside ones)

A humble demonstration:

 

 

I'll make a stud.io file soon and link it here.

Edited July 31, 2021 by Attika

[Igor1]

@Attika Thanks for the detailed description, your approach has always been deeply worked out. How nice it's to read technical calculations, and not just squeals about how bad things are with Lego. And welcome back!

[Johnny1360]

So glad to see all these issues getting worked out before I can even get my hands on the set, lol. That is why I like this forum so much, even if I do have to read all the useless complaining, that often includes no alternatives to the issues. 

Thanks to those who make all their hard work available to others.

Edited July 31, 2021 by Johnny1360

[Bartybum]

@Attika Hell yeah good buddy, excellent work 

Now for the next redesigns lol  What are your thoughts on the possibility of an axle design with drive, diff locks AND steering?

[Attika]

15 hours ago, Igor1 said:

@Attika Thanks for the detailed description, your approach has always been deeply worked out. How nice it's to read technical calculations, and not just squeals about how bad things are with Lego. And welcome back!

Hi, I'm glad you appreciate my brainf4rt.  Sorry for not reacting back then when you posted the G-wagon. I had the look, I just didn't react. My bad.

15 hours ago, Johnny1360 said:

Thanks to those who make all their hard work available to others.

It is my pleasure. 

9 hours ago, Bartybum said:

@Attika

Now for the next redesigns lol  What are your thoughts on the possibility of an axle design with drive, diff locks AND steering?

wow, hold your horses!  You're going to regret this question, I promise.

Sooo, diff lock on the front is virtually imposible at the given width of the axle. Because of the steering, the hubs need to be held by the ball joints. That is only possible by using the 5L ball socket ended steering arm which is going to collide with the differential if -like at the rear axle- the 7x11 frame is being used to brace the mechanics. Even if this problem would go away, there is not enough room beside the 7x11 frame to connect any steering rod to the hub.  My opinion is that by keeping the original dimensions of the axle, it cannot be done (properly, at least).

I've scheduled to rebuild the whole chassis, using the bigger tyres and conveniently use the defender rims. That offset rim gives me another 2 studs on width and beside I'll give an additional 2 studs to compensate for the tyre size and make sure it does not rub against the chassis when the steering is at full lock. That seems to be quite manageable (says my Dunning kruger).

Up to the news:

Today I woke up and made the front axle even shorter by a stud. (If I had a month probably I could make it disappear as a whole )

That's how it looks:

Also some hastily made instructions:

https://drive.google.com/file/d/1Ql2Sd7b39LiXBh8JkXBaguNJQFYa1XMq/view?usp=sharing

Some additional changes I made on the front of the chassis:

1. Eliminated the panhard rod, changed the 3L axles with stop to 4L ones and by sticking them from the outside, could connect the two sides with that red connector assembly. It stiffened up the whole structure making the panhard rod redundant in it's function.

2. changes the springs to soft ones (all 4 of them) it lets the axles flex way more than the hard ones. The front end therefore is "floating", siting in a bit more than half on the suspension travel length. I'm happier with it this way, but it is mainly a personal preference. Do as you like.

3. Lifted the panel on the bumper by one stud (by taking out a 9L beam). It meant to compensate for the slightly lower stance caused by the softer springs.

That's it. Have a good time with the set guys, I go back to my cave now. Bye

Please keep in mind: This is my take on the subject, made in a very limited time window. The design may contain some solutions which satisfy my taste, yet can disturb others. Time and users (preferably @Didumos69) will tell if it is good enough or not. 

Edited August 1, 2021 by Attika
can't spell

[efferman]

A nice Progress i see here.

My Axle has now a instruction. (mirror error is fixed)

https://bricksafe.com/files/efferman/vehicle-axles/42129 rearaxle fix.pdf

Edited August 8, 2021 by efferman

[Bartybum]

Again, excellent post as always hehe.

50 minutes ago, Attika said:

Sooo, diff lock on the front is virtually imposible at the given width of the axle. Because of the steering, the hubs need to be held by the ball joints. That is only possible by using the 5L ball socket ended steering arm which is going to collide with the differential if -like at the rear axle- the 7x11 frame is being used to brace the mechanics. Even if this problem would go away, there is not enough room beside the 7x11 frame to connect any steering rod to the hub.  My opinion is that by keeping the original dimensions of the axle, it cannot be done (properly, at least).

Yeah I figured that might more or less be the answer. This is beating a dead horse by now but since the reviews I've always thought that independent steering in both axles would be far better than lockable diffs, especially since there's no high speed gearbox to accompany unlocked diffs. Do you think there'd be room for a two speed gearbox linked to the diff locks in the chassis?

50 minutes ago, Attika said:

I've scheduled to rebuild the whole chassis, using the bigger tyres and conveniently use the defender rims. That offset rim gives me another 2 studs on width and beside I'll give an additional 2 studs to compensate for the tyre size and make sure it does not rub against the chassis when the steering is at full lock. That seems to be quite manageable (says my Dunning kruger).

I assume by bigger tyres you mean the CLAAS ones yeah? I've seen pics of that from Kbalage (I think?) and I gotta say that they do look odd in my eyes, but hey. If you're interested I might be interested in designing an offset rim for 3D printing with the regular tyres. It would end up going against a purist solution though obviously.

Edited August 1, 2021 by Bartybum

[Attika]

1 hour ago, Bartybum said:

If you're interested I might be interested in designing an offset rim for 3D printing with the regular tyres. It would end up going against a purist solution though obviously.

The inner diameter of this rim is less than 5 studs, so an offset rim could not take the hub. 

[efferman]

14 minutes ago, Attika said:

so an offset rim could not take the hub

Sure?

[pleegwat]

2 hours ago, Attika said:

That is only possible by using the 5L ball socket ended steering arm which is going to collide with the differential

Only if you mount them parallel to each other. What if you mount them in a trapezium shape - 3u between them at the wheel but 5u between them at the innermost mountpoint. Of course, I have no idea how you would formlock that, and it's going to cost you in ground clearance.

I'm trying to think of other pieces with a ball socket, but all other options I can think of offhand are even longer.

[Attika]

15 minutes ago, efferman said:

Sure?

Confess, you heretic! 

That tyre of yours has an unnatural belly. It is forced to a slightly bigger rim than it supposed to be.

Now I know I'm the last one who should comlain about that (I've got a past ) but that printed rim isn't the same size as the lego counterpart, is it? 

[keymaker]

25 minutes ago, efferman said:

Sure? 

 

Yes. Rims from Zetros are less than 5 studs, sadly.