[MOC] 4x4 Off-Roader with unusial drive system

[SimplyLegoTechnic]

A lightweight and compact model for moderate off-road fun. The main feature is its unique all-wheel drive system that powers both rear wheels and the front axle differential.

This MOC is my take on what Lego Technic should be. It skips unnecessary details of non-working features and focuses on making the driving and steering systems as effective as possible.

Weighing about 1.2 kilograms, this model has a speed similar to the stock Lego Technic 42160 set but with way better off-road capabilities and playability, all with the same number of motors.

The free detailed instructions https://reb.li/m/186485 include a step-by-step guide for setting up the control profile in the free Controlz app, which I think is the best free app out there for this.

A short demonstration video:

I hope you enjoyed it!

[pow]

How many electric parts where harmed during filming? You're a brave person! Nearly making a submarine out of your build!

I like that drive train and the look of your build. Nice that you built a proper exterior! The speed and climbing abilities are amazing especially considering the motor configuration!

[SimplyLegoTechnic]

7 minutes ago, pow said:

How many electric parts where harmed during filming? You're a brave person! Nearly making a submarine out of your build!

I like that drive train and the look of your build. Nice that you built a proper exterior! The speed and climbing abilities are amazing especially considering the motor configuration!

Amazingly, none of the motors or hubs got damaged)))

The model came out simple and fun to play with, which is the best part.

[CK28]

Great build and nice performance! Although looks weren't the main goal it still looks very clean and technic like! I am kinda amazed that it uses the grey shock absorbers with this kind of weight. 

Don't you think adding a little gearing would improve the climbing capabilities even more? Not that they disappoint tho 

As Pow already said you are brave man going down those watery roads while testing!

[SimplyLegoTechnic]

44 minutes ago, CK28 said:

Great build and nice performance! Although looks weren't the main goal it still looks very clean and technic like! I am kinda amazed that it uses the grey shock absorbers with this kind of weight. 

Don't you think adding a little gearing would improve the climbing capabilities even more? Not that they disappoint tho 

As Pow already said you are brave man going down those watery roads while testing!

Thanks so much for your kind words!

The grey soft shock absorbers are more than enough, though they're a bit on the stiff side.

When I was developing this model, I tried three different gear ratios from the motor to the wheels:

1. 20:20, or 1:1 - the fast version;
2. 12:20 - the main version;
3. 12:28 - the slow version, which is the one climbing the 35% incline in the video.

The fast version doesn't have enough power with just two Lego PU L-motors(((

The 12:20 ratio works well for moderate off-roading.

The 12:28 ratio is a bit slower, but I was really impressed with its off-road capability and playability.

The instructions have detailed photos of this version. I'll be making extra instructions for the rear axle with this gear ratio soon.

Edited June 14, 2024 by SimplyLegoTechnic

[SimplyLegoTechnic]

This version with three Lego PU L-motors works with the original Lego Control+ profile for set 42160.

But it takes 10-15 tries to get the calibration right. The free instructions: https://reb.li/m/186574

This might be because the steering L-motor in my MOC is positioned differently than in the original 42160 set.

It's much easier to control with the Controlz app...

[SimplyLegoTechnic]

https://reb.li/m/186875

And little video with Climbing:

 

Edited June 17, 2024 by SimplyLegoTechnic

[gyenesvi]

It's an interesting concept for the rear axle / drivetrain. But the middle link one at the bottom looses ground clearance, and it's not really necessary to have a third link if the axle has positive caster and doesn't have CV joints at the axis end. Is there any reason why you don't use proper wheel hubs on the rear?

[SimplyLegoTechnic]

55 minutes ago, gyenesvi said:

It's an interesting concept for the rear axle / drivetrain. But the middle link one at the bottom looses ground clearance, and it's not really necessary to have a third link if the axle has positive caster and doesn't have CV joints at the axis end. Is there any reason why you don't use proper wheel hubs on the rear?

Advanced question!))) This is a bit tricky, so I'll try to explain it clearly.

1. Why do we need three links, or three attachment points, for the axle to the frame? The shock absorbers don’t count since they have two degrees of freedom and there's some play and bending. We need three links to keep the axle fixed relative to three axes: the two from the motors and one from the rear axle differential to the driveshaft on the front axle. This prevents the situation where one or two of these axes act as the connection point where they enter the axle. Previous versions with two-link mounting caused this issue, and the axles at the friction points didn't look good. The third link at the bottom almost completely solved this problem. Plus, the model performs better in situations needing maximum power, like steep climbs.

2. Sure, the bottom link reduces ground clearance at the approach to the axle. Even though the attachment point isn't below the level of the large yellow gear of the rear axle differential, it still affects clearance. I tried to make this part of the model as smooth as possible for potential obstacles like tree roots.

3. Why didn’t I use wheel hubs at the rear? Initially, I wanted to build the model without using rare and expensive parts. Unfortunately, that didn’t work out))) But I didn’t use hubs in the rear axle because the construction was strong enough even in extreme conditions. Nothing slipped or fell off.

[gyenesvi]

2 hours ago, SimplyLegoTechnic said:

Advanced question!))) This is a bit tricky, so I'll try to explain it clearly.

1. Why do we need three links, or three attachment points, for the axle to the frame? The shock absorbers don’t count since they have two degrees of freedom and there's some play and bending. We need three links to keep the axle fixed relative to three axes: the two from the motors and one from the rear axle differential to the driveshaft on the front axle. This prevents the situation where one or two of these axes act as the connection point where they enter the axle. Previous versions with two-link mounting caused this issue, and the axles at the friction points didn't look good. The third link at the bottom almost completely solved this problem. Plus, the model performs better in situations needing maximum power, like steep climbs.

2. Sure, the bottom link reduces ground clearance at the approach to the axle. Even though the attachment point isn't below the level of the large yellow gear of the rear axle differential, it still affects clearance. I tried to make this part of the model as smooth as possible for potential obstacles like tree roots.

3. Why didn’t I use wheel hubs at the rear? Initially, I wanted to build the model without using rare and expensive parts. Unfortunately, that didn’t work out))) But I didn’t use hubs in the rear axle because the construction was strong enough even in extreme conditions. Nothing slipped or fell off.

Huh, this took me to a deep rabbit hole..

So now that you mention this detail, I remember seeing this as a drawback of this construction before, and I think the 3rd link (or any method except using joints) cannot completely solve it. Without joints, I think it's impossible to completely avoid situations where one of the shafts enters the axle not completely perpendicular. This is because as the axle articulates (tilts sideways), it cannot articulate around all the 3 shafts as axes at the same time, only around one shaft (mostly the middle shaft). So the side shafts will necessarily have some contact with the frame.

I understand that the third link helps to reduce the problem; without the third link, the axle can move in two ways that would result in some of the shafts contacting the frame: the whole axle could rotate forwards / backwards as the motors spin the wheels, and the axle can move sideways. In general, a third link can prevent both, but only because it has a single degree of freedom (limited sideways, kind of a simulation of a triangulated 4-link in lego form).

However, this third link solution also has a problem in your case; depending on the relative arrangement of the links, as the axle moves up/down, its caster either stays constant or may change differently as the angle of the driveshafts would require. It could in principle change together with the driveshafts if the 3 links would rotate around the same axis at the chassis end, but that's not the case in your build. But having a third link may still be better in your case than not having it, I think in that case the wear at the contact points without 3rd link came from rotational motion and maybe also from sideways motion. However, if the range of articulation was bigger (longer springs) then it would not work so well, as the angle discrepancy of the driveshafts would necessarily have to become more.

The reason I said it's not necessary to have the 3rd link is because there are other ways to prevent sideways / rotational motion, without loosing ground clearance and without the problem I detailed above.

The sideways motion of the axle can simply be avoided using a panhard rod (cross link).

But, the really interesting part is avoiding the rotational motion of the whole axle. My first axle I built without double CV joints in an older model was similar to yours in one aspect; I put the springs to the same position in the middle (longitudinally), right above the driveshaft to the wheel and I also got some lego axles that did not look good at the friction points, and in some high torque situations the driveshaft got torn apart :) Later, in another model, I learned that it is much better to put the springs behind the whole axle, for multiple reasons. First, they can be lowered, leaving more free space above the axle, and it also looks more realistic. Second, and most importantly, in lego form, it is enough to prevent the axle from rotating, so you don't need to use a third link for that.

Btw, in my model I did not solve the problem this way back then, but rather by limiting the degrees of freedom of the link at the axle end, only allowing the axle to tilt, but not rotate forward/backward (and using a panhard rod). Since then I learned that that's actually a solution used in reality, even on front axles.

So in the end, you do need a third link, a panhard rod, but that's easier as it's not in the way under/above the axle, but can be placed behind it and is more realistic. And all this can only provide a really solid solution that would work for high articulation angles as well when there is only one driveshaft, and it has to be in the middle (symmetric for axle tilt). In reality it's called torque tube suspension btw.

As for the wheel hubs, the ones I linked are not rare or expensive, they are in production, used in many models. Can make things a bit more stable, especially if you run the lego axle all the way through them to the wheels.

Edited June 17, 2024 by gyenesvi

[SimplyLegoTechnic]

21 minutes ago, gyenesvi said:

Huh, this took me to a deep rabbit hole..

Oh, absolutely! You’ve got tons of experience in this area! It's awesome to read such insightful comments.

In this case, I worked with a friend who wanted a competitive LEGO MOC for competitions. I aimed for simplicity and reliability over a theoretically perfect design. If you look closely, you'll see the rear axle even uses the play in LEGO parts to adjust the axles precisely, so nothing wobbles. It’s not exactly by the book, but practical tests often show that compact and reliable solutions win out.

After kilometers of testing, the most obvious takeaway is simple: the model’s effectiveness depends more on weight than on the perfection of the build. Of course, I’m not talking about amateur-level building. I always want to make it look good, perfect, and efficient. It’s a tough challenge! That's why I love LEGO Technic!

Thanks again for your comments!