[WIP] 4x4 Unimog Trial Truck

[Lukes_Brick_Studio]

20 minutes ago, Teo LEGO Technic said:

Sweet truck! Very nice system for the front axle, I saw the same portal axles used by Functional Technic on his Unimog, they're very compact and sturdy https://www.functionaltechnic.com/2020-08-30/unimog-406-with-diff-lock-and-portal-axle

Thanks you very much, yes those portal axles are very compact and study, the only problem is the turning circle radius.

 

24 minutes ago, Teo LEGO Technic said:

I can also probably stick an XL in each and see if it holds.

It seems lots of people use pneumatics for diff locks, that might be something I should look into.  

Really good ideas.

[2GodBDGlory]

8 hours ago, 1gor said:

I don't know if you have pair of defender wheels (and if portal axles hub from 8110 set fits in it); that could (perhaps) help you to gain stiffness

Those portal axles sort of fit in the defender rims. They do rub at the top, so some may not prefer it, but I'm personally fine doing it, and have done it in a Zetros MOD I have yet to post.

 

Also, are you sure you can't do the "old-school" diff lock with the new differential? In my experience it rubbed just a little, but was (in my opinion) workable.

[Johnny1360]

Great choice for a moc, lol, I am currently working on a very similar model myself, although I have decided to omit many of the features you will be using, for now. Mostly just so I will actually complete it, rather than quit in disgust as often happens with my mocs.

I am having a hard time with steering right now, having decided on a large actuator for the control. So I will be following with great interest.

[Teo LEGO Technic]

On 8/14/2021 at 7:14 PM, 2GodBDGlory said:

In my experience it rubbed just a little, but was (in my opinion) workable.

It may work, but the issue is the triangular shape of the diff causes uneven friction, which will be a worse issue for the MOC at higher speed (when the gearbox is in higher gear). 

On 8/14/2021 at 9:46 PM, Johnny1360 said:

I am having a hard time with steering right now, having decided on a large actuator for the control. So I will be following with great interest.

It's a challenge for sure, hopefully I have a solution for you soon   

On 8/14/2021 at 10:33 AM, 1gor said:

I don't know if you have pair of defender wheels (and if portal axles hub from 8110 set fits in it); that could (perhaps) help you to gain stiffness

I don't sadly, gonna have to work with the regular rims.

BTW more examples for driven, steered, diff locking axle builds are welcome!!! Thanks @Lukes_Brick_Studio for your input so far

[Teo LEGO Technic]

So here's the current progress. The front axle continues to be a challenge, but once I figure it out, the gearbox, diff lock mechanism, and chassis will be relatively easier. As far as the steering axle is concerned, I've broken it down into drive, steering, and differential lock, to investigate and compare my options:

Drive:

1. New differential in a 5x7 frame 
2. Zetros style, without frame, and geared on the side to allow space to for the clutch to enter the differential on the side

Of these two options, I chose the 5x7 frame, because I want the perpendicular gears to be as secure as possible to take the toque I'm planning to put on it.

Steering:

1. Using an actuator:
   1. Small actuator - more space efficient, but has to be within two studs of the wheel hub pivot centre in order to have enough range of motion 
   2. Large actuator - takes up A LOT of space, but can be much stronger, and has much more travel
2. Worm gear - use a rack and pinion connected to a worm gear 

Either way, the motor powering the steering can either be on the axle or chassis. Of these options, the large actuator simply takes up too much space, so I plan to go with either the small actuator, or the worm gear.

Differential Lock: (given 5x7 frame)

1. Clutch:
   1. "Classic" lock system with clutch directly connected to differential causes some friction between diff and clutch, but is workable like @2GodBDGlory said
   2.  Use more gears to push the clutch further out, and have less friction 
2. Clutch movement:
   1. Small pneumatic actuator
   2. Zetros - style with orange screw piece 

Of these choices, all are available, as none have any clear advantages or disadvantages. 

My one question to you guys is: what do we think of reducing the gear ratio further after the diff, and increase the ratio before the diff, to take pressure off of it in terms of torque? Is this worth the effort, or does the added friction and complexity not make it worth it overall? Hope I haven't rambled on too much

 

 

[Mechbuilds]

Use the small actuator. Wormgear will be slow AND if you oversteer it, it'll break the model because it won't stop. the small actuator has a clutch to prevent this. 

[Teo LEGO Technic]

After a lot of prototyping (and I mean a lot ) I finally have a working version that is pretty close to the final version of the front axle. Drive is using the new differential, in the 5x7 frame for maximum strength, steering is using a small actuator, placed at 2 studs from the pivot point for sufficient steering range of motion, and differential lock is using a clutch operated by a small pneumatic piston, shifted away from the differential with two sets of gears, to eliminate friction between the clutch and the differential hub.

The portal axles are secured very well, both top and bottom, and the only thing still needing some work is some reinforcement on the gears at the front that transmit power to the M-motor for steering to prevent slipping, and some work on the diff locking system, perhaps strengthening it or moving it in a new position to increase ground clearance.

Thoughts?    

Edited August 18, 2021 by Teo LEGO Technic

[1gor]

My friendly advice is to try to make additional bracing for those gears used with small linear actuator.

At least on blue 20 toothe double bevel gear with clutch put bushes and axle with stop instead of pin ...

[Teo LEGO Technic]

16 minutes ago, 1gor said:

My friendly advice is to try to make additional bracing for those gears used with small linear actuator.

For sure yeah, it's currently slipping under pressure so I was planning to strengthen it.

Do you have an idea for a better diff lock setup? If it was more compact, I could turn flip the axle around and that would remove the need for the gears entirely, just stick the M-motor directly into the actuator following the perpendicular gears. As it is, the diff lock system protrudes too much and would ruin the truck's approach angle if it was at the front. 

Edited August 18, 2021 by Teo LEGO Technic

[nerdsforprez]

On 8/17/2021 at 12:11 PM, Mechbuilds said:

Use the small actuator. Wormgear will be slow AND if you oversteer it, it'll break the model because it won't stop. the small actuator has a clutch to prevent this. 

This has not been my experience.  Wormgears have been successfully used by others with no problems for steering.  I mean, once you hear a change in the motor pitch as long as you stop you should be okay.  If you keep power going and going, sure, it will break, but there is sufficient warning for you to stop....

[1gor]

For differential I have only idea to put itside frame, but honestly I didn't try it...

[Mechbuilds]

15 hours ago, nerdsforprez said:

This has not been my experience.  Wormgears have been successfully used by others with no problems for steering.  I mean, once you hear a change in the motor pitch as long as you stop you should be okay.  If you keep power going and going, sure, it will break, but there is sufficient warning for you to stop....

yeah but with a linear actuator it doesn't matter if you hold the button for too long as it won't damage anything. 

[functionalTechnic]

Hi guys
@Teo LEGO Technic saw your e-mail and thought it’s better to answer here. I disassembled the Unimog already a while ago. But I made some renders for you of the front axle if you still want to use it for your model. the steering angle is with Lego always a problem if you want to have propulsion in the front axle and that is due to the old and new cv-joints. You cannot change that but only if you use cardan joints. In my opinion cardan joints do not fit for steering in front axle because of the pivot point of the steering axle. In real cars the pivot point of the steering axle is close the middle of the tire if you look on the tire from above. With Lego it is normally outside of the tire like in my axle. with cardan joints it is even further away from the tire. Another problem my axle has is that you cannot use Lego wheel hubs if you want to have a portal axle and keep the pivot point of the steering axle close to the tire. Without a Lego wheel hub, the tire is more lose on the axle but turns better. Now everybody has to decide how you choose. The performance you can see in my video of the Unimog.
Regarding how you can make diff-lock: With pneumatics it is easier on the axle but you need in the model a compressor and valve. With mechanics like in the Zetros form Lego it is much harder on the axle but easier in the model an you need only one motor. With pneumatics you need at least on motor if you use a auto-valve on otherwise two motors.

 

[Teo LEGO Technic]

Hey guys, sorry for the long break I took from this MOC, but school has been very busy. Silly real life priorities kept me busy 

But back at it! My choice right now is about what wheel-base to use: normal or long. The image below shows both:

I have to figure out how to fit in the heavy-duty gearbox as well. My thoughts are that the long base gives me more room and more climbing ability, but the added weight may reduce performance, and the small actuator used for steering may start to run out of power. The smaller base will be lighter, but requires a more compact design, and also won't climb as steep slopes.

Thoughts? Let me know guys, I'm going to get back to building now I have more free time! 

[1gor]

Long wheelbase gives you more room, but at the same time model will be heavier. I personally like simplicity

[Teo LEGO Technic]

UPDATE:

Finished bracing the front axle:

Built the rear axle, similarly to the front but much sturdier since it doesn't have to be steered:

And here's the beginning of the gearbox. Design uses Sariel's principle (http://sariel.pl/2011/02/2-speed-heavy-duty-linear-gearbox/) but with 2 XL's.

Final gear ratios from the gearbox outputs are:

1. 1:5.00

2. 1:1.80

This ensure that even in the slow gear, only the torque of 1.11 XL motors is driving the front and rear diffs, and so no gear damage will occur. The slowing down at the wheels will be:

1. 2.33: 1

2. 6.48:1

This gives it more than enough torque on the low gear, and quite good speed in high gear. 

Thoughts? :)))

Edited December 18, 2021 by Teo LEGO Technic

[Lukes_Brick_Studio]

Good progress, The front axle seems to be very solid, which is very good. I really appreciate, the idea to built a gearbox inside of this trail truck. Well done!

[Teo LEGO Technic]

Hey guys, it's been a long time since I had time to work on this project, I was just too busy with school. But with the semester over, it's finally time for an update:

The gearbox is installed in the chasses and the beginnings of the chassis are started, complete with strong bracing and diagonal beams. The M-motor for actuating the gearbox is installed, as well as the M-motor that drives the pneumatic pump and auto-valve (based on @functionalTechnic's design here). The job now is to install the suspension mounts to the chassis, and to position the Panhard rods that center the axles. 

An issue I've found is that the wheels on the front axle start to develop an unwanted negative camber angle when the car drives with the steering all the way to one side. This may be because the tires on the front aren't braced as strongly as on the rear (due to the custom-made portal axles), or maybe it is due to the lack of Ackerman steering geometry (the wheels pivot equally when steering). It's not a very serious issue, but I would like to be able to resolve it. 

Otherwise, a preliminary test worked like a charm. The truck gains pretty decent speed on high gear, and has a ton of torque on low gear, without any gear slippage happening or other issues. It's just so great to have time to work on this project again after so much time!

[gyenesvi]

This thing is getting huge, especially in the middle! Are you sure you are not loosing the ground clearance provided by the portal axles? It seems to be quite low in the middle. About the negative caster, as you are using the ball joint to mount the front axle as well, that can only result in a negative caster at the front, unless you mount it horizontally when the springs are fully uncompressed, and go up from there, but that’s again bad for ground clearance. This is why I don’t use it at the front, I think it’s mainly good for the rear axle (maybe for an SUV type car, but not for a trial truck).

[Teo LEGO Technic]

2 hours ago, gyenesvi said:

This thing is getting huge, especially in the middle! Are you sure you are not loosing the ground clearance provided by the portal axles? It seems to be quite low in the middle. About the negative caster, as you are using the ball joint to mount the front axle as well, that can only result in a negative caster at the front, unless you mount it horizontally when the springs are fully uncompressed, and go up from there, but that’s again bad for ground clearance. This is why I don’t use it at the front, I think it’s mainly good for the rear axle (maybe for an SUV type car, but not for a trial truck).

Thanks man! Yes, it is getting very large  In terms of ground clearance, the photo shows how the suspension will look when it is fully compressed - normally it will be much higher:

 

On the front axle, I didn't mean negative caster I meant camber. As you say, there is no way to get rid of the negative caster - that's part and parcel of the ball joint mount - the issue is that the wheels tilt and sag outwards as the car drives when steering. I can post a photo to show what I mean:

I don't think there's much I can do to fix it - implementing Ackerman geometry is too painful, and either way the custom portal hubs aren't as robust as the LEGO ones.

On another note I could use some advice - how do you guys think I should mount the axles? My initial plan was to use Panhard rods, but even with the long linkage (below), there is quite a bit of side-to-side movement due to the long suspension travel, and the fact that Panhard rods naturally inscribe an arc. Should I use a different centring method? Like in the 9398 Rock Crawler?

 

[gyenesvi]

2 hours ago, Teo LEGO Technic said:

On the front axle, I didn't mean negative caster I meant camber.

Oh, sorry, my bad, I totally misread that. I guess then, as you write, it's due to the custom built portal hubs not being that strong. I really wish the Lego portal hubs were more useful and there would be no need for custom building them. They are so bad with the steering geometry.. which causes problems with the wheel hitting everything nearby.

2 hours ago, Teo LEGO Technic said:

In terms of ground clearance, the photo shows how the suspension will look when it is fully compressed - normally it will be much higher:

Wow, that will be quite high and steeply angled when not compressed. Is that going to be symmetric in the front, with similarly high negative caster? That could also cause problems with the steering geometry. Furthermore, what's strange for me in your build is that the driveshaft is routed very high in the chassis. The axle builds are interesting in that the driveshaft comes in quite high, and I thought the purpose is that that way you could allow the ball joints to sit horizontally, while still having enough clearance. Why did you need to put the ball part of the ball joint so high in the chassis end? Was it not possible to place it to the very bottom of the chassis? That way the whole driveshaft could be horizontal, which could eliminate the negative caster in the front.

As for the panhard rod, its geometry matters a lot when trying to avoid sideways movement. You need to place it in such a way that is is horizontal when the suspension is compressed halfways, in order to minimize side movement. Also, the longer the rod the better, going from one side to the other. You can actually build a longer one from towball arms connected by a liftarm.

[Teo LEGO Technic]

50 minutes ago, gyenesvi said:

I really wish the Lego portal hubs were more useful and there would be no need for custom building them. They are so bad with the steering geometry..

You're right, definitely would be nice to see some better steerable portal axle options from LEGO

52 minutes ago, gyenesvi said:

Why did you need to put the ball part of the ball joint so high in the chassis end? Was it not possible to place it to the very bottom of the chassis? That way the whole driveshaft could be horizontal, which could eliminate the negative caster in the front.

The entire drivetrain is indeed, as you say, quite high in the build. The reason for this is that, when building the front axle, the only solution I could find to build the front axle to be steered by a linear actuator, powered, and have a pneumatic differential lock system, involved building it with the large ground clearance. After that, I built the rear axle similarly as there was no point in modifying it. Next, I placed the motors lower than the drivetrain in order to reduce the extra ground clearance, which does not match the clearance in a real Unimog, and the extra height would have made the truck fall over easier when climbing over obstacles.  

Do you think there is a solution for keeping the drivetrain flat, but also avoiding excessive ground clearance? It would require a rebuild of the chassis so far, and although it would reduce the negative camber, the steered axle was causing issues in testing even when placed flat, so I don't know if much would be gained

1 hour ago, gyenesvi said:

As for the panhard rod, its geometry matters a lot when trying to avoid sideways movement. You need to place it in such a way that is is horizontal when the suspension is compressed halfways, in order to minimize side movement. Also, the longer the rod the better, going from one side to the other. You can actually build a longer one from towball arms connected by a liftarm.

The issue with the rod is the tight space I have to work with. After lots of different iterations, here is the best I could come up with. I just need to brace the shock absorbers better: 

[gyenesvi]

10 hours ago, Teo LEGO Technic said:

Next, I placed the motors lower than the drivetrain in order to reduce the extra ground clearance, which does not match the clearance in a real Unimog, and the extra height would have made the truck fall over easier when climbing over obstacles.

What I meant is that the overall height and clearances would look the same, it's just that the driveshaft would come out from the chassis lower, and the ball joints going to the axles would be laid out horizontally.

10 hours ago, Teo LEGO Technic said:

The issue with the rod is the tight space I have to work with.

You could put the panhard rod to the very end of the axle, there you have a lot of space, and also it does its job better there as there it takes less force to hold the axle in place.

[Teo LEGO Technic]

6 hours ago, gyenesvi said:

What I meant is that the overall height and clearances would look the same, it's just that the driveshaft would come out from the chassis lower, and the ball joints going to the axles would be laid out horizontally.

Oh ok, that makes sense . What would that improve, the negative camber angle? Because the steering is equally solid with or without the camber angle it seems. It would be a pretty major redesign I wonder if it's worth the trouble...

6 hours ago, gyenesvi said:

You could put the panhard rod to the very end of the axle, there you have a lot of space, and also it does its job better there as there it takes less force to hold the axle in place.

That's a good idea . At the rear, the challenge is that the pneumatic pump gets in the way but I will try to get around it. The other issue on the rear has been that I can't move the diagonal beams any further inward as they will touch the axle, so the suspensions have to be mounted further out than I would like, stiffening the side-to-side movement of the suspension. I will be mounting the suspension closer together on the front however so that the front axle has more side-to-side movement. This is consistent with the LEGO Unimog, and I think it's useful for keeping the vehicle more stable - the heavier front of the car can tip over more easily.

[gyenesvi]

2 hours ago, Teo LEGO Technic said:

What would that improve, the negative camber angle?

Not the camber, rather it would improve the caster angle at the front, it would become neutral, not negative.