[WIP] Praga Trial Truck 6x6

[Teo LEGO Technic]

Hey everyone!

For the first time Toronto, Canada is going to have a Truck Trial race . This is my WIP topic for my entry - a 6x6 truck based on the Praga trial truck, as seen below:

The goals for this model are as follows:

1. Performance prioritized over looks - a bulkier or uglier design that is more robust will be favoured over a cleaner design. The primary goal is the best possible offroad performance
2. Highly robust transmission and steering - I want to make sure that none of the mechanical functions are going to fail under pressure
3. 6x6 drive with diff locks on all 3 axles
4. 2-speed gearbox
5. Winch

My plan for this MOC is to reuse many of my successful design ideas from my most recent Unimog U5000, but with a couple of key improvements:

Reusable ideas

1. Robust transmission setup - by gearing down the transmission close to the wheels, pressure is reduced on sensitive components such as universal joints and differentials
2. Gearbox - the gearbox design, based on  @Sariel's heavy-duty gearbox idea, was very reliable
3. Pneumatic diff lock mechanism - this was also working very well

Features needing improvement

1. Steered hub robustness- the steering hubs on the Unimog were too weak
2. Steering strength - the small linear actuator used for steering turned out to be too weak under the weight of the finished model, as @nerdsforprez originally anticipated (if only I had listened  )

The first decision I have to make is how to power the model. Although I was happy with the power output of 2 XL motors with BuWizz on the Unimog, this MOC will be even heavier, so I can either gear the motors down further or power it with 4 XL motors and 2 BuWizz (which is the more fun but more expensive option). 

I also have to decide how I am going to build the robust front axle I want. I'm leaning towards using the LEGO Unimog portal axle hubs. Despite their poor turning radius, they are probably going to be more robust than any custom portal axle I could possibly build, but if anyone disagrees I would love to hear your arguments . As for the steering mechanism itself, I am thinking of either using a large linear actuator, although it will be difficult to fit in the limited space without limiting turning radius, or alternatively, a large rack and pinion with a worm gear setup. I am leaning towards the latter, but this will be a bit more tedious due to the extra gearing to reduce the speed and the fact that I would like to add a clutch mechanism of some kind so as not to break any components.

Here is some WIP for the front axle:

 

I am looking to make this my best trial truck yet, so all comments are welcome and appreciated! I'm looking at you @Zerobricks, you said there was room for improvement on the Unimog and you could give me some advice on my next MOC , so please feel free to do so!

 

Edited April 25, 2023 by Teo LEGO Technic

[TexasEngineer454]

This looks like it'll be an interesting build! I do have a few questions and comments. 

Are you planning on powering each axle with it's own XL motor? 

Are you thinking about adding a working winch? 

I'd guess the rack and pinion would provide better control but the locked differential and low gearing may make turning at low speeds difficult. You may want to consider an independent suspension front drive axle and solid rear axles. As long as the rear axles can float independently of each other this should make off camber crawling a bit easier.  

Edited April 25, 2023 by TexasEngineer454

[Zerobricks]

Like I said in the previous post, I'd use the brick-built portal axles to get the steering pivot closer to the wheels. Also use the 7x11 frames and contain the differential inside them along with the lock. If you are worried about the differential slippoing, I'd suggest using a 28 tooth gear to drive it, or the ones from the Daytona. I would also use the new 12 tooth straight gears for steering, since t hey have less slack. Also use big CV joints instead of U-joints which may fail at high torque.

Edited April 25, 2023 by Zerobricks

[Daniel-99]

1 hour ago, Teo LEGO Technic said:

Hey everyone!

For the first time Toronto, Canada is going to have a Truck Trial race . This is my WIP topic for my entry - a 6x6 truck based on the Praga trial truck, as seen below:

The goals for this model are as follows:

1. Performance prioritized over looks - a bulkier or uglier design that is more robust will be favoured over a cleaner design. The primary goal is the best possible offroad performance
2. Highly robust transmission and steering - I want to make sure that none of the mechanical functions are going to fail under pressure
3. 6x6 drive with diff locks on all 3 axles
4. 2-speed gearbox
5. Winch

Hey! That will be another project for me to follow! Keep Up! 

I appreciate your goals, especially, the first one. This is actually my main principle of engineering ;-) Though I would not use legoish portal hubs with a long wheel base to keep "decent" manoeuvrability.

[2GodBDGlory]

Nice to see your progress! I've got a decent bit of work done on my model for that race, so maybe I should start a WIP thread sometime too.
I would wonder about how important the differential locks are. In my experience, permanently locked ones are more reliable for crawling, and don't hurt steering too much on low-traction surfaces.

[Teo LEGO Technic]

47 minutes ago, TexasEngineer454 said:

Are you planning on powering each axle with it's own XL motor? 

Are you thinking about adding a working winch? 

No, I don't plan to power the axles separately, as I want to hook them all up to a single gearbox. Yes, I will have a working winch.

47 minutes ago, TexasEngineer454 said:

I'd guess the rack and pinion would provide better control but the locked differential and low gearing may make turning at low speeds difficult. You may want to consider an independent suspension front drive axle and solid rear axles. As long as the rear axles can float independently of each other this should make off camber crawling a bit easier.  

Do you mean it'll be better for climbing sideways on a slope? My concern with independent suspension is that the ground clearance isn't as good, and it's hard to put a lot of torque on it.

43 minutes ago, Zerobricks said:

Like I said in the previous post, I'd use the brick-built portal axles to get the steering pivot closer to the wheels. Also use the 7x11 frames and contain the differential inside them along with the lock. If you are worried about the differential slippoing, I'd suggest using a 28 tooth gear to drive it, or the ones from the Daytona. I would also use the new 12 tooth straight gears for steering, since t hey have less slack. Also use big CV joints instead of U-joints which may fail at high torque.

Sounds good, thanks . Ideally yes, I would like to use the new Daytona bevel gears as they are thicker and could take more torque, but they are priced at $3.50 each on Bricklink, so unless someone knows a cheaper option that's off the table. I will try custom portal axles like you say, probably the ones from your Tiger 6x6, along with the 7x11 frame, and post progress when I'm done.

9 minutes ago, 2GodBDGlory said:

Nice to see your progress! I've got a decent bit of work done on my model for that race, so maybe I should start a WIP thread sometime too.
I would wonder about how important the differential locks are. In my experience, permanently locked ones are more reliable for crawling, and don't hurt steering too much on low-traction surfaces.

Thanks, and yeah you should start a WIP! 

As for differential locks, I was also considering eliminating them. Aside from the loss in power in corners, especially at higher speeds, my main question is what perpendicular bevel gear combination can handle the most torque? Is it the 12-tooth and 28-tooth gear?

34 minutes ago, Daniel-99 said:

Hey! That will be another project for me to follow! Keep Up! 

Sounds good! Hope you'll enjoy the process

Edited April 25, 2023 by Teo LEGO Technic

[TexasEngineer454]

24 minutes ago, Teo LEGO Technic said:

Do you mean it'll be better for climbing sideways on a slope? My concern with independent suspension is that the ground clearance isn't as good, and it's hard to put a lot of torque on it.

To answer your question, yes. It should be better when approaching an incline at an angle. 

Torque only seems to be an issue for the older style differentials 62821b in independent suspension rigs. I am currently running my WIP Land Rover (full independent suspension and all wheel drive to four corners) without any kind of differential and have noticed the better torque for crawling. Turning and handling isn't very good though. So there are some trade offs. 

[2GodBDGlory]

1 hour ago, Teo LEGO Technic said:

Sounds good, thanks . Ideally yes, I would like to use the new Daytona bevel gears as they are thicker and could take more torque, but they are priced at $3.50 each on Bricklink, so unless someone knows a cheaper option that's off the table.

I mean, there's always AliExpress ones, depending on how you feel about that. I haven't got any of those, but I have been tempted.

1 hour ago, Teo LEGO Technic said:

As for differential locks, I was also considering eliminating them. Aside from the loss in power in corners, especially at higher speeds, my main question is what perpendicular bevel gear combination can handle the most torque? Is it the 12-tooth and 28-tooth gear?

My typical strategy for high-torque bevel gear setups is to do a sort of "square" setup to split load among more gears. For example, in my current WIP, I've got a 12T double-bevel gear on the input part of the O-frame, driving a tan 20T double-bevel gear that drives the output shaft. This is where a typical MOC would leave it, but I go a step further, and put a second 12T gear on the opposite side of the O-frame, so that it is meshing with the tan 20T, and then put in a blue 20T clutch double-bevel gear on the output shaft so that it meshes with both 12T gears. It rotates in the opposite direction of the tan one, but because it's an idler that's not a problem.
A similar strategy can be used with 28T gears, by using just the red gear off of the new differential and a standard 28T gear.

[Teo LEGO Technic]

On 4/25/2023 at 12:27 PM, TexasEngineer454 said:

Torque only seems to be an issue for the older style differentials 62821b in independent suspension rigs. I am currently running my WIP Land Rover (full independent suspension and all wheel drive to four corners) without any kind of differential and have noticed the better torque for crawling. Turning and handling isn't very good though. So there are some trade offs. 

I think overall the most robust is still the mighty live axle, but that's definitely a good solution for a medium-sized MOC

On 4/25/2023 at 12:53 PM, 2GodBDGlory said:

I mean, there's always AliExpress ones, depending on how you feel about that. I haven't got any of those, but I have been tempted.

Definitely an option, but do you think they would ship in time? I wasn't able to find sooner shipping times than mid-June.

On 4/25/2023 at 12:53 PM, 2GodBDGlory said:

My typical strategy for high-torque bevel gear setups is to do a sort of "square" setup to split load among more gears. For example, in my current WIP, I've got a 12T double-bevel gear on the input part of the O-frame, driving a tan 20T double-bevel gear that drives the output shaft. This is where a typical MOC would leave it, but I go a step further, and put a second 12T gear on the opposite side of the O-frame, so that it is meshing with the tan 20T, and then put in a blue 20T clutch double-bevel gear on the output shaft so that it meshes with both 12T gears. It rotates in the opposite direction of the tan one, but because it's an idler that's not a problem.
A similar strategy can be used with 28T gears, by using just the red gear off of the new differential and a standard 28T gear.

That's an interesting idea. I think overall the benefits of lockable diffs are outweighed by the drawbacks for a heavy trial truck, so I will go with a no-diff setup. By leaving them out you save space, increase robustness, and save weight as there is no need for any mechanism, pneumatic or otherwise, to power the locks. At the slow speeds that this heavy truck will be driving it seems like an unnecessary addition, and as much as it pains me to let them go, it's a necessary sacrifice in the name of performance.

On 4/25/2023 at 11:03 AM, Zerobricks said:

I'd use the brick-built portal axles to get the steering pivot closer to the wheels.

I built your custom portal axles from the Tiger 6x6, and their robustness is amazing, I am thoroughly impressed! It will operate even nicer than the Unimog portal axles as the steering pivot is closer to the center when combined with the Defender rims. 

Since I am eliminating a differential, there is no concern anymore about how to position the locks. In terms of your other suggestions, I will implement them all once I buy the necessary parts, but I had one question. Do the new CV joints enable a large enough angle to be used at the hub for steering? I'm thinking it may be worthwhile sacrificing a bit of strength there to have a better turning radius, and only use the large CV joints at the axle input location, what do you think?

Edited April 26, 2023 by Teo LEGO Technic

[lmdesigner42]

Looks like a great plan for an improved truck trial! Some thoughts: I tried the 28 tooth idler setup as @2GodBDGlory described, and I found it had a lot of friction. Maybe that can be reduced through lubrication though. Also, be careful applying too much torque to the rack and pinion setup shown in your picture. I drove one like that with a PF M-motor geared down 9:1, and rather than stall, the 12 tooth double bevel gears skipped on the rack at full lock, even with bracing. Reducing the gearing to 5:1 fixed the issue.

Anyway, best of luck with your build, and I look forward to seeing what you come up with!

[gyenesvi]

Really nice target to build :) About the usage of a diff-lock vs a permanently locked diff, I have been wondering about that a lot too. On one hand I did build some models with locked front diff as well, but I had the impression that it does impede steering performance more than I would like. For example my Unimog alternate of the Zetros had that locked diff, and it felt like it was kind of trying to push itself straight instead of wanting to turn, if you get what I mean. Before that I did not think that an open diff would make such a difference, but afterwards I opt more for open front diffs and I don't observe that behavior.

@Zerobricks's planetary construction seems quite a good one, I have checked it out and plan to use it in the future. However, I did not see an easy way to incorporate a new CV into it if the goal of that is to get the pivot point even closer to the wheel; one problem is the size of the CV joint's head being too big, but another difficulty will actually be building the support for the pivots due to space constraints. Maybe not impossible, I have seen such constructions in smaller scale with the old CV joint, but those were much less robust ones (only supported at the bottom and only using a plain axle not a wheel hub) that would not really fly for this large scale. If however the point would be not to move the pivot point closer to the wheel but only use a CV joint which is supposed to be stronger, then it might be possible but not sure if it's worth it. BTW, the CV joints have quite a good max steering angle, though not as extreme as the U-joint. Did anyone ever had a problem with a U-joint breaking in a portal construction? I have the impression that the reduction after the U-joint takes enough stress off of it.

[2GodBDGlory]

4 hours ago, Teo LEGO Technic said:

Definitely an option, but do you think they would ship in time? I wasn't able to find sooner shipping times than mid-June.

In my experience, stuff from there usually arrives in about a month, though it's hard to say, and sometimes my orders have missed some stuff. It's probably not ideal if you're counting on something to come on time; I've always just been stocking up in advance.

4 hours ago, Teo LEGO Technic said:

Since I am eliminating a differential, there is no concern anymore about how to position the locks. In terms of your other suggestions, I will implement them all once I buy the necessary parts, but I had one question. Do the new CV joints enable a large enough angle to be used at the hub for steering? I'm thinking it may be worthwhile sacrificing a bit of strength there to have a better turning radius, and only use the large CV joints at the axle input location, what do you think?

Yeah, the new CV joints allow for quite acceptable steering lock.

2 hours ago, lmdesigner42 said:

Looks like a great plan for an improved truck trial! Some thoughts: I tried the 28 tooth idler setup as @2GodBDGlory described, and I found it had a lot of friction. Maybe that can be reduced through lubrication though.

I first used this setup on high-powered MOCs with a non-Lego motor, and the main purpose was to try to avoid destroying parts! I absolutely had to lubricate everything in that application, but I generally don't otherwise.
It definitely is more friction than just a single meshing, but in some models reliability has to be prioritized! It is somewhat hard to assemble, because if one of the gears gets a tooth off, it won't go together well, and will add a lot of strain to the assembly. It's possible that that could have happened to yours if the friction is really excessive, though I'm guessing you did it just fine!

[HorcikDesigns]

Oh, glad to see another Praga trial truck being made! Definitely looking forward to see the finished version! 

I have built similar vehicle some time ago for our local TT event, and the best performance was achieved with HUBs made from big old turntables (1:7 reduction saves many u-joints), they ensure also excellent ground clearance. 

[gyenesvi]

Oh, one more thing came into my mind about locked front diffs. Once I tried to build an axle with those lego portals and locked diffs, and I was shocked to see that it simply cannot steer in place. That is because of the large scrub radius, as you steer, one wheel is actually rolling forward, while the other is rolling backwards, and that counter rotation is twisting the drive axle between them if they are hard coupled, and the axle twist simply stopped the steering mechanism at a fairly low angle. Of course you could force the steering mechanism more, but I think that's not the right solution (rather improving the scrub radius). So curious how it will work out in your case!

[Teo LEGO Technic]

21 hours ago, lmdesigner42 said:

Also, be careful applying too much torque to the rack and pinion setup shown in your picture. I drove one like that with a PF M-motor geared down 9:1, and rather than stall, the 12 tooth double bevel gears skipped on the rack at full lock, even with bracing.

I will definitely brace it very well. For your setup, did you have it braced on either side with beams? I'm hoping that will be enough.

9 hours ago, HorcikDesigns said:

I have built similar vehicle some time ago for our local TT event

Just looked up your truck, it's really great looking and performs quite well  . Mine will be a bit bigger, not quite to scale with the CLAAS tires, but close.

20 hours ago, gyenesvi said:

Really nice target to build :) About the usage of a diff-lock vs a permanently locked diff, I have been wondering about that a lot too. On one hand I did build some models with locked front diff as well, but I had the impression that it does impede steering performance more than I would like. For example my Unimog alternate of the Zetros had that locked diff, and it felt like it was kind of trying to push itself straight instead of wanting to turn, if you get what I mean. Before that I did not think that an open diff would make such a difference, but afterwards I opt more for open front diffs and I don't observe that behavior.

 

Yes the steering will definitely suffer a bit, I know exactly what you mean by "pushing itself straight."

20 hours ago, gyenesvi said:

 @Zerobricks's planetary construction seems quite a good one, I have checked it out and plan to use it in the future. However, I did not see an easy way to incorporate a new CV into it if the goal of that is to get the pivot point even closer to the wheel; one problem is the size of the CV joint's head being too big, but another difficulty will actually be building the support for the pivots due to space constraints. Maybe not impossible, I have seen such constructions in smaller scale with the old CV joint, but those were much less robust ones (only supported at the bottom and only using a plain axle not a wheel hub) that would not really fly for this large scale. If however the point would be not to move the pivot point closer to the wheel but only use a CV joint which is supposed to be stronger, then it might be possible but not sure if it's worth it. BTW, the CV joints have quite a good max steering angle, though not as extreme as the U-joint. Did anyone ever had a problem with a U-joint breaking in a portal construction? I have the impression that the reduction after the U-joint takes enough stress off of it.

 I think I will stick with the regular CV joint, as the stronger ones indeed take up too much space, and I think they can take enough torque for my needs.

4 hours ago, gyenesvi said:

rather improving the scrub radius

What solutions would you use to improve the scrub radius? I don't see any reasonable way to bring the steering pivot any closer to the center of the wheel rim. Perhaps there is some solution involving kingpin inclination, but that goes beyond what I currently understand, so if anyone wants to explain a solution I would be interested

[Zerobricks]

What about using the planetary hubs with Defender's rims? That should give you a nice low scrub radius and you can use old 6,5 L suspension arms on top and 6L ones on bottom angled up to get a very high ground clearance.

[Daniel-99]

1 hour ago, Zerobricks said:

What about using the planetary hubs with Defender's rims? That should give you a nice low scrub radius and you can use old 6,5 L suspension arms on top and 6L ones on bottom angled up to get a very high ground clearance.

Indeed! I was going to suggest the same idea here, but you made it quicker! ;-) First time I saw this design in Attikas pickup: 

 

[gyenesvi]

Indeed, I was about to suggest the same. At least this is how I will want to build and test a trial truck myself at some point :)

[TexasEngineer454]

2 hours ago, Zerobricks said:

What about using the planetary hubs with Defender's rims? That should give you a nice low scrub radius and you can use old 6,5 L suspension arms on top and 6L ones on bottom angled up to get a very high ground clearance.

That may solve one issue and create another. The backspacing on the 49294 defender rims may bump the tire into the axle. This was an issue I discovered on my Rover MOC. 

[Teo LEGO Technic]

21 hours ago, Zerobricks said:

What about using the planetary hubs with Defender's rims? That should give you a nice low scrub radius and you can use old 6,5 L suspension arms on top and 6L ones on bottom angled up to get a very high ground clearance.

That's a great idea! I saw a similar design recently from you, @gyenesvi and noted it down as an idea to try in future: 

I guess I will be using this technique sooner than I thought . It looks like the new CV joints have a very good range of motion, unlike the older ones, which is more than enough for steering purposes, as well as more robust than U-joints as you pointed out before @Zerobricks.

Because this will enable me to make the axle narrower still, I wonder if it isn't a bad idea to scale down the MOC, perhaps choosing a 4x4 model instead? This would enable me to use BuWizz + 2 XL power setup on a lighter build, and so improve the power-to-weight ratio. I could still make the build large enough to use CLAAS tires, perhaps just having them a bit oversized for the chassis, similarly to what you did @HorcikDesigns on your Praga. This also will mean I don't have to invest more money into additional batteries and motors which is a plus . I would probably go with any 4x4 chassis that has front and back live axles, perhaps a Jeep Wrangler or something similar.

18 hours ago, TexasEngineer454 said:

The backspacing on the 49294 defender rims may bump the tire into the axle.

Why do you think this will happen? It seems to be working fine on Attikas pickup?

Edited April 28, 2023 by Teo LEGO Technic

[Zerobricks]

You can still make it a 6x6 with 3 motors, one per axle and you can connect them together via locks so they can help each other and also drive a possible fake engine.

[TexasEngineer454]

2 hours ago, Teo LEGO Technic said:

Why do you think this will happen? It seems to be working fine on Attikas pickup?

I have not seen Attikas truck? I can only speak from my own experience that the defender rims have a very deep offset and required longer arms when incorporated into a steer axle. I believe this is why the original LEGO 42110 set had IFS/IRS. 

[gyenesvi]

31 minutes ago, TexasEngineer454 said:

I have not seen Attikas truck? I can only speak from my own experience that the defender rims have a very deep offset and required longer arms when incorporated into a steer axle. I believe this is why the original LEGO 42110 set had IFS/IRS. 

I don't really get this problem either, what do you mean by longer arms? A wider axle? Of course that would be required. The set 42110 has IFS/IRS because the real new Land Rover has that, so simple.

@Teo LEGO Technic, the design of Attika is even more angled than mine and it has 1 stud more ground clearance!!

[TexasEngineer454]

3 minutes ago, gyenesvi said:

I don't really get this problem either, what do you mean by longer arms? A wider axle? Of course that would be required. The set 42110 has IFS/IRS because the real new Land Rover has that, so simple.

@Teo LEGO Technic, the design of Attika is even more angled than mine and it has 1 stud more ground clearance!!

I meant a wider axel. I didn't mean to de-rail this thread I was just offering my own experience with the rims. I had issue using them on a narrower steer axle. 

[Teo LEGO Technic]

On 4/28/2023 at 1:47 PM, Zerobricks said:

one per axle and you can connect them together via locks

How do you mean that I can connect them with locks, do you have an example that uses that? 

On 4/28/2023 at 2:28 PM, gyenesvi said:

the design of Attika is even more angled than mine and it has 1 stud more ground clearance!!

Good point! One issue I have with Attika's design is that the greater ground clearance is achieved using the old CV joints at the differential, which may not take the full torque as well, and also pop out sometimes. As well, the dark grey side only has a 2-stud axle, which works for connecting it to a differential housed in a 5x7 frame, but would not work for a solid axle that I plan to use since I will not have differentials. This makes me wonder if perhaps I should either use your design and sacrifice ground clearance, or I may modify Attika's design to use universal joints (although those are also weaker under high torque). 

On 4/28/2023 at 2:33 PM, TexasEngineer454 said:

I didn't mean to de-rail this thread I was just offering my own experience with the rims

No worries, it's a good point to bear in mind :)

Given the new design, I will order some parts today. Progress is going to have to wait until they come, at least in terms of the physical build. 

Edited April 30, 2023 by Teo LEGO Technic