[WIP] Praga Trial Truck 6x6

[Zerobricks]

18 minutes ago, Teo LEGO Technic said:

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

Personally i would connect the rear axles fixed together on let's say rear drive-axle. Than I'd use a differential to connect the front and rear drive-axles to drive the fake engine. And you can lock the same diff so the rear and front axles can help each other. Just my 2 cents.

[Teo LEGO Technic]

Just now, Zerobricks said:

Personally i would connect the rear axles fixed together on let's say rear drive-axle. Than I'd use a differential to connect the front and rear drive-axles to drive the fake engine. And you can lock the same diff so the rear and front axles can help each other. Just my 2 cents.

That's a clever idea, I like it. It will make it so that it's not possible to incorporate a gearbox, but on the plus side it will make it so that every axle will have more power than just one XL, when it needs it. Do you think 1 BuWizz 2.0 is capable of powering 3 PF XL motors?

[Zerobricks]

4 minutes ago, Teo LEGO Technic said:

That's a clever idea, I like it. It will make it so that it's not possible to incorporate a gearbox, but on the plus side it will make it so that every axle will have more power than just one XL, when it needs it. Do you think 1 BuWizz 2.0 is capable of powering 3 PF XL motors?

Thanks. Yes, 3 XL is doable, but nothing more that for now (FW update with current limits is in the works).

Edited April 30, 2023 by Zerobricks

[Teo LEGO Technic]

2 minutes ago, Zerobricks said:

Thanks. Yes, 3 XL is doable, but nothng more that for now

Sounds good  That will keep the weight down a bit, and save me some cash

[Zerobricks]

Also I suggest you orient the motors longitudally on the axles and use the red diff with a combination of 28 gears to drive the planetary hubs. Also a diff lock activated via wave selector. To lock diffs you can use a centralized system which locks them all, or use a motor for it's own axle like I did with my Wildcat's 6x6 gearbox motors. Might be a good idea to have seperate axle and central lock, sometimes you just need a central lock to help climb up without losing the mobility of whole axle locks.

Edited April 30, 2023 by Zerobricks

[Zerobricks]

Been thinking a bit more about this project and connecting the 2 rear axles will be tricky and a weak point, since the propshafts can slide out when the axles move up and down. How about using a bit of an unusual solution, independent rear suspension with new CV joints and both axles connected like leaf suspension? When one wheel goes up, the other goes down in tandem. This would give you an emulation of a solid axle without having to worry about driveaxles popping out. And you can have the rear axles locked without a diff in order to have suspension arms as long as possible. I don't think steering would be major issue as long as you have a central and front differential with lockers.

[gyenesvi]

3 hours ago, Teo LEGO Technic said:

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

Actually Attika did the same, his latest version is with U-joints and I think it works fine :) But obviously it would depend on the weight of the whole model.

[Teo LEGO Technic]

23 hours ago, gyenesvi said:

Actually Attika did the same, his latest version is with U-joints and I think it works fine :) But obviously it would depend on the weight of the whole model.

The only model I've seen Attika make since his Ultimate Pickup is this, and it's only a teaser: https://www.youtube.com/watch?v=BpIwocFrzKI Can you attach the link for the build you're referring to?

 

On 4/30/2023 at 12:42 PM, Zerobricks said:

Also I suggest you orient the motors longitudally on the axles and use the red diff with a combination of 28 gears to drive the planetary hubs

Do you mean I should brace the red diff in a 7 x 11 frame and drive it with a 28-tooth gear? Just so we're on the same page, longitudinal mounting means perpendicular gears are required:

On 4/30/2023 at 3:16 PM, Zerobricks said:

And you can have the rear axles locked without a diff in order to have suspension arms as long as possible. I don't think steering would be major issue as long as you have a central and front differential with lockers.

I very much like the idea of having a diff lock only on the front axle, as this makes it easier to create a more robust perpendicular gear system on the rear axles, where most of the torque is going anyway for steep uphill climbing, while also maintaining a good steering radius so the front wheels don't 'drag' when turning with locked diffs. It also simplifies the system by reducing the number of diff locks required.

On 4/30/2023 at 3:16 PM, Zerobricks said:

How about using a bit of an unusual solution, independent rear suspension with new CV joints and both axles connected like leaf suspension? When one wheel goes up, the other goes down in tandem. This would give you an emulation of a solid axle without having to worry about driveaxles popping out.

This is an interesting idea, although it would look a little odd with a live axle front suspension and independent rear suspension. At that point might I not be better off just using pendular suspension on the rear, with small turntables to take the friction off the drive axle?

I ordered the parts today, including the planetary hubs, new CV joints, 28-tooth double-bevel gears, and 2 more CLAAS tires for a total of 6 (among others). Parts won't arrive until end of the month, so meanwhile I will experiment on Studio.io with different solutions. 

[gyenesvi]

2 hours ago, Teo LEGO Technic said:

The only model I've seen Attika make since his Ultimate Pickup is this, and it's only a teaser: https://www.youtube.com/watch?v=BpIwocFrzKI Can you attach the link for the build you're referring to?

I don't think there is a more recent link than this, I saw his build in the Buwizz camp in person. But as a start you can simply swap the inner CV joint to a U-joint, it is the same length, only the flex point will be different but that does not matter as the axle is a solid one. Then with the U-joint in, it is possible to use different drive axle lengths, so it becomes possible to make the whole axle wider if one wants, but maybe for your build you just want to keep it narrow.

[Teo LEGO Technic]

36 minutes ago, gyenesvi said:

But as a start you can simply swap the inner CV joint to a U-joint, it is the same length, only the flex point will be different but that does not matter as the axle is a solid one

Oh, I see, that makes sense. I'm used to thinking that the flex point has to remain stationary for independent suspension, and I forgot Attika uses an independent suspension setup repurposed as a live axle. 

[Teo LEGO Technic]

Update May 13

It's been a busy few weeks, but it's time for an update. My parts shipped, so I could experiment, beginning with the front axle. I decided to forget about incorporating a diff lock, as it's hardly necessary for a trial truck, and instead to focus on robustness and a good range of motion. I currently have two designs that I'm choosing between.

The first one is wider, heavier, and slower, but is more robust, using  @Zerobricks' custom portal axle design from the Tiger 6x6, combined with a 24-tooth worm gear attached directly to the steering rack for maximum precision, and driven by an axle-mounted M-motor so that the worm gear limits any play in the system. It looks like this: 

While this is a good option, my main complaint is that it is quite bulky, the steering is quite slow (and obviously not return-to-center), and I wasn't able to implement a clutch system, so the parts are put under a lot of tension when the steering is fully turned in either direction. Despite the tension, there is no slippage anywhere. 

Option 2 is my favourite. It uses @Attika's live axle design using the planetary hubs but modified to work with the Defender rims, allowing for a steering pivot that is very close to the center of the tire. I also removed the differential and substituted the steering system with a rack-and-pinion system, plus a servo motor. 

The main disadvantage of this system is that the steering has more play, but aside from that it practically has all advantages. It is narrower and lighter than the other one, so I could build a truck at a smaller scale with it, which would be nimbler. The steering is fast and return-to-center, and the planetary hubs simplify the transmission, so there is less power lost to friction. The ground clearance is the same in the center and only slightly worse close to the wheels. Overall I love the elegance and efficiency of this design. 

I attempted to reduce the play by using a longer steering arm, but there wasn't any solution that I could find. While it is possible to extend the hub with 2x4 liftarms when using regular rims, they don't fit in the Defender rims, and using 3x3 liftarms that can protrude enough from the rims doesn't work either, as they touch the driveshaft when steering and severely limit the steering radius. As it is, the steering radius is excellent on both axles.

The plan for the rest of the truck is to use a 2-speed gearbox powered by 2 XL motors and install a winch. This will take up all 4 available BuWizz ports - drive, steering, gearbox, and winch. Using the second design, the truck would be significantly smaller, and so the power-to-weight ratio would be better. As for the rear axles, I was looking at using a bogie-style suspension to allow lots of articulation, but in real trucks, they are usually made using leaf springs. Does anyone have any suggestions? I was looking at the 6x6 Wildcat for reference, but it uses independent suspension instead of live axles: https://www.youtube.com/watch?v=cEBgLdNghpQ&t=91s

Suggestions are welcome as always!

Edited May 14, 2023 by Teo LEGO Technic

[Daniel-99]

I would prefer OPTION 2 as well. Moreover I am interested to see how you adapted Attika`s design to defender rims in more details! Currently the steering rack has not been fixed in place properly. I would attack some sort of "slider axle" to prevent unwanted wobbling. 

[Teo LEGO Technic]

9 hours ago, Daniel-99 said:

I would prefer OPTION 2 as well. Moreover I am interested to see how you adapted Attika`s design to defender rims in more details! Currently the steering rack has not been fixed in place properly. I would attack some sort of "slider axle" to prevent unwanted wobbling. 

Hey! I can gladly provide more detail, what were you wondering about exactly?

Can you give me an example of what you mean by a "slider axle"? Do you mean attaching an axle to the steering rack, which in turn is attached to the axle along sliding pinholes?

Edited May 14, 2023 by Teo LEGO Technic

[Teo LEGO Technic]

I think I solved the rack wobble issue. I added a slider axle as you recommended, and since it is fitted directly underneath the rack, it provides a lot of rigidity. I also added some liftarms on the sides for a bit of extra bracing. Overall, it works excellently 

Edited May 14, 2023 by Teo LEGO Technic

[gyenesvi]

This solution will result in a slight toe out of the wheels as the steering rack is now slightly shorter than the axle due to the links being at an angle. Not sure if it will be noticeable, but such an imperfect geometry was noticeable in the Ford Raptor for example, though that's a manual build.

[Zerobricks]

Just now, gyenesvi said:

This solution will result in a slight toe out of the wheels as the steering rack is now slightly shorter than the axle due to the links being at an angle. Not sure if it will be noticeable, but such an imperfect geometry was noticeable in the Ford Raptor for example, though that's a manual build.

Agreed. I would use a 2x4 L arm liftarm or a perpendicular connector with a towball pin (like 42099's steering arm) to raise the point of the steering arms and to give the steering system extra leverage and stiffness. Remember, the longer the steering arms, the more stable the system and less slack which can cause issues or overload the steering motor. If possible also use double steering racks, that's what I do with my 1:10 and 1:8 cars where there's simply no space to for longer steering arms.

[Teo LEGO Technic]

6 hours ago, gyenesvi said:

This solution will result in a slight toe out of the wheels as the steering rack is now slightly shorter than the axle due to the links being at an angle. Not sure if it will be noticeable, but such an imperfect geometry was noticeable in the Ford Raptor for example, though that's a manual build.

You're right, it does indeed cause a slight toe out at the wheels. I will test it out soon in a basic chassis to see if it is an issue, but I'm hopeful it won't be noticeable on a trial truck. Furthermore, I've always found that on LEGO trucks, with steering play, the torque of the wheels pulling the car forward tends to "toe-in" the wheels, so if they start out a bit "toe-out", they may end up actually parallel.

5 hours ago, Zerobricks said:

Remember, the longer the steering arms, the more stable the system and less slack which can cause issues or overload the steering motor.

Good point, I would have liked longer steering arms. The issue is I want to use the servo motor for compactness, and it only rotates 90 degrees in each other direction. If I were to lengthen the steering arm, that will reduce the turning radius of the truck, which already won't be amazing on a 6x6 with a long wheelbase. 

5 hours ago, Zerobricks said:

I would use a 2x4 L arm liftarm or a perpendicular connector with a towball pin (like 42099's steering arm) to raise the point of the steering arms and to give the steering system extra leverage and stiffness.

These are also great ideas. I tried to use 42099's strategy actually, but unfortunately, the steering axle touches the rims with that configuration when using Defender rims. When I tried using a 2x4 liftarm, it wouldn't fit in the rim in Attika's configuration, and placing it with the long arm inward makes for a really bad Ackerman geometry. I also tried a 3x3 liftarm, but when steering it touches the driveshaft, again limiting the steering angle. One modification I could make to fix the "toe-out" issue is to raise the tow ball position by one stud and move it one stud out to fit. The issue with this is it gets farther from the hub, and the precision is reduced, so I think the better compromise is to have more sturdiness. 

5 hours ago, Zerobricks said:

If possible also use double steering racks, that's what I do with my 1:10 and 1:8 cars where there's simply no space to for longer steering arms.

Do you recommend double racks to improve rigidity/reduce play? Or to stop the gear from skipping on the rack-and-pinion steering? As it is currently built, the steering motor runs out of torque without skipping gears, but I will try using double racks to see if I get more precision. Appreciate the tips as always

Edited May 15, 2023 by Teo LEGO Technic

[Teo LEGO Technic]

I implemented the double rack, and rigidity is indeed improved a little, although most of the play comes from the inherent play of the steering components, and without a longer steering arm it cannot be reduced. As I mentioned earlier unfortunately a longer steering arm doesn't fit in the rims. I did, however, find a solution to the "toe-out" problem - by placing the steering links on 3L axles with flat pin-stops, I managed to bring the link 1/2 stud closer to inline, so that the issue is almost non-existent now. 

Here's a closer view of the new connection:

Edited May 16, 2023 by Teo LEGO Technic

[TexasEngineer454]

12 hours ago, Teo LEGO Technic said:

I did, however, find a solution to the "toe-out" problem - by placing the steering links on 3L axles with flat pin-stops, I managed to bring the link 1/2 stud closer to inline, so that the issue is almost non-existent now. 

Here's a closer view of the new connection:

This looks like a great solution. How does it do with turning the wheels under load though? I'd imagine this would actually hold better than the ball end joints. 

[gyenesvi]

1 hour ago, TexasEngineer454 said:

This looks like a great solution. How does it do with turning the wheels under load though? I'd imagine this would actually hold better than the ball end joints. 

I have never been a big fan of such a solution, because when I tried it seemed to have more slack than the ball joint, and it also seemed to be able to disengage when twisted around (on the other hand the ball joint is pretty hard to disengage from the link). Though in this configuration it might not have enough freedom of movement to be able to disengage.

@Teo LEGO Technic, you are right that one big difficulty of the deep rim is that the steering link must be inside as well, so it becomes impossible to make longer steering arms and move the steering linkage further away from the differential for example.

[Zerobricks]

If you're using a solid axle, why not just use a 1x6 half beam? Or such?

[LegoTT]

I love this thread.

But there is a thing I didn't understand :

TexasEngineer454 said: 

"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. "

Are we saying that independant on the front and solid axle on the rear is a better combinaison for trial truck than solid axle on both axles ?

 

 

Edited May 16, 2023 by LegoTT

[Teo LEGO Technic]

7 hours ago, TexasEngineer454 said:

How does it do with turning the wheels under load though? I'd imagine this would actually hold better than the ball end joints.

As you say, it holds very well there is no play at all.

6 hours ago, gyenesvi said:

I have never been a big fan of such a solution, because when I tried it seemed to have more slack than the ball joint, and it also seemed to be able to disengage when twisted around (on the other hand the ball joint is pretty hard to disengage from the link). Though in this configuration it might not have enough freedom of movement to be able to disengage.

You're definitely right, using a steering axle with the ball joint end attached to an axle has a lot of play, in the range of a few millimetres even. The way I braced it in this configuration however, it is no longer able to pivot in all 3 planes of motion, but rather only hinge on the axle. It is basically acting as a liftarm at this point, so it would not work for say independent suspension, but it works fine for a live axle where the wheel hub remains in the same plane relative to the steering rack. It doesn't provide a lot of strength if you were to have a force pushing it up, as it can slide off the axle end when you push on it, but this is not a danger in this case because all the force acting on it will be side to side, where it is rigid.

6 hours ago, gyenesvi said:

you are right that one big difficulty of the deep rim is that the steering link must be inside as well, so it becomes impossible to make longer steering arms and move the steering linkage further away from the differential for example.

I agree, it seems to be an inherent limitation of the part itself. The only way to make a longer steering arm realistically is to use 3x3 half-stud liftarms, which as I said before will knock into the driveshaft, and reduce range of motion.

5 hours ago, Zerobricks said:

If you're using a solid axle, why not just use a 1x6 half beam? Or such?

That's not a bad idea, but the issue is that there is still an ~1/2 stuff offset, which the steering link can compensate for, unlike a beam. As well, the narrow midsection of the steering link enables it to still fit when it gets closer to the suspension arm, as the steering hub inscribes an arc at either end of the steering motion.

[Teo LEGO Technic]

4 hours ago, LegoTT said:

I love this thread.

Glad you're enjoying it!

 

4 hours ago, LegoTT said:

Are we saying that independant on the front and solid axle on the rear is a better combinaison for trial truck than solid axle on both axles ?

Yes I believe that's what you were saying right @TexasEngineer454 ? Personally that wasn't an option for me on this build because I'm modeling a trial truck that has live axles on all 3 axles. In general my issue with independent suspension in offroad vehicles is that the ground clearance isn't as good as live axles with portal hubs, and the ground clearance under the centre of the axle is reduced when the suspension is pressed, which is not the case with live axles. But it can definitely be made to work, as @Zerobricks did for example on his Wildcat 6x6.

[LegoTT]

I don't understand why it's better