[WIP] Praga Trial Truck 6x6

[Zerobricks]

Personally I mostly prefer high-travel independent suspension, especially in the front for fast vehicles where stability is important. Independent suspension allows you to build a much lower and stable frame, and the steering system tends to be more responsive than linking the power to a live axle. Also there's less chance of drivelines popping out under stress. Independent suspension is generally much more responsive and smoother since there's no weird perpendicular conenctors needed to connect 9L shock absorbers at non-axial mounting points.

Having said that a live axle with integrated drive and steering motors is a good way to go too, but than again you need a high, stiff frame to hold it all together, increasing center of gravity.

I think a compromise with Independent front suspension and live axle in the back might be a good way to too.

Edited May 17, 2023 by Zerobricks

[TexasEngineer454]

18 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 ?

I say this as the current MOC @Teo LEGO Technic is working on is based on a 6x6 drivetrain. With an independent front suspension and solid rear axles you could more easily tackle off camber climbing. The geometry of the front axle would be more complicated and the clearance would be limited, but the ability to approach obstacles at different angles is better for this type of build. Both types of front axles have their benefits and cons. Either way I'm eager to see the completed vehicle!

[gyenesvi]

3 hours ago, Zerobricks said:

Personally I mostly prefer high-travel independent suspension, especially in the front for fast vehicles where stability is important.

I agree with you for the case of fast vehicles (I guess that is why buggies like Ultra4s and UTVs have them), but I think for a slow trial truck the front live axle setup can be better because of the more ground clearance it provides. Also, another thing to consider is that IFS is only really useful if it is high travel, but that's something hard to achieve in lego on a smaller scale or in a narrow vehicle (like a truck) when it is driven, because the diff already forces the A-arms to be far apart and shorter (unless you drive left-right wheels independently, which complicates the drivetrain). So in some sense the live axle is even simpler to build. For medium scales, like 13 wide axles, a (clean and solid) driven IFS is just impossible (maybe with a floating diff), we are only left with live axles as a possibility.

[Daniel-99]

14 hours ago, gyenesvi said:

For medium scales, like 13 wide axles, a (clean and solid) driven IFS is just impossible (maybe with a floating diff), we are only left with live axles as a possibility.

Check out this genius solution with IFS and IRS: 

 

Edited May 18, 2023 by Daniel-99

[gyenesvi]

28 minutes ago, Daniel-99 said:

Check out this genius solution with IFS and IRS: 

Well that is indeed a genius solution (love the steering as well)! Thanks for showing! However, it does not really fit my idea of clean and solid, neither is it long travel, and it does not have a diff either :) But for a small model like this, it's really nice.

[Teo LEGO Technic]

On 5/17/2023 at 12:20 PM, gyenesvi said:

So in some sense the live axle is even simpler to build.

Good point, I agree that this is usually the case in LEGO. 

On 5/18/2023 at 2:44 AM, Daniel-99 said:

Check out this genius solution with IFS and IRS: 

This is a very interesting solution, I like it! As @gyenesvi points out it isn't very robust, and for example wouldn't fare well in a trial truck scenario, but as a proof of concept it's a very creative solution

Update May 27

It's been a busy time, so little progress has been made. I'm a bit blocked currently deciding how to properly link and brace the 2 rear live axles, both to each other and to the chassis. The main movements I'm trying to focus on bracing are longitudinally so that the force applied by the axles pushing the car up a slope has minimal play, and transversely so that the axles don't move side-to-side when the truck is climbing sideways over obstacles. Here is a picture of some progress:

I like very much @Zerobricks's solution on his Wildcat which allows the rear axles to articulate respective to each other without pressing on any shock absorbers, so that the wheels make maximum contact with the ground, and yet still have shock absorbers used to absorb impact if, for example, all 4 wheels are to be pressed at the same time. To implement this, I plan to replace the grey linkage bars in the image below with this LEGO approximation of a leaf spring suspension:

Please post any examples you guys know of where other builders have achieved something similar, I'm trying to get a better understanding of how this should be done. More progress to come soon hopefully!

[lmdesigner42]

Those rear axles are looking good so far! How strong is the flexible "leaf spring" you built?

Going off the first picture, I would strongly attach the central pin holes of the grey links to the chassis as the main pivot points, and maybe lower them and add some braces on the axles to keep the ball joints from popping out. Then add some transverse panhard rods and extra longitudinal links to stabilize everything.

That's just a suggestion though, there are many ways to make a great set of live axles!

[Teo LEGO Technic]

Update July 10

It's been some time since the last update, but unfortunately, I was too busy to make any progress. Finally, I made some time this weekend to make some headway, and here it is - a working prototype for the suspension on the rear 2 axles. 

After several different strategies, I settled on this bogey-style strategy as the best one, mostly because it keeps the distance between the axles closest to constant so that the connecting drive shaft doesn't fall out or get squeezed in, allowing the mechanism to function properly. It is braced very rigidly both longitudinally and transversally, and the high and low mounting points of the links keep it from bending sideways under the torque of the drivetrain, of which there will be lots. 

The only issue I would like to fix happens when the axles tilt sideways respective of one another. When this happens, the distance between the axles starts to decrease, which applies pressure and friction to the central driving axle, which causes power loss. Currently, this is the only part of the mechanism limiting the side-to-side motion, causing all the more friction. One solution I was playing with was to add torsion bars, but the issue I found was I wasn't able to make the torsion strength soft enough without having ridiculously long arms connection the axle links to the axle under torsion. 

Any advice on how to fix this problem, as well as any other suggestions, are welcome as always 

Edited July 10, 2023 by Teo LEGO Technic

[lmdesigner42]

Those axles are looking pretty capable, would a sliding CV joint help (77590)?

[Teo LEGO Technic]

25 minutes ago, lmdesigner42 said:

Those axles are looking pretty capable, would a sliding CV joint help (77590)?

Thanks! Yes that could work very well, do you know how deep the hole goes inside that piece, is it 4L? If so I could replace one of the light grey CV joints with that sliding joint and put a 4L axle between them, which would give a whole stud of play to the system, which is more than enough.

Edited July 10, 2023 by Teo LEGO Technic

[lmdesigner42]

According to Sariel the hole is 3.5 studs deep (Zetros review), which should fit your setup.

[Teo LEGO Technic]

13 hours ago, lmdesigner42 said:

According to Sariel the hole is 3.5 studs deep (Zetros review), which should fit your setup.

Great, that fits perfectly. Time to order some

[Teo LEGO Technic]

Update August 2

Back from vacation, I'm resuming work on the truck, and planning to complete it by our scheduled race in 2 weeks' time. 

An issue I'm facing is on the rear bogie suspension system. While my setup is keeping a fairly constant distance between the two rear axles, and any variation is easily compensated for by the sliding CV joint piece, I'm finding that when the inner axle is lowered fully the sliding CV joint can fall out when the axle slides out too far. One option to fix this is to lower the universal joint coming out of the gearbox, but I don't like this solution as it reduces ground clearance. Another option is to add a set of two 16-tooth gears to raise the existing exit point from the inner axle by 2 studs, which again would allow the driving axle to stay more horizontal and fix the issue, but I'm not a fan of this solution either.

Because the effect is so slight, my ideal solution would be to restrict the range of motion of the inner axle slightly to ensure the axle doesn't fall out. It only really falls out because of play at the bottom of the range of the suspension anyway, so this wouldn't reduce the suspension travel by much, rather just brace it better. Does anyone have any recommendations on how to build such a stopping mechanism without being too bulky? How do such suspension mechanisms restrict the range in reality? The following illustrates the issue: 

Edited August 2, 2023 by Teo LEGO Technic

[gyenesvi]

If you already see that shaft falling out, then it will definitely fall out under torque. It's actually worse than that :) What can also happen is that the axle slides into the sliding shaft and out of the U-joint on the chassis end given that it has enough play. That's what happened with my model. I could not imagine it could fall out, but under torque the axle twisted enough to allow things to be pushed apart (though that happened with 2 Buwizz motors). Also one problem I see on the image is the negative caster angle of the axle. That is making the CV joint break more and get more elevated, which pulls the shaft apart. Changing that to have zeros caster could also help.

[Teo LEGO Technic]

2 hours ago, gyenesvi said:

If you already see that shaft falling out, then it will definitely fall out under torque. It's actually worse than that :) What can also happen is that the axle slides into the sliding shaft and out of the U-joint on the chassis end given that it has enough play. That's what happened with my model. I could not imagine it could fall out, but under torque the axle twisted enough to allow things to be pushed apart (though that happened with 2 Buwizz motors). Also one problem I see on the image is the negative caster angle of the axle. That is making the CV joint break more and get more elevated, which pulls the shaft apart. Changing that to have zeros caster could also help.

Yeah, that's a good point, I had thought of that as well but hoped it wouldn't happen lol. Frankly, I'm not too keen to redesign the rear axle suspension after all the effort that went into the current design, but if it must be done I guess I will.

My question is how is it possible to keep a constant (or near-constant) distance between both the two axles, as well as from the gearbox to the inner axle? 

Edited August 2, 2023 by Teo LEGO Technic

[Zerobricks]

Have the trailing links connect the axles in same orientation and length as drive axles. 

[Teo LEGO Technic]

On 8/2/2023 at 2:22 PM, Zerobricks said:

Have the trailing links connect the axles in same orientation and length as drive axles. 

I looked at your Mercedes 6x6 and I see what you mean, you linked the middle axle to the chassis, and the rear axle to the middle. I guess the only issue with this setup is that it does not implement the bogie design, but rather uses regular shocks on both axles, but it would appear to make the design more robust. 

In the end, this is the design I settled for, with a custom sliding axle design that is robust enough and allows more travel:

With the competition only two days away I'm going to rush to make this thing drivable and add some bodywork, and I think I may continue to work on it afterwards to try to fix a few things.

In terms of the robustness of the chassis, what techniques do you guys recommend to make it as strong as possible with the fewest parts, to keep weight down? One technique I always use is two have a 2-stud thick build made of beams and 5x7 frames linked together, but in this case, I would like to do things a bit differently as this does not fit above the portion of the chassis where the motors are, where the chassis is only 1 stud thick above and 1 stud thick below.

Edited August 9, 2023 by Teo LEGO Technic

[Zerobricks]

1 hour ago, Teo LEGO Technic said:

In terms of the robustness of the chassis, what techniques do you guys recommend to make it as strong as possible with the fewest parts, to keep weight down? One technique I always use is two have a 2-stud thick build made of beams and 5x7 frames linked together, but in this case, I would like to do things a bit differently as this does not fit above the portion of the chassis where the motors are, where the chassis is only 1 stud thick above and 1 stud thick below.

I suggest using 3x19 frames

[Teo LEGO Technic]

3 hours ago, Zerobricks said:

I suggest using 3x19 frames

That would be ideal, sadly I don’t have any. I’ll get some perhaps for the next build.

[Zerobricks]

16 minutes ago, Teo LEGO Technic said:

That would be ideal, sadly I don’t have any. I’ll get some perhaps for the next build.

Than build 2 large perpendicular triangles, which meet where the rear of the cab is.

Edited August 9, 2023 by Zerobricks

[Teo LEGO Technic]

On 8/9/2023 at 1:47 PM, Zerobricks said:

Than build 2 large perpendicular triangles, which meet where the rear of the cab is.

In the end I did something similar to this, yes.

[Teo LEGO Technic]

Update August 16

This past weekend, Toronto had its inaugural Truck Trial! Thanks to @2GodBDGlory and @lmdesigner42 for participating, here is a photo of the 3 trucks that competed: 

All the trucks performed well, with different strengths and weaknesses. @2GodBDGlory's Lada was a monster of torque and power, speeding through the course but also destroying a few parts along the way, more info about it can be found here: 

@lmdesigner42's truck was slow but powerful and its winch really helped it out of some tough situations, but the added weight made some of the gears crunch under high torque as well.

As for my truck, I was very satisfied with the performance. The drivetrain never had any breaking gears anywhere, and the only issue is once, under the worst conditions, the drivetrain slipped out of the middle axle, and this is something I will fix by adding some stoppers to limit the range of motion on that axle with respect to the chassis. The gearbox worked like a charm, as expected, as did the suspension. The steering, which I was worried wouldn't be quite strong enough, turned out to have plenty of strength, and the speed of it, and return-to-center provided by the servo was useful. I was also concerned that the lack of differentials could hurt the steering radius, but it turned out to have quite a good steering angle despite its long wheelbase due to its large steering angle, and the fast steering meant it wasn't time-consuming to steer back and forth several times for really tight corners. 

Overall it is definitely my most capable trial truck to date, and thanks a lot to everyone who helped with advice and feedback along the way, particularly @Zerobricks and @gyenesvi. As a next step, I'm going to redo the bodywork to look nicer, as it was just a temporary build for the truck to be ready for the race, and as I said before I'm going to fix the escaping drivetrain axle as well. As well, a video with the race will follow as soon as I have time to edit the footage. Below are the results of the race:

Results

Track 1

2GodBDGlory - 1:54

lmdesigner42 - 6:00

Teo - 0:42

 

Track 2

2GodBDGlory - not completed

lmdesigner42 - 8:00

Teo - 0:20

 

Track 3

2GodBDGlory - 0:17

Teo - 0:19

lmdesigner42 - 5:00

 

Track 4

2GodBDGlory - 3:19

lmdesigner42 - 12:00

Teo - 2:00

 

Track 5

2GodBDGlory - 1:45

lmdesigner42 - 5:00

Teo - 1:43

Edited August 16, 2023 by Teo LEGO Technic

[2GodBDGlory]

The event was a lot of fun, thanks for organizing it!

I definitely learned a lot from looking at other people's trucks, and from the shortcomings of my own!

[Teo LEGO Technic]

On 8/16/2023 at 10:08 AM, 2GodBDGlory said:

The event was a lot of fun, thanks for organizing it!

Yeah, it was awesome! Hopefully the first of many. 

Update August 18

The final video is edited and finished, enjoy!

Next step is to finish up the Praga truck and made a video with it as well.

[Teo LEGO Technic]

Update September 27

After a busy couple weeks, finally had time to redo the cabin and take some photos. Video to follow soon.

More here: https://bricksafe.com/pages/Teo_LEGO_Technic/praga-6x6-trial-truck