[TC25] 41999 4x4 Crawler Exclusive Edition

[Thirdwigg]

Great job finishing this. It turned out great. I'm happy you kept the white stripes a full liftarm thick. 

[gyenesvi]

11 hours ago, Mikdun said:

It looks very nice, closely resemble the original. So great job! 

8 hours ago, Thirdwigg said:

Great job finishing this. It turned out great.

Thank you guys!

8 hours ago, Thirdwigg said:

I'm happy you kept the white stripes a full liftarm thick. 

Indeed, the half would have looked odd..

11 hours ago, Mikdun said:

Looking forward for the instruction, will have to build it.

Instructions are finished, published on Rebrickable for free:

https://rebrickable.com/mocs/MOC-156786/gyenesvi/shrunken-41999-4x4-crawler

 

Now the only thing remains is a motorization. I already have a possible plan for it.. but will have to see if it actually flies..

[SAM_KTM_304]

insrtuctions?

[gyenesvi]

8 hours ago, SAM_KTM_304 said:

insrtuctions?

It's literally two lines above your question.. Welcome

[Seasider]

If you work out a version of this with electronics that would be great.

[gyenesvi]

18 hours ago, Seasider said:

If you work out a version of this with electronics that would be great.

Well the concept is worked out already, just need to see if/how it actually works in real life. There are some tight spaces, the steering linkage is close both to the drive motor and to the axle's gearing (there's an 8:24 down-gearing in the axle), but it might just fly. Also, the right speed/torque ratio is a question for a single L motor, but that is somewhat adjustable by using old/new differential. Unfortunately I won't have any time to build it at least for a week (or more)..

 

Edited August 24, 2023 by gyenesvi

[SAM_KTM_304]

sorry i didn't see that when posting

 

[gyenesvi]

On 8/23/2023 at 2:36 PM, Seasider said:

If you work out a version of this with electronics that would be great.

So last week I had the time to build and test the above RC version. The good new is that the build actually worked out fine, things don't collide and the gearing is good for a crawler. The bad news is that the axles don't hold up for the job, they fall apart quite easily when driving on real, somewhat demanding terrain (on flat terrain it works okay).

The axles have 3 problems that were actually anticipated, and all of them manifested themselves quite quickly. For one, the 8T gears on the portal hub are only supported from one side, so they can crack, and the upper gears can slide off, and both happens. Second, the built portal hub, comprising of a 3x3 T beam held by pinhole-pins can slide off to the side, and as the model steers and the tires get caught up in rocks, it does slide apart. Third, the old CV joint has a 1.5 deep axle hole in it, but the 3L axle coming out of the differential is only 1 stud deep into the CV joint, so that 3L axle can slide out of the differential and into the CV joint, effectively detaching the drive, and it also happens after a while under some torque.

So altogether because of these problems, these axles are not usable for an RC build in this form. Even one of these issues alone would hurt playability on the longer run, but the 3 together just make it pretty unusable. So I started to think about how this all could be fixed, and came to the conclusion that dropping the portal hub and using a regular wheel hub and widening the axle could solve all issues. However, these balloon tires don't play well with the regular hubs and rims, as the tires are too wide, they rub the steering arms. So it could only be done by changing the tires to non-balloon 68mm ones from the Batmobile. Also, dropping the portal hub lowers the model by 1 stud, and that way the wheels can slightly rub the fenders when the axles are steered and articulated to maximum. Nonetheless, I still thought I could give it a try, just to see how it works. But when widening the axles by two studs, I bumped into another problem: 7L steering links would be required instead of the current 6L ones, which does not exist. The only option is to build a 7L link from suspension arms, but that requires more space, which is already quite tight around the steering linkage. In the front I could find a way to make some space, but in the rear I could not do the same, as the steering servo itself is in the way and cannot be moved. In the end, I found a solution by making the steering asymmetric; the rear one steers a bit less.

Here is the redesigned axle. At the bottom I used old 4L technic plates to fix the wheel hub without sacrificing much ground clearance.

And here is how the complete model looks with them and the alternative tires.

I think it actually looks cool, even though it diverges quite a bit from the original model. After indoor testing, it seems that the build is okay, the axles don't bind and can articulate and steer properly. Some outdoor testing will follow soon to see if they hold up. I am really curious how a single PF L motor for drive can perform with a good amount of down-gearing, it seems powerful enough at first sight!

Edited September 4, 2023 by gyenesvi

[Lego Tom]

Could you use the original tires/wheels by putting the wheels on inside out? I know they wouldn't look proper but the wheels appear to have quite an offset and reversing them would make them stick out more and might help with clearance.

[gyenesvi]

1 hour ago, Lego Tom said:

Could you use the original tires/wheels by putting the wheels on inside out? I know they wouldn't look proper but the wheels appear to have quite an offset and reversing them would make them stick out more and might help with clearance.

Then the situation is somewhat better, but it still rubs. The problem is that in this axle, the steering linkage is coming from above, that's why it's in the way, and that's a key feature of this lifted axle. With a different suspension/steering mechanism the wheel hub and tire combination could work when used inside out, but not here.

[gyenesvi]

I have built the above axle and tested it on rocks. First of all, now it is solid, nothing falls apart. Second, I was pleasantly surprised by it's performance, a single L motor with enough down-gearing can do a lot of crawling. The tires are very slippery though, so I put some proper RC tires on it. I have even recorded a video of it but just when I started editing it, I realized that at this size, I could build it with planetary hubs as well :) So I went back, redesigned the axle again.

Besides more down-gearing, this axle has slightly more ground-clearance in the middle, no 24T gear sticking out behind the axle.

I went out again and tested it, and I was just blown. Plenty of torque from a single L motor. It practically never stopped, it was either spinning the wheel under itself, or flipping itself backwards (even with more sticky RC tires, if both wheels of an axle were on the ground). Besides that, the main limiting factor was the open differentials, but I did not want to close those as both axles are steered.

Action video coming soon!

 

[msk6003]

On 9/5/2023 at 8:49 PM, Lego Tom said:

wheels appear to have quite an offset

This wheels's offset is same on both side

[gyenesvi]

Here is the promised action video :)

Also, free building instructions are available on Rebrickable, and more photos are available on Bricksafe.

 

And 68mm RC4WD Brick Edition tires are perfect fit for this model, especially with white rims :)

[TexasEngineer454]

Awesome work! The dual steer axles are great!

Is it a single servo to steer both front/rear axles?

[gyenesvi]

On 9/22/2023 at 3:25 PM, TexasEngineer454 said:

Awesome work! The dual steer axles are great!

Is it a single servo to steer both front/rear axles?

Thanks! It is a single servo placed in the middle, you can see it on the renders above.

[phool]

So I recently finished bricklinking all the parts for this (just missing the 2L CV Joint axle which has not been available locally) and took a few bad photos of it alongside the real deal. Figured I should post the pics here of the two alongside one another for general interest. Only minor parts swop I did was using replacing the old Hinge Plate parts (which were tricky to find) with plates with bar handles and clips which are much more common.

 

[gyenesvi]

Oh thanks for these photos, they look quite nice together :)

5 hours ago, phool said:

Only minor parts swop I did was using replacing the old Hinge Plate parts (which were tricky to find) with plates with bar handles and clips which are much more common.

Indeed, plates with bars and clips are a good alternative for hinge parts that I am starting to discover and use as well!