[MOC] Ultra 4 Buggy - Zetros alternate

[gyenesvi]

Hi All,

I am here again with yet another Zetros alternate and yet another buggy; this time it's an Ultra 4. The story behind this is the following. I have been tinkering with the idea of a small and fast buggy from the Zetros since the beginning, but couldn't come up with the right form factor and technical details. Originally I was thinking of something RWD by gearing up the motors, and independent suspension on all wheels, but it would have been only possible with quite wide (23 studs) wheelbase due to the lack of enough CV joints of the shorter type, some of the new 2 studs longer ones would have had to be used like in @Didumos69's Hot Trot and I thought it would have been too wide for the wheel size and proportions (I wanted something short, but actually it does not look too wide on the Hot Trot). Also I could not figure out an interesting gearbox and motor layout, as my initial idea was to connect the drive directly to the rear axle without any complicating factor to minimize friction. So I put the project aside for a while.

Apart from this, I have also been lamenting lately how nice it would be to build an actually useful RC gearbox :) I mean something that has a visibly different speed ratio, something like 3:1 not the simple 1.666:1. As we all know, that's not easily (simply) possible as there's no 24T clutch gear, but @Zerobricks noted in another thread that the old differential could also be used in a gearbox, as it has clutch on both sides (thanks for that). So I thought I'd try that idea to see if it works reliably, and since the Zetros has one of that differential (and one 8T gear), I started to think about it as an alternate build. The gearbox itself is simple to build, only it takes quite some space, but at least can be sandwitched between two motors. The high gear ended up being 4x faster than the Zetros.

So at this point I started to think about the form factor. Recently I have been learning about Ultra 4 buggies, that they are designed for both high speed off-roading in the desert and rock crawling at the same time (as the King of Hammers races contain both types of terrain). So I thought this could be a nice match to the gearbox concept; fast and powerful at the same time (well, fast, given the parts of the set (planetary hubs), nothing like BuWizz fast). Another advantage of the Ultra 4 concept, is that it typically uses independent front but live axle rear suspension, for which there are enough short joints and links to be achieved with 19 wide wheel base (just okay for the wheel size). Furthermore, an Ultra 4 should be AWD, which eliminates a problem with the planetary hubs if used in RWD vehicles; if the front hubs are not driven, the wheels back-drive the planetary gears (and the further drivetrain if any), and that creates excess friction; my Jeep Wrangler alternate had noticeable friction resulting from this back-drive in RWD mode. So I settled with the Ultra 4 concept.

Making the independent front suspension did mean some challenge here. First there is only 1 steering link in the set, so those had to be built as well in a tight space. Second, I did not want to loose much ground clearance, I wanted to build it in a way that only a few really necessary parts go below the level of the differential. Luckily it was possible due to connecting flip-flop beams with a frame that creates very strong basis for mounting the A-arms. Even the steering motor could be nicely placed on top of the front axle. And that leads to the bigger difficulty: placement of the motors and the hub.

With the steering motor on the front axle, and the drive motors on the floor, only the gearbox motor and the hub remained to be placed. I have elaborated on this in another thread: if you want access to the batteries, there aren't many options for placing the huge hub even in a medium sized model; either on the bottom in the middle (that would block the drivetrain), or in the trunk (if the model is large enough) or the last resort is using it as a B column (or cab wall in case of trucks). I resorted to this last option again with no other real choice. Luckily, due to the length and triangulated nature of the links of the rear suspension, they could easily go around the hub.

But that made the gearbox motor placement really hard, as it also needed to be routed to the gearbox somehow with the appropriate gearing ratio and a safety clutch included. In my previous alternates I used a 1:3 gearing from the gearbox motor which was just enough to result in a 180 degree turn. But this time I could not as I used the only 8T gear of the set in the gearbox itself. So I had to make some more complicated gearings, and I also had to go around the hub somehow as there was no way to place the gearbox motor close to the gearbox, only somewhere in the rear part, on the other side of the hub. I fiddled with quite a few candidate solutions, until I realized that if I push the drive motors to the front as much as possible, the gearbox motor just has enough room under the hub, and can be mounted on the hub itself, with its axle coming out towards the rear. So I only had to route it back to the middle of the chassis somehow, and could just make some space for that under the hub without pushing the roof too high. The final chassis looks like this, it's a pretty compact and dense build and I am quite satisfied with it in the end.

Here is an x-ray view of the drivetrain and the gearbox itself:

And the same with the motors included:

Somehow magically, I even had enough space left in the cab to put two larger seats (the seats in the set were too small) that avoid touching the gears, but their mounting was more difficult than building them with the limited parts. They look nice I think, and they rely on a few bars instead of axles due to the connection angles. I intentionally put a bunch of stop axles in them to make them look like they're bolted on the side :)

For the exterior, I wanted something light-weight this time, and the flat panels of the set were not useful anyway for a buggy shape. So I went for a tubular design, with only side panels. I was able to use the seats as fenders in the front, and I found that the flip-flop beams create a nice rough look on the top and around the edges. Shaping the front to make it look something like a buggy also had me fiddle around quite a bit, but I think the result is okay, at least the lights make it look nicer. On the rear, I wanted to add some details, but I had to build it in a way that makes it easy to access the batteries (the frame and rear suspension is not in the way!). So I made it such that the exhaust looking decoration can be removed easily. To remove the batteries, the large gears of the gearbox motor need to be taken out as well, so that also had to be built such that it is easy to remove them and put them back (but they don't fall off easily).

Here are some images of the final model:

As for the suspension, it articulates pretty well, it can bridge a 9 stud high obstacle without all the other wheels leaving the ground:

Check out the video to see how it performs in different conditions and let me know how you like it!

Building instructions are available on Rebrickable.

Cheers,

Viktor

Edited January 16, 2022 by gyenesvi

[Zerobricks]

Very well done indeed. Having a gearbox is a great function, so you can make a fast model with enough torque to climb. Only thing that worries me is the way the rear shock absorbers are attached to the control arms, they seem a bit loose.

[Jundis]

Oh man this is soo good. I really never thought about buying this set. But with so many great B/C-Models, it's now tempting.

The crosscountry quality is fantastic considering the possibities of the motors. Bravo!

[1gor]

You know, you're making really cool Zetros alternatives

[Permo]

Well done !

[GerritvdG]

Cool model Viktor, again a high quality alternative. 

[Pattspatt]

Wow, another great alt. I’m amazed you were able to make such complex suspension work so well, in a 4x4 RC alternate no less! The motors and gearbox arrangement is impressively compact - really well done.
 

Also, I always enjoy learning about your design process and the foibles encountered on the way - thanks for another great write-up! 

[Pato Sentado]

That was the set I was asking for!!!

[gyenesvi]

On 1/15/2022 at 1:14 AM, Zerobricks said:

Very well done indeed.

On 1/15/2022 at 7:58 AM, Jundis said:

Oh man this is soo good. I really never thought about buying this set. But with so many great B/C-Models, it's now tempting.

The crosscountry quality is fantastic considering the possibities of the motors. Bravo!

On 1/15/2022 at 8:18 AM, 1gor said:

You know, you're making really cool Zetros alternatives

On 1/15/2022 at 10:46 AM, Permo said:

Well done !

On 1/15/2022 at 11:01 AM, GerritvdG said:

Cool model Viktor, again a high quality alternative. 

23 hours ago, Pattspatt said:

Wow, another great alt. I’m amazed you were able to make such complex suspension work so well, in a 4x4 RC alternate no less! The motors and gearbox arrangement is impressively compact - really well done.

Thanks guys, glad you like it!

23 hours ago, Pato Sentado said:

That was the set I was asking for!!!

Do you mean as an alternate build, or would you like something like that as an official set?

On 1/15/2022 at 1:14 AM, Zerobricks said:

Only thing that worries me is the way the rear shock absorbers are attached to the control arms, they seem a bit loose.

Glad you pointed this out, I forgot to mention that bit. So originally, the shock absorber was fixed to the lower control arm with a 2x4 L beam, which would be solid. However, the rear of the model was sitting a bit too high, so I tried a couple of geometries, to get the right position (and softness). I wanted it lower, but too low would not work either, as then the driveshaft could touch the 24T gears of the gearbox motor when pushed down or articulated. So I was playing around with the geometry and tried this just to see where it could be ideal and this was a good middle ground between higher and lower. I also thought it's a bit loose, but I realized that it's solid agains pushing down because the shock absorber is firmly stopped by the control arm under it, and it actually increases the articulation! In a sense, it works like the official Wrangler's rear articulation, that the spring is fixed on a lever on the top end such that it can follow the articulation (although in that setup it's necessary to make it work at all), except this one has the low end loose. I thought it's an interesting concept to try, so in the end I kept it to see how it works, and it seems fine. Sure that the spring is not pushing the wheel down to the ground at max articulation, so the ground contact is not so strong in that case, but I guess it's still better than lifting the wheel.

Edited January 16, 2022 by gyenesvi

[Daniel-99]

Well done! Especially I enjoy 4x4 configuration and the rear axle, while the front axle is quite arguable for me: in the initial position joints are angled too much... Especially on the high gear car will have struggles with maximal steering angles. I would lower a car a bit to reduce extra stress form the joints.. Or even made the front suspension with 2 positions: lowered for high speed and lifted for low speed (for crawling). But all in all, this will be only a small "improvement" of initially good car (cant say this about improvements of Zetros set  ) 

Also I would really enjoy seeing this buggy with different wheels: RC tires of motorcycle one!

P.S. I think this car has no problems with extra weight as the original set 

Edited January 16, 2022 by Daniel-99

[gyenesvi]

2 hours ago, Daniel-99 said:

in the initial position joints are angled too much

This only looks so on the renders where I had to angle it as if it was in max position to let the springs fit in (it would have looked awkward otherwise). In reality the spring has a bit of preload because of the model's weight, and that makes the angle lower. I have actually tested this, and there was no problem for the joints, they were running smooth. But you are right that care must be taken with this as a too high angle could become a problem for the joints. Anyway, thanks, glad you like it!

BTW, it's only 1.2 kgs. I think the Zetros is over 2 kgs.

[agrof]

I really love how much passion and knowledge You put into alternatives. Ultra4's have a special place in my heart, the introduction is also a very enjoyable reading.

The sensitivity of the suspension looks spot on!

[lmdesigner42]

Another great alternate build of this set! I feel like the B/C models people have come up with are more interesting and fun than the original Zetros. Do the larger Claas tires fit on this buggy?

[gyenesvi]

On 1/20/2022 at 11:05 AM, agrof said:

I really love how much passion and knowledge You put into alternatives. Ultra4's have a special place in my heart, the introduction is also a very enjoyable reading.

The sensitivity of the suspension looks spot on!

Thanks a lot, actually, your ultra 4 model raised my curiosity and awareness of this class of vehicles :) I like the concept too, and I do plan to build another one in the future with unlimited inventory, something faster.

On 1/21/2022 at 4:33 PM, lmdesigner42 said:

Another great alternate build of this set! I feel like the B/C models people have come up with are more interesting and fun than the original Zetros. Do the larger Claas tires fit on this buggy?

Thank you, glad you like it! I did try putting larger wheels on it. On the rear, it works without any problem, but on the front it can touch the fender when it’s steered. But it can easily be solved by taking the fender off. To me they looked too large though (it’s a short wheelbase).

 

BTW, I just put out all of my Zetros alternates in a package for a better price than buying separately:

https://rebrickable.com/mocs/MOC-99251/gyenesvi/3-in-1-42129-c-model-pack

[Bony_em]

Very cool, one question, what is part 46834 and 46835, used for to force the shaft?

 

 

 

[gyenesvi]

11 hours ago, Bony_em said:

Very cool, one question, what is part 46834 and 46835, used for to force the shaft?

Thanks, that's a good question actually. The Zetros has a PU M motor for the gearbox. That motor does not have position sensor, so it is much more cumbersome to make a gearbox switch with it. Instead it is programmed to rotate for some time, say half a second. During that time, it rotates more or less 540 degrees as I have measured. First, it has to be geared down to amount in a bit more than 180 degree turn, and one must use a physical limiter to precisely limit the movement to 180 degrees; that is done by a dbg connector in front of the orange rotary catch, it can be seen in an upright position on the full chassis image, but kind of hard to notice as it blends in. On the image it is in neutral position, and turns left or right for the two speeds, where it is limited. Since there is a physical limiter, and the motor turns the axle more than 180 degrees (especially during calibration), there must be a clutch (parts 46834 and 46835) in between the two to prevent breaking gears or hurting the motor. Also, the clutch has to be after the down-gearing, otherwise the motor does not have enough torque to make the clutch work. It's quite cumbersome to build a gearbox with all these limitations, a motor with a position sensor (as in the Volvo hauler) would be much simpler, not sure why this was used in the Zetros.