[MOC] Blue Bird Ultra4 Buggy with Buwizz

[gyenesvi]

Hello Eurobrickers,

Let me share my first finished model of the year, even though this one has been long overdue from last year. This time it is a fast buggy inspired by Ultra4 buggies, a variant of which I have built for last year's Buwizz camp, but I didn't get to publish it so far. The idea was to build a buggy with responsive independent suspension, but with something out of the ordinary on the rear: trailing arms with radius rods; something I have never seen implemented in Lego form. At the front the goal was to build a long travel (driven) suspension with long shocks and positive caster angle, which is not easy to do. The first implementation I built for the race was not very satisfying a bit weak and full of half stud offsets. Recently, I managed to redesign it to a more realistic and more robust form.

Originally, I have designed this model with 2 Buwizz motors and one Buwizz 3 unit for the unlimited race category. However, after comparing to my fellow hungarian competitors' builds that used 4 motors, I decided to upgrade it and use 4 motors as well (at the chance of melting some axles). I managed to do an upgrade, which performed very well speed-wise, but turned out to be very hard to control (both speed and steering), so I did not fare well at the race unfortunately. I did not have much time for the upgrade, and the placement of the extra 2 motors was not ideal, was in the way for the steering motor, which I solved by adding one more CV joint to the steering actuation. This made the steering response even slower than usual and the whole car was hard to control at the race. So in the end I chose to publishing the 2-motor variant, which is more controllable, safer to use without melting and also cheaper to replicate.

So without further ado, here is the usual performance video.

 

Features

• Fast and powerful 4x4 drivetrain (2x Buwizz motors)
• Steering by PU L motor
• Powered by Buwizz3 unit
• Long travel double wishbone independent front suspension with positive caster angle
• Long travel independent trailing arm rear suspension with radius rods
• Complete interior with two seats
• Sleek curvy body on a tubular frame

 

Suspension, drivetrain, chassis

The whole experiment started with the rear suspension. Real high speed off-road buggies often have trailing arm suspension, which are stabilized by radius rods. I have already designed one such suspension in my Polaris RZR alternate of the Audi set. However, in that case I could only approximate the actual trailing arm and radius rod geometry with the available parts; the result was a bit of a fake variant, that's similar but not quite how it is in reality. So I wanted to build a more realistic variant of it too. On a larger scale this became possible, however, the space was still quite tight.

In such a suspension, the radius rods keep the wheels moving on an arc, just like in case of a double wishbone independent suspension. At the same time, the trailing arm itself also moves the wheel on another, perpendicular arc. This is only possible if there is some wiggle room in the system, and the longer the parts (the trailing arm and more importantly the radius rods), the more flat those arcs become and less wiggle room is needed. So I had to maximize the length of the radius rods, and the 9L links were just good enough at this scale, but they had to be mounted as close to the center of the vehicle as possible, which made the space around the rear drivetrain very tight; no space for differential. No problem, it would be closed anyway on an off-road vehicle.

Here is the linkage geometry I managed to implement. The planetary hubs are mounted onto the trailing arms, which was challenging to make solid, but the end result is quite good, thanks to using Defender rims, which can hide the hub assembly. The radius rods (9L links) cannot be pushed further back, that's why there's no room for differential. The 12T gears hold up surprisingly well, but knob gears could also be used, although they might not be efficient for high speeds. I didn't try.

An advantage of this setup is that there's space in between the trailing arms for motors.

 

At the front, I went for 9L suspension arms with 9L steering links, which just resulted in a feasible suspension geometry at the same width as the rear axle. However, there are a couple complications. First, I wanted to use long springs for realistic looks, which need to be mounted on the lower arm, and then somehow avoid both the upper arm and the driveshaft. This is not such an easy puzzle given the limited number of lego parts available for such a purpose. For the second try, I managed to come up with a geometry in which the shock absorber is just one stud off center, but the driveshaft passes safely next to it, and the upper arm is split in two going around the shock absorber, just like in real rigs.

The other difficulty is the positive caster angle. To achieve this, I tilted the whole front of the chassis, which means that the drivetrain had to contain a CV joint. That's not an issue, but due to the single-sided nature of the 9L steering links, the steering rod could only go relatively low, close to the driveshaft, due to which it was not possible to integrate the steering motor into the tilted assembly because it would have collided with the driveshaft. So I had to add a CV joint to the steering axle too, and move the steering motor further behind.

The final chassis came together like this; quite minimal but solid. The Buwizz unit is easy to access for charging or for taking it out of the model.

The driveshaft itself is very simple to remain efficient. Joined Buwizz motors at the fast output, 16:16T gearing underneath and straight to the (locked) differentials (1:1 ratio), then to the planetary hubs. There would even be space for a 2-speed gearbox in the middle, but I did not want to complicate it for this one.

 

Interior and exterior

The interior has lots of space left, so I added two large seats and a dashboard / steering wheel.

The shaping of the exterior was loosely inspired by the Ultra4 KOH buggy of Lazernut Racing:

 

Besides trying to resemble the paneling using some simple flat and curved panels available in blue, I also aimed to replicate some of the tubing, both under the panels and the visible ones around the windows. The model can actually be lifter by the roof thanks to the two pillars in the middle of the windshield.

Here are some renders and photos of the complete model from all angles.

 

All in all, the model is quite fun to play with, it's fast enough but has good torque as well to climb some rocks, and the suspension is really responsive. It sits in a bit under its own weight, but is bouncy and floats nicely on uneven surface.

 

More pictures are available on Bricksafe.

Building instructions are available on Rebrickable.

Let me know what you guys think of it!

Cheers,

Viktor

[Toastie]

3 minutes ago, gyenesvi said:

Let me know what you guys think of it!

I am not into cars, nor any of this business you guys are in - but I am totally impressed here.

I rarely dive into "another" BUWIZZ or off-road, or trial truck or whatever post - not because of disinterest, but mostly because of not comprehending what you guys accomplish. I take a peek, take a deep breath and then think "another league", what I want to reply will sound stupid.

Same thing with regard to your post - but the elegance, the "light" feeling of the build, the power, the functions just blew me away. This is such a nice model you created, I am in awe.

Congratulations - and all the very best
Thorsten

[Aurorasaurus]

I love this model, I think it is a great balance between price and performance, using accessible parts. How is the driveability with the CV joint in the steering train? Does the caster angle feel like its compensating for the U joint? Would you keep the caster angle and slight slop in the steering for the realism if you did it again?

[Michael217]

The model is beautiful, but you should redesign the rear suspension, because in reality it doesn't work that way) Irregular geometry.

I see that the lever and the shock absorber are offset by half, but that doesn't change the situation much)

 

Edited February 18 by Michael217

[Zerobricks]

Only thing I would recommend is changing 12 tooth double bevel gears for 20 tooth thin bevel ones to increase efficiency. Other than that, amazing job. 

[Aurorasaurus]

1 hour ago, Zerobricks said:

Only thing I would recommend is changing 12 tooth double bevel gears for 20 tooth thin bevel ones to increase efficiency. Other than that, amazing job. 

I thought those were prone to cracking and breaking? I guess 12t double bevel gear teeth arent particularly strong either though.

[2GodBDGlory]

Looks impressive as always! Good, clean looks, and an interesting suspension setup!

[N1K0L4]

21 hours ago, Aurorasaurus said:

I thought those were prone to cracking and breaking? I guess 12t double bevel gear teeth arent particularly strong either though.

That's why I use this:

Both half 12 and 20 bevel gears. This also reduces slack a bit. 

[gyenesvi]

On 2/17/2025 at 7:03 PM, Toastie said:

Same thing with regard to your post - but the elegance, the "light" feeling of the build, the power, the functions just blew me away. This is such a nice model you created, I am in awe.

Thanks for appreciating it!

On 2/18/2025 at 4:03 AM, Aurorasaurus said:

I love this model, I think it is a great balance between price and performance, using accessible parts. How is the driveability with the CV joint in the steering train? Does the caster angle feel like its compensating for the U joint? Would you keep the caster angle and slight slop in the steering for the realism if you did it again?

Thanks! The steering with one CV joint is okayish, but with two (the version in the Buwizz camp) it was getting sloppy. The caster is a good question. I have the feeling that it is not doing so much compensation, so I was thinking of trying a variant without caster and direct steering for next year's model. But I like the realism aspect :)

On 2/18/2025 at 8:29 AM, Michael217 said:

The model is beautiful, but you should redesign the rear suspension, because in reality it doesn't work that way) Irregular geometry.

I see that the lever and the shock absorber are offset by half, but that doesn't change the situation much)

Thanks! Yes, I know about the irregular geometry of the rear suspension, this is exactly what I tried to explain above. Do you know how this is resolved in reality and how that could be implemented in lego? Because this is something that I don't quite understand in reality either. My guess is that it is actually irregular, but it is so slight that it is not too noticeable. So if you know how it works, let me know!

BTW, the purpose of the half stud offsetting is that in the neutral position it is close to parallel, so there's not torsion in the parts.

On 2/18/2025 at 11:56 AM, Zerobricks said:

Only thing I would recommend is changing 12 tooth double bevel gears for 20 tooth thin bevel ones to increase efficiency. Other than that, amazing job. 

Thanks, glad you like it. Aren't those 20 tooth ones more prone to break?

4 hours ago, N1K0L4 said:

Both half 12 and 20 bevel gears. This also reduces slack a bit. 

That is actually an interesting idea worth trying! Thanks!

On 2/18/2025 at 3:24 PM, 2GodBDGlory said:

Looks impressive as always! Good, clean looks, and an interesting suspension setup!

Thank you, glad you like it!

[Michael217]

3 hours ago, gyenesvi said:

Thanks! Yes, I know about the irregular geometry of the rear suspension, this is exactly what I tried to explain above. Do you know how this is resolved in reality and how that could be implemented in lego? Because this is something that I don't quite understand in reality either. My guess is that it is actually irregular, but it is so slight that it is not too noticeable. So if you know how it works, let me know!

This buggy has the same suspension as yours. But the levers are positioned at an angle, which probably makes up for it. You need to try to assemble in lego and see what happens)

 

[gyenesvi]

12 hours ago, Michael217 said:

This buggy has the same suspension as yours. But the levers are positioned at an angle, which probably makes up for it. You need to try to assemble in lego and see what happens)

Yes, indeed, I have seen that on real buggies the trailing arm is positioned at a bit of an angle, and was thinking whether that is making up for the geometry. But I am not convinced, I don't see how that would fix things. I mean it does not change the fact that the radius arms will push out the trailing arm and the wheel will have some toe-in. Or do you see if it changes anything?

Besides it would be quite hard to implement such a slight angle in lego in a robust way, for one it would need quite a bit more space right where the Buwizz motors are in the buggy, for two, the same angle would have to be compensated at the wheel end of the trailing arm, and that would be really hard I guess (mounting the wheel hub was hard even the straight version),  so I decided not to bother with that in this model.

[deehtha]

16 hours ago, gyenesvi said:

Yes, indeed, I have seen that on real buggies the trailing arm is positioned at a bit of an angle, and was thinking whether that is making up for the geometry. But I am not convinced, I don't see how that would fix things. I mean it does not change the fact that the radius arms will push out the trailing arm and the wheel will have some toe-in. Or do you see if it changes anything?

Besides it would be quite hard to implement such a slight angle in lego in a robust way, for one it would need quite a bit more space right where the Buwizz motors are in the buggy, for two, the same angle would have to be compensated at the wheel end of the trailing arm, and that would be really hard I guess (mounting the wheel hub was hard even the straight version),  so I decided not to bother with that in this model.

The real buggy has bushings at the frame end of the trailing arms. These allow for some sideways motion of the trailing arms.

[gyenesvi]

9 hours ago, deehtha said:

The real buggy has bushings at the frame end of the trailing arms. These allow for some sideways motion of the trailing arms.

Yes, but you mean sideways tilt, right? That doesn't change things, it still turns the wheel as well (toe in). And in lego form, the parts have enough slack to simulate that (liftarm on an axle).

[Michael217]

On 2/20/2025 at 1:58 PM, gyenesvi said:

Or do you see if it changes anything?

I spent the whole night creating a working version. But no matter at what angle and at what mounting point the lever is located, the wheel still changes its position a little, but I think it's better than fastening with a stitch)

This is the very first version, I have not photographed other versions =) Pay attention to the attachment points of the rods and lever, their height, this helped me minimize the change in wheel angle when the suspension is working.

 

[gyenesvi]

2 hours ago, Michael217 said:

I spent the whole night creating a working version. But no matter at what angle and at what mounting point the lever is located, the wheel still changes its position a little, but I think it's better than fastening with a stitch)

Interesting experimentation, thanks for doing that! Unfortunately I cannot experiment now with this as I am abroad and away from my lego for two months to come..

2 hours ago, Michael217 said:

This is the very first version, I have not photographed other versions =) Pay attention to the attachment points of the rods and lever, their height, this helped me minimize the change in wheel angle when the suspension is working.

Great that there may be an optimal angle that minimizes the wheel movement. It's nice for an initial concept version, but for a working heavy duty version I see several problems that need to be addressed:

• first, it has to be built with planetary hubs, because otherwise the drivetrain with 2 Buwizz motors is not possible, the speed is too much and torque is too little. That means for example that you cannot use the pinholes on top and below the wheel hub, because those have permanent balls on the planetary hub.
• the trailing arm cannot arrive to the point where it arrives now because the rim would collide with it (the buggy is using a deep Defender rim to be able to move the wheel further inwards). Of course a non-deep rim could also be used, but that would make the buggy look a bit too wide, and front suspension would also have to be adjusted accordingly.
• the current construction will be weak, fall apart in seconds. For example the radius rods would easily disconnect and also the structures that create the angle are weakly connected and I believe it would be hard to strengthen up without compromising ground clearance for example (my current version is form locked, it literally cannot fall apart). Just to illustrate the amount of forces it needs to take, when I built the initial 4-motor version and started making really hard turns, it broke one of the trailing arms (at that time it was not completely form locked though)..
• there will be an additional problem of mounting the shock absorbers, as they will also be at an angle. I guess that may be compensated for at the top, with maybe a ball joint, at the cost of complicating the chassis structure there as well.

By the way, the fact that my current version is on a half stud offset is important. Even though the wheel angle changes a bit, it ensures that it is never too much off from the straight line. It starts out with a bit of toe out and it transitions into a bit of toe in through the straight position. I found that as a good compromise.

Edited February 21 by gyenesvi

[Michael217]

45 minutes ago, gyenesvi said:

45 minutes ago, gyenesvi said:

Interesting experimentation, thanks for doing that! Unfortunately I cannot experiment now with this as I am abroad and away from my lego for two months to come..

Great that there may be an optimal angle that minimizes the wheel movement. It's nice for an initial concept version, but for a working heavy duty version I see several problems that need to be addressed

Yes, first of all, need to assemble a working prototype, and then assemble the full version, taking into account all your needs)

Have a nice trip!

Edited February 21 by Michael217