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Solaris Urbino 18

December 29th, 2019 Leave a comment Go to comments

Model of a Polish articulated bus. Features drive, steering, remotely opened doors, remotely controlled kneeling mechanism, automated lights, dot matrix screens and custom stickers.


Completion date: 28/12/2019
Power: electric (Power Functions)
Remote control: SBrick
Dimensions: length 111 studs / width 20 studs / height 19 studs
Weight: 3.624 kg
Suspension: none
Propulsion: 2 x PF L motor with 2.33:1 gear reduction
Motors: 3 x PF L motor, 1 x PF M motor, 1 x PF Servo

Solaris Urbino 18 is a very popular articulated bus in Polish towns. I’ve seen it and rode it so many times that it eventually started to appeal to me and I’ve decided to build a model. In theory it’s a simple box on wheels, but mechanically it’s much more complicated than it seems.

Having no prior experience with bus models, I wanted to start small and simple. I’ve chosen 1:20 scale with a 16 studs wide body because of the recently released 30x14mm narrow LEGO wheels, which made it possible to build a 16 studs wide axle with a kneeling mechanism and drive/steering. It was still too narrow for double wheels, though.

Just like the real Urbino 18, my model had driven axle #3 and steered axle #1, and a kneeling mechanism in all axles. I’ve used small linear actuators for kneeling rather than pneumatics, because pneumatics would require more space and be less stable and not as synchronized. The model was fairly rear-heavy, so with pneumatics there would be a risk of front axle standing up sooner than the rear axles. With actuators, all axles were synchronized. The actuators were placed horizontally to save space, and I was also hoping that this would help them handle the model’s weight. All three kneeling mechanisms were controlled using a single PF L motor driving a long driveshaft in the floor that went through the bottom of the central joint.

Atop the central joint there was another similar driveshaft powering the door opening mechanisms. I’ve tried to make the doors open exactly like in the real bus, but it required placing some parts at quarter-stud offset and this was difficult at this scale. Instead, I have achieved the same movement by pulling one side of the door inwards and mounting the other side on an offset hinge. My goal was to use as little space inside for the doors as possible, so I didn’t want any guides on the floor and essentially fit the entire mechanism under the roof. All four doors were opened by a single PF M motor, and each door wing was opened using its own clutch gear. This way if doors opened at various rates or one door got stuck on something (like a passenger 😉 ), the other doors were unaffected. Given that all four doors use many gears and are connected with long axles, I’ve decided that it’s better to accept some backlash between them and be ready for it rather than to try and synchronize all doors perfectly and encounter problems if one door is out of sync. The front door only opens in half because of the room required for the front dot matrix display and for the front right traffic signal. Had the bus been just 1 stud longer in front of the front doors, I would be able to make them open entirely. In the real buses half of the front door is often kept shut for driver’s visibility (to keep passengers from obstructing it), so this is actually fairly accurate.

The model was driven by two PF L motors behind the third axle, driving it with 2.33:1 gear ratio, while a single PF Servo motor in front of the front axle controlled steering and moved the steering wheel. Yes, it was my goal to use as few motors as possible. This also kept the model reasonably simple, since there was little room available and many, many wires that needed to be put somewhere. The entire exterior lighting was done using LEGO LEDs and the interior was lit using Brickstuff products, and all in all there was approximately 26 meters of wires inside.

The lighting included turn signals, reversing lights and brake lights, all automated using SBrick sequences. This was why the model used two SBricks despite having just 4 motorized functions – the second SBrick was used solely to control the lights.

The main challenges were building the interior and the central joint. I wanted the bus to have as realistic interior as possible since the beginning, hence my drive to keep the number of motors and the size of mechanisms down. To make things harder, the model didn’t have any actual body frame – its bottom was just a single layer of plates with a single layer of tiles on top. Instead, everything was kept together and kept rigid by the side walls, which is why I’ve fit them all with glass for extra stiffness. The only part resembling a body frame was a framework of beams connecting the central joint with the top and bottom of walls on its either side. The finished interior is completely covered and included 46 individual seats, which roughly corresponds to the number of seats in the real bus (depends on a version).

The central joint was the chief challenge. I was ambitious and I wanted the joint to work like it does on the real bus, with an opening through it, with decent range of movement and without a canvas or paper cover which just felt cheap to me. Instead, I’ve built bellows using similar solution I’ve used for heavy duty tracks: overlapping 2L liftarms on axles. Unfortunately, ugly openings were required to ensure a decent range of movement. The biggest problem was the central joint section – for the bellows to work properly, you need a central section that always bends only half the way the entire joint bends. So if the joint bends 30°, the central section bends 15° and it keeps the bellows moving properly because they’re attached to it. Real buses mostly use hydraulics for that, and I didn’t want to use pneumatics nor rubber bands or shock absorbers because it felt like cheating. I’ve experimented with a system of gear wheels between the front and rear half of the bus, but I could’t get it to work. Eventually, I found a simple geometric solution by bending an axle with two u-joints on it: the short axle between the u-joints will always bend just half the way the rest of the axle does. It’s hard to explain, but it’s show clearly in the video below.

Finally, the model would not be complete without parts from Brickstuff. These included LED strips used to light up the interior (LEGO LEDs work too much like spotlights to work well here) and three dot matrix displays used to show the bus number, direction and route. The displays were custom-built and programmed by Brickstuff using dot matrix displays I’ve found. The rear number board, which I didn’t want to use a dot matrix display for, was built using trans-yellow 1×4 bricks with a semi-transparent sticker, illuminated from the inside using two LEGO 9V light bulbs. All the lighting and all electronics were powered by a single LEGO 8878 rechargeable battery, which really shortened its battery life and which sometimes caused the Brickstuff electronics to blink when the battery was less than full and when the PF L motors were stressed. This could have been avoided by adding another battery just to power the Brickstuff elements, but I really didn’t see any extra room for it.

In the end the model was a nice first try at building buses. It provided me with plenty of experience to try and build a bigger, 20 studs wide bus one day.

Work in progress photos:

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