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Dear readers, For a new project that started a while back (about a year ago), I was investigating a certain track configuration for a tracked vehicle. It pointed out that I needed a lot of ground clearance from the track wheel, but the track wheel could not be two studs wide: the tracked vehicle is only 15 studs wide and a lots of gears, motors and linkages need to be in between. As you can see, the standard wheel above is two studs wide due to a ring of 1/2 a stud that supports the track. This ring is not always necessary. In fact, the tracks barely even touch this ring under normal driving. A lot of you will recognize the difficulty with this wheel: it protrudes half a stud to both sides. In many designs, the number of horizontal studs is uneven: an M motor, XL motor, frames, differential, everything has an uneven width when measured in studs. This enables a symmetrical design as Lego Technic has no beams with even lengths (only 3, 5, 7, 9 ..). That's why many Technic designs of this day are an uneven number of studs wide. This is in contrary to the olden days, where all Technic components and widths were even. Check for example the old style liftarms, differential, frames, motors and even battery boxes. With this theory lesson over, it is easy to see why this track sprocket introduces a problem: when the wheel is mounted in the uneven Technic system (to get a nice symmetrical design with uneven Technic beams), the wheel size is even! So the wheel can not be connected to a liftarm at the place where the axle connects to the wheel. There needs to be a distance of 1 stud to any supporting liftarm. This is mostly done with a bush or a 3L thick liftarm. This also means that to mount this sprocket very strongly, the minimum width of the structure (when fit in an uneven system) is 5 studs. Then I started thinking: how nice would it be if there would be NO distance to any supporting liftarm? In that case, the sprocket has to be only 1 stud thick and the supporting structure is only 3 studs wide. Also, the support ring has to go. This is an enormous space saver when the maximum width of your vehicle is 15 studs.. I decided to draw the part in Solidworks and print it with my own 3D printer. Out of curiosity, I checked GrabCad to check whether someone got the same idea too. To my amazement, somebody had already tried this part, with the internals to be the 40 teeth gear. This is done with a reason. A standard 3D printer is generally very bad at printing axle holes. If you draw the outline of a standard Lego Technic part (that has a nice axle hole) into your 3D printed part, the part will be connected firmly inside the model(nice) while the axle is inserted into a Technic part, so no troubles with cracking plastic will occur. In short, I printed that part. It didn't work. (See image below). It turned out that every 3D printer is a little different. A tiny offset can cause a diameter difference, which then will cause friction, because the track doesn't fit around. The wheel needs to be perfect to make the solution work. That's why I decided to design my own custom sprocket wheel from scratch. This was my first 'from scratch' attempt. As you can see, the pin and axle holes do not have great quality. A revised 3D design is in the image below. As you can see, there is a large hole in the middle. This hole fits two 4185 pulley wheels (shown right) very tight. This means the vehicle is connected to a nice and round axle hole, while the 3D printed part is also fit very well to the vehicle. The sprocket diameter is not arbitrary as the number of 'lobes' is fixed: there are no half lobes, otherwise the wheel won't work.. That means that for the same tracks, a limited number of wheel diameters exist. As you can see, there is a slight ring in the 1 stud wide sprocket wheel, not all track support is gone. The 3D printed results: I was really happy with the quality of this wheel, the drawbacks from 3D printing don't show up and the wheel is very strong and straight. In the image below, is is visible that the tracks fit very smoothly around without additional friction. It is also visible that the wheel is just as wide as two pulley wheels, so one stud. I was so amazed when the 42095 set was launched. There was my not-published-about part, exactly the same size, in a real Technic model! And now there will be 4 DBG ones in the 42100 Liebherr, which will drop the price. There is one 'slight' drawback though. My 3D printed wheel is only one stud wide, needing only a frame of 3 studs wide to support it. The new 42100 sprocket wheel has the 1/2 stud ring protruding out of it on both sides, taking the drawback of the old wheel onto the new wheel. That's a pity. It could have been so nice. Then comes the question: why would you 3D print parts? That's cheating. And I agree. It is cheating. I never do this in my models. You are allowed to check my YouTube. I even built of a 3D printed way of getting three axles through a Lego Technic turntable but never used the solution as I do like pure models. Then, why am I doing it now? Because currently, I'm using Lego Technic as a strong platform for a scale model that requires 100% of the quality of Lego Technic, but cannot do without adding custom parts. Even the most skilled builder cannot work his way round sprocket wheels that don't exist. That's why I see it as an engineering project, and from that prospect, creating your own parts (when all else fails or doesn't exist!!) does not show lack of skill but broadness of view.. Surprisingly, some other minds thought alike and brought a Lego Technic wheel of the same size on the market. Too bad.. Let's start building and stay thinking!
