Self-steering car chassis: how, when and why?

As many have probably already noticed, the NXT set is great for a whole lot of things, but isn't the simplest start for a large remotely controlled car. Mobile phone control applications allow easy motor speed control, but not the absolute movements (at least not without some extra programming, development, etc. what perhaps isn't everyone's cup of tea). The traditional approach to chassis design, i.e. the one providing independent drive and steering axles, is thus difficult to implement: while there's no problem providing the driving power, accurate movements and precise recentering required for useable steering are very tricky. Hence, it is logical that the standard NXT vehicle models rely on the differential drive, or in other words, steering the vehicle by driving the left and right wheels at different speeds.

While such differential drive works nicely if the model is not too massive or is running on tracks, it becomes an issue with large cars. Regardless of whether the car is AWD or RWD, wheels will tend to skid in the curve, making the steering very unreliable and unstable. (The tracks actually skid as well, but they have more uniform contact with the surface, making them easier to handle.)

One of the simple solutions of this problem is a self-steering chassis. It's perhaps not as refined and sophisticated as some other solutions, but it takes little space, is simple to build and implement, and does not put parts under any serious strain.

This type of chassis is only RWD ― or more precisely, each rear wheel is powered by its own NXT motor. The front wheels, however, can rotate freely and are mounted on the standard steering mechanism, just like usual Lego Technic models. The difference is that the wheel axes are mounted at least a stud or two behind the vertical axes of their control arms/hubs (in automotive terminology, we'd say they have a certain amount of caster). Whether are the control arms connected by a standard equidistant beam or an Ackermann system, is up to you. Theoretically it could work even without a connecting rod, though it would be shaky and unstable, reviving all the problems we are trying to avoid in the first place.

The principle of a self-steering chassis is that the small amount of turning (tangential) force caused by the different speeds between rear wheels should suffice to push the front wheels sideways in the desired direction and turn them, especially while the model is driving when the turning friction is small. Of course, according to the same idea, when the model is driving forward (i.e. the rear wheel motors are driving at identical speeds), the force will straighten front wheels, not much unlike the wheels of a shopping cart.

Additional advantage of a self-steering chassis is its driving power: as opposed to the classic construction where steering and drive are handled by one motor each, most of the time here the model will be powered by both motors, significantly increasing the maximum uphill model can climb against.

However, it's not flawless either; but let's summarize these pros and cons...

▪ GENERAL PROS & CONS
+ Very simple construction
+ Uses no exotic parts
+ Minimum part strain, can withstand serious load
+ Works on any scale, with any wheel and chassis size
+ Does not require a motor itself (allows both rear motors to dedicate entire power to driving the car forward)
+ Very stable drive forward (recenters quickly)

- Requires plenty of grip on rear wheels, and at least a significant amount on front
- Can be unreliable on long, narrow chassis
- Usually impossible to manuever the vehicle in tight, cramped areas (it's difficult to turn the front wheels while stationary!)
- Due to caster, wheels move a lot while steering, requiring an adequate chassis and generous arches
- Difficult to drive straight backwards
- Without adequate special parts, difficult to build with full suspension

Decoration in Lego Technic: How much, and how much is too much?

The role of looks in Lego Technic has always been sort of a gray area. While the requirement of good looks is much more clearly pronounced in some other Lego themes (e.g. especially Creator), finding and implementing an adequate dosage of decoration in Technic is rarely easy.

I like to think of Technic approach to aesthetics similar to the one somebody once described about high-end sportscars: their good looks are always a side-effect of technical solutions. It's form following function, and their aesthetical roots are in the engineers' laboratories and wind tunnels, rather than clean sheets of designers' paper.

Most original Lego Technic sets follow this principle. Except for a few partial exceptions (such as the 8051 bike reviewed here on Legoism), those sets contain very little dedicated decorative parts, if at all. But despite that, their bodyworks, fuselages and similar structures usually look great. Where is the fine balance, or in other words, what is the guideline for making useful technical things look good as a side-effect?

As far as I can judge, there are few points worth considering during planning and building. They are somewhat vague as they vary among models and purposes, but in general, most good-looking models obey them.

▪ Use reinforcements as decorative elements or body contours.
Check out the 8865 Test Car. While its side beams between the front and rear wheels clearly serve as reinforcements and support for the seat plates, they also nicely define the bottom side contours of the bodywork. The same counts for almost all outer beams on the car: while each increases the structural strength of the bodywork and the underlying chassis, they are all cleverly positioned to define the outer edges. Many models follow this rule strongly, but on 8865 it is particularly evident.

▪ Go minimal with plates; try to keep as much inner mechanics and controls easily visible and accessible as possible.
One of the advantages of aforementioned reinforcement design is its transparency. Seeing "the things happening" inside is a part of what makes Technic what it is. Although attaching the plates to the beamy skeleton underneath may provide more pronounced or strictly realistic bodywork, that is rarely desired in Technic. Though, even some early original Technic models do not follow this rule ― but neither are they among the Technic's nicest, if you ask me.

 ▪ Small details are welcome, particularly if they emphasise the various components' functions.
If not hindering mechanical functions or hiding controls, occasional small details can nicely enhance the model. For example, mounting a small rotating fan on the car engine will add to looks and realism, even if it doesn't really move any air. Just as little wing mirrors could be a nice touch if it's obvious that the car is otherwise missing them. But in all these cases, decorations should strive to make the model look more realistic, rather than just put an unnecessary gizmo somewhere for the sake of making things more complex. Example? Look at the recently reviewed 8053 Mobile Crane ― it has several lights on the front of the cabin. The model would be just as functional without them, but the front mask could look somewhat bland. Well-positioned and subtly fitting into the overall design, they are a perfect little touch. But the concentration of such details is here wisely judged, too: adding the front air intakes and wipers would just make the model look to fiddly.

▪ Try to follow the structure of the models' real-world counterparts as closely as possible.
There will always be variants and controversies, but in general, most machines in the real world have their proportions and structure following some well-established guidelines. Let's again use cars as typical examples; there is a certain ratio between the chassis length, width, engine size, wheel size, cabin volume and many other parameters that almost all cars follow within certain extents. If you're building one, plan staying within those limits ― otherwise the disproportional structure might look inelegant, even if there is actually a good sane reason for doing so. You might often find yourself having to choose between mechanical functionality and realistic look. Check out the majestic 8880 ― every major component's size roughly matches the general car dimensions, and it's approximately where one would expect it in a real supercar.

▪ Don't make it look like a carnival in Rio.
This one comes the last, as it's the most obvious and ― due to the possible lack of choice ― often difficult to follow consistently. Namely, technical functions (which should be a clear №.1 in Lego Technic) will be easiest to distinguish and understand if the viewer is not distracted by twenty different colours around. Basic grey shades in which most Technic parts are made work nicely, and often adding one extra color can make it more intriguing. But if you can choose, resist making a colour blindness tester, like I did:

Not too big, not too small, but just right: Goldilocks' principle of Lego scale

Arguably, almost every single thing we use in life has a range of optimal dimensions. For example, a laptop must not be too large as it degrades portability, neither small enough to affect usability; too large car will be difficult to drive and park, and too small offers insufficient space for practical things; too long fork will exhaust the hand and be impractical to handle, while a short one may be useless for some specific meals... the list goes on and on.

The same approach of optimal range of sizes can be applied to Lego models. It has been already briefly mentioned on the 'Technic Tips' page, but it deserves some additional attention as it is often overlooked by the newcomers. So, the principle says you should have at least a general idea about how large will the model be, even before you have assembled the first two parts. Ideally, this idea will be clear and accurate enough to guide you in choosing the correct part sizes already from the beginning of construction, but often it is not necessary to go that far, as the precise scales will crystallize on-the-fly.

However, determining the scale of a desired model is not easy ― it depends on many factors. Generally, as a rule of thumb, the model should be the smallest possible as long as it allows all technical, decorative and other elements you want to implement.

In certain themes and models, the scale will be predetermined according to some "fixed" factors: e.g. a Lego City garage will have its scale preset by the figures and standard-size cars. However, in many other situations you will have a choice. This choice can be quite wide and vary according to the complexity ― great examples are the original Lego Technic cars that range from small functional go-karts to XXL supercars packed with features. However, you will notice that all these cars (as well as most other models) are tightly packed with features, and usually have no unused space at all. That is the guideline worth sticking to.

Of course, for any desired design and a list of desired features, there is a range of possible sizes that will work nicely; for example, a Technic car with a 2-cylinder engine, steering and trailing suspension could be super-densely packed into cca 24x12 studs chassis, but it may look fine in 32x16 format as well, especially if there will be some decorations. However, going beyond these extents will usually lead to dead-ends sooner or later, and get the builder back to the drawing board. Here are some headaches that might arise:

If the model is too small
▪ difficult to build sturdily
▪ likely collisions and limitations between moving parts
▪ inelegant mechanics
▪ strong limitations to the spectrum of usable parts
▪ complicated decorations, if at all possible
▪ difficult to modify

If the model is too large (much more common!)
▪ excessive weight needs extra reinforcements, in turn again increasing weight
▪ final model might look "poor" with features
▪ increased forces put parts under unnecessary strain, often risking breaks
▪ number of required parts increases dramatically, long construction and deconstruction
▪ often boring to build (many routine components)

Learning to judge the optimal size is a complex and subtle process that takes some experience, but paying particular attention to that parameter on some great models will certainly help just as much.

That brings us to the second point ― why are some systems just impossible to build without specialized parts (whether are they available or not). Namely, there is a certain minimum size that a mechanical system with a specific purpose can have. For example, it is perfectly possible to build a double-wishbone car suspension with steering and axle using just regular parts, but even in the smallest possible design, it could not occupy less than, let's say,  8x10x10 studs hypothetical block. With such dimensions, a whole car would be as large as a typical cocktail table. The forces in such a huge construction would be very difficult, if not impossible to handle by our standard beams.

Realizing the problem, Lego has arrived with a solution: dedicated parts that provide the suspension functionality in a smaller package, allowing the whole car to be within the dimensional constraints acceptable for plastic beams, axles, etc. The same can be said for specialized cylinders and pistons, differentials, gearboxes, and similar parts: they all could be made with the usual bricks, but just too massive and impractical to fit into reasonably (or bearably) large vehicles. Admittedly, some clever Legoists have built rather small and advanced components such as automatic gearboxes, but many remain out of reach, except as isolated showroom models.

But on the other hand, the available constraints are still quite generous, so don't despair. After all, sets 8880, 10188 and 8421 are out there alive and well, so if you can cram your mobile idea in some three or four thousand parts, you'll be just fine. And of course, if it's static, the only limit is the... ceiling.

Lego Creator 5893 'Offroad Power' Review: Blue Sporty Nirvana

Despite the recent trend among the posts, this site is not strictly dedicated to Lego Technic. So finally, here we go with something slightly different ― a review of a Creator set. It's the 5893 'Offroad Power', a relatively large set consisting (in the primary model) of a pickup truck, a quad and its carrier trailer.

Creator is an interesting theme as its objective is to combine a solid mechanical functionality with pretty and realistic look. Exactly this sort of combination is where this set excels at: like all great machinery in the real world, it's both great to use and to look at (and in this case, to build). Its size is respectable too, boasting over a thousand parts in a span of almost half a meter. To be precise, there's 1061 parts in the box, but not everything is used.

The pickup truck is the largest and the most complicated vehicle on board, but then, it has the most to offer too. It combines independent suspension with springs on the rear wheels with the freely swinging steering axle at the front, allowing the chassis to move (absorb) both vertically and sideways. The truck is easily steered with a gear on the roof and is very stable to drive. There's more: it has a functional hook-winch (a special brick for the purpose) placed in the center of the chassis, operable by easily pushing the front radiator grille via an extremely clever and subtle sliding frame. It features an automatic locking mechanism under the openable bonnet, where one can find a nicely modeled engine, too. A fantastic demonstration of efficient space and part usage! Both doors open on a hinge, the tailgate can too, and even the wing mirrors' angle is adjustable.

But, listing the technical functions, I'm doing unjustice to the overall design which deserves just as high marks. Namely, this must be one of the nicest vehicles in the arsenal: its proportions are nicely judged to match the real real trucks and there are tons of attention put into every visible surface ― slightly angled bonnet edges, headlight orientation, doors with inner compartments, a flashlight in the cabin... One can find such precise little details everywhere around the truck.

The quad follows the strong guidelines set by the truck, altohugh ― by the limitation of its size ― it cannot possibly offer much more functions than it does already: steering combined with independently freely swinging front and back main axles. However, exactly that independence makes it seriously agile if one really tries to ride it on some bumpy surfaces. It's nicely designed too, with cute details like exhausts, footrests, etc., and also fits the truck in overall scale.

Finally, it can be parked on its trailer with a retractable ramp, and let stand by itself on an extendable front mini-wheel, or attach it to the truck through a ball-joint. The trailer is very simple and is not packed with functions, but then, neither are the real-life quad-carrying trailers, are they? Thanks to the inclined "seats" for the front quad wheels to sit into, it stays nicely in place during transportation ― if it's not too steep.

It's nicely done, but let me once again stress the general point: there's rarely a set that combines as much clever and rich functionality with strong construction and high level of designer detail, as does this one.

▪ BUILDING, IDEAS AND CONCEPTS
The set (as well as the alternate models) is not too difficult to build ― most of the time it's simply from the bottom upwards, relying on the studs, though it takes time to fiddle with all the details and tiny parts if you want to make it perfect. But it's really much fun, and there's a lot to learn from it too: a subtle sliding winch-operating frame, advantages of a combined spring-loaded and free-swinging suspension, ultra-efficient usage of space and bricks, especially the hinges that the model uses abundantly. Great role model for the realistic medium scale car builders!

▪ PARTS SUPPLY
5893 is a goldmine of bricks! There are some valuable special parts, such as the automatic push-operated winch, but much more, this set is a great source of standard building material: bricks, beams, inclines, joints, plates, curves, etc., and plenty of other generally useful parts. There is a small amount of Technic too ― including the rare white studded beams. It's mostly in monochromatic and blue colours, though there are some other colours in lesser amounts as well. And it is a massive source of wheels: four large and four medium-size offroad versions, and two sportscar wheels. (And a tiny one for the trailer, if we'll be so precise.)

▪ EXPANSIONS
As the truck's chassis is by far sturdy enough to withstand extra force, I reckon it could be nicely upgraded using the available space in the back. A radio listening station, perhaps? Or a mobile water desalinizer? Of course, as the truck has a ball-joint for a trailer, it's ready to accept all kinds of specialized trailers. However, as the interior is very tightly packed with parts, I suspect there is not much room for changes, at least not simple ones.

▪ OPERATION & PLAY VALUE
As said before, controls are easily accessible, and its really great to play too, with its many moving parts and construction that will drive smoothly over all kinds of surfaces. And even when not in use, its refined looks make it valuable as a display model as well.

▪ GENERAL PROS & CONS

+ Well-judged proportions and very detailed design

+ Plenty of moving parts and functionality

+ Not too difficult to build, nice to play

+ Good role model for efficient and nice construction

+ A massive goldmine of building material

- Somewhat pricey (but then, you do get what you've paid for)

- Takes much patience to build small details uniformly and perfectly

▪▪ VERDICT ▪▪

5893 is an excellent set ― great to build, play and learn from, and is a heavy source of parts. While some very dedicated Technicians could probably stuff more functionality within the constraints, it would never have such looks and intricate details. So, a nice compromise between "pretty" and "technical" sets, taking many important points from both and combining them into a tightly packed, cleverly designed model. Two thumbs up!

Red Highway Chile: Lego Technic 8051 Review

Due to the laws of scale, it was never possible to build really, really large Technic motorcycles that would classify them among the season flagships, or at least the upper-echelon models. However, their specific and somewhat unique design deserves constructor's respect, and should be a good source of ideas and a role model of efficient, compact design. That is, at least, what I was expecting while unpacking the Lego 8051 Technic motorcycle.

Instead of featuring a main model and a significantly less efficient "B", here we find two models that are almost equivalent ― though there is a tiny bit more focus on the racing bike that we will be dealing with here, than on the Easy Rider-style cruiser.

The first easily noticed parameter regards its looks: as opposed to many other strictly functionality-driven Technic sets, this motorcycle puts a lot of effort in its form and decoration: body panels, exhaust pipes and branches, radiator grille, plenty of stickers, dashboard... but yes, despite none of these having mechanical functions, they all make it look great and without them the bare bike would look just like some poor skeleton. (Though, in that case it would be a little bit easier to see the engine in operation).

However, it is nicely packed with features for a model of that size and a very limited dimensional constraints: fore and rear suspension, three-cylinder inline engine with transmission and a chain drive to the rear wheel, openable fuel tank, steerable nose and a retractable kickstand.

Such a density of features, combined with high strength of the supporting frame and the need to look realistic means the structure is quite complex, which reflects to the building. Despite the building instructions being quite clear themselves, the model is not the easiest to build, with many parts being loose or delicately balanced throughout the process. It takes about an hour and a half to build for an experienced Legoist.

▪ IDEAS AND CONCEPTS
One can find a nice demonstration of the flexible axle connector usage, along with the clever positioning of the engine and many other components at an angle, which is generally quite difficult with Lego. The clever bodywork structure itself is a great idea, and if you are into building motorbikes, you will definitely get some great hints from the 8051. Among moving parts, there is no particularly new system involved.

▪ PARTS SUPPLY
For a medium-sized set, quite a nice amount of useful parts: two large wheels, two large and one heavy-duty small spring, several dozens of chain links, a large 40-tooth gear and a couple of smaller ones, three engine cylinders with its pistons and stuff, and some custom bodywork panels. There is also an amount of beams, though insufficient for large experiments. And ― tons of 3-stud-width axles!

▪ EXPANSIONS
None officially supported, though I suppose it should be relatively easy to modify the bodywork and the overall design, and perhaps modify it to look like some specific motorcycle. However, since the model is tightly packed with parts, there isn't too much freedom to change in its interior.

▪ OPERATION
No specifically designed manual controls on this one; roll it on the floor, look at the engine running, steer and occasionally stop (and extend the kickstand).

▪ WEAK SPOTS
It is not the easiest to build and requires some patience with parts loose during the construction. Also, it would be quite difficult to build almost anything else substantial out of this set, than an another motorcycle. It would be nice to see the engine operation more clearly, but since it's so constrained by design, construction and form, it would probably make it look somehow awkward.

▪▪ VERDICT ▪▪

A fine piece of mid-sized Technic, though perhaps a little bit more designer-oriented than usual models, with plenty of purely decorative parts. However it looks undeniably great, and without those parts, it would not resemble a motorcycle in the first place. Some cool technical parts to add to the collection, especially for the road-going projects, and some clever construction tricks to learn, though applicable primarly on ― other bikes. It is nicely operable and fun to play, but doesn't have a power feature that makes your neighbour's children start packing stuff for Legoland.

Lego NXT Plotter... And Why Did It Fail

Admittedly, having noticed that the NXT set includes three precisely controllable motors, building a Lego plotter is sort of a "natural idea" to try sooner or later. Two axes, a mechanism to lift and place the pen on the surface, and not too much programming; as Jeremy Clarkson would say facing the bucket of bricks ― how hard can it be?

So I've decided to pursue the idea myself, certainly not being the first neither the last one to do so. The concept of a Lego plotter is actually quite older than the NXT; still in 1990 Lego Group released a Motorized Control Center, set No. 8094, that featured a simple plotter among other models. (More about it on Blakbird's great site.) It used only two motors for X-Y movements, while the pen had to be operated manually.

So, what about this one? Building a plotter offers a couple of choices in design approach, among which the most important is its form, i.e. whether the pen moves along one or two axes.

With the second, "classical" approach, the paper is fixed while the pen moves in both X and Y axes. The aforementioned 8094 model utilizes this approach, as do many commercial plotters. However, its serious drawback is its size, as the drawing board ― and thus the plotter itself ― needs to be larger than the maximum size of the paper.

Trying to avoid large, cumbersome and part-demanding construction, I opted for the other form. Its pen moves along one axis on a rail, while the paper moves underneath perpendicularly. The resulting construction is smaller (as it only has to be as wide as the narrower side of the paper), and less fragile as there are no moving structures that have to provide support for further moving structures. The most common paper format here is A4, so I took its width (21 cm, a bit over 8 inches) as a target for the plotter size.

Having done these basic decisions, the construction is fairly straightforward. The drawing head moves along a rail, driven with a rack & pinion. It carries a small mechanism that grasps the pen, and another rack & pinion to lower or lift the pen from the surface.

Underneath, the paper is moved by two pairs of small touching wheels near the edge, and an extra one in the middle bottom, for stability. The area where the pen lands on the paper is reinforced from the bottom with a long cascade of studless Technic beams to provide a firm drawing surface, rather than wobbling the paper in mid-air.

All NXT motors are driving the pinions and paper wheels directly ― I've avoided gearboxes to reduce the gear slack that necessarily reduces precision. The 8-tooth gears serve nicely as pinions as they tightly sit in the rack teeth and there's very little slack.

Programming the plotter for the basic drawing is fairly simple ― though one could go to great lengths optimizing the movements, angled lines and curves, as many others in Lego community do.

So, after several work hours, the plotter, or at least its prototype, was finished, and equipped with an off-the-shelf Staedtler black technical pen (needs significantly less pressure than a biro to draw, and offers a more consistent line width).

Results? Bad. Pretty bad.

The first obvious trouble is with the behaviour of NXT motors ― as they tend to "zero in" on the desired position with some oscillations. But it is actually rather easy to solve reducing the motor speed, or introducing a tensioning and damping system, as often done with high-precision NXT machinery.

More troublesome weakness is the aggregation of errors that quickly throws the entire plotting off target. When the pen is drawing, it offers a slight tangential resistance to the paper moving underneath, which is enough to rotate it by a couple of degrees after a dozen drawn lines, and jam it into the feeder after a hundred or so (by what time the drawing is useless anyway).

Reducing the pen pressure was not an option as it was already adjusted to the smallest possible force that draws a clear line. Increasing the contact pressure of the feeder wheels might help, but it would only delay rather than solve the problem, and make the mechanism much slower and more difficult to operate for the NXT motor.

One solution could be to introduce the side rails that hinder the paper rotation (like those in the printer feeder trays), but that would complicate the design and make it less "automatic". Nevertheless, though, the first question is now answered: it's not as easy to design as it seems. Q.E.D.

It could probably be improvised and rebuilt to handle these problems, but the "real" solution is to switch to the other, large design form. So that is the direction I will take for the plotter version 0.2, rather than to overcomplicate this compact form, making it anything but compact in the process.

A Few Tips for Building Lego Technic

If you have recently started experimenting with Lego Technic and your models suffer from mechanical weaknesses, over-complex systems and cumbersome construction, check out the new Technic Tips page. As it will probably be updated over time, I've posted it as a separate text rather than a typical blog post.