Hrafn

Maybe a Technic Disk or a 36t gear in black would work? Or a 3-blade thin liftarm?

The wheelbase looks long to me, too. Most cars seem to have a wheelbase approximately equal to 4 wheel diameters; this looks more like 5 or so. But if it's what you want (and if it turns OK!) then it's not an issue.

Is it this one?

Do you have a particular real-world vehicle in mind? If so I'd try using whatever that vehicle uses for center suspension. Otherwise, pendular suspension is relatively straightforward and robust, especially if you use turntables for the central pivot. Take a look at the Unusual Locomotion site, too - they have pages for 6x6 trucks, both medium and heavy, plus a lot of other information about different kinds of vehicles. I'd also take a look at some 6x6 armored personnel carriers, though the ones with 2-axle steering seem mostly to steer the front two axles.

Are you looking to do 6x6 or 6x4? 6x4 would be easier, since integrating steering, suspension, AND power into a robust front axle can be tricky. There are a number of possible suspension types out there: Tatra (swing-axle), live axle, etc. Some real-world trucks (like my favorite ugly duckling, the Scammell Explorer/Pioneer) have a "walking beam" suspension for the rear 4 - on each side, a beam is mounted on an axle, and the wheels are then mounted at the ends of the beam. This allows for considerable vertical motion, but then the wheels aren't independent. Sariel's book (The Unofficial Lego Technic Builder's Guide) has some good instructions, and his site (sariel.pl) is good for inspiration.

That's a beautiful space, and excellent photography. I love the spare, modernist lines.

I agree - the thread is a good idea but problematic in the implementation. It's already becoming hard to follow the different topics.

Thanks for the correction, I copied and pasted from the wrong tab.

There are a number of different sized train wheels as shown here. Diameters vary from 23 to 37 mm (~3 to ~5 studs).

The servo motor and L-motor may not yet be up in S@H, but they're on the main Lego PF page, so hopefully they will come to S@H shortly: http://powerfunctions.lego.com/en-us/products/88004.aspx

Maybe he's thinking of how some inexpensive RC cars use only one motor for both propulsion and steering. When the motor spins one way, it drives the vehicle forward; when it spins the other way, it turns the vehicle (always in the same direction, say, left). It works, but it's awkward, and you sacrifice a lot of maneuverability.

Do you have enough beams (in yellow, red, white, or tan) to do twin racing stripes?

Same here - and I'd love to see the rest of the photos, since this is an amazing creation!

For the ball and socket, we have to wait until these come out in the spring of next year: http://www.lego.com/mixels/

I personally shy away from modding parts, because I like the challenge of working with the same parts everyone else has; but I share and understand the frustrations people have with the limitations of those parts, and I understand why some people do prefer to modify some parts. What I'm suggesting is another option - instead of modding parts ourselves, what if there was a third-party company that produced "beyond Technic" LEGO-compatible parts for really advanced building? That could be useful for AFOLs who wanted to build more realistic MOCs, and potentially for educational purposes (and who knows, even as a rapid-prototyping tool for engineers).

And if not, how much interest is there in such a product, what parts would people most like to see, and what would they be willing to pay for each? It sounds like the main categories are 1) suspension (steered wheels rotating about their centers, CV joints with a large range of motion, suspensions other than double wishbone) 2) stronger differentials (and maybe 3L ones that are slightly wider so they don't skip under high torque) 3) gearbox improvements (clutch gears with other than 16 teeth) 4) rim gears for planetary/epicyclic gearing (other than the large turntable and the Power Miners wheel) 5) more realistic (i.e., thinner) wheels and tires 6) RF control instead of IR 7) new liftarm and connector designs (3L thin, 2x3 L-shaped, 3L pin and axle connector perpendicular with the axle and pin holes switched, etc.) TLG also doesn't make helical gears, but I don't think there would be much advantage to making them and they would likely be fragile at this scale. What about flex splines for harmonic drives? Beveled clutch gears? Gears with other numbers of teeth (since we are missing 28 and 32, except for the 28 teeth on the 3L differential)?

Anyone who can see my profile pic will not be surprised that this one is one of my favorites: It's very useful for custom kingpins in vehicle suspensions.

I really like the idea of continuously variable transmissions, where the transmission can shift smoothly and continuously along a range of ratios instead of moving from one fixed ratio to another. Inspired by Zblj's differential-based CVT and its development by Nico71, I wondered if it would be possible to build a spherical CVT similar to the NuVinci CVT, using Technic ball joints as the spheres. For those unfamiliar with spherical CVTs, the NuVinci site has a decent explanation. The goal is the same as for other CVTs, but the mechanism is different. The answer is: yes, such a thing can be built, but it is so limited in the torque it can transmit that it's of little practical value. In theory, one could get over the torque problem by gearing up between the motor and CVT's input, and then gearing down again between the CVT's output shaft and the wheels. In practice, I had limited success with this approach because of frictional losses in these two additional geartrains. However, I'm new to Technic and maybe one of you can solve this problem. If you do, please let me know! There are plenty of other features of the CVT that could probably stand improvement. For one, the spheres contact the side of the tires, pushing the tires away from the sphere. Ideally, the spheres would contact the edge of the tires, keeping the wheels from dislodging sideways. However, in practice there are two problems. The first is that this is hard to do while building using parts with standard stud- and half-stud distances (8mm and 4mm). The second is that if the two wheels are too close to one another, their faces tend to rub together. I tried using Bionicle Zamor/Chima spheres, which are much larger and therefore might allow a better wheel/sphere contact geometry without having the wheels too close to each other, but didn't find a good way to get them to work. I also tried a wide selection of small tires and wheels; none of them worked better than the wedge belt wheel and its tire, though maybe someone else can find a better solution. An early version of the CVT was also an Infinitely Variable Transmission (CVT-IVT), at least in theory. The sphere could rotate enough that the radius of rotation of the sphere where the output wheel contacted it was zero; this gave, in theory, infinite speed reduction and infinite torque multiplication. In practice, the output sphere stopped rotating - and stopped transmitting any torque - well before the sphere reached the theoretical infinite-torque angle. One happy accident I encountered while building the CVT is that while I planned for the shifting to be manually controlled, the CVT as built is actually automatic. As you apply a resistive torque to the output shaft, the spheres rotate of their own accord to slow the output and increase its torque. Note that if the tan axle pins in the pictures were not present, the spheres would keep rotating until the flat face of the ball joints (where the axle pin attaches) faced one of the wheels, which would then cease to be in contact with the sphere and cease to transmit rotation. OK, on to the pictures. Since I don't have a Flickr or Brickshelf account to host photos, I can only upload 3 small photos in this post; hopefully they'll be adequate. For some reason they upload out of order, so the first one is of the CVT partially assembled; the second shows the parts needed; and the third shows the completed assembly. Note that 8 of the lime connectors are connected to the 5x7 frames using black friction pins; the central 4 lime connectors use 3/4 pins instead, with the 1/2 stud portion inside the lime connector. This is to prevent the pins from rubbing against the ball joints.

Large hardware stores are usually good places to look, at least here in the US. I've gotten a lot of mileage out of one of these, though with a collection your size you may want several, in addition to bins for infrequently used parts. These bins aren't the sturdiest, but they work - and at $20, they're affordable.

Zblj, I'm not sure I understand what you're asking. Some "wobble" is OK, though not ideal - I think it indicates motion that's not being smoothly transmitted from the input to the output wheel. Icks, Your basic idea of adding bearings to stabilize the wheel is a good one. In this particular configuration, adding technic ball joints as bearings doesn't work because it introduces too much friction. Each wedge belt wheel is slightly more than 1/2 stud wide, and each ball joint is about 1.4 studs wide, so they jam up against each other too much and there is too much friction for the wheel to spin smoothly. With different wheels, or a different overall CVT geometry, it may well work. There are other parts that can also be used as roller bearings - non-friction pins, pin joiners, small wheels with or without tires, etc. This is definitely a good avenue to explore.

It's hard to tell, but my guess is that issue is that the tires are contacting the ball at the wrong point. If they are contacting the ball exactly (or close to) 180 degrees apart from each other, then no matter how the ball is tilted, the radius of rotation about the sphere's axis is the same for both tires. I think that if you move the balls in or out a bit it may work. What holds the sphere in position in your version? Another possibility is that the tires are contacting the ball over too large an area. In theory, you'd want the tires to contact the ball at an infinitesimally small point; in practice the contact happens over a small area. Different parts of this area want to move at different speeds, so you get "contact spin" and friction. The bigger the area, the worse this problem becomes. The more the tires are pressed against the sphere, the more they're deformed and the larger the contact area gets. The wikipedia article on NuVinci transmissions says: This is because, for any given contact patch, parts of the ball are going in a slightly different direction and at slightly different speeds than the disc (this phenomenon of traction-type CVTs is referred to as "contact spin"). "The spin velocity (or drill speed) is defined as the difference in the rotational speed of the driving and driven rollers in a direction perpendicular to the contact patch plane. It is caused by the relative difference in surface speeds of both elements across the contact patch and is a major source of power loss in traction drive CVT’s." :P That does make sense! Unfortunately I only have 3 wedge belt wheel tires at the moment, so I can't fully test it myself - but maybe I can test one side and see if it makes a difference on that side. Probably not until tomorrow, though - today's busy :( Yes, that might work with the right mounting, especially if it includes a central idler wheel that encourages the spheres to spin properly and not just move about crazily. Getting the right amount of contact friction is important, too - my limited experiments with using 6 Zamor spheres held by two large balloon tires (and using an idler wheel) suggests that with low friction, the spheres just act as ball bearings and don't transfer any significant torque. I'm trying to stay "purist" and use only unmodified Lego pieces, but you're right, glass on rubber has a really high friction coefficient - that's a great idea, and very tempting! Maybe I'll pick up some marbles at the dollar store.

Hm, I'm not sure I understand what you mean. The way spherical CVTs work is that a wheel transfers motion to a sphere, which transfers it to another wheel. Since the axes of the wheels are fixed, they can easily be used as inputs and outputs. You seem to be suggesting we use the spheres themselves as inputs and outputs? Is that right? The difficulty there is that the axes of the spheres move, so you need a mechanism to capture the spheres' rotational motion no matter what their orientation is. I've played a bit with using u-joints, constant-velocity joints, and gears to use the sphere as the output (and to get rid of the output wheel) but I haven't worked on it much yet.

That is a big problem with them, yes. There is a very narrow hole in them but I haven't found a part that would engage that hole - it's much narrower than a bar. The sports balls would have the same problem.

Thank you! I'm glad the second video worked. I agree that the geometry of where the tires contact the sphere is an issue. In the NuVinci design, they sandwich the spheres between outer wheels and an inner idler wheel; that geometry probably will not work with Lego. Barring that solution, I'd like to see the wheels much closer to one another, so that the force on them from the spheres is mostly perpendicular to the axes of rotation of the wheels. That would keep the force pushing the wheels outward to a minimum. It would actually increase the force pushing the balls outward, but in practice I've found it's easier to keep the balls in place than to keep the wheels in place, at least with these particular wheels. Unfortunately, so far I've found that if the wheels get too close they tend to rub against each other and the whole mechanism reverts to what you originally thought it was - a kind of linear clutch. Yes, aol000xw mentioned the same part previously, and I did get some from Bricklink while searching for alternate solutions for the CVT. Those tires have a bit more grip than pure ABS, but much less than the current, soft tires do. They also have some disadvantageous characteristics. For one, they are not truncated spheres, so when you tilt them around their center point, both sides don't stay in contact with the wheels. For another, they are odd sizes - not quite 2 studs tall - which makes them difficult to keep in position using standard stud- and half-stud-length pieces. Still, they're probably the best bet I can think of. Thanks! I'd actually prefer to keep it as small as possible, so I can use it in a vehicle; but since the small version doesn't transmit enough torque, maybe I do need to make it bigger just to get a version that works better. I do have some of the 43.2 x 22 ZR tires, and have been experimenting a bit with them them. No real successes yet, but they're promising, and while they're wider than the wedge belt wheel tires, they are sturdier. The existing spheres are pretty good, though they vary to a shocking degree - some are noticeably wider or rougher than others, and some have prominent or misaligned mold lines that make them unsuitable. I tried using tow balls, which are even smaller - but they're too small, and the mold release divot on the end makes them lose contact with the wheels too easily. I also got a few Zamor/Chima spheres (which are about 2 studs in diameter), hoping that they'd be softer and "grippier", but sadly they seem to be made of the same hard, slick ABS. Maybe some of the sports balls / GBC balls would work? I don't have any and don't know what they're made of. Also, if anyone knows a "purist" way to get the rubber end off of old Technic competition arrows, let me know! Those things are pretty close to being hemispheres, and they're very "grippy."

If nothing else, posting this has made it clear to me that I am not cut out to be a technical writer, since I haven't done a good job of explaining the CVT. Nazgarot, there is a change in the relative contact points, but it's very hard to see in the photos. If the wedge belt wheel tires contacted the sphere on exactly opposite sides, that is to say if a line drawn between the contact points went through the center of the ball, then you're right that the speeds of the two wheels would always be identical. However, because of the way the tires do contact the spheres, there is some difference in the speeds of the wheels. It's difficult to explain with words, so here's another video; this one is of a modified, cut-away version of the CVT with manual control of the sphere's tilting. Hopefully this video is viewable. Unfortunately, by cutting away much of the structure, I also made the CVT even less reliable than it already was, so it stalls a few times in the video. As you can (hopefully) see, the speed does vary. Torque also increases some as the speed drops, but I'm not sure how to show that, especially since the transmitted torque is so low anyway. If you can't see the video by clicking on the link above, try going to my photostream: http://www.flickr.com/photos/100476839@N08/with/9541362712/. The video is titled "CVT cutaway in operation".