pdw

You can't power something with 20A; you can connect it to a power supply capable of delivering 20A at 9V, but that doesn't mean it will draw 20A. The current draw depends on the load. The SBrick has overcurrent protection and thermal protection. I think the overcurrent protection is set at 3A per channel, which would mean 12A in total, although actually drawing this would trip the thermal protection very quickly. A PF train motor draws 1.3A @ 9V when stalled, and much less in normal usage, so you unlikely to trigger the overcurrent or thermal protection, even with several motors connected.

1000 mAh is a measure of battery capacity, so is not relevant. What matters is the input voltage. There's a definitive answer from one of the SBrick team on this thread on the SBrick forum. Basically, 10.8V safe upper limit, 11.8V absolute upper limit. So if you're planning to use 2 x LiIon cells and step up to a regulated 9V you should be absolutely fine, and could safely go a bit higher.

I don't think it can be done directly, but there's a bit of a hack you can use. Assign a button to the same output, set it to "simple toggle" and set the maximum output to the offset that you want, inverted if necessary. Then press the button when you start the profile and it should offset the servo, as the two outputs will be added together. Bear in mind that the PF servos only have 7 positions in each direction, so you won't get very precise control over the offset.

I decided to give this a go, but skipped the body work in favour of a bit of structure to hold it together: And a quick video: A lot of fun for not a lot of parts. Thanks for the idea!

Very nice. It looks like the distance between the rack and gear for the steering changes with movement of the axle. How does that work in practice? I assume that the axle movement is small enough that it's not a problem?

No, my idea is that the shim would slot into the end of the next one. As @pleegwat says, I don't think there's enough diameter, at least on the 2L ones, and you'd only get a whole number of studs with a single unstacked gear.

I've not posted anything yet, but at a high level it's similar to this one: https://rebrickable.com/mocs/MOC-7530/1963maniac/adjustable-spirograph-v9-by-pg52/#details Mine takes a different approach to the gearing, using a differential to slow down the second arm relative to the first to introduce the phase shift. The differential can be run at (3 x 2 x N) times slower than the first arm, where N is set by a gear box, which gives a ratio between the arms of -1 * 3N : (3N - 1) [we can mostly ignore the -1] The gearbox can set N to any combination of 1, 2, 3 and 5, so it can do a number of curves ranging from 3:2 to 90:89 - my previous post wasn't quite right as the diff is actually running in the other direction. Obviously using two arms like this means the axes aren't independent, although it should be a reasonable approximation with long arms and a small amplitude. In the spirograph, the interesting factors are number of turns per phase change, and number of phase changes per rotation of the base. Generally you want the first to be quite high to give a nice dense pattern, which is why the gearing has the built-in factor of 3.

Sorry, I missed that bit. In that case, no, I don't understand either. In fact, if you made a small unthreaded shim on one end, and a corresponding indentation on the other, you could make one that was both stackable and a full 2L long. Maybe not enough diameter to play with on the 2L worm gears, but should be possible on the newer 1L ones, I think.

As mentioned above, I think it's so that you can stack multiple worm gears on the same axle and have the threads line up. The tooth pitch is 2 * pi / 16 studs (a 16T gear has a radius of one stud, circumference is 2 * pi * r) which is 0.393 studs. A 2L worm gear has 5 full turns and so has a length of 1.96 studs. If you want to be able to stack gears you need the worm gear to be a multiple of quarter turns, so the next length you could use is 5.25 turns which is 2.06 studs.

Interesting project. [edit sorry - just noticed you've done exactly this] It looks like you're gearing down then gearing up on one axis. Could you switch to gearing down on both axes? i.e. Have 1:1, 1:3, 1:5 on one axis, and 1:1, 1:2, 1:3 and 1:4 on the other? I think that will give the same effective ratios, but gearbox friction should be much less of an issue. I've been working on a spirograph recently, and this has made me realise that part of it is close to being a Lissajous curve generator, except I have the ratios setup with a different goal. My gearbox gives various ratios between 3:4 and 90:91, and the mechanism is more like the one in the video you linked to, with a simple pair arms on rotating bases.

If you're happy with a non-Lego solution, then there are various options. I did these using SMD LEDs: The LEDs are glued into a short piece of silicon tubing that's just the right diameter to plug into the back of half pin. SMD LEDs are also small enough that they'll fit in the hollow on the underside of a stud, which is what I did for the front lights. The 3rd party lighting kits have similar options, but they're quite expensive for what they are.

Front wheel steering is naturally stable, whereas rear wheel steering is unstable. If a rear wheel steer vehicle starts to turn, its momentum will act to make it turn more. A front and rear-steered vehicle is likely to close to neutral, depending on where the CoG is relative to the mid point between the steering axles. This means that if it starts to turn it'll carry on turning, but it won't get tighter as a RWS vehicle would. A vehicle that is positively stable will be easier to drive. Front-and-rear steering is used where manoeuvrability is essential, such as telehandlers, and even there they often have a 2WS mode for driving along - even on a vehicle with a 40kph max speed. The Alvis may not be very quick but it is about getting from A to B, rather than constant low speed manoeuvring. The additional axle for improved off-road ability, and the second axle is steered to avoid tyre scrub. Turn radius is the same as if the middle axle wasn't there. A front-and-rear steered 6 wheeler has the same turn radius as if the rear axle wasn't there i.e. it has half the wheel base, and thus a tighter turn radius.

If you have an Android phone with an IR output and a bluetooth game controller then I think you can use BrickController 2 directly with your IR receivers.

I didn't have that particular issue because the LA pivot is forward of the upper arm pivot, but the flip side of that is reduced reach. I came to the conclusion that the hydraulic cylinders on the real thing have far less "overhead" than a Lego LA i.e. they get much closer to doubling the length between pivots when extended, so it's always going to be difficult to replicate the geometry.

As a parent, and a user, it's not the cost of batteries that bothers me, as we use rechargeables for everything, it's the inconvenience. I've not got any PU stuff, but the PF battery boxes are both an utter pain to get the batteries in and out of, and most chargers will only do four batteries at a time. My BuWizz powered models I just plug into a USB cable like pretty much every other battery powered device in the house. Buying something as expensive and sophisticated as the 42100 and then having to deal with AAs seems bizarre to me. That's true of the XL motor, I think, but the L has exactly the same, and the M has one fewer hole and more studs. The interesting one for me is the servo. The PF servo is flawed and has an awkward shape, but the ability to get an axle out of both ends is very useful, particularly for 4 wheel steer. You can also mount them back-to-back and get two independent servos in less than 10 x 7 x 3 studs. Compact 4WS is much harder with PU L motors if you want to avoid gears in steering.