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.

I did this as a Christmas present for my daughter, and it was put together fairly quickly as I needed to get parts ordered. It was my first attempt at doing something to scale, and was done entirely digitally - my daughter received a box of bits and instructions for a model that had never been physically built. A bit of a risk, but aside from a quick redesign to get the battery one stud forwards for balance it went pretty well. It's got two L motors for drive, behind the rear wheels, two M motors for the arm, controlled by an SBrick and the AA battery box. I was happy with result, but there's definitely things I'd improve or do differently with a bit more time. I realised too late that I'd messed up the scale, and both the roof and rear section are too high. The large LAs are too long, but I couldn't get sufficient height with the medium LAs. Looking at some of the details, I like how you kept the arms to 2 studs wide. I wanted to drive the tilt mechanism from underneath the arm pivot, as you have done, but the angle of the arm when lowered was just slightly too steep and the gears would collide. The system bricks for the mudguard and curved section behind the rear wheels work really well - I recognised it instantly as a Cat before I saw the topic title! Filling in the lower section behind the wheels with yellow beams also makes a surprisingly big difference to the appearance.

Nice job. I built something based on a 216B at the end of last year so very interesting to see how you tackled it. I struggled particularly with the shape of the rear, and I like what you've done there. I also struggled with balance, although I had the battery box over the wheels which helped. I was struggling with this in another model, and discovered that if you drive them with a lower PWM frequency they work much better. Not sure which app you're using, but I have a custom build of BrickController 2 for Android that allows me to set the PWM frequency on an SBrick.

The tyres look great. So much more suitable than any of the Lego options. I've managed to order some from a supplier in Croatia. I actually ordered a few sets as the shipping was more than the tyres, and I may also get an import duty handling fee. Your build is looking good. Interesting positioning of the lift motor. Are you planning to put both of the other motors in the boom?

How does the on/off button work on the BW 3.0? On the website photos it looks like it's recessed into a stud.

That looks better. Thank you. It actually came together surprisingly easily, except for sorting out that drive line and motor bracing. The end result looks simple, but those few pieces took me hours! Tyre clearance is a real issue. I think the Fischertechnik tyres would be a big improvement, but I can't find how to get them in the UK.

Yes - the 16 to 20T. I am a little surprised, as it seems it relies on friction in that axle and any bend in the axle or lower chassis will allow separation. I only commented because I struggled with the bracing in a very similar situation after rearranging the servos in my model (see here). I have to deal with a bit more power from the BuWizz, but I was surprised at what was needed to secure it. I continue to follow your build with interest

It's looking good. Do you plan to add more bracing to the drive motor? It looks like the gears are only held together by a pin and axle in tension.

I read the subject line of your post and thought "spirograph"! I was inspired by some of the recent spirograph posts on here, and built something similar to what you have and quickly discovered the limitations of the standard gears so it's very interesting to see what you've done here.

Thank you very much for the positive comments. It is very hard to improve anything in this model - pretty much all my changes come with some compromise, so I understand the decisions you made to keep the design buildable, reliable and affordable. I've actually just spent a few more happy hours modifying. The play in the front steering from the bevel gears was annoying me so I decided to try something which I'd previously declared impossible: This arrangement is much better than my previous effort. As well as getting rid of the play, it gets rid of the unsightly servo below the cab. I was concerned that removing the beams between the drive motor and servo would make the chassis too flexible, but the end result is actually very strong. The only issue is losing the T pieces that brace the steering axles which means the gears slip if the servos force the steering against the stops. Not an issue for use with SBrick/BuWizz as you can simply limit the servo travel electronically, but would be a problem for bang bang steering. I had an "aha" moment when I realised why that bracing was needed. Getting 3 IR receivers in the cab is very impressive in itself! I've got one of my SBricks in the "upper" position, and one in the middle at the bottom. Now that I have the BuWizz, I could get rid of one of them altogether. Thank you. Sadly, I've discovered a bit of a problem with them which is that at full lock and full speed, the play in the hubs allows the tyres to move really quite a lot and they rub enough to be a problem :-( I think it's the lack of Ackerman geometry that puts quite a lot of sideways force on the tyre as there's no rubbing on full-lock crab steering, only on four wheel steer. I'll carry on experimenting, but I'm running out of ideas. I had the same dilemma, it's actually 2 studs forwards. For me, practicality won out. The fact that I have impatient children may be a factor in this :-) Agreed! The reliability has been very impressive. The one question I did have is the use of the half bevel gear to drive the front diff. Is there a reason for this? My model eventually spat out the gear and I replaced it with a full 20T gear. In the rear, there's a stopped axle so it's fine. Interesting. You may be interested in some of the mods I've made for controller modes. For example, with the increased speed of the BuWizz, I have a button that toggles a "slow speed" mode which halves the drive speed, but there's lots more options for changing what controls do on the fly. The problem I had was when using the fork rather than the bucket. In order to get the fork tip under a palette accurately needs a very tiny movement. I have it set up on the "hat" on my controller, and use quick taps to get small movements, but even that is too much. So I reduced the power that the tap uses to slow it down, but then when you put a load on the fork it hasn't got enough power to consistently lift the fork. I've discovered that lowering the PWM frequency on the SBrick increases the torque so I can now run the motor slowly with a wider range of loads. There's one more thing I want to try which is that the SBrick has the ability to drive a motor for a specified amount of time. This should allow you to get short "blips" of drive more consistently, as it's not subject to Bluetooth jitter. The shortest period is 0.2s which I fear may be too long. As you can see, there's nothing holding the 4x2 beam except the side panel, and the 13L beam is only held by a single full pin at the rear. I was doubtful about relying so much on the plates for structure, but it's proven completely reliable and very stiff. Another tiny mod is just visible on the grey pin on the arm just in front of the light is a small elastic band holding the motor cable to the underside of the beam. This solves the problem of the cable not feeding into the arm correctly which is more of an issue with the extended reach.

I started fiddling with the bracing for the arm extension worm gear, as with too much load on the fork the driveshaft will bow upwards and the worm gear will slip. As part of doing this I realised that it was fairly straightforward to increase the extension of the arm to get an extra 4 studs of usable reach. . Lift height is now 41cm which is bang on scale - 7m at 1:17. I've also had some success with making the fork tilt more controllable by dropping the PWM frequency on the SBrick controlling the tilt motor. This seems to give much more torque at lower speeds.