Eurobricks Vassals
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About jam8280

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    Colorado, USA
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    Technic, Mindstorms, motorized boats, spinning tops


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  1. The only thing worse than a hungry meat-eating dinosaur on your tail is one with a tank! (Well, getting caught is pretty bad, too.) Battery: 7.4V PF LiPo rechargeable in place (was AA box) RC receiver/controller : SBrick/custom joystick on Android phone (was PF with bang-bang handset) Motors: L front and rear (were Ms) Drive train: Direct drive (no change) Steering: Differential power to the tracks (no change) Tracks: Lugs every link for traction (new) Overall dimensions: 206x170x146 mm (LxWxH) Mass: 546 g (down from 599 g) Installed power: 4.3 W (was 1.5) Power to mass ratio: 7.9 W/kg (was 2.5) Stalled torque: 0.52 N m (was 0.18) The biggest and most important advance over 42065 was in outdoor play value -- by far my best yet in any MOC or MOD. The biggest contributors there were the L motors and the red rubber track lugs. Dinotrack performance was generally traction-limited without the lugs. Photos and write-up at http://www.moc-pages.com/moc.php/441699
  2. [HELP] Buggy Motor Price

    If everything really works, that's a fantastic bargain! Heck, if only one motor works, it's still a good deal.
  3. One more practical point about Buggy motor thermistors: Current flow is determined only by the voltage applied and the temperature of the thermistor itself. How that temperature is reached doesn't matter. On a hot day last summer, I absent-mindedly put the yellow car (2 Buggy motors) in the trunk of my car to show off its speed to a friend. When I got there 10 minutes later, the yellow car wouldn't budge. Both motors had shut down just from being in the hot trunk! Once the car had cooled off at home, it ran just fine. Since a thermistor throttles current somewhat gradually as its temperature rises, a car with warm Buggy motors for any reason could appear to work when it's actually running at reduced current despite being at full voltage.
  4. Some kind of malfunction is a possibility, as I bought all 5 of my Buggy motors used, with no idea of their prior histories. When I overheat any of them to the point of shutdown, though, full recovery takes a lot longer than 15 sec, even at room temp or in the fridge. (Not a word to my wife!) Three of the 5 motors are installed in the red and yellow cars I showed earlier, and all behave pretty much the same. Brown-outs are not infrequent after several runs on hot rough asphalt at 30+°C air temperatures -- especially uphill. But on an air-conditioned gym floor, the 2 motors in the yellow car will run at full throttle till the PF LiPo dies. Keeping motor thermistors as cool as possible for official top-speed runs might be an argument for push starts, as current draws (and therefore thermistor heating rates) are highest when accelerating from a standing start (effectively, a stall). Though I have yet to try them, I see push starts as perfectly legit when top speed is the only goal.
  5. Yes, I'm aware of all that. I thought it might be the battery, too. But while the car wouldn't move (i) the battery light stayed on, (ii) the battery toggled off and on normally, (iii) the battery could still power a test motor through the SBrick just fine, and (iv) the still car still wouldn't budge with a fresh battery installed. So the SBrick and battery weren't to blame, and that leaves only the motor. Philo's dissections have confirmed that the Buggy motor's only overcurrent protection is a thermistor, and the motor "brown-outs" and shutdowns I've observed with my cars and in other settings have all been entirely consistent with the heating and cooling of a thermistor. For those who haven't had the pleasure, when I've tripped the overcurrent protection in PF LiPos in my boats, the battery lights always went out, and the batteries wouldn't turn back on until they'd been recharged. Others have reported the same behavior. But none of that happened here.
  6. You've clearly faced problems that I have yet to encounter. The tire ballooning issue isn't just theoretical. For the record, that 17 kph figure is the red dragster's average over an 11 m rough asphalt course from a standing start. My top speed has to be higher and could be higher by 50%. Still no match for your top speed, but closer to your territory. I'm aware of the relationship between drive wheel diameter, gearing, and drive wheel torque. When you guys change drive wheel diameter, do you actually do the regearing you described? And with available gears, how close do you come to the final drive ratio exactly compensating for the change in wheel diameter? Ideally, to maximize top speed, you want your motor shaft speeds to level off at 40-60% NLS (no-load speed) to get maximum mechanical power out of them. In the absence of tire ballooning, if smaller wheels were to take the final motor shaft speed from, say, 30% to 40% NLS, the added final power might outweigh the added bearing friction due to higher shaft speeds throughout the drive train. If the smaller wheels were to take your motors from 60% to 75% NLS, on the other hand, they'd definitely be robbing you of power at the motors. Has anyone run into thermistor heating inside the Buggy motors? Gear too high or use drive wheel diameters that are too large, and the thermistors will start to heat up as the motors draw more current to supply the additional torque at full throttle. In my experience, the thermistors don't shut down all at once. Instead, they limit motor currents gradually as they heat, and that could impact top speed as well. I ran into this problem with the red dragster when I switched to the larger new motorcycle wheels and a AAA box with disposable lithiums (total 6 x 1.8 = 10.8V) in lieu of the PF LiPo battery (7.4V). The thermistor in the motor started limiting current after 2-3 speed trials. When I put the car in the fridge for a while, the thermistor revived.
  7. Thanks, that's certainly an important data point. I'd like to know a little more. Q1: Besides drive wheel diameter, what else changed when you changed the wheels? Q2: Did you regear to compensate for the wheel diameter change? Q3; What was the motor shaft speed near top speed with each drive wheel type?
  8. Added power lost to bearing friction is certainly a valid concern with a smaller drive wheel, but if that wheel also puts your motors closer to the peaks on their power-speed curves as the car approaches top speed, the smaller wheel could be worth the extra loss. My point is that in playing trade-offs like this with so many interrelated parameters involved, guessing isn't as safe as testing in a carefully controlled fashion. Hmmm, hadn't thought about floating tires on their wheels. Was the tire in the video spinning at a speed comparable to that when the car is near full speed?
  9. In rubber tires, rolling resistance is mainly a matter of sinkage, sidewall flexure, and contact patch area. Your balloon tires may have less rolling resistance than you might think -- at least on cold, hard, smooth surfaces where sinkage isn't an issue. The contact patches are largely limited to the tread's narrow central ridge on such surfaces and are therefore quite small. In addition, there won't be much sidewall flex in a vehicle this light as long as the tires stay cold and the surface stays flat. These ideas are borne out by the testing I've done with different wheel/tire combinations on the 2 cars below. (Details on the red and yellow cars at http://www.moc-pages.com/moc.php/430234 and http://www.moc-pages.com/moc.php/429695 , respectively.) The 279 g red dragster is my fastest no-frills top-speed car to date at an average speed of ~17.3 kph over a 9.1 m course with rather rough pavement. If acceleration were constant -- and I'm sure it's not -- top speed would have been twice that, or ~34 kph. Still a far cry from 40 kph, I know, but I'm sure my top speed, whatever it was, would have been faster on a longer course with a smoother surface. Bottom line: If you generally run on cold, hard, smooth surfaces, and if balloon tires give the drive wheels a diameter that lands your motors near the peak of their power-speed curves as the car tops out, then you may want to keep them. The red dragster's top speed fell dramatically when I switched the drive wheels to the newer motorcycle wheels and tires with smoother treads -- probably because the larger drive wheel diameters took the single Race Buggy motor out of its sweet spot and added too much to the drive wheel moments of inertia. v All that said, the 559 g yellow car is faster on asphalt with the big ZR wheels seen here, but traction becomes a big issue on smoother surfaces.
  10. You've touched on a very complex topic with no easy answers. But there are some valuable rules of thumb. At the risk of getting too technical and way too long-winded, I'll share what I've learned from real-world engineering and from extensive experience and experimentation with working LEGO gizmos of all kinds, including vehicles operating on land and in the water. I'll focus on vehicles here. May have missed it, but I didn't see anyone mention the critical role of mechanical power in a vehicle's top speed. In short, a vehicle reaches its top speed when the mechanical power collectively delivered by all the motors equals the power consumed by all losses between the motors and vehicle's interfaces with the real world (e.g., the ground and the air). That's because one way or another, total loss invariably grows with both speed and total vehicle mass. Once this power balance is reached at full throttle, no further acceleration is possible, and speed levels off. So, there are 2 main strategies for increasing top speed: (A) Know your batteries, RC receivers, and motors and increase the total mechanical power coming out of the propulsion motors where you can. (B) Know your losses and reduce them where you can -- starting with the biggest losses -- while being mindful of the trade-offs involved at every step. Understanding all the losses in play and which ones are worthy of attack gets technical in a hurry, but Wikipedia is a great resource. (A) On the motor end, the goal is to arrange -- through gearing and other choices -- for each and every propulsion motor to be operating near the peak of its power-speed curve as the vehicle approaches top speed. For reasons I won't go into, you'll know that you're near peak power in a LEGO motor when its shaft is turning at ~50% of its no-load speed (NLS) with the battery/receiver combo you'll be using. The good news: The power vs. speed peak is broad enough that 40-60% of NLS will be good enough. If you do nothing else to maximize top speed, do this while trimming weight where you can! For example, the NLS of an XL at 7.4V (PF LiPo battery voltage) is ~180 RPM. So you'll want that XL to be running at 72-108 RPM when the vehicle hits top speed. If it takes more gears to bring this about, so be it. The triple-screw powerboat below reached its highest top speed (~1.1 m/s) when each XL was turning at 90-100 RPM when the speed flattened out. That required a 3-stage 1:8.33 overdrive transmission between each XL and its prop, for a total of 3 gear pairs per prop. When I used fewer gears, the boat when slower. NB: Finding the motor/gearing/prop or motor/gearing/wheel combo that maximizes top speed in a particular vehicle is an iterative process based on exhaustive guess-and-check testing, changing only one thing (say, final drive ratio, or prop or wheel diameter) per trial. You'll need a cheap laser tachometer to get at motor shaft speeds and a way to measure or at least rank top speed results as well. (B) When taking stock of your losses and what you can realistically do about them, it's helpful to divide them into internal and external losses. The internal power losses in LEGO vehicles have several important sources: (i) Gear-to-gear and shaft-to-bearing friction -- the latter especially at the drive wheel or prop bearings. (ii) Continuous distortion of axles that bend or twist while turning under load. (iii) Unwanted motions of motor and bearing mounts during operation. So, use the fewest gears possible (preferably double-bevels) to attain the optimal motor shaft speeds at vehicle top speed (see above). Keep axles as short as possible and reinforce the ones that have to be long with bushes or axle joiners. Make all motor and bearing mounts as rigid as possible while keeping an eye on total mass. These measures are often worth the added mass that most entail. Count on having to run top speed trials to find the sweet spot in the rigidity vs. structural mass trade-off. The external losses come mainly from total rolling resistance (not the same thing as friction) , water resistance, and even air resistance. Rolling resistance grows with weight and surface softness but generally varies little with speed. It increases sharply with tire or tread sinkage into the ground and to a lesser extent, with sidewall flex. Water and air resistances, on the other hand, grow very rapidly with speed. Water resistance also grows quickly with displacement (total mass), but displacement vs. resistance trade-offs in boat top speed are very complex. Finally, whatever you decide to do about top speed, test, test test! The trade-offs involved are seldom straightforward and often counter-intuitive.
  11. I've found recent pull-back motors to be useful for lots of things besides vehicles. The studded one with the built-in push-button release is too weak for most purposes, but the others are versatile, compact, high-speed power sources with lots of torque. v This flying rotor launcher with reversible handle (http://www.moc-pages.com/moc.php/418125) can send the rotor shown to a height of 15 m. v A similar design makes a great high-speed starter for LEGO tops -- e.g., at 2:21 below. v These white non-LEGO biconic dice have lots of potential as spinning tops, but they're too slippery to spin on their poles by hand. This high-speed starter with ratchet, trigge,r and safety catch (http://www.moc-pages.com/moc.php/438782) turns them into long-spinning tops. v This spoof particle accelerator (http://www.moc-pages.com/moc.php/430714) sends wheels flying around inside wall of the blue bowl at high speed. v This wind-up mug mixer (http://www.moc-pages.com/moc.php/439024) stirs cream into coffee very efficiently with only a few half-turns. At full power, it can empty the mug. After building it, I found out that it shouldn't be used in hot liquids you actually plan to drink, as the US Food and Drug Administration considers ABS plastic unsafe for contact with hot foods. I made this pull-back rat (http://www.moc-pages.com/moc.php/429782) for my dog (part rat terrier) to chase when she was about 1 year old. It looks more like an armadillo than a rat, but she was still happy to run it down and pull the ears off.
  12. Very kind! Lots of photos and written info on the design at http://www.moc-pages.com/moc.php/441459 . Coming up with reliable string guides was the hard part.
  13. String is evil, no doubt about it! No one's more surprised than I am at how rarely the Stringatron jams. When it does, it's nearly always because I was messing with the outbound limb of the loop -- never with the return limb or turnaround. And no jams of open-ended strands at all with the current strand guide. Very kind, Teflon! Not a lot of jazz behind YouTube videos, period. A LEGO toilet paper minigun, eh? I love it! You know, a single-shot toilet-papering machine is not entirely outside the realm of possibility with LEGO. The wheels are turning... Thanks! Now that you mention it, the tires are showing a little wear. But I guess that shouldn't be too surprising, as they'd already put in 8-10 high-speed kilometers by then. Thanks!
  14. Purism

    I got my silicone tubing on Amazon. It's much more flexible than LEGO tubing but just as air-tight at valve and cylinder fittings. It's also much less expensive.
  15. Thanks! Yes, silly names are half the fun. At 5:11 in the 2nd video down, a breeze blows the loop over the edge of the table at 90° from the direction the Stringatron is pointed, and it just keeps on going. The outbound and return limbs are both at the same level at that point but manage not to get tangled. I've seen the same thing when the Stringatron is pointed directly over the edge of a table.