PorkyMonster
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Everything posted by PorkyMonster
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I've been thinking about this... while I agree that it allows for very low drag correction, I seriously doubt the shopping cart caster works the same way as a car's caster... because when direction changes, the shopping cart has absolutely no self-desire to revert back to its original (straight) direction, unlike a car with caster. Here's what I found from wikipedia - One major disadvantage of casters is flutter. A common example of caster flutter is on a supermarket shopping cart, when one caster rapidly swings side-to-side. This oscillation, which is also known as shimmy, occurs naturally at certain speeds, and is similar to speed wobble that occurs in other wheeled vehicles. The speed at which caster flutter occurs is based on the weight borne by the caster and the distance between the wheel axle and steering axis. This distance is known as trailing distance, and increasing this distance can eliminate flutter at moderate speeds. Generally, flutter occurs at high speeds. -
Oops!! u'r right, this chart is based on other unknown set of factors. Can't take short cuts...
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Nice video - something I'm way poorer at producing. Can't comment much about the modeling part, as I've no experience building tracked vehicles, but I think yours looks rather cute because as a tank, it risks toppling over... too short and too tall... and if for real, firing at -41 degrees depression will send your tank flipping backwards. Also, are you able to show some pictures on how your cannon works? I'm exploring some possibilities for my next build...
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That's true . I did some googling and found this chart: Apparently air resistance is the biggest culprit... So to break the next frontier - looks like its either adding paneling or to build loooooong and narrow... now i see why dragsters are built that way
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Out of boredom, I did some calculations to determine the acceleration gain (if any) from adding motors (here, each pair of Lego RC motor is assumed to consist of the weight equivalent of 1 RC unit, 2 RC motors and 6 AA Alkaline batteries - ~450 grams including slight overheads) for models of different overall weight (when using 1 pair). And the higher the acceleration gain, the more likely that gearing up will give better max speed. Here's the result: The chart indicates that the heavier a model is, the more it stands to gain (in relative terms) from adding motors... however, adding motors will not allow them to accelerate any faster than the lighter ones. However, playing with momentum, wheel sizes and gearing, its hard to tell whether they can reach greater max speed though... Also, does look like most of the models here fall within the orange and yellow weight range... and adding 2nd to 3rd pairs of motors can be useful, but the gain from adding 4th or more pairs will be negligible...
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Did you gear up further as you add more motors? You need to gear up each time you add motors, because otherwise you won't benefit much (speed-wise) from the added torque. Edit: Let's see... assuming that with 2 motors, you model weighs W, and you get torque T and top speed S using gear-up ratio of G. Now, when you double the motors, you get 2T, your model won't weight more than 2W (hence your top speed definitely won't be lower than S given the same G), and if you double your gear-up to 2G, you'll be able to get 2S!!! And you'll be able to break 40 kph - no sweat . And after that someone will definitely raise the need to limit the number of motors...
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its a balance between power and weight... heavier models can go faster if it has the necessary power... which comes to my point - is there a limit on the max number of motors in this competition?
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Nice... especially the "100% compatibility with lego" part , because that is what really matters to the people here - they'd rather spend much more on Lego's RC unit/motor than on the cheaper, more widely available and more powerful 3rd party motors, because without the necessary pinholes/axle holes they don't know how to proceed. Of course, there are also people with different ideas of fun, so speed and torque are not something that they crave . I think your idea is highly feasible, but I'll just share my thoughts on three specific areas: (1) There is probably no point working with Lego's PF receiver (perhaps you already know this, since you didn't mention it), because even if you get the signalling through, the control range would be pathetic. (2) You probably need to add "something" between Sbrick and the brushless motor - something ESC-like, that converts simple forward/reverse and speed signals into brushless' equivalent, while at the same time handling different modes of acceleration and braking, among other great functions of typical ESCs. So this ESC-like component should ideally be able to plug into Sbrick just like Lego's motors. This ESC-like component should also be the one that feed power to the brushless motors. (3) I'm not sure about your experience with brushless motors... but gearing down might be necessary (unless you use small brushless motors or feed it low voltages) to ensure that the rpm through EVERY Lego pinholes do not exceed 4-5k (which is already very good, considering that a model using the porsche wheels can reach ~77km/h!!!! if you get the weight/power ratio right, and bigger wheels -> even higher speed!!!)... below this, brushless motors won't melt Lego (and btw, gearing down is nothing new - Lego PF motors are all geared down within their packaging). I would have done something similar if the people here were interested, and if my focus wasn't so much on having fun myself .
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Looks like you guys are going from already-quite-small wheels to even more flimsy options... I always thought that if we're relying on sheer engine (*ahem* motor) power, then the lighter the wheels the better... however, if we want to build up momentum and accelerate gradually over a long distance, then larger wheels are better since they'll store energy and keep going with less and less "push".
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[MOC]4x4 offroad pickup
PorkyMonster replied to Toni-23's topic in LEGO Technic, Mindstorms, Model Team and Scale Modeling
Only playable off-roader I've seen... the rest using PF motors are too sloooooooow... alternative to buggy motors will have to be 3rd party motors for such models to be lively. -
Just wondering... how do you guys judge whether the car has reached its top speed? I mean, at low speed you can probably tell that it is still accelerating... but at 200m and already quite fast, what if it can still go even faster further down? Trial and error? Or do you set a fixed max distance just for the sake of comparing? I was playing around using L motors and realised that 10m (in door) is far far far from enough...
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which brings to mind the reality that going light is very much limited by the collective weights of motors, power source, and pieces of Lego plastic... not to mention that lightness must not result in losing traction too often... whereas if you go the other way - adding more motors, more power, etc., the sky's the limit!! ok that's just my view Edit: Of course, I do understand the fun involved in pushing the frontier while limiting resources.
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Agree with your reasoning that there is probably no need for suspension in this case. For the sake of discussion, I suspect that there is close to no spinning (effect wise) probably because (1) there is zero bumps? (quite unlikely), or (2) those bumps you hit were not large enough to impact your car's momentum significantly, such that even if lifted off the ground, the "flight" time is very short, and perhaps coupled with that, (3) your wheels are probably turning at its max capacity already while on the road, so it has no "motivation" to spin any faster when traction is lost momentarily.
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Did you test out that model with springs (gotta be hard springs though...)? Weight gain should be quite negligible... no? I do suspect that at that speed (which is "dangerously fast" relative to its size), tiny bumps (which can be many for a car that small... even on surfaces we think are flat) will send the wheels spinning the in air and lose traction momentarily regardless of weight distribution - and suspension might help to regain all that lost traction and get you to 40 km/h .
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I see two different aspects here - (1) how weight differences of different power sources affect the overall weight and therefore speed, and (2) how different power sources serve out power when the model calls for it (e.g. under stress conditions). I don't think (1) is as useful as (2), since if we know the result of (2), we don't really need to know (1), but not the other way round.
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Some times, weight can be a necessary evil - in the form of more power and better traction. And talking about traction, have you tried doing with suspension?
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When different power sources produce similar top speed under the same condition, it means that condition is not tough enough... so it is time to raise the stake - toughen the condition (as @Marxpek suggested) - gear up, or go up-hill... that'll force more current to be needed, and we'll then see which power source is able to meet the demand in a sustainable way.
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Wonder if you guys feel the same - the steering wheels from Lego are either too small or too big - so for a supercar this size, none of the steering wheels fit naturally...
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Right, that's where my point on "some complications" come into play . Naturally, the spring will gain more compression the closer it is to the wheel, given the same wheel travel. However, because of non-static top end, this gain in compression will be negated to some extent. And in your case, the parallel nature of the wishbones cause a negation that equals to the gain, hence you get the same effective compression. This being the case, your determination of the arm-length based on the image on the right will be meaningless, since there can be infinite different arm-lengths giving the same compression. Instead, the rightful arm-length should be based on the original image on the left, where the top of the spring is static.
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Hmm... had you actually positioned the spring as in the right half of the image, you would have had stiffer suspension... So I think they (the left and the right setups) don't behave the same way. Of course, the fact that the top of the spring in the right image setup is not static adds some complications.
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My personal gut feel is that Lego motors are too weak, so we'll need either smooth flooring, smooth tires (both resulting in low traction), or impossibly tiny and/or light model , in order to drift...
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@Didumos69 and @Erik Leppen, thanks for explaining how this suspension works - I get it now . And tweaking this setup can even allow for stiffness beyond what compressing a shock perpendicularly can give... that's something I've never thought of...
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At 25%-33%, weight is about 700 gm... which came as a surprise to me... I've always thought that spring rates are either linear, or hardens progressively (for variable rate springs). But for this Lego yellow shock, its seems to soften progressively! So I did a google search to find out more... didn't find anything that answers my SHOCK (LOL), but I found this thread by @Doc Brown about 3 years ago, and his results corresponded to mine as far as the yellow shock goes... But bear in mind that all these measurements assume 100% direct/vertical compression.
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Looks like your wheels (especially front) have too much traction . But as @TechnicRCRacer mentioned, a 4WD (hmm... I think AWD better) should give better result... And just for fun, here's a link to a video I found some time back - seems to drift very well! That fellow used two RC motors (sort of a hybrid of AWD and 4WD)... but not buwizz... wheels taped though...
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Hmm... even if 1 stud vertical wheel travel translates into 1 stud shock compression (doesn't look like it though...), I wonder if the fact that the shocks are mounted horizontally weaken the spring strength (lever effects)... and if there is no 1-1 correlation, only the weaker parts of the springs will be used before bottoming out... Now assuming that spring strength is unaffected, and based on the shocks I have currently, I just measured that each yellow shock can take on 1.5 kg before compressing fully - (so 4 x 1.5 = 6 kg!!!) not bad indeed...