nbTMM

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  1. nbTMM

    measuring flexibility/stiffness

    I can see how a bike would benefit from a flexible chassis because when the bike is tilted the motion ratio of the suspension has changed and the suspension is significantly stiffer. Allowing the chassis to flex in the horizontal direction only lowers the effective spring rate when leaning without affecting the suspension operation when the bike is upright. In an off road RC car I can't see this being the case nearly as much because flexible chassis such as those cut from CFRP/GFRP sheet are mostly flexible in the vertical axis and almost ideally stiff in both horizontal axes. The chassis only wants to flex significantly in almost exactly the same axis as the suspension hinges, so it doesn't really change much in the case of the wheel being compressed at an extreme angle such as landing off a jump crooked. The only significant difference is that the chassis will flex across it's entire length/width, effectively acting as a really long suspension arm so it will give more effective camber gain versus the suspension arm pivoting with a softer spring/damper. Dial in more camber gain and a softer spring/damper with a rigid chassis and you achieve a similar spring rate and geometry as the flexible chassis however it is now a properly damped system.
  2. nbTMM

    measuring flexibility/stiffness

    My train of thought is that flex in the chassis acts as part of the suspension however it is an undamped spring which is not good for car control. A rigid chassis allows the wheel movement to be fully controlled by the intended suspension which can be properly damped. If the car seems to drive better with a flexible chassis compared to a rigid chassis then either the suspension spring rate and/or damper rate is too high, or the geometry of the suspension (particularly camber gain) needs tweaking. Lower spring and damper rates, and more camber gain should make a rigid chassis feel more like a flexible chassis however with more control (less bouncy) Same goes for flexible suspension arms, links, strut towers etc - they add undamped springs to the suspension.
  3. If there is a gap then it'll leak and there is always a gap. 1M cst will still leak through a hairline crack, just 1000 times slower than 1K cst, which may make the leak rate acceptable in a poorly sealed diff. Having O-rings and gaskets just reduces the leak rate substantially so thin oils can be used with an acceptable leak rate. There's no such thing as perfectly sealed, especially when outdrives have to rotate inside an o-ring, and hydraulic pressure can force the oil past a flexible seal.
  4. nbTMM

    Slash 4x4 heat on 3s.

    Both esc and motor both have resistive losses which increase with the square of electrical current (if current doubles, heat quadruples). In the motor, the resistance of the windings, in the esc the resistance of the MOSFET transistors. Current is highest at stall, so the slower the motor is turning the hotter BOTH motor and esc get. When you increase the FDR (smaller pinion), the motor rpms increase for the same road speed and acceleration increases so you spend more time at high motor rpms where the current is low, therefore both motor and esc run cooler. Additionally, the esc has switching losses, which occur due to the transistor resistance being at an intermediate value between on and off resistances. Technically if operating at higher RPMs, more switching is occurring therefore there are greater switching losses in the ESC and it heats up a bit more, although I suspect this effect is negligible compared to the current as long as the ESC is designed properly and doesn't spend too much time with the transistors 'half on'.
  5. nbTMM

    Slash 4x4 heat on 3s.

    A low kv motor will have less torque than a high kv motor on the same battery. A lower kv motor will be slower in every way. The only advantages of a lower kv motor is that it runs cooler and doesn't drain the battery as fast, and well... it is slower which is advantageous if you want to crawl or run an entry level racing class. As far as high kv low voltage vs low kv higher voltage, it doesn't make too much difference as long as kv*volts is the same. More important is the overall power output and gearing. Motors have horrible efficiency down low in their rpm range so if you have the car geared for 100kmh unloaded wheel speed, but spend most of the time full throttle at 20kmh the efficiency will be terrible, probably less than 20%. That means that 80% of the power coming from the battery just gets turned in to heat. If you change your gearing so the unloaded wheel speed is only 50kmh then it'll ran way cooler when you're driving at 20kmh due to operating the motor at much higher efficiency, perhaps 60% - now only 40% of power is turned into heat and everything runs much cooler. The sacrifice is in top speed. A high power setup (higher kv, lower turns, more cells battery) will run hotter than a low power setup, so it makes sense to try to operate it at higher efficiency to manage heat. That is why with a high kv / low turns motor, it is usually recommended to run a higher FDR than a low kv / high turns motor. If you want to run a high power setup with a low FDR, either you need to increase the amount of cooling (use a physically larger motor e.g. 550 size motor instead of 540, ESC with lower resistance MOSFETs and/or larger heatsink/fan), or just accept that it's not going to be able to run for very long before it overheats. Melted as in warped, or just discoloured? If the former, it sounds like something has gone horribly wrong. Plastic doesn't melt until >120*C, and I would think the velineon esc should go into an overheat protection mode before it gets to that. Perhaps your motor and/or ESC have been damaged.
  6. Ignore them. They are just looking for a reaction. If you give them nothing they will get bored and find something/someone else to amuse themselves. They are probably killing time in the street because their druggy parents haven't provided them enough resources to amuse themselves at home.
  7. Lesson learned: never close a paypal dispute unless the dispute has actually been resolved. If the seller wants to compensate you outside of paypal, then they have to accept the risk of not being able to prove that they did anything and the buyer fraudulently gets a full refund via paypal in addition to a store credit etc. If the seller doesn't like that, their only choice is to refund via paypal or do nothing, which results in you escalating the dispute and getting your money back anyway. If they aren't willing to accept that as a solution they shouldn't offer paypal as a payment method. That said, I've used Hobbyking many times and never had an issue. I see their site has a dispute system, they probably want you to use that as their 1st tier customer support staff probably don't have access to their paypal account. I assume that they had good intentions telling you to close the paypal dispute as they wanted to handle it via their own internal system, but incompetence has resulted in no further action being taken. I would continue bugging them via chat or email and if you get no where contact paypal and get them to reopen the dispute. For paypal transactions outside of ebay, I personally try to contact the seller about the issue via the contact details on their site (contact form, email, live chat) first. If I get no where with that, I'll then open a paypal dispute. If they promise a solution outside of paypal (e.g. store credit or send a replacement) I'll give them a reasonable amount of time for that to arrive and if I don't receive anything the paypal dispute gets escalated. If they request that I close the dispute before they send me compensation I'll refuse. The worst that can happen is that the dispute becomes a stalemate and you wait the requisite amount of time to escalate it to paypal, who will issue a full refund to the buyer approximately 99% of the time if you have proof that the item was damaged/incomplete or the seller can't prove that they sent anything (no tracking).
  8. nbTMM

    Tamiya Chassis Lipo compatibility

    3.4V is over 95% discharged, I wouldn't worry about seeing and imbalance below that voltage. A lot of new packs would be lucky to have the cell capacity matched within 5% of each other. From the literature I've seen, internal resistance increases at about the same rate in % as capacity decreases, so if you aren't seeing a significant decrease in capacity, I doubt the pack has increased in internal resistance appreciably. Given that LiPo internal resistance is incredibly low to begin with, I doubt you'd notice any difference in punch if the internal resistance increased by even 50% unless you have a really powerful system (perhaps 3S+, <10T brushless), unless you're putting it head to head against identical cars with new batteries. Not saying it's not possible, just that I was under the impression that you'd notice a significant decrease in capacity first.
  9. nbTMM

    Old NiCd batteries

    If they still perform satisfactorily then just keep using them. NiCD and NiMH is pretty safe, so pretty much the worst that can happen is they leak and you have some electrolyte to clean up (just wash your hands after / don't eat it) before tossing them.
  10. Remove the hardcase or shrink wrap (with caution!). If the pack is not fixable, but you can still temporarily attach some cables (alligator clips?) then hook it up to a load to drain it slowly. I like the idea of a car headlight bulb. If attaching cables isn't possible, put a big nail on the end of a long piece of wood and pierce the cells from a distance. Stand back and watch the fireworks.
  11. nbTMM

    So, What Have You Done Today?

    Could do, just need to hook up a current clamp to the second channel of my scope. I burnt half a pack bashing on the 30*/1000rpm/10000rpm settings and the motor can hit about 60c, top speed 36kmh. Then I tried 40*/1000rpm/12500rpm for a few minutes and got 41kmh, then 45*/1000rpm/13500rpm for a few minutes which got 42kmh then the ESC went into over temperature protection for the motor (85C) and the can was ~70C. I feel peak power must be in the 30-40* range but 45* went fastest because I'm gear ratio limited, not peak power limited. I'll have to change the spur/pinion to dyno >30* because the rpm goes way up and the speed that the acrylic flywheels rotate makes me uneasy.
  12. nbTMM

    So, What Have You Done Today?

    Received a second Trackstar Gen II 120A esc for my 21.5t car, did some dyno runs and collected some data: Ignore the wiggles (and power spiking back up, lol) in the plots near the max rpm for each run, this is an artifact of me backing off the throttle before the rpm maxxed out fully and became constant, and because I'm doing some curve fitting because the rpm data is a bit noisy. I expected less torque down low when the timing was wound up. Instead, there is almost the same torque down low and significantly more rpm and a bit higher maximum power. This should pick up a decent increase in top speed without sacrificing acceleration. Another thing to consider here is that we do expect the dyno runs to be the same at very low rpm because I'm using the ESC's boost function to try to add a static amount of timing. I set the boost start rpm to minimum values; start 1000rpm end 3000rpm; so even in the '30*' run the ESC is starting at 0* then ramping up to 30* between 1000 and 3000rpm then holding 30* from there. I'm not actually sure what the bell timing is on the motor because the Trackstar software was bugging out on me trying to measure the bell timing, I would hazard a guess that it is 25-30*, so perhaps the actual timing at the motor is more like 30*, 40* and 60*. Maybe the effect of higher torque at low timing/rpm would be more dramatic if I used a motor with an adjustable bell that can go down to 0* and did the dyno runs by rotating the bell and keeping the ESC in blinky mode. I think I want to try a Trackstar 21.5t V2 for curiosity's sake, just have to wait for them to come back in stock :). The green traces were with the ESC programmed to boost = 30*, start rpm = 1000, end rpm = 10000. This gets more or less the same power curve as 'constant' 30* boost but it's going to run much more efficiently and cooler when the motor is operating below 10000rpm. Next step is to run the car and see how hot it gets driving around with those settings.
  13. Standing up the shocks stiffens and reduces suspension stroke. Laying them down makes the suspension softer and gives more stroke - for the same stroke of the suspension arm the shock isn't compressed as much because some of the motion of the suspension arm just pivots the shock instead of compressing it. Imagine the most extreme scenario of the shock mounted fully laid down, with the 'upper' shock mounting point at the same spot as the lower suspension arm mounting point on the chassis (ignore the fact that in reality it is probably too long to actually do this) - now the shock won't compress at all when the suspension moves, it'll just pivot and the suspension arm flops around freely as if there were no shock at all.
  14. nbTMM

    So, What Have You Done Today?

    I built up a pretty decent electronics 'lab' while studying electrical engineering. I get my fix of electronics at my day job so now I tend to tinker more towards the mechanical side at home - RC cars and building speakers/audio
  15. nbTMM

    Turbofans for wheels - Talk to me!

    The fans have plastic clips on the back that match the shape of the fifteen52 spoke design: https://images.amain.com/images/large/hpi/hpi114637_1.jpg I guess you could cut the tabs on the fan and glue them to any wheel you want, perhaps even drill some small holes in the fan so they can be attached with some fine gauge wire and avoid needing glue.