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  1. #1
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    motor KV higher or lower?

    what's better a 3500kv or a 5600kv? witch would be faster the 56 or?
    thanks

  2. #2
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    The more the better, 5600kv.
    To upgrade or not to upgrade,is that even a question?

  3. #3
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    If you're going for speed runs, higher kv is better. However, the higher the kv, the fewer LiPo cells you can run. This is because BL motors have a redline. For instance, a Neu 1515 motor can only spin to 60,000 rpm. The rotors are balanced for a certain rpm and beyond that, you risk damaging them.

    Back to lower vs. higher kv. The lower the kv, the more torque that motor will produce. Higher kv is less torque, but higher top speed.
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  4. #4
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    thanks..

  5. #5
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    Quote Originally Posted by Rem.308
    The more the better, 5600kv.
    Have to consider gearing too along with the Kv choice.
    ------
    hmmm, I've never seen it spelled that way.

  6. #6
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    And heat!!!!
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  7. #7
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    What batterys will you be running?
    If i was running 3s Lipo i'd go for the 3500kv
    If i was running 2s Lipo or 7 cell NiMH packs i'd go for the 5700kv

  8. #8
    RC Turnbuckle Jr. TAT2's Avatar
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    Quote Originally Posted by SavagePede
    If you're going for speed runs, higher kv is better.
    Only to a point. IMHO, a lower kv motor is better suited for speed runs because you can run more power through it, which, in the end, will result in higher speeds. A high kv motor is limited in the number of cells it can tolerate whereas a low kv motor will be able to handle way more voltage without overheating. More voltage = more speed. You can also typically gear a lower kv motor higher than you can a high kv motor. But don't take my word for it; instead take a look at the motors and gearing the speed guys are running--they're almost always low kv setups, geared VERY high.

    Quote Originally Posted by SavagePede
    Back to lower vs. higher kv. The lower the kv, the more torque that motor will produce. Higher kv is less torque, but higher top speed.
    Not entirely true. Brushless motors don't work like brushed motors do in this regard, where lower turns equals more torque. I've posted this before, but it's worth posting again: "A high power capable brushless motor in electrical engineering and physics terms, has unlimited torque. We live in “the real world” so technically for us that’s not totally true, but – a brushed motor has a torque level that due to its design has an upper limit, regardless of how much power is being applied to it. That limit is low enough that you can see it clearly on an average track. On the other hand, a high power brushless motor’s limit to torque in an RC vehicle is not within the bounds of the motor itself so much, but rather falls on the ability of the battery to deliver current to it. We generally don’t describe these motors in terms of “one has more torque than the other”, but rather “the 7700Kv motor is faster and draws more current than a 5700Kv motor in the same vehicle”. It draws more current, because it’s making the car go faster and doing more work than the 5700 motor is. As long as the batteries used are very good at supplying current without an excess of voltage depression (low internal resistance is good) both motors will appear to have the same torque, even though one is much faster than the other. Battery technology is constantly improving, and the first thing you’ll notice when you use a very good battery pack (or perhaps trying a Lipo pack for the first time) with these systems is a more “punchy” feel when you accelerate. The faster you set up the car to go at full throttle, the more reliant you are on good batteries to flow that current into the motor and maintain acceleration performance. So think of torque as a function of battery capability only." That was pulled directly from Castle's website, in case you wondered.

    Quote Originally Posted by Nitro 21
    witch would be faster the 56 or?
    With equal battery packs the 5600 would be the faster motor. However, since you can run more power though the 3500, ultimately, in terms of top speed, the lower kv motor has a slight edge by virtue of the power it can handle. You can run the 5600 safely on up to 3S, and 4S if you want to push the system to it's limits. The 3500 you could safely run on 4S if you geared it correctly, and you could run up to 5S (which again is pushing the system's limits) if you have the nerve, and the ESC that could handle it.
    A low kv setup is the better all around choice imo. They are much more flexible than high kv motors are. If you want to run off-road, it will happily do that all day long--just adjust your gearing accordingly. If you want to do speed runs; throw in a larger battery pack, gear up, or both, and tighten your chin strap. It's the best of both worlds, really; at least imo--torquey when you need it to be, or fast when that's what's called for.
    Last edited by TAT2; 06-14-2009 at 04:35 PM.
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  9. #9
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    i think i know what your trying to say because my losi 18.5 i gear very high like 24 pinion but my mamba 5700 is wild geared 15 pinion
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    I know this is an old thread, but I'm still having a hard time finding good info on this topic, high vs low kv.
    Here's the issue that I'm having with this. For example, I have a 3800kv motor, when running on 3S lipo, that's 47,880rpm. Now, if I was running say a 1900kv motor on 6S lipo, that's also 47,880rpm. So both motor can hit the same rpm but the low kv motor requires twice the battery power to do so. Which also adds twice the weight in batteries to the car. Of course these #s are based on the theory that 'rpm=voltage x kv' formula.
    So, since castle has thrown out the window the whole 'lower kv=more torque & vice versa', then I'm kinda at a loss here with what's the real reason high speed runners use low kv motors. Since I cat get a higher kv motor to spin at the same rpm.

  11. #11
    RC Qualifier GotNoRice's Avatar
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    Quote Originally Posted by Pirullee74 View Post
    I'm kinda at a loss here with what's the real reason high speed runners use low kv motors. Since I cat get a higher kv motor to spin at the same rpm.
    It's about the amount of power (Watts) sent to the motor. Watts is Volts x Amps. The higher Voltage you run, the less current (Amps) you need to achieve the desired wattage. You would need twice as many amps at 3s compared to 6s to produce the same wattage. ESCs can only handle a certain amount of amps, so at some point, you have to increase the voltage in order to increase the wattage.

    Quote Originally Posted by Pirullee74 View Post
    For example, I have a 3800kv motor, when running on 3S lipo, that's 47,880rpm. Now, if I was running say a 1900kv motor on 6S lipo, that's also 47,880rpm. So both motor can hit the same rpm but the low kv motor requires twice the battery power to do so. Which also adds twice the weight in batteries to the car.
    "Lower KV = more torque" is misleading. Once again it's about the amount of power (wattage) that the motor can handle, it's KV is simply a means to an end. The lower KV allows a motor to work with higher voltages without exceeding it's max RPM. That higher voltage reduces the amount of amps that the ESC has to handle in order to produce that desired wattage. So there is a trend of higher-torque motors being lower KV, but it's an indirect link, not a rule.

  12. #12
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    Quote Originally Posted by Pirullee74 View Post
    I know this is an old thread, but I'm still having a hard time finding good info on this topic, high vs low kv.
    Here's the issue that I'm having with this. For example, I have a 3800kv motor, when running on 3S lipo, that's 47,880rpm. Now, if I was running say a 1900kv motor on 6S lipo, that's also 47,880rpm. So both motor can hit the same rpm but the low kv motor requires twice the battery power to do so. Which also adds twice the weight in batteries to the car. Of course these #s are based on the theory that 'rpm=voltage x kv' formula.
    .
    Heat is another thing to consider, in your example yes the two motors would be spinning the same rpm, however the lower kv motors will be physically larger and able to handle more heat generated. This allows you to gear up the lower kv motor much more than the higher kv motor which would equate to more speed.

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  13. #13
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    Quote Originally Posted by FnFancy View Post
    ...however the lower kv motors will be physically larger and able to handle more heat generated...
    Misleading, not always true. Using Castle creations 1/10 scale 1406 motors as an example. One motor you could get at 4600kv and another at 7700kv, both are physically sized @ Length: 49.5mm/Diameter: 36mm.
    Heat generated is from amount of amps being used/needed. The less the amps you need the less heat generated.

  14. #14
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    Thanks,
    Yes, I didn't even think about the wattage. Yes, higher voltage will give me a higher wattage. I ran the #s & yes, a hobbywing quicrun 2000kv motor for example can push ~2600W/max (yeah baby!! :P) and a hobbywing quicrun 3800kv motor 460W/max.

    So yeah, that's 'slight' difference lol.

    I just wanted to make sure I wasn't gonna blow crazy $ on a motor/ESC combo as I continue to push my rustler to the limits. I'm currently running a 120A Quicrun ESC with a 3800kv castle C. 1410 motor, mod1 gear, 24/36. And the thing is a beast, can barely make it to 1/2 throttle before she spins out of control, may have to dif lock her. I have her on my youtube channel, RC Bustanut.
    Thanks again for taking the time to reply & help

  15. #15
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    Thanks,
    Yeah, I hear ya on that, gotta love the heat factor lol.

    So far, I'm pushing a mod1 gear setup 24/34 (1.5 ratio) on my CC 3800kv 1410 motor, it is running super cool. I ran a couple 3S in parallel fully, a 2S & then another 2S all back to back & the motor & ESC didn't get even warm. I was seriously shocked. I thought that at that gear ratio I would heat up for sure. I was doing speed runs on a pretty large school parking lot, got top speeds of 48mph cuz I couldn't get past 1/2 throttle or she would spin out.

  16. #16
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    This topic should include reference to motor winding configuration, can size, gearing, etc., not just the obligatory kv=rpm per volt. Holmes will tell you, scientifically, can size is the best way to reduce heat, because they are simply more powerful. Look TP, they’re silly long but have insane output.


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  17. #17
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    Replying to the pack size question above... Watt hours is a factor in selecting lipo size too.

    A 2S 5000mAh lipo is 37Wh (7.4V x 5Ah)
    While a 3S 3300mAh lipo is the same at 36.63Wh (11.1V x 3.3Ah)

    This mostly balances out pack size for similar system wattages. Again, the higher voltage system will be drawing less current for the same wattage.

    If both lipos above are connected to power systems that produce the same wattage, they should also have a similar run time. (Even though the 3S is smaller in mAh it is the same in Wh).

  18. #18
    RC Champion FnFancy's Avatar
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    Quote Originally Posted by grizzly03 View Post
    Misleading, not always true. Using Castle creations 1/10 scale 1406 motors as an example. One motor you could get at 4600kv and another at 7700kv, both are physically sized @ Length: 49.5mm/Diameter: 36mm.
    Heat generated is from amount of amps being used/needed. The less the amps you need the less heat generated.
    This is true, however the OP used an example that would most definitely be different can sizes. I suppose I should have mentioned that.

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  19. #19
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    Quote Originally Posted by FnFancy View Post
    This is true, however the OP used an example that would most definitely be different can sizes. I suppose I should have mentioned that.

    it's alright son, I'll fix it
    Yeah, sorry I probably should have said that my post was a side note to the main conversation.

    There was one obscure question buried in there asking on the size of the battery so just thought to add a bit of info on that too.

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