# Kv



## RCDawg83 (Nov 30, 2007)

I know this has probably been discussed many times in the thread, but what are the reasons to pick a higher or lower KV motor? If you know you can only run a ceratain number of LiPo cells in series (say 3) will you get more poop out of a high KV motor? If you can run as many cells as you want, is low KV and lots of cells better than high KV and fewer cells? 

My application is a Losi 8ight-T which I'm going to run brushless. Any suggestions for motor? I'm partial to Novak, but the Excelorin setup looks pretty clean.


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## swtour (Oct 8, 2001)

The easy thing to use Kv for is motor RPM

The Kv works out to the 'unloaded' RPM "per volt" of supplied power.

a 3 cell lipo - 11.1 volts

a Novak 10.5 motor is rated at 4200 Kv (rpm per volt) = 46,600 'unloaded' rpm.

The loaded rpm will vary depending on surface, gearing, etc.

a Novak 3.5 motor is rated at 10,500 Kv so the rpm it would turn w/ no load is like 116,000 RPM

A high number motor like a 21.5 motor is rated at 1800 Kv, so with a 11.1v pack it's closer to 20,000 rpm's. (again, with no load) 

So the motor of choice would depend on if you are looking for bottom end torque, or top end...and what kind of gearing you can achieve.


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## RCDawg83 (Nov 30, 2007)

Thanks. The 'unloaded' part was the piece of the puzzle I was missing. So the big variable is going to be how much energy gets transferred to the wheels under certain conditions and how much gets dumped off as heat from the ESC and the motor can.

Is KV a reasonable way to compare different manufacturerer's motors (Novak vs Mamba vs Excelorin........)? The highest KV Excelorin (2100) in the 1/8 scale series is rated lower than the lowest Novak HV (3000 for the 7.5). NEU/Castle comes in at 2650 for the one they intend to use in 1/8. Does unloaded KV always predict how the motor will perform under load?


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## pup1970 (Apr 2, 2008)

The kv rating determines only the rpm per volt. The size of wire in the motor will determine the performance. I have a mamba 5700 and a vxl rated at 3550. The traxxas has alot more torque than the mamba but I have to run a higher gear ratio to acheive the same speed, so they prett much equal in terms of performance, but the traxxas runs cooler under same driving conditions. The 1/8 motor setups have huge wire size in the motors. they also draw alot more amperage nbut are better suited for the larger scales due to the torque they put out.  trying to run a mamba 5700 on 3slipo will work but it will more than likely thermal or burn up trying to move that much weight.


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## RCDawg83 (Nov 30, 2007)

*Motor Winds*

Thanks fot the input. I'm only considering motors that are in the larger can and designed for 1/8. I'm getting that lower KV and more cells can have comparable output to higher KV and fewer cells. One of the more experienced guys at my club was telling me the same thing today. My buddy was also telling me that running more cells on a lower KV motor keeps the heat down.

What you said about wire size makes sense. My dad (an EE) showed me how to re-wind armatures (with bigger wire) in my slot cars in the 60's. I had a 'hot' motor, but the solder joints in my controller kept melting  Too bad he's not still alive, or he could explain all this stuff to me. 

I'm trying to look ahead and guess what kind of specs are put on 1/8 electric for competition. It seems to me that a battery spec may me the only real way to ensure fair competition. I'm hoping that 1/8 electric will get a big following. It makes sense, given noise issues with tracks and cost. Brushless and LiPo does have a big up-front investment, but at least you're not constantly buying/rebuilding/tuning your motors.


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## ta_man (Apr 10, 2005)

RCDawg83 said:


> I'm trying to look ahead and guess what kind of specs are put on 1/8 electric for competition. It seems to me that a battery spec may me the only real way to ensure fair competition. I'm hoping that 1/8 electric will get a big following. It makes sense, given noise issues with tracks and cost. Brushless and LiPo does have a big up-front investment, but at least you're not constantly buying/rebuilding/tuning your motors.


A battery spec has nothing to do with fair competition, a point that is apparently lost on ROAR, limiting the class to 4S LiPos.

A guy running a 1000KV motor on 8S LiPo has no racing advantage over a guy running a 2000KV motor on 4S LiPo. But the 8S setup runs cooler, has slightly more runtime (because less energy is wasted as heat) and the electronics (assuming properly designed and rated) will last longer because they are not running as hot.


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## RCDawg83 (Nov 30, 2007)

*Specs for Racing*

I understand your point, but I'm thinking economics and operational issues as well as the technology. If you spec a motor characteristic, then you also have to spec the number of cells, or someone with more money to spend buys more cells and wins all the time. ROAR or somebody like them has to do a bunch of testing to see to it that both batteries and motors have equivalency across brands. 

Motors cost the same, so if you simply spec the battery (number of cells and/or capacity) why not leave the racer to choose the most appropriate motor? High dollar tweaking and motor cheating would be irrelevant since anyone could just buy the next higher KV motor. The only thing that ROAR would have to do is certify certain packs as race legal. It would be pretty easy to tell if someone is running too many cells by simply looking at the size of the pack.

Does this make any sense?


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## ta_man (Apr 10, 2005)

RCDawg83 said:


> I understand your point, but I'm thinking economics and operational issues as well as the technology. If you spec a motor characteristic, then you also have to spec the number of cells, or *someone with more money to spend buys more cells and wins all the time*. ROAR or somebody like them has to do a bunch of testing to see to it that both batteries and motors have equivalency across brands.
> 
> Motors cost the same, so if you simply spec the battery (number of cells and/or capacity) why not leave the racer to choose the most appropriate motor? High dollar tweaking and motor cheating would be irrelevant since anyone could just buy the next higher KV motor. The only thing that ROAR would have to do is certify certain packs as race legal. It would be pretty easy to tell if someone is running too many cells by simply looking at the size of the pack.
> 
> *Does this make any sense?*


No, it does not make sense. Once you get to a certain point (and 1/8th scale cars on 4S already exceed that point) off-road racing isn't about straightaway speed and more cells won't buy you a win.

The problem is that people making these rules and making these statements haven't driven the cars in races. I've run a whole season with a Neu 2.5D motor on 4S. Another guy ran most of the season with a Neu 1.5D on 4S. He always beat me but it was on driving. We were both going down the straightaway at the same speed. And guess what? He is switching to a 2.5D next year and will probably beat me by more because he won't have to contend with having too much power for the track.


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## RCDawg83 (Nov 30, 2007)

ta_man said:


> No, it does not make sense. Once you get to a certain point (and 1/8th scale cars on 4S already exceed that point) off-road racing isn't about straightaway speed and more cells won't buy you a win.
> 
> The problem is that people making these rules and making these statements haven't driven the cars in races. I've run a whole season with a Neu 2.5D motor on 4S. Another guy ran most of the season with a Neu 1.5D on 4S. He always beat me but it was on driving. We were both going down the straightaway at the same speed. And guess what? He is switching to a 2.5D next year and will probably beat me by more because he won't have to contend with having too much power for the track.


Well that was my point. Although I'll admit the part about more cells buying a win didn't make any sense. Leaving motor choice up to the driver lets the more experienced people who understand what you just said make good choices about matching the motor to the track, and less experienced people choose too powerful a motor and learn (hopefully) for next time.

But given that, what does make sense? At a high level of competition, your mains are going to be long enough that battery capacity will become an issue, hence the case for a battery spec. At club racing level (5-10 minute mains), maybe any spec at all is irrelevant.


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## ta_man (Apr 10, 2005)

RCDawg83 said:


> But given that, what does make sense? At a high level of competition, your mains are going to be long enough that battery capacity will become an issue, hence the case for a battery spec. At club racing level (5-10 minute mains), maybe any spec at all is irrelevant.


What high level of competition will have the capacity be an issue? Mains long enough for battery capacity to be an issue are a nitro "thing". Electric is a different class.

Experienced people that run these things know it doesn't make sense to run the packs down and ruin them. If you tell them the race is 30 minutes and no battery swap, they just won't bother to enter. If the race is 30 minutes with a battery swap, no problem, and you don't need lots of $$$ for batteries to run like that. But most guys want to run around 10-15 minutes, which eliminates the need for a battery swap and can be done with economical batteries.

If the pros want one-run batteries, let them have them. I can run near 20 minutes on my SMC 5000s, over 20 on my MaxAmps 6000, and the guy with the 1.5D got 17 minutes on SMC 5000s.


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## RCDawg83 (Nov 30, 2007)

ta_man said:


> What high level of competition will have the capacity be an issue? Mains long enough for battery capacity to be an issue are a nitro "thing". Electric is a different class.
> 
> Experienced people that run these things know it doesn't make sense to run the packs down and ruin them. If you tell them the race is 30 minutes and no battery swap, they just won't bother to enter. If the race is 30 minutes with a battery swap, no problem, and you don't need lots of $$$ for batteries to run like that. But most guys want to run around 10-15 minutes, which eliminates the need for a battery swap and can be done with economical batteries.
> 
> If the pros want one-run batteries, let them have them. I can run near 20 minutes on my SMC 5000s, over 20 on my MaxAmps 6000, and the guy with the 1.5D got 17 minutes on SMC 5000s.


You made the point to me. I can't disagree with any of the above. My original head-scratcher is this. Say several guys start showing up at the first of dirt season with electric 1/8 scale buggys/truggys and monster trucks. Is there a fun, fair, and enforceable way for them to run with nitro guys? If there are enough electrics to run together, same question. 

The reason I think this is an important issue is that a) Lots of 1/8 electric stuff is coming out, b) Some guys still just love nitro, and c) In the current tough economy, small clubs are going to have a tough time keeping car counts up this year. Anything that could be done to help the guys who do show up to have fun and fair competition would be good. Personally, I don't care that much what the pro's do.

Thanks for your input :thumbsup:


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## hankster (Jan 1, 1998)

Locally if there is not enough electrics to make a class, they can run with the nitro guys and are allowed to change batteries if needed. At the local track nitro mains are either 20 or 30 minutes. If the mains are 20 minutes, most of the electric guys will just drive easier and not change batteries, if 30 minutes they just go as hard as they can and change the batteries once. Either way, in the end, driving skill determends the winner, not what "fuel" is being used.

At this point the battery/motor rules are open. There doesn't appear to be much advantage to one combo over the other on the track.


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