KV and runtimes

[mjderstine]

hey guys, I wanted to ask a question to clear this up in my own head, and since you have all been doing this for a long time, i thought you could put some of this in layman's terms for me.

(I am using fictional numbers, because its easiest, and i hope that will stop people from just correcting my numbers, and not answering the questions)

Say i have a 4c 5mah lipo, around 30C for starters.
my speed goal is around 37-40 mph. (what i think is standard for a 1/8th buggy)
I have the option to buy a 36-70 medusa motor but which KV do the best for runtimes? (that is my main question)

2300, 2000, or 1600. i assume gearing will be able to be adjusted for each motor to result in around the same final speed on each.
will gearing for the same speed on all the motors overstress any of them with only using a 4cell lipo?
would you think the ESC may get over stressed if the 1600 was geared to the same speed as a 2300, (approx. 37-40mph)?
keep in mind that i am planning to use a MM, not a MMM. (i have a MM on hand already)
and then will the runtime be different for each motor geared at similar speeds. i was under the impression that a lower 1600KV motor is more efficient, thus allowing for longer run-times, but is there a trade-off if i gear for the same stock speed at a 2300 motor?

i would go with the 2000

[hootie7159]

Lower kv+ higher voltage = longer runtimes and more efficiency (less heat).... But that depends on your gearing as well...I'm looking to do this exact same thing, I currently run an rc8t on 4s 30c 5000 mah flightmax on the 2200kv MMM combo (42mph on large track 15-20 min run time and ok temps...I have to have a cool down period after back to back packs) but I'm switching to 5s 4500 mah lipos and 1700kv motor for longer runtimes and less heat...but this is all because of the possibility of having 20min+ qual's and mains......

[mjderstine]

okay, that is a great answer. thanks,

one limitation i have is that my track limits racing to hardcase packs. and i do not know good company that is making hardcase 5 cell packs yet. SMC will have their 4 cells soon, but would that limit me to the 2300 motor? or would i be able to gear a 2000 up for good runtimes 20+

[_paralyzed_]

if you must run that large of a motor on an MM you are close to or at its limit. the 1600 will tax your MM the least, and go slow as possible on gearing.(30 is an acceptable top speed on a tight track) Also, getting that now will allow you to run 6s when you upgrade to an MMM.

btw: for a four cell pack we say, "4s" not "4c'. It stands for four cells in Series, s for series

[mjderstine]

thanks, i always wondered why you guys put 4s when cells clearly starts with a 'c' whoops.

i would say my track is pretty tight. the straight cant be more than 80-100 feet max, its a quick blip of the throttle and its over. so perhaps 30mph would be fine. and as you say, upgrades in the future always tend to happen...
these are the numbers i got on 16 pinion. probably not a bad place to start.

Differential Ratio: 3.3076923076923075
Transmission Ratio: 1
Other Ratio: 1
Spur Tooth Count: 46
Pinion Tooth Count: 16
Total Voltage: 14.8
Motor KV: 1600
Tire Diameter (inches): 4.1
Tire Ballooning (inches): 0
Motor Current Draw: 0
Motor coil Ω: 0.007
Spur/Pinion Ratio: 2.88 : 1
Total Ratio: 9.50962 : 1
Tire Circumference (inches): 12.88 inches (327.17 mm)
Total Motor Speed: 23680 RPM
Vehicle Speed: 30.37 mph (48.79 km/h)
Effective KV Value: 1600
KT constant: 0.85 oz-in/A
Motor Torque: Amperage not specified...
Final Torque: Amperage not specified...
Final Power: Amperage not specified...

[BrianG]

There should be a sticky about this type of topic since I know this has been covered numerous times, but here goes:

For a given voltage and speed, it does not matter if you use high kv and gear down or use low kv and gear up; the amount of work being done is the same. Low kv motors geared up will pull roughly the same current as a high kv geared down.

The advantage to using low kv is you have a little headroom to run with higher voltage if you want to go faster. If extended runtime is the goal, use higher voltage and lower gearing. Every motor has an efficiency curve, and most of the good models are pretty good all the way up to 50k. GriffinRU's program demonstrates this.

The advantage to using higher kv and low gearing is that there are more rpms per mph, so at really slow speeds, the chance of cogging is reduced since the ESC can get more back-EMF pulses for efficient commutation. You may be physically limited by the size/availablitity of the spur/pinion too.

There are limits to each setup. You can gear up a low kv system so much that the batteries and ESC run hot trying to deliver the acceleration current. You can gear down a high kv system so much that the motor is running over its peak efficiency.

Personally, I prefer higher kv and lower gearing for lots of start/stop style of driving. Starts and acceleration seems smoother. I shoot for ~40k-45k rpm on the good motors (Medusa/Neu). If I was doing just speed runs, I would probably go the other route since starts aren't as important.

BTW: mjderstine, I'm not trying to be short with you, I know sometimes it's hard to search when you don't know the exact search phrasing...

[mjderstine]

brianG: no need for the apology, i have been on this site long enough that i realize this info is here somewhere, weeding through the searches was making my eyes cross. maybe we can get a sticky on this thread so that its easier to find!

i understand the goods and bads about each one, both high and low KV motors will have their specific applications. i suppose the best thing for me is to just buy something, and start my own testing.

my track is a pretty small windy track=, with lots of humps and bumps to keep the speeds relatively slow. slow enough that stock slashes are about equal when driven correctly (except for the straights).

i can then deduce from your statement

The advantage to using higher kv and low gearing is that there are more rpms per mph, so at really slow speeds, the chance of cogging is reduced since the ESC can get more back-EMF pulses for efficient commutation. You may be physically limited by the size/availability of the spur/pinion too.

that since my track is rather small and windy, a higher KV motor might have less change of cogging at low speeds.
but what are these speeds? under 5 mph? because i am sure there will be lots of wheel spin that can limit the cogging effect.

i am still coming down to be ing torn between the 3 motors. the 2300 will work best for a standard 4s pack but may stress the ESC too much, MM.

1600 may be limited by the physical limitations of the spur and pinion to gear it to a higher speed, and may be prone to more cogging.

the 2000 is in the middle of the road, and this may seem like the best bet, but the 1600 also has the added advantage of being able to perform at much 6s levels "better" than the other two higher KV motors.

maybe i should just buy all 3...

[BrianG]

Yeah, I'm talking under 5mph mostly. Like I said before, a 2300 motor geared for the same speed as a 1900kv (same voltage) will not work the ESC any harder.

If the purchase decision was mine:

- If I thought I might use 5s once in a while, I'd go with the 2000kv model.
- If I never will go beyond 4s, I'd pick the 2300.
- I probably wouldn't pick the 1600kv at all unless I was planning to run 6s all the time.

But that's just me...

[mjderstine]

well i solved my dilema and just went with the MMM 2650. no worries now.

thanks for all your help.

[rootar]

did you get the combo?

[mjderstine]

i ordered it from Mike, but i had made two orders, asked to get some things changed around, long story short, i think i confused mike. havent gotten word on shipping yet, but im waiting to see that he finalized my order.

[littlegiant]

Originally Posted by BrianG

The advantage to using low kv is you have a little headroom to run with higher voltage if you want to go faster. If extended runtime is the goal, use higher voltage and lower gearing. Every motor has an efficiency curve, and most of the good models are pretty good all the way up to 50k. GriffinRU's program demonstrates this.

U forgot to mention that heat loss from I^2*R is lesser in a low KV high voltage system since the voltage makes up a higher part of the wattage as compared to higher kv systems. :D And i totally agree with u on that high KV systems run smoother.

[BrianG]

Yeah, but it's usually not that much in practicality. High kv motors, having fewer turns, have less resistance. So, current is higher, but resistance is lower. There are more losses to be sure, but it's not that much.

Take a monster truck; judging by the average runtimes people get, the average power required is around 300w.

Let's say we are using a 1515/1y (kv 2200) on 4s. With that 300w average power figure, the average current is around 20A. Losses are around 2.4w, which is 0.8%.

Same vehicle and average power, but with the 1515/2.5d (kv 1650) on 6s will have an average current around 13.5A. Losses would be around 1.8w, which is 0.6%.

See? Not really enough to consider IMO. Yeah, bursts currents/power will be higher, but those are short in duration. Even so, the percentage of losses will be in about the same proportion.

But we aren't really talking about high kv/low V vs low kv/high V comparisons really. The question seems to be: Given a specific voltage, what is better; high kv geared down or low kv geared up. But really, the end is the same.

[lincpimp]

Also worth some mention is vehicle weight and tire size...

I have found that most 4s combos are just not capable of handling a large MT with big heavy tires... Just going up to 5s makes a huge difference. Seems like that knocks off about 20amps draw, and makes the batts and esc alot happier. In an 1/8 buggy it does not really matter but in the heavier vehicle it does. Plus it all depends on battery quality too. If you plan to run a 30+c lipo you may get away with 4s, but going up to 5s will make it easier on the battery.

I do agree that the 2000kv motor will give good performance on 4-5s and if you are gearing for around 35mph it will run cool and produce great power on either voltage.