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Running high-speed but cool

smirkracing

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EB410, EB48, ET48
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Hi all, didn’t get enough detail over on RCTech so thought I would post here since the motor I am selecting is for the EB48 2.0.


I am running a 6s pack in my 8th scale buggy (bashing, not racing) and I could use some advice about which motor to select.

My current choices are either 1800kv, or 1300kv. Other than the kv rating, the motors are identical (same mfg, same length and diameter, same weight, etc). Both motors have the same max RPM rating (50k).

With a 12 tooth pinion, the 1800kv motor gives me the top speed I'm looking to achieve, which is about 45mph. However, the motor is running quite hot.

If I bought the 1300kv motor, I could increase the pinion to a 16 tooth pinion and I'd get the same top speed (more or less, at least in theory).

The big difference I see is that the 1800kv motor spends a lot of its time at about 90% of max RPM (about 45k rpm, again at least in theory) whereas the 1300kv will spend most of its time closer to 65% of its max RPM (theoretically). For the sake of discussion lets assume running on wide open spaces with mostly wide open throttle.

Any advice on this? Which will run cooler (which is my primary concern)? I can (and will) use cooling fins and fans later, but for now I just want to get the motor selection right.

Thanks in advance.
 

Billl DeLong

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What you are asking for is information that would otherwise be made available from a dyno which will answer all of your questions:



If your only goal is to run 45mph, then why try to reinvent the wheel? There are plenty of setup sheets posted by pro drivers who have already gone through the painstaking effort that you are re-hashing.
 

smirkracing

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Tekno RC's
EB410, EB48, ET48
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Basher
Bill, thanks. I am finding that what people post for racing (usually very limited in time, only a small amount of time on full throttle) does not map to bashing around on open fields. I spend a lot of time on WOT.
 

Billl DeLong

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Racer
I started out as a basher myself and slowly migrated over to a racer. I can assure you that racing off-road is by far the greatest amount of stress you can place on your electronics. Hard acceleration on corner-exit pulls more current and generates more heat that maintaining full speed on an open field. Add more heat/stress when accelerating up the ramp of a jump! Add more heat to a well prepped track with water which adds traction which in turn increases heat more so.

Many bashers find that they have opportunity to gear far more aggressive than a racer's setup because they aren't putting near the stress on the electronics that is put on a prepped race track.

Rule of thumb is to increase voltage to improve efficiency, so running 6S is definitely the right idea. For speeds of 45mph, then going 1300KV will be sufficient.

I used to race a GT8e many years ago on 6S with a 4268-2350KV motor, and my speeds were in the 60mph+ range on a prepped parking lot track at the local HobbyTownUSA in my area. I would eventually settle on 5S being the sweet spot because 6S caused the car the spin out on hard acceleration for the higher KV motor. My motor temps averaged about 160° after a 5 minute race.

I highly recommend you read the following thread:
What gearing for upgraded electronics?

Good Luck!
 

smirkracing

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Tekno RC's
EB410, EB48, ET48
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Basher
Bill, thanks. I hadn't realized that racing conditions were actually more stressful on the electronics than bashing.

I have glanced through the thread you linked - very helpful.

I guess to bring this down to a single question, it would be this:
Should I combine battery+motor kv+gearing such that the vehicle reaches at 90% of max RPM at top speed, or only reaches about 60% of max RPM at top speed? (the top speed will be roughly the same in both cases, and the exact number shouldn't matter much within reason - it is all about what RPM the motor will reach when the vehicle hits top speed).

Thanks!
 

Billl DeLong

Well-known member
TF Excellence Award
Supporting Member
Build Thread Contributor
Messages
629
Reaction score
535
Location
Austin, TX
Tekno RC's
EB410, EB48, ET48, ET410, MT410, SCT410
Driving Style
Racer
Your question is VERY complicated that nobody can possibly answer without a DYNO, which is what I mentioned above in post #2.

The issue is you are wanting to know the "efficiency" of a motor but you are including multiple variables such as voltage and gearing which in turn will effect the efficiency.

It's entirely possible that you could get 2 different KV motors with different gearing and different voltage to get the same efficiency, though this is unlikely.

I welcome you to read the following experiment I recently performed in my my quest to improve efficiency while not giving up any speed:

(Battery-Can Size-Motor KV-Pinion)
Original Setup: 2S-3660-4300KV-15T --> 160°F
Current Setup: 3S-3660-2500KV-18T --> 135°F

https://www.teknoforums.com/threads/build-review-sct410-3.1389/post-14872
 

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