Medusa 36-80 or 36-70

Ok, I have seen some people running the 70mm in the EMaxx with no problem. I am wanting some advise on what I might need. I have the 3905 with MMM ESC, aluminum bulks, RPM arms, Moab 40 series tires, Axial beadlocks and hope to get the FLM V2 chassis soon. I am running 3S 5000mah Zippy Lipos and will also be running 4S. All I do is bash, my delima is that the 80mm (2200kv) is on backorder and the 70mm (2300kv) is in stock. I want to go about 40-45mph (that is fast enough). So, If anyone has a recommedation I would greatly appericate it.

Tim

[MetalMan]

The 70mm will be plenty for an E-Maxx, and the 80mm will only ruin diffs even faster

What about temps??? Does the 70mm run fairly cool??? I live in Texas and it gets pretty warm here sometimes.

[MetalMan]

It will run just fine, unless there is binding in the drivetrain (which is bad for any motor).

Ok, thanks. One more question.....what about the weight, if I remember right with the Moabs it weighs about 13lbs still ok???? With the stockers it is just a hair over 11lbs.

Tim

[MetalMan]

If it's not geared for any faster than 40mph, it should run great. But your truck will be a handful at that speed with those tires anyways. I'd be more worried about the diffs

Ok, I thank you for the input.

[BrianG]

If it doesn't really matter to you, get the 80mm. The extra length will just keep it cooler. And if it's too powerful, you can always turn down the power. Better to have it and not need it, than to need it and not have it.

Originally Posted by BrianG

If it doesn't really matter to you, get the 80mm. The extra length will just keep it cooler. And if it's too powerful, you can always turn down the power. Better to have it and not need it, than to need it and not have it.

Do you think that the 2000kv would be sufficent on 3-4S???? Because that is all I can find in the 80mm no one seems to have the 2200kv and I called Medusa and they said that it could be 2 or more weeks I guess I could always wait, but man going 2 or even 3 weeks with out running that would be tough......I know I could always gear up on the lower kv but then I might run into some heat issues, right???

[BrianG]

2000 kv will be perfectly fine. If you gear up too much, yeah, there will be heat issues, but more on the ESC than the motor. But, I would go no lower than 4s on the 2000kv motor. If you think you'll be running 3s, I would get a higher kv motor around 2600-3300. Really, you shouldn't be running 3s on anything like a MT or Truggy (even buggy is pushing it IMO) because the current and inefficiency will be high.

General rule of thumb, go with as high voltage as the ESC you have supports, then select the motor kv based on that, then select your motor size by what vehicle it is going in, then gear for the speed you want.

Alright, I can make a harness and run my 3S in series to get 6S but that would only give me 5000mah, basically cut my runtime in half. Now say I do this what kv would be good for running 4S and 6S??? the 2000kv or lower than that and change gearing to make up for the change in batteries???

I like the long runtimes that I am getting now, that is why I was looking at running only 3-4S with the 36-70-2300.

[BrianG]

It wouldn't cut runtime in half. Actually, you'll probably gain runtime. It takes X amount of power to launch and run your vehicle the way you want. And power is volts * amps. Whether this power is developed with low V and high A, or high V and low A makes no difference, except low currents are more efficient (less heat).

So, if you find you need 350w average power to run like you want (roughly typical for a truggy), with 3s that's 31.5A, which equates to around 19 minutes of runtime (assuming 5Ah * 2). If you go with 6s at 5Ah, that's 15.76A, which equates to the same 19 minutes of runtime. So, same runtime, but the lower current of the 6s setup will be more efficient, which means less heat. Of course, the motor kv will be different.

A good kv for 6s is somewhere between 1600 and 2000. Just gear up or down to get the desired speed (down if using 2000kv, up if using 1600kv).

Thank you for the info. I should have specified that I am running my 3S in parallel and have a total of 10000mah, but I am sure the same ohm's law principles apply. So, 2000kv would be good??? I really dont think I would be to happy with 1600kv.

[BrianG]

I figured they would be in parallel at 3s. The middle paragrah in my last post compared the runtime for a 3s parallel config and a 6s series config. Same batteries, different hookup. And yeah, the ohms law still applies.

1600kv on 6s would be 35000 rpm, right in the sweet spot. And as long as you're looking for 40-45mph speeds, it would be perfect IMO. You could go with the 2000kv too, just gear down, but 2000kv on 6s would be almost 45k rpm, which is higher than most people like to run.

Originally Posted by BrianG

I figured they would be in parallel at 3s. The middle paragrah in my last post compared the runtime for a 3s parallel config and a 6s series config. Same batteries, different hookup. And yeah, the ohms law still applies.

1600kv on 6s would be 35000 rpm, right in the sweet spot. And as long as you're looking for 40-45mph speeds, it would be perfect IMO. You could go with the 2000kv too, just gear down, but 2000kv on 6s would be almost 45k rpm, which is higher than most people like to run.

Which would equate to more (higher) temps. Hmmm well I am courious as to why the Castle Neu is 2200kv and most people will run 4S (32000rpm), it seems to me that castle would offer it in a lower kv so we all could take advantage of the 6S rating on the speed control.