1350kv...Why did I buy this?
 

Before I damage anything else, I figured I would ask for some advice here. I hate my first post to be "Hi, I did something stupid.." but I have no choice.

My old Novak HV (it's a 6.5, but this was before they labeled them as such) was in my old 14.4 Emaxx. I decided to put it into my new Emaxx 16.8, but the factory heatsink won't let it happen. So, I went with a new Tekin solution, the RX8 combo w/1350kv motor.

My goal here was to make my Emaxx bullet-proof, maintenance free and something that would rarely need to be on the bench. I went with the lowest kv motor so I could gear it just high enough to get okay speeds, long run times and cool running electronics. Well, so far that is not the case at all.

What happens is the truck runs okay for about 2-3 minutes, then there is no power at all. It feels like a stock Emaxx running on a single battery. I am using two 7 cell trinity 4600mah nimh. I have 5 of these batteries and have tried a different set with the same outcome. I have tried a 15t, 17t and 21t pinion, all came out with the same results.

I just got some lipos recently, but haven't tried them yet. My concern is that this motor requires some serious battery power and I don't want to detonate my truck by smoking two Lipo's because I don't understand what this motor wants from me.

I have a new 2250kv sitting on my bench, waiting to be swapped with the 1350kv, but what could I even use this 1350 for? Do I need a 6s? I hear all these stories about people going to lower kv for more torque and control on a smaller track, but have I gone too far?

Thanks!!

That 1350 will definitely need 6s to be responsive! On 6s, thats still only 29970 rpms, which is pretty conservative! You could run that motor on 8s no problems!

And Nimh packs just dont cut it either.

Swapping in that 2250 motor will really wake it up tho! But gear conservatively with those NimH packs otherwise, you'll fry those packs in no time from the massive amp draws. Or use the Lipo's you have and hold on!

According to mike at rc-m, the t8 motors like being driven at lower rpm.


Bump up to 6s lipo... makes all the difference.

Hobbycity.com <<<<<good lipos for dirt cheap

Is anything getting hot after running the 1350kv with the NiMh? What kv is the Novak6.5?

I bet the nimh are hot.... The T8 is gonna want a ton more juice than the novak system..

"My goal here was to make my Emaxx bullet-proof, maintenance free and something that would rarely need to be on the bench."

Welcome to the forum. A bulletproof Maxx is a tall order, especially the 3906. The 3905 is much better in some respects, but about the same in a few other areas.

The 1350 is definitely too low, but you'll have no issues with "overpulling" your lipos with that motor, if I had to guess, that motor will pull a maximum of 50-60 amps, well within almost any decent lipo's discharge capabilities. A bl motor will only use what it 'needs', whereas a brushed motor will try to take whatever you throw at it in most cases.

The 2250 is too high for use on 4S IMO, (not sure what you have for lipos) but proper gearing will make it useable.

Don't feel bad, you can unload whatever you don't end up using here.

See my 3906 mute in my sig for some ideas.

You could still be geared too low.... I run a 1390 12XL with a 24T pinion on my old FLM maxx....

as said above, i would go lipo before anything else!


6s would be a good combo

Thanks for all the responses, I guess I will try the Lipo's first. I am not done with my Lipo bunker yet, but I will give it a shot soon regardless.

Those nimh batteries may be on their last leg anyway, I had 6 of them until one of them blew a cell in those oven-type battery holders in my Erevo. Sorry Traxxas, that was a crappy design. I will run them with the 2250kv in the Emaxx after I try the Lipo's with the 1350kv.

With the 1350kv, the nimh's were getting pretty warm. Since they are getting old, the cells are way out of sync now. Will be breaking them apart soon to cycle each cell and rebuild them as side-by-sides. If the 2250kv hurts them, well, they had a good run.

bruce750i: The Novak 6.5 HV is a 3400kv. It made my old Emaxx fly (standing back flips) and never had any heat issues. I just put the 5mm rotor in it, which was a pain, but it doesn't have a home right now. May go back to my old Emaxx when it is running again.

redshift: The Emaxx I am using is a 3905. I have a 3906 as well, but its in the middle of a never ending project. It doesn't even look like a monster truck anymore. I will take some picks when I get done...whenever that may be. The "bullet-proof" remark was meant to, more or less, just mean a truck I don't have to jack with too often. Just throw in 2 charged batteries, get some modest speed out of it and chase my dogs.

aqwut: I have a roll-cage on it, the motor is sitting in between two plates, I may be able to get a 22 or 23 in there at the most. I may try doing that.

I can´t be sure, but for me looks like a too small pinion. I´d go with a hobbycity 6S 30C lipo and a larger pinion. Ever tried brian´s calculator?

"I am not done with my Lipo bunker yet"

:lol: listen, forget the horror stories. If you have a decent charger, and are charging at the right rate, you have nothing to be concerned about. As soon as you run the lipos you will forget Nimhs ever existed.

"I have a 3906 as well, but its in the middle of a never ending project. It doesn't even look like a monster truck anymore. I will take some picks when I get done...whenever that may be."

Well now that has me (and a few others I'm sure) very interested......

BTW if you need charging info, post up your equipment. :yes:

Havoc,

The rotor in your motor may be unbonding from the shaft. It is a very common problem with Tekin T8 motors and Tekin will send you a new improved rotor ASAP. If you can still hear the motor spinning, but the truck is barely moving, then the rotor is going bad.

+1 if you need to have a battery bunker you are not doing something right.
all the youtube stories are scare tactics. the people with fires and lipo explosions dont know what they are doing. Let us help out.

+1 those people are dead shorting and charging on insane rates on a lead acid battery charger for a long time before anything happens

sabongi: I will take a look at Brian's calculator. Not rdy for 6s yet, the only Lipo I have even used was in a Micro Desert Truck, those little Losi 1/36 scales. Need to get some use out of these 2s's first.

I know the chances of having problems with Lipo's are slim, but you are talking to a person who has the worst consumer luck. I could go on forever with stories about dead/stuck LCD pixels, bad motherboards, beaten-to-death guitar amps courtesy of UPS, etc. Even then, I make very few mistakes when charging, but stupidity can strike anytime.

My main concern is just storing them in a good place when I am not at home. I am paranoid enough about coming home to a kicked in door to have to worry about a pile of ashes where my home use to stand. I have an old gas BBQ pit that I may sand down, repaint, and remove all the gas fixture crap from. It would be perfect for storing and charging, could be a fun project too.

Regardless, I will become a Lipo user this weekend as long as it doesn't rain again. Stupid South Texas, we get a drought in the spring and rain in the fall.

For batteries I have 2 of these:

http://www3.towerhobbies.com/cgi-bin...&I=LXYCB5&P=ML

2 of these:

http://www3.towerhobbies.com/cgi-bin...?&I=LXVHA3&P=7

And 1 of these:

http://www.teamepiconline.com/shop/i...p?item=TEP2000

I am using this charger:

http://www3.towerhobbies.com/cgi-bin...&I=LXPYT8&P=ML

With this balancer, which I haven't opened yet:

http://www3.towerhobbies.com/cgi-bin...&I=LXTSA5&P=ML

I have been using the charger on my nimh's for the last 2 weeks. Its pretty awesome, much better than the Integy and MRC Superbrains I am use to. Wish I would have gotten it a long time ago.

I think I need some adapters for the balancer though, the one included with the balancer is for Checkpoint batteries with mini-banana plugs. All my batteries have the wired balancing plug already. I will research this more tomorrow while I am bored at work.

And redshift, OMG, that poor old Emaxx!! You already win, my old Emaxx will never be that modded, that is pretty awesome. Good job. I am going for a different look, just not sure what it is yet.

GO-RIDE.com: I don't think there is a problem with the rotor, but I could be mistaken. I will see how it does this weekend on Lipo's. Thanks for the info.

You want to power the motor so that it spins in the 30k to 40k rpm range. The higher kV motors tend to like to spin faster (35k to 40k) while the lower kV ones work well spinning a bit slower (28k to 35k). The smaller motors (36x50mm size) will spin even faster (50-60k being ok for them as long as the bearings hold).
With a 1350kV motor, you NEED 6s. You can get it to work ok on 5s, but you NEED 6s if you really want to run that motor in it's sweet spot.
IMO the 2250kV is perfect on 4s. On 4s, it's spinning at 33k rpm...perfect! And your E-Maxx will absolutely fly! Gear it slow (30mph) if you run the 2250kV on NiMH. With 4s LiPo, depending on the discharge capability of your packs (not the C rating, but the actual discharge capability), you should be able to plenty fast.
If you still don't like the 1350kV, I'm looking for one. Let me know if you'd be interested in selling it.

your motor is showing the same symptoms that my de-bonded KB was. once the motor warms up, it slips more. it was like my slipper clutch was loosening up during runs, but i found out it was the motor as it got worse with each run and after talking the motor off the truck, i can make the rotor spin while i hold the shaft still. if that is the case, send it in to teken, but in the meantime, you can use loctite to re-bond the rotor. just be sure to clean both surfaces with rubbing alcohol to get a better bond.

to tell you the truth i store my lipos in the cars. they are not doing anything they sit dormant. if we have a party or something i move them to the fireplace where nothing can touch them or hurt them. or we go on vacation but, it is better to store and charge in a safe place. i charge mine right here next to my cpu. if the charger finds a problem it sends the data to the cpu via fma usb and it beeps like a m-f'er and annoys the living crap out of everybody

Was looking at the link for your packs, seems Tower has put out some bad info.

"NEVER allow to Discharge below 2.5V per cell"

NO. The absolute minimum is 2.8V, and the widely used cutoff is 3.0, for a buffer zone.

Many people (myself included) use 3.2V per cell for cutoff, usually anything below that you will notice the power drop off.

I am using Hyperion Swift Classic 4000 2S 20C packs, which are very similar to the first set of Orions you linked. You'll get 15-18 minutes in a proper setup. Maybe use that for reference until you get a feel for things. The 3800 hardcase packs will be good for 12-15 probably, but will be stronger.

You can charge the 4000's and the 3800's at 4 amps for a 1C charge rate, technically you would charge the 3800's at 3.8 amps, but 4 is close enough. And just so you know I have been charging my Hyperions at 8 amps (2C) for about 9 months now, they take it just fine. Incredibly good packs, nothing but praise for 'em!

Okay, so, the 1350kv can move on 14 nimh cells. It appears that only two of my Trinity batteries are good. I decided to charge both on that Checkpoint charger paying close attention to the discharge and recharge. Both stopped charging at around 2000mah, so I forced them to start again, both were very warm at the completion. With some time to cool, I prepped the Emaxx, had to resolder one of the connections on the ESC.

Anyway, slapped em in, walked outside and WTF!?!? I was riding wheelies, couldn't do a standing backflip, BUT, could get one in a few feet. It was like a different truck. The 1350kv can push this thing just fine, and the temps were great:

motor can = 115 - 125f
esc heatsink = 110 - 115f (fan kicked on once for a few seconds)
batteries = 115 - 128f

I was worried about the possibility of a bad rotor, but it looks like on each test run, one or both of the packs had bad charges due to out of sync cells.

Well, now I have a spare 2250kv I guess, there is no point is switching this out. However, I am considering a complete resolder job with better solder and using 6mm connectors instead of 4mm for the motor connection.

So, it was my crappy batteries, but the right combination of the two least crappiest yielded a good run (finally). I know, I know, ditch the nimh and get the lipos going. I will.

Need to make a few more test runs to make sure everything is solid. I got about 10-12 minutes out on that run, could have been longer except for the newly grown grass from all the stupid rain. Deep grass = power drain...

One other quick question. Am I destroying these nimh batteries by using them with this RX8 1350kv combo? I would think the higher kv motors would cause more wear, but I read different stuff all over the place.

the lower kv will be easier on nimh but they will fry quick. need lipos imo

Actually you are putting both the Nimhs and your ESC at risk, see here-
http://www.rc-monster.com/forum/showthread.php?t=17200

Do keep in mind this is straight from the president of Castle, I know you have a Tekin but this still applies.

There is also a little thing called ripple current, the simplest way I could explain would be a propeller cavitating, the ESC has very sudden gaps where it needs to bite onto the power, but it can't. This can take out your ESC in a hurry.

Not to sound like an @ss, you need to run BL with Lithium, period.

The best thing you can do with your Nimh is ziptie them together and use them for a boat anchor. Stop wasting your time, you have the lipos....:yes:

One other thing, compared to your 7 cell terminal voltage of about 9.6-10 volts each, the lipos may feel slow. Keep in mind any voltage advantage with your Nimhs will quickly burn off. The 2250 is worth holding onto, you may find it is a better match for the lipos. As you see the potential of the lipos you will likely want to bump up the speed.

Remember the main advantage of lipos over any other chemistry is it's voltage holding ability, which spells consistency over a full run. I am very used to my 4S Hyperion setup snapping my truck skyward until cutoff. And it's not a light truck by any means...

So the point is, get a few runs with the lipos. Keep in mind any loss in speed will be made up either by gearing the lower kv motor up, or using your 2250. You have the advantage of having 2 motors to experiment with, that is a good thing.

Hope that helped...