motor temps

[tonka truck]

Question for anyone that answer. Just been having great fun at the local bmx track for about 20min, but forgot to check temps of lehner 4200 (basic) when I came in v. hot only able to touch end for 2 sec and actual end shaft that buts out was v. v. hot.

problem once off? what do high temps do to the motor? and gearing it down?? at present 17/88 or just reduce EPA on Tx so that not as much top speed?

thanks

[Re~Mix]

Thats odd, I run 19/86 without heat problems pretty much, unless i run it really hard in all grass, but even then i can still touch it for about 5 secs.

Sometimes gearing up actually reduces temperatures, but i'll leave mike to this one.

[Batfish]

I used to run the Lehner basix xl4200 in my XXX-T MF, geared 19/86.
One thing to remember is that everyone is a little different when it comes to the 'touch' test for heat. You're much better off getting a low-cost IR temperature gauge for more accurate (and less-blistering) readings. OFNA, Duratrax, and a few other companies make these deviced. You should check with Mike and see if he can get one for you.
When I first ran my XXX-T with the xl4200, I had it geared 15/86 and the motor topped out at 190F, which was too warm for my liking, and burning to the touch. Once I geared to 19/86, the motor topped out at 165F, which is much more acceptable, but felt no cooler to the touch than 190F. Either one would burn my finger red in less than 3 seconds.
When the temps get that high, it's hard to differentiate between another 10 or 20 degrees.
I always strive to keep the motor temps below 160F. Most posts you will find about maximum temps will state that the motor innards will start to fail at 220F.

Go w/ Batfish on this one and purchase an IR temp gauge (should be around $25). As they aren't 100% accurate you should take a couple readings and use the average.

If you're running the gearing that you want but still need it to run cooler I suggest a DC fan (computer case fan, I use an 80mm fan (others use smaller fans)), thermal grease (I use Artic Silver 5), and a heatsink.

Prices are approximated
$25 temp gauge
$25 heatsink
$15 fan
$9 thermal grease

I was hitting somewhere north of 170f before adding the aforementioned items, now my truck runs 125-140f (140f when batteries are drained)...even when ambient temps are 90f+.

Keeping the temps down will improve run time and motor life.

[Sneeck]

So gearing up can make your motor run cooler? Does it affect your run time?

[Batfish]

From what I'm learning, just about every motor has a "sweet spot" for RPMs and load/gearing, when it comes to brushless.
The more techinically inclined folks should jump in and correct me if I'm wrong here.
The RPM spot looks to be between 35,000 and 40,000 RPM. You should look at the voltage you're going to put through it, then find a kv that suits the voltage to get you in that RPM range.
The gearing I'm used to comes from running on a track, but it's a rule of thumb that can be used for bashing as well. Generally, you set your gearing so that you are only using full throttle at the longest straight, if you use full throttle at all. Full throttle should be reserved for short speed bursts. If you're geared so that you're running full throttle most of the time, you're making your motor spin much more than it needs to to get to the speed you're vehicle is moving. This will cause excessive heat since there isn't enough of a load on the motor. Add a tooth or two to the pinion or get a spur with several fewer teeth.
Oppositely, if your driving area/track never lets you get past half-throttle, you should adjust your gearing to where you can blip full throttle on occasion and drive about 3/4 throttle most of the time. Not being able to let your motor "stretch it's legs" keeps more of a load on it, which can cause excessive heat. Subtract a tooth or two from the pinion or add several to the spur.

I hope that makes some sense.

To sum it up, there are two steps:
1 - Find the motor that suits your voltage.
2 - Find the gearing that suits your driving area.

As far as how this affects your runtime, when you find that "sweet spot" for the RPM and gearing, you will have achieved the maximum runtime for your vehicle in your driving area/track.

[Sneeck]

That does make sence. I'm running 16/72 now in a bl emaxx, should I try 18 or 19/72? Cause it does make a hell of a lot rpm to get to speed.

[Re~Mix]

It never hurts to try things out, I might even gear up to 21/86 for a run and see what happens.

[Sneeck]

86? Where do I find a spur for an maxx that big? Or do you mean 76?

I'm giving 19/72 a try tomorrow at the track. I got up to 22t in pinion's so i'll try some. Thank's for the info.

[tonka truck]

temp gun on the way as well as more pinion gears to get right ratio,

thanks for your help

- sneeck just to clear the air, I was talking about bl xxxt in this thread.

[Sneeck]

Yes I understand, but I just wanted to know more about this gearing up stuff. Sorry for the hijack.

[Serum]

A sweet spot has got nothing to do with the RPM range where a motor works in.. basicly a high KV motor on less cells gets hotter than a low kv motor on much cells.

every motor knows a sweetspot, which is the direct result of the maxximum amount of energy the magnet on the rotor can translate in rotation. If the truck is overgeared, the magnetic field will jump over the rotation speed of the rotor, with which a big lost of energy will happen.

Small motors with lower quality magnets need to run on an higher RPM than bigger motors with high quality rotor magnets.

From what i experienced, that segmented rotors, from high end motors, such as the plettenberg bigmaxximum, get hottor when geared taller. though i don't know if this experience is adequate, because the bigmaxx has got a fan.. (normal things with a self powered fan, such as powerdrills, get less air when they most need it (motor rotates slower when they are fully loaded.. )

With the feigao types of motors they get very hot when ran with allmost no load..

A motor can get less hot when used on more cells too

A IR tempgun gives you the proper information with this tuning.


I am a personall fan of big motors (such as the 7XL) with a taller gearing. Dafni had some temp issues (at least his car had) he tried some setups, but he found his grale with the 7XL on 14 cells.

In germany they run the biggest available Lehner motor (2280) on 'only' 20 cells at 63 mph.

Asmll note though; smaller motors/rotors take less energy to reacht the top RPM than larger types.. (but the larger ones can produce more rotation energy when the spin up..)

[tonka truck]

hey sneeck, that's cool. I didn't think you were hijacking at all:). Gearing is one of the few areas that I am still grappling with in this hobby...:C:

Serum, so what your saying is that running the 4200 on 7 cells I will need to gear taller to account for its qucikness in getting to RPM range...?

Would this helpgear ratios

[Serum]

Try it, and use a tempgun... It's hard to tell what the sweet spot is, depending on weight and ratio.. Only way to find out is to try/observe and measure..

[Batfish]

Thanks, Serum - I was hoping someone would jump in with a more technical explanation :)

All things considered, though, you still need to make sure your vehicle is geared properly for your driving conditions, but it's great to know more specifics in regards to motors.

From your post, I believe we're talking about the same thing with the "sweet spot" of a motor - I just took a stab at guessing the RPM range had something to do with it. From just about all of the posts I've read where advice is given on which motor and which voltage to use, the kv x volts generally ends up in the 35000 to 40000 range, so I assumed that meant something. :)