Revo BL Conversion questions
 

Theres a few things I don't quite understand - everyone seems to steer away from lower turn motors because they generate more heat, but I thought surely if your gear say a 7XL and 10XL for the same speed all other things equal, they would draw the same amount of power?

Secondly, I have a Revo 3.3 and I own a Mamba Max (early one) my current purchase plan is a RCM Revo motor mount, and a 10XL and a heatsink to suit. I was hoping to run 5s2p A123 cells, and gear the truck for about 45mph - so about 20/36 or something - does that sound right? I plan to do heatsinking and cap mods the my MM - but is it fully necessary?

And Thirdly, if I don't lock it in second, is it putting undue stress on the motor when it snaps into 2nd in the direct drive design of the RCM mount (as opposed to the TeknoRC clutch system...)?

Cheers! I'm sure I'll think of more stuff to ask! :P

Welcome to the forums!

To reach the optimum max rpm of lower turn motors (high kv), you need lower voltage or they'll spin too fast and overheat. However, to get high power at the lower voltage, you need high current. Higher currents are inefficient and are harder on all electronics and batteries. So, most people prefer a higher voltage system coupled with a lower kv motor. Power is the same, but efficiency is higher, and higher-end batteries aren't as important.

5s2p A123s would be a good choice for an early MM. But, I would probably go with a 9XL instead since it is a closer match kv-wise for the voltage you want to use. For gearing help, you can calculate an estimate of top speed using these web pages.

I wouldn't worry about stressing the motor with first to second gear snap. The drivetrain is already in motion when it shifts, so it's not the same force as when it's trying to accelerate from 0mph. If anything, the stress may be too much for the arm and lever design of the clutch system. And, the Tekno clutch system only works from 0rpm up to the engaging rpm - it's still fully engaged during 1st to 2nd gear shifting so it has no effect.

So how come you say the 9XL is a closer match kv-wise? I figure if the trx3.3 is really revving out to 40k like it supposedly does - actually call it about 38k, because by the calcs in your link if it did get to 40k the revo top speed would be above the traxxas claimed 45mph, which is probably unrealistic anyway. So say about 38k, then that means at the stock 15/38 gearing, the tranny input is at about 15k.

Now as an aside, I noticed in the rcm store, the 9xl (or 10xl) kv is 1667 (or 1502) respectively, but on your calcs page, it's 1853 (or 1668) I'm not sure why the difference, I'm guessing the rcm are the loaded values? so assuming that - to get the same speed as the stock nitro setup, at 5s A123 = 16.5v nominal, the motor would be spinning at 27.5k (or 24.7k). So I guess the closest possible match would be to run the 9xl with 23/38 gearing, but would it be bad to say get the 10xl at run 22/40 to get a near similar result?

This is really for interest sake I guess, I was tossing up between the two motors anyway... depends what (if anything) is left in stock by the time I order :P

I would say a 9xl would be the best bet with your battery choice. You really want to run an xl motor at around 35k rpm. That seems to be the max speed to spin it to, any more and it just gets hotter. No need to compare the nitro gearing and the brushless gearing. I usually gear for 40 mph and it easily takes the nitro trucks.

The 9xl listed kv value in the store is not correct.

To get to 35k I would need to run nearly 19V! which I'm pretty far off... or did you mean that you dont want it up that fast? I'm confused :P

Is it even worth going A123? I haven't got any yet, I just thought it seemed like a good idea though because the high discharge ratings on them. my other option would be a 4s2p lipo, which I guess I might even have to go down to 8xl... I'd like to stick to the high side of things (high turns that is) incase I want to upgrade later...

You could run a 8xl, 5s a123 is similar to 4s lipo. I would personally go with a pair of trakpower lipos over the a123 cells. The a123 would require a 5s2p configuration if you want the voltage to stay up under load and decent runtimes too. I find that the stick pack sized trakpowers lit better than the a123 cells, maybe that is just me. The 8l is a good motor on 4s plus you are leaving in the 2 speed so that will help with the lower gering at takeoff!

At the same voltage (ie rpm), what would be the difference in having an 8XL and a 9XL geared for the same speed? would the 8 turn heat up more or something? this whole concept confuses me, I understand what BrianG said before, but what if the voltage is equal and you just change the gearing to make the speed of the truck the same.. surely the current draw would be the same and motors and electronics would heat up just as much?

Heat production appears to be based on two things. Motor effecientcy, and load. I feel that the feigao motors are best between 30k and 35k rpm. Inside of that rpm range gearing for 40mph seems to be the sweet spot for these motors (in a revo/maxx sized truck). As long as good quality batteries are used this wil be a good setup.

I prefer to run the motors near 35k rpm. This leads to lower gearing, and less load on the motor. Plus having a larger motor speed range and lower gearing seems to work better over that range. The 9xl seems to be a good 5s motor, I have one but have not tried it yet. It will do in my savage conversion on 5s. I have run the 7xl and 8xl on 4s and they both work fine for me.

Cheers thanks for the info. I was a little afraid of getting a lower turned motor because there seems to be a lot of people saying they are prone to running very hot. I understand that they are going to run a little hot being a lesser quality motor and all anyway..

If I have the older mamba max (yet to crack it open and check, but I'm pretty sure it is I bought it the day they released the 6900 motor) are they known to run on 5s ok? If i do the 'stage 1' mods that is..

I am holding out for MMM, but in the meantime I'm wondering if it'll work without frying it :) I read a post of yours (i think) elsewhere stating that most 4s setups wouldn't have a continuous draw over 80A, based on that I'm guessing a 5s1p lipo pack constructed from 4000mah cells (rated to 25C continuous, 30C burst) would give pretty respectable performance? being 5s lipo I guess I'd go for the 9XL - which would give about 34k rpm... and gear it from there (16/38 or 17/38 perhaps?)

I've read a bit on running 5s with the MM but nothing conclusive yet...I guess I should just buy the cells and try it, im pretty confident (and competent hopefully :wink:) at pack building so I could always just pull apart and make a 4s2p or something...hmm just speaking my thoughts now lol, any comments from those more experience appreciated :)

Well the mm definately works on 4s, not sure about 5s. Personally I like 5s setups, and I run a 2000kv motor in a revo with a tekno kit on 5s 5000mah 20c lipos. I get about 35mins of runtime and everything stays cool. I use a quark 80 airmaster, with the stock mech brakes. If you are staying with the mm I would go with a 8xl and 4s lipo, possibly a pair of trakpower lipos wired in series, 4s 4900mah. Real good lipos, I run them with both a 7xl and 8xl in revo based trucks.

I wish I wasn't so indecisive lol... now im even thinking of keeping my MM in the rustler and upgrading my esc to something HV to go 6s... bah so many options... well I'll wait to see if hobby city (formally united hobbies) will cancel my order and refund my $$ from a stupid impulse order i put in a while back (still hasn't even finished processing...) then spend it on some more useful stuff...

For 6s you can even use 4000 20c cells, 80amps at 22+ volts is quite a bit of power. Not sure what esc you will use, MGM? Sounds pricey!

I was thinking someone like this one that was posted in another thread. I'm usually weary of the cheaper chinese made stuff, but it has a 12 month warranty on it so maybe its ok? someone in one of them comments says its a OEM controller of something brand name... doesnt mention what though

Metalman has one of those escs and aqwut uses them too. They seem ok, I might give one a try myself, just waiting for them to get more feigao motors in!

In large lines your theory is right; with two different KV motors, at the same voltage and geared for the same speed;

but there is one detail; the lower the KV of a motor, the higher the torque. (which is needed to pull the sled with the lower final ratio)

all and all the startup-burst in current a lower wind (higher KV) motor makes is relative higher than the startup of a higher wind (lower KV) motor. the internal resistance of a lower turn motor is higher than the higher turn one.

Artur (griffinru) made a motor calculator. those motors you mention are in his database.

Why is that? I thought I higher turn motor would have higher resistance? since with more turns the length of wire is longer and thus so is the resistance...

Also I couldn't find GriffinRU's calculator, but I found BrianG's datasheet which includes the specs of those motors, which seems to agree with my understanding.

Or is the internal resistance something different to just the normal resistance that I'm thinking of?


Also, with lower kv motors being higher torque, wouldnt that cancel out the extra current burst because you would have to gear a the lower kv motor higher, so it would be using more torque but at a lower rpm = same power...?

sorry for the confusion;

a higher KV motor (lower turn) has a lower internal resistance than a lower KV (higher turn)

and yes, you are right; the internal resistance of a lower turn (less length of wire with a larger diameter) has a lower internal resistance. That's why it can take more current.

Like i said; it simple takes a larger current burst to spin up a high kv motor, than a low kv motor. gearing would compensate the difference in gearing at the same inertia/speed. but bottom line is that the startup burst will be higher of a higher KV motor.

But that's not all;

One of the reasons why these motors heat up is because they are less ideal for partial load; this seems to be due to eddy currents that disturb the movement of the magnet. with thin sliced magnets (segmented or sintered) the eddy currents on partial load are remarkable lower, and makes it more ideal for partial load. magnetism is a weird thing.

it also takes an ideal gearing per wind of motor to get
into its own sweet-spot (read the story in BrianG's signature ' is my setup too hot ' this would illustrate why you can't simple say 'geared for the same speed with a different KV'

I can see how it can be confusing for a person newly into brushless.

It comes down to this: it takes X amount of power to launch your vehicle with the acceleration and top speed you want. But you have to take gearing, weight, etc into consideration.

Power is volts X current. Looking at this formula, it's probably easy to see that for a given power, increasing the voltage decreases the current and vice versa. However high power is easier and more efficient to obtain with higher voltage and lower current. Lower currents are drawn when using higher turn motors (higher resistance).

Torque is dependant on motor kv and current. Torque (in ft-lbs) is 1352 / kv X amps / 192. So, decreasing kv or increasing current creates more torque. For the same kv, larger motors will generate more torque compared to a smaller motor. This is because they will draw more current (lower resistance).

For example, lets say you think you need 1500w to make the vehicle perform like you want. Let's say you have a 6s and 4s battery. 6s (22.2v) requires 67A. 4s (14.8v) requires 101A. The 6s option is better to use 6s than 4s because of the lower current. You generally pick the highest voltage your ESC and wallet will support. Then, you pick your motor kv based on that voltage. Some motors have a higher max rpm than others. Cheaper motors lose efficiency at higher rpms. Feigao/Wanderer motors typically operate best at around 35k max rpm. Better motors operate better at higher rpms (like up to around 58k for Neu).

You also have to pick the proper size motor (for torque) based on the size and weight of vehicle. Larger vehicles like buggies, truggies, MTs, etc work better with larger motors like XL series, or Neu 1512 or 1515 series, or an equivalent size motor.

So really, when designing a system, you start by picking the size motor you'll need (for the torque). Then, decide the voltage (higher is always better). Then, pick the motor kv based on that voltage by dividing the max rpm by the voltage you decided on.

Well put Brian, I could not have said it better myself. The cheaper motors also seem to prefer lower current draws (lower kv models). Case in point, a feigao 9l on 3s and a 15l on 5s. I have tried both in my 2wd emaxx, both geared for the same speed. The 15l does not get much more than warm, but the 9l runs hot, and requires good batteries to perform.

Yeah, along with the hard formulas, there IS a certain amount of user experience that helps when making motor selections.

Brian do you think that there is any specific reason that the lower kv feigao motors work cooler than the higher kv motors? Is it just the extra torque and lower current demands, or am I just seeing the battery perform better due to less load on it? Setups were almost identical, but I have about 80amps cont on the 5s, and more like 120 amps on the 3s.

Hey Linc,

a motor with lower kv is running cooler because the efficiency of electric motors is going down with rising amps. So if you got 120A on 3s, the efficiency will be around only 70%-75%, that means from the around 1100W you battery delivers only around 800W go to the pinion and 300W are going into heat. At 80A on 5s, the efficiency will be at around 85%, so from the 1300W your battery delivers around 1130W will go to the pinion and only 170W go into heat.

Thanks BL, makes sense to me!

I read somewhere (one of the mamba manuals perhaps?) something that basically said that brushless motors will use all the power you can throw at them. This agrees with my experience with brushless systems - it implies that either your batteries or your motor is going to be overloaded and overheating - and thats a full throttle - if your driving around half throttle then your esc will just cook. That follows my experience pretty well. If I put some crappy batteries (3s 3000mah 15C) in my rustler it doesn't go very hard and batteries will cook. If I put some relatively good (3s 4000mah 25C) batteries in, the batteries barely get warm, but the esc and motor cooks.

Does this mean you have to match your batteries with your esc/motor combo as well? and according to people here there is an optimum gearing as well, so then you can't change gearing or something will overheat? that doesnt sound like theres a lot of versatility in these systems? something just doesn't seem to add up right right... I don't want to spend $600 on a brushless conversion to have a truck that I have to stop and check temps and let cool down every 5 minutes.. I crave the power of brushless in my truck, but to me nitro engines (although very messy and expensive to run) seem so much more versatile and reliable (or have been in my experience anyway...)

Basically I guess is there a balance with brushless/lipo combos? If I run quality nimh packs in my rusty then nothing overheats and its got nice controllable power good speed and is fun to drive. If I put lipo's in, either it goes like a dog and the batteries cook, or it absolutely insanely overpowered to the point where you can't give it full throttle without it swapping ends or flipping, which usually results in the esc overheating trying to limit all the power from the batteries, or if u let the power through the esc, the motor cooking from dealing with all the power..

Sorry bout the long post guys :P I hope I'm kind of making sense.

There are limits to how many amps a motor can pull, but generally it should be fewer than the battery can supply, and the esc can handle, otherwise thats when things start to cook as you discovered. If you use the most efficient motors like Neus, and a 6s setup, you should get plenty of power yet run at a safe temperature. THe MM motors arent all that efficient -they seem to be similar to feigaos in design and construction- , so that would be why they run so hot, and suck the life out of whatever battery you hookup; they can pull a rather large current for their size, and thats down to the insanely high Kv ratings. The MM esc just gets stuck in the middle, and suffers from the high current being pulled through it.

If you switched the MM motor in your rusty to an S can feigao, or VXL even, you should see much improved temps and run times- use BrianG's calculator to determine your best voltage/motor selection choices; the same rules apply governing gearing/voltage/motor kv as has been previously mentioned.

As for batteries, much depends on your budget, but buy the best you can afford for the specific application (size, weight, capacity); an efficient system wont draw so many amps, so you can get away with a lesser capacity or C rating, but you want atleast 80amps on tap for any setup I believe (4000mah 20c, or 8000mah 10c etc etc)

OK So if I get a low kv motor (say 11S - 3200kv) that will sit at around 35k on my 3s it should run cooler, even If I gear it up to get some decent speed (25/76 or something in the rusty).

So back to the revo anyway :P If I get that cheapo turnigy 100A HV esc, say run 6s in the revo then I could perhaps find a feigao 11XL somewhere I should be sweet - it would rev out to about 33k, then gearing it say 18/38 I could pull 45+mph. What kind of burst currents would you expect from 6s in a revo? That esc is only rated to 110A burst, so I could fry it...either that or triple my esc expenditure :P

If you gear it reasonably, I doubt an 11XL will pull anywhere near 110A, even burst. I would expect 10A average (from runtime and batt capacity calculation) and 70-80A bursts. 80A @ 6s is the same power as 120A @ 4s.

I would expect it to be around something like that too.

120A bursts might be possible, but my guess is that it requires a tall gearing for ~70mph or so.

Well according to the speed calc the 18/38 gearing should top out at about 47mph (in a perfect world...one so perfect that my wild guess at tire ballooning is accurate :P).. hmm should be insteresting to see how this esc works.. only thing that worries me is there doesnt seem to be a settable lipo cutoff. "low" "mid" and "high", 3 settings for a 12s controller? somethin fishy goin on...

LVC is based on the single cell voltage (total voltage divided by number of cells), so low might be 2.8v per cell, mid would be 3v, and high 3.2v, at a guess...