Revo BL Conversion questions

[Sammus]

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

[BrianG]

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.

[Sammus]

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

[lincpimp]

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.

[Sammus]

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...

[lincpimp]

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!

[Sammus]

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?

[lincpimp]

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.

[Sammus]

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 ) 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 :)

[lincpimp]

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.

[Sammus]

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...

[lincpimp]

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!

[Sammus]

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

[lincpimp]

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!

[Serum]

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.