Best "Direct to diff" motor for MT2 ?

[Guile]

Hi all,

I search what motor can be the "best" for an HPI nitro MT2 ?
With big torque and a good speed.

Can you help me ?

Thx for your advice

[MetalMan]

Good question. There are a few things that need to be considered when searching for the appropriate motor for a "direct to diff" conversion.

-overall RPM of the outrunner (kv x voltage)
-size of the motor (too small, and it will over heat, too large and it will be too heavy and require too much space)
-cost of motor
-whether or not it's rebuildable, since it pretty much needs to be
-if the shaft isn't the right diameter, would you be able to turn it down to the appropriate diameter?
-will this be used off-road? I don't think outrunners like dust/dirt, so an enclosure or shroud or something might be in order

That said, I'm the only person on this forum (so far) who has converted the MT2 with "direct to diff". Others have converted similar trucks (like the LS10), so their opinions would also be valuable.

This is the motor in my MT2:
http://www.hobbypeople.net/gallery/108521.asp
It's a big 'un, but I chose it since it would have enough power to get the truck up to 70mph and beyond with the appropriate voltage. The maximum voltage I have run through it is 10s A123, which produced a RPM that was too much for the truck (it took flight at ~60mph, and there was more throttle left). On 4s A123, it wasn't quite fast enough, but on 6s A123 it was pretty much perfect.

Since you didn't state what voltage you will run, I have come up with an example I think is suitable. On 3s Lipo, with regular 4" tall tires you will want a motor with a kv of about 900 to achieve 40mph. On 4s with the same speed and tires, the motor will only need to have 700 RPM/volt.

With 4s and the above-mentioned info., this motor should be more than enough:
http://www.hobbypeople.net/gallery/108520.asp

I use the KMS brand of outrunners myself (have 3 of them) since they are sold at the LHS I work at, and they're cheap in general, without seemingly-poor quality.

[Guile]

Ok! Thx for the answer !
I will run with the badlands 2.2" and the savage tires (gt tires). I plan to run with lipo 4s ou 6s, i don't know.
What do you think about the AXI 2826/10 ?

[MetalMan]

You might have a tough time with those larger tires. The MT2 diffs certainly aren't the strongest out there, and any tire over about 4" tall is going to stress out the diffs. My MT2 is the 18SS kit with the hardened ring and pinion gears, and even still they would click with enough torque running Proline Road Rages (2.2). This was fixed by getting some aluminum differential housings (reduce flexing between ring and pinion), but I'm not so sure aluminum diff housings would help the stock non-hardened diffs.

But if you do have 18SS diffs and aluminum diff housings, then you're at least on the right track. Badlands 2.2 are 5" tall, and the Savage tires are at least 6" tall. With Badlands and 4s Lipo and 40mph top speed, you need a motor with a kv of about 560. On 6s and Badlands for 40mph, 370 kv. Savage tires + 4s Lipo and 40mph requires a motor with 460 kv. 6s and Savage tires at 40mph, 310 kv.

You can of course go faster than 40mph, but the motor needs to be sized appropriately. I wouldn't push a 2826 faster than 40mph, but even the slowest of them (2826/12) has a kv of 760, which on 4s and with the Badlands tires would result in a theoretical top speed of 55mph. That pretty much means you need to bump up to a larger motor, which would be the Axi 4120 series:
http://www3.towerhobbies.com/cgi-bin...RCH=AXI+++4120
These motors have more suitable kv ratings, but even still might be a bit too fast based on your choice of tires and voltage. For Savage tires, the 4130 series would be best:
http://www3.towerhobbies.com/cgi-bin...ARCH=AXI++4130

[Guile]

Thks very much !
I think the axi 4120 is the better choice for me.

Another question, the A123 cells are betters than lipo ?
Wich center shaft do you use ?

PS : the graphit diff housing aren't good than the aluminium ?

[MetalMan]

A123 cells are physically more robust than Lipo, but they are heavier than Lipo. You can charge A123 faster than Lipo, and A123 is usually less expensive.

I haven't tried the graphite diff housings, but they would flex more than the aluminum, and generally wouldn't be as strong. Graphite is lighter than aluminum, though.

[cart213]

Regarding tire size vs. diff stress, I used to have an MT2 that I ran with Masher 2000's a lot. With the 18SS gears and aluminum housings, I didn't have any problems running the big tires.