Outrunner question - RC-Monster Forums

lincpimp · **Outrunner question -** 12.11.2007, 07:20 PM

Heres one for you Sike, or anyone else with some experience with Axi outrunners. I have a hb lightning street, which is basically a stretched cassis 1/8 buggy with street tires. I am planning on doing a direct to diff conversion, but with a twist. I would like to run two outrunners, one for enach diff. I would connect them to the diff with short dogbones, or something similar. I would like to run 2 4120/14 motors and a pair of the OEM Setilon HV escs. I plan to run somewhere between 6s and 10s lipo.

Now, I am wondering if the motors would be up to the task. On 10s they would be spinning around 24k rpm. Is that too much for an outrunner? These are supposed to have 3 bearings, so I am guessing that they will hold a load better than a two bearing model.

This will be a strictly on road car. And I will mainly be doing high speed runs with it. So what do you guys think?

sikeston34m · 12.11.2007, 07:39 PM

Quote:

Originally Posted by lincpimp

Heres one for you Sike, or anyone else with some experience with Axi outrunners. I have a hb lightning street, which is basically a stretched cassis 1/8 buggy with street tires. I am planning on doing a direct to diff conversion, but with a twist. I would like to run two outrunners, one for enach diff. I would connect them to the diff with short dogbones, or something similar. I would like to run 2 4120/14 motors and a pair of the OEM Setilon HV escs. I plan to run somewhere between 6s and 10s lipo.

Now, I am wondering if the motors would be up to the task. On 10s they would be spinning around 24k rpm. Is that too much for an outrunner? These are supposed to have 3 bearings, so I am guessing that they will hold a load better than a two bearing model.

This will be a strictly on road car. And I will mainly be doing high speed runs with it. So what do you guys think?

I want to do a Hotbodies Lightning Stadium Pro Conversion! *drool* LOL

The 4120/14 works great with 4S to 6S. If you're hard set on the 4120 and plan to do HV, go with the 4120/18 or the 4120/20.

The 4120/14 would probably work on with higher voltage, all the way to the point where heat becomes an issue, BUT anything really high speed becomes hard to control.

Here's my suggestion, Do some math.
You need:

1. Tire Size
2. Diff gear ratio
3. Decide on your "target" operating voltage.
4. Decide on your desired top speed.

My Revo conversion with the 4120/14 tops out around 45mph. But remember, with outrunners, the lower the kv, the More torque it will have.

I'm spinning the Diff pinion gear at 10,000 rpms on 4S Lipo with a 5.75" tire.

If I were running HV, I could spin the Pinion gear at 10,000rpm with 294KV motor at 36 volts! Same top speed, but MUCH more torque. Much more acceleration!

Consider this. The AXI 4130/20 is 305kv. How much more power would it make, than the 4120 on 36 volts? The 4130/16 is 385kv. Pick your top speed and go figure.

The 4120's are good. There are some other kv options in the 4120 line also.

Ummmm, remember to get the cooling fan.

lincpimp · 12.11.2007, 11:47 PM

Ok Sike, I have ordered the cooling fan for my redcat, I will do the same if I go with the dual outrunner hb ls.

Here is the math from briang's speed calc with the 4120/14 on 6s:

Differential Ratio: 3.3076923076923075
Transmission Ratio: 1
Other Ratio: 1
Spur Tooth Count: 1
Pinion Tooth Count: 1
Total Voltage: 22.2
Motor KV: 660
Tire Diameter (inches): 4
Tire Ballooning (inches): 0
Total Ratio: 3.30769 : 1
Tire Circumference (inches): 12.57 inches (319.19mm)
Total Motor Speed: 14652 RPM
Vehicle Speed: 52.71 MPH (84.68km/h)

and with 10s:

Differential Ratio: 3.3076923076923075
Transmission Ratio: 1
Other Ratio: 1
Spur Tooth Count: 1
Pinion Tooth Count: 1
Total Voltage: 37
Motor KV: 660
Tire Diameter (inches): 4
Tire Ballooning (inches): 0
Total Ratio: 3.30769 : 1
Tire Circumference (inches): 12.57 inches (319.19mm)
Total Motor Speed: 24420 RPM
Vehicle Speed: 87.86 MPH (141.13km/h)

Just wondering if you think the motors can hold up to 25k rpm? I am sure that the rest of the drivetrain can.

sikeston34m · 12.12.2007, 12:43 AM

Quote:

Originally Posted by lincpimp

Ok Sike, I have ordered the cooling fan for my redcat, I will do the same if I go with the dual outrunner hb ls.

Here is the math from briang's speed calc with the 4120/14 on 6s:

Differential Ratio: 3.3076923076923075
Transmission Ratio: 1
Other Ratio: 1
Spur Tooth Count: 1
Pinion Tooth Count: 1
Total Voltage: 22.2
Motor KV: 660
Tire Diameter (inches): 4
Tire Ballooning (inches): 0
Total Ratio: 3.30769 : 1
Tire Circumference (inches): 12.57 inches (319.19mm)
Total Motor Speed: 14652 RPM
Vehicle Speed: 52.71 MPH (84.68km/h)

and with 10s:

Differential Ratio: 3.3076923076923075
Transmission Ratio: 1
Other Ratio: 1
Spur Tooth Count: 1
Pinion Tooth Count: 1
Total Voltage: 37
Motor KV: 660
Tire Diameter (inches): 4
Tire Ballooning (inches): 0
Total Ratio: 3.30769 : 1
Tire Circumference (inches): 12.57 inches (319.19mm)
Total Motor Speed: 24420 RPM
Vehicle Speed: 87.86 MPH (141.13km/h)

Just wondering if you think the motors can hold up to 25k rpm? I am sure that the rest of the drivetrain can.

They "probably" can.

If the ESC can keep up. The switching rate for 25,000 on an outrunner is like 100,000 rpm on an inrunner.

I believe they are balanced well enough, BUT I also believe that it comes down to a matter of keeping heat in check.

I wrote AXI another email asking about the best way to remove the stator from the bearing holder. Here's what they said:

"Dear James,

I must say that to remove the stator pack from bearing holder without damage is not easy but even mode difficult is to rewind the stator succesfully.

If you want to try to remove the stator pack than use heat. About 160 degrees of Celsius. At this temperature the glue can be released.

Usually this temperature dmage the grease in bearings so bearings need to be replaced.

In case you succesfully pull out the stator from bearings holder you can try to rewind. I really don't recommend this. Many people tried but no one succesfully."

Anyway, My point is, if you get it too hot, it's gonna come apart!

10S exceeds the ratings of the 4120/14. If you wanna "sneak" up on it, checking with a temp gun and taking baby steps, I'm sure you can find the point where heat really starts to be an issue.

I wouldn't just sock it on 10S and "see" though. You might be disappointed. If you are sure you want to run 10S, then I would choose a lower KV.

PS.......plan on changing that diff ratio to 4.3:1 Redo your figures with that and see what you come up with.

sikeston34m · 12.12.2007, 12:46 AM

Are you sure about that tire size?

lincpimp · 12.12.2007, 12:47 AM

Why the change to 4.3:1?

I know that you had to do it on the revo, but I am running a smaller tire than you, 4" versus 5.75" on your revo. My overall gearing to the road is lower than yours is on the revo. Or is there some other reason I am not thinking of (very likely!)

lincpimp · 12.12.2007, 12:48 AM

Quote:

Originally Posted by sikeston34m

Are you sure about that tire size?

Yeah, my onroad 1/8 tires are 4 inches tall.

sikeston34m · 12.12.2007, 01:14 AM

Oh ok, I'm sure that gearing will work then.

I was thinking the truggy tires would be 5.75"

Cool

lincpimp · 12.12.2007, 01:26 AM

I shall start gathering parts, and maybe make the motor mounts. Did you find dogbone cups that would fit a 6mm shaft? If so let me know, as that will be easier than try to drill them out!

sikeston34m · 12.12.2007, 01:30 AM

Quote:

Originally Posted by lincpimp

I shall start gathering parts, and maybe make the motor mounts. Did you find dogbone cups that would fit a 6mm shaft? If so let me know, as that will be easier than try to drill them out!

All the E maxx and Revo stuff is 6mm.

lincpimp · 12.12.2007, 01:33 AM

Thanks, I think I have some left over cups from my emaxx builds!

MetalMan · 12.12.2007, 02:41 AM

Just FYI, I'm spinning my KMS 4120/6 (610 kv) to ~20,000 RPM max. at 10s A123. I'm sure this motor isn't nearly as good quality as AXI motors, but it seems fine. But as sikeston34m said, the switching rate could become an issue. I think that for a 14pole outrunner (what I'm assuming you're using), 25,000 RPM would be to the ESC like spinning a 2pole motor at 175,000 RPM. Unless my logic is wrong, that will take quite an ESC!

lincpimp · 12.12.2007, 11:13 AM

Actually, I was planning on using the same esc that you are using. Do you think that it will hold up? Also have you got the startup better on your thru programming?

MetalMan · 12.12.2007, 02:44 PM

I don't know the specs for maximum 2pole RPM, but I guess it could handle another 5,000 RPM on a 14pole motor. The startup is really good with 4s A123, but at 10s A123 it can be rather rough, considering it is a 4wd truck set up for ~70mph. Also, if you read it in my thread, the ESC will make this weird crow-like screaming sound if the truck is moving along at ~1/4 throttle and I yank WOT. I don't know if it would do this with other motors and voltages or not, though.

lincpimp · 12.12.2007, 02:49 PM

For my purposes I can always give it a little push start! I may try a pair of HV85 castle controllers. Price is a bit more! on them, but I am sure the quality has to be better! The OEM esc is a great deal, I may purchase the motors soon and get the construction going, and the dual disk brakes too. Then I can pick and choose what esc I want.