Rear motor stall fwd only

[jmindek]

Howdy!

I am new to the hobby and over the last 6 months have built an XR10.
About 5 weeks ago, I finished soldering all my electrics together and have been running my rig as I have the chance.

Here's a description of my XR:
Stock XR10
Novak Rooster Crawler ESC
Integy 55t lathe (front axle)
Venom 55t (rear axle)
Solar 280oz servo (forget the model number)
GensAce 2S 1300mah
Deans Ultra plugs

My motors are wired to each other. Then, I tapped the ESC wires into one of the sets of motor wires.

I pretty quickly realized that my rear wheels were not turning when I would go forward.

This behavior happens about 99% of the time when going forward climbing and rolling in grass, small rocks, etc...

If I hold the rig in my hand and pull the throttle, both motors turn.
So I know that the connection between the motors and the ESC is delivering power.

If I pull the throttle with the front wheels down on the ground and leave the rear wheels in the air the rear wheels don't turn.
If I put the XR on my wood living room floor and pull the throttle I can watch the front wheels pull the rig across the floor dragging the frozen rear wheels.

I pulled the servo out to make sure it wasn't that. The rear wheels were still not turning.

It never happens going backward. I can do all the same tests as I mentioned above in reverse and this never happens. It climbs like a mo-fo in reverse!

I have searched around for threads regarding clod stall and delayed response. None of those seem to address the fact that it happens in one direction but not the other.

One popular suggestion for fixing clod stall is to step up to a 4PL with two ESCs.
I am going in that direction but not for a while as I don't have the cash for that right now.

Some other suggestions were to use a different pinion gear in the front than in the back.

Before I apply any fix, I want to make sure that it applies to my situation. Could you suggest why it happens in one direction and not the other and a suitable fix?

It's much appreciated. "thumbsup"

 

[JeremyH]

My motors are wired to each other.

Describe this a little more. Wired in series or parallel?

 

[jmindek]

It's wired in series. Motor #1 positive is wired to motor #2 negative. Motor #1 negative is wired to motor #2 positive.

I am not running a BEC.

What other details would you like?

 

[JeremyH]

Humm....that should be just fine. The lack of BEC will really just affect your servo during motor stall.

Did you calibrate the ESC to the radio when you first plugged it in? Is there any expo setting on your Rx or Tx? If so, are both curves the same?

 

[jmindek]

I bought the ESC used and did not calibrate it.

I read the instructions for doing so but haven't done it.

I hesitated because step 2 of 3 says to put the throttle at full brake. I didn't know what that was. If it was the neutral position why doesn't the instruction say neutral?

Do you know if "full brake" implies neutral?

 

[JeremyH]

Anytime you change the ESC or Rx, they need to be calibrated. Full reverse/brake implies pushing the trigger as far forward as possible. Neutral would be you not touching the throttle at all.

 

[jmindek]

But it says "full brake". What throttle position does that coorespond to?

FYI: Here are steps 6 through 8 of calibration:
6. Pull throttle to full-on
7. Push throttle to full brakes
8. Return throttle to neutral.

 

[JeremyH]

Full throttle/forward means pulling the trigger toward the handle....full reverse/brake means pushing the trigger away from the handle.

 

[jmindek]

Ah, brake and reverse are the same position. Told you I was new at this.

I just completed the calibration. Then put it on my hardwood floor, and gave a little forward throttle. The rear wheels still do not spin.

 

[JeremyH]

Ok...hummm.

Did it fix the issue of it only happening in one direction?

Have you tried pulling the motor away from the gears and making sure that you dont have any binding in the axle somewhere?

 

[redcard]

THe motors should be wired in parallel. Try wiring it like this: from your esc motor + terminal or lead: split into 2 leads, one to the rear motor and one to the front motor. then from your esc motor - terminal or lead: split into 2 leads, one to the remaining rear motor tab and the other to the remaining front motor tab.

run it with the wheels off the ground and make sure the wheels turn in the same direction. if not, then reverse 2 leads on either the front or rear motors.

 

[JeremyH]

THe motors should be wired in parallel.

That's how I've always wired single ESC setups, but I didnt see a problem with his setup...other than increased resistance over the longer wire which really should be minimal.

 

[JohnRobHolmes]

In series the motor with more load stalls out, it is very bad for crawling with a MOA rig. Parallel is the way to do it.

 

[AddictedtoRC1]

It could be something in your radio setting also. I am not sure what radio you are using, but make sure all the throttle epa's are set at max and any expo is set to 0 before calibrating it with the esc. Check trims and make sure they are at 0 also. Even sub trim if your radio is capable of sub trims.

 

[JeremyH]

In series the motor with more load stalls out, it is very bad for crawling with a MOA rig. Parallel is the way to do it.

Is that because of the high draw from the first motor in series?

 

[JohnRobHolmes]

When motors are in series they each drop a portion of the voltage and share the same current. Whichever motor is higher resistance will get a greater portion of the voltage. As a motor speeds up, the back EMF increases the effective resistance. Thus, as one motor gets slowed down it sees less and less of the voltage in the circuit. At full stall it practically has no voltage left to drive it while the spinning motor has it all. Everything is fine until one motor breaks traction, then the motor with traction has no motive force.


In parallel the motors both see the full voltage, and pull whatever current they need for the respective load. Doesn't matter if one motor is loaded and the other unloaded.




Sounds like there may be another issue going on though, like motor timing or a motor needing some care. It should act the same forwards and reverse unless the weight bias is causing better performance with the series motors in reverse.

 

[JeremyH]

Thanks for the explanation JRH! "thumbsup"

 

[jmindek]

No, it didn't fix the issue of happening only in forward.

I haven't tried pulling the motor away from the gears. It will be a day or two before I can do that.

Ok...hummm.

Did it fix the issue of it only happening in one direction?

Have you tried pulling the motor away from the gears and making sure that you dont have any binding in the axle somewhere?

 

[jmindek]

Hi redcard.

I soldered my motors to the ESC the way Brandon did.
He blogged a basic XR10 build here:
Axial Racing - Budget XR10 Build

I will throw your recommendation on the stack though of things.
Would someone else recommend changing my wiring too?

As a side note:

I don't think I understand the difference between parallel and serial. I actually meant to say that I have them wired in 'parallel'.

I will have to search the forums for that unless someone feels generous and would like to post the explanation and an example to illustrate that here.

THe motors should be wired in parallel. Try wiring it like this: from your esc motor + terminal or lead: split into 2 leads, one to the rear motor and one to the front motor. then from your esc motor - terminal or lead: split into 2 leads, one to the remaining rear motor tab and the other to the remaining front motor tab.

run it with the wheels off the ground and make sure the wheels turn in the same direction. if not, then reverse 2 leads on either the front or rear motors.

 

[jmindek]

Hi John, thanks for weiging in on the issue.
Someday, I will buy some of your motors. "thumbsup"

Until then:
My motors are "brand new".
I bought them at a LHS. I wanted two Integys but they only had one. My other viable choice was the Venom.

Is it possible that the Venom needs maintenance already?

I do have more weight in the front wheels than the back (which is normal right?). I will try and take some of the weight out of the front then report on the result.

When motors are in series they each drop a portion of the voltage and share the same current. Whichever motor is higher resistance will get a greater portion of the voltage. As a motor speeds up, the back EMF increases the effective resistance. Thus, as one motor gets slowed down it sees less and less of the voltage in the circuit. At full stall it practically has no voltage left to drive it while the spinning motor has it all. Everything is fine until one motor breaks traction, then the motor with traction has no motive force.


In parallel the motors both see the full voltage, and pull whatever current they need for the respective load. Doesn't matter if one motor is loaded and the other unloaded.




Sounds like there may be another issue going on though, like motor timing or a motor needing some care. It should act the same forwards and reverse unless the weight bias is causing better performance with the series motors in reverse.