Granite Audible Snap

[GuyFromCanada]

I'm taking bets on the audible snap I heard at the end of my bashing session. I was (perhaps foolishly) reversing, and hitting forward to try to get it to wheelie, then SNAP. Something still occasionally engages, and the wheels are connected to the drive shaft, so I'm thinking maybe something with the spur gear? I will find out soon.

As a side note, the motor was hitting 72 C or so...that's a little over 180 F. Just taking the shell off helped drop temps a few degrees, so maybe I will toss that fan on that I have sitting around.

 

[GuyFromCanada]

Whelp that didn't take long to see...I shredded my spur, and I guess my attempts to drive with it mangled deformed my pinion teeth as well....boo! But I had fun before that happened. I'm wondering if my slipper is too tight? Once it is all sealed up, how do you tell the slipper is "too loose"?

 

[Jerry-rigged]

On the brushed motor, I could adjust the slipper to find the 'sweet spot' On brushless, the spring is too weak, so it is either too loose, or locked.

 

[GuyFromCanada]

When you say the spring is too weak, you mean it doesn't have enough adjustment range? I can see the slipper has some visible wear but it seems pretty minor.

How should the ideal slipper behave, and how do you consistently tighten it each time a spur change is done?

 

[Jerry-rigged]

Yes, I mean the spring is too weak. The only way to get the slipper to hold power is to tighten it down so the spring is locked, then a bit more. But maybe that is just because I am running that huge 4074 BLX185 motor. Maybe with a more sane motor size, the spring will work OK.

To me - proper slipper adjustment means, you sometimes hear it slip a bit when accelerating hard with good traction - but not more than a second or so. If it is slipping for several seconds, you risk glazing the pads, which will make them slip more, or stick, which leads to melting the spur, or stripping it when it sticks.

 

[GuyFromCanada]

I'm surprised my diff input didn't break. I think I'll try going a little loose than i ihave been... The pads are cheap to replace when the rest of the drivetrain is considered...plus, I want to see if I can tell the difference between slipping and locked.

I find between the stock tires and the relatively light weight the wheels spend a lot of time looking for grip.

 

[Mentolio]

I adjust mine so that I can hear/see slippage for approximately two feet under hard/full acceleration on pavement from a stop. Slippage sounds like a high pitched whine, and you'll see when the slipper is fully engaged, as the car just suddenly starts accelerating really hard. I usually lean towards just a bit more slippage, as I feel this tends to be less strain/shock on the rest of the drive train, especially when landing jumps under throttle (which does occaisionally happen...).

I watched a video once of a pro racer adjusting the slipper on his 2wd race buggy. He set it so that if he held the rear wheels down on the table, full throttle would just lift the front tires off the ground. Of course, I don't remember what track surface he was setting it for, and a 4x4 likely won't behave like that, so the point is moot other than to demonstrate some slipping is generally considered necessary.

 

[GuyFromCanada]

I adjust mine so that I can hear/see slippage for approximately two feet under hard/full acceleration on pavement from a stop. Slippage sounds like a high pitched whine, and you'll see when the slipper is fully engaged, as the car just suddenly starts accelerating really hard.

Is it easier to adjust it from tight to loose or the other way around? I'll bring my tool with me and increment it in half or full turns or something like that.

My gear mesh on brushless seems to be pretty whiny but that is true well after the truck gets moving so I wouldn't think it is slipping for that long. Maybe I'll need to try setting it really loose to see extreme slipping.

Thanks for the tips and ideas.

 

[Jerry-rigged]

I watched a video once of a pro racer adjusting the slipper on his 2wd race buggy. He set it so that if he held the rear wheels down on the table, full throttle would just lift the front tires off the ground.

Yeah, I've seen that video, and done that years ago. Works great for brushed motors, and maybe for sensored Brushless, but does not work at all for sensorless brushless motors.

 

[GuyFromCanada]

Yeah, I've seen that video, and done that years ago. Works great for brushed motors, and maybe for sensored Brushless, but does not work at all for sensorless brushless motors.

Care to expand on why it wouldn't work for sensorless brushless?

 

[Jerry-rigged]

Lock rotor and a heavy load = just a lot of cogging. Unsensored motors pretty much always have issues with this, some so bad you can't even drive them at less than about 5mph. (This is also why 90% of crawlers use bushed motors.)

If the slipper is set pretty loose, the motor can overcome the cogging and spin the slipper. But once you tighten it down close to right, most unsensored motors just stall and cog.

 

[GuyFromCanada]

Clear as day, thanks. I'm using Smart Sense with the Mamba X and I didn't even think about the possibility of cogging at low speeds. It runs smooth as butter at slow speeds, so maybe I'm winning more than I thought with this ESC...nice.

 

[Jerry-rigged]

FYI,In case you don't know -

(From RCGroups FAQ's) -

1: What's the difference between brushed and brushless motors?

Simple answer would be, one has brushes, one hasn't. But that's a little too simple perhaps. In a brushed motor, the brushes deliver current to the commutator, a series of metal strips, that each are connected to a coil in the motor. When a battery is connected to the 2 motor terminals, current starts flowing through those brushes, to the coils, making these magnetic, causing the axle to spin. Reverse the polarity of the terminals, by switching the wires, and the motor starts turning the other way. This type of motor has simple wiring (2 wires, that's all) and is quite cheap usually. Performance can be pretty good too, there is just one catch. The friction between commutator and brushes causes these to wear down, and eventually the brushes need to replaced, or the motor needs a new rotor, as a worn commutator can't be replaced. It's possible to clean the surface of the metal patches of the commutator some times, for instance by using a special lathe tool, but as this removes material from the surface, at some point it's over. Also, the efficiency of this type of motor isn't very high, as the friction turns energy to heat.

Brushless motors don't have this problem, as there are no brushes and commutator to switch on and off the current to the motor's coils. Instead the motor is connected to the speed controller with 3 wires, and the switching of the respective coils is done electronically, in the speed controller. This makes a brushless speed controller more complicated, and also more expensive. But in return you get almost zero wear on the motor, just the bearings that may need replacement sooner or later, depending on track conditions, and the motor can run more efficient, converting more of the battery's power into motion, instead of heat.

17: What's the difference between sensored and sensorless brushless motors?

Short answer, one has sensors, but that still doesn't explain much, naturally. To make a brushless motor run in a certain direction, the ESC needs to activate the coils inside the motor in a specific order and frequency, to make the motor run smoothly. In a sensorless motor, the ESC uses some clever trick, to "guess" the position of the rotating part of the motor (rotor) and adjusts upon that. This system is more prone to error, especially when the car is coming from standstill, there is a change the motor will start to run choppily, as we will see covered in the next question, below this.

With a sensored motor, there are some small sensors inside the motor, and there is an extra set of wires between motor and ESC, for the sensor data. With that the ESC knows the exact position of the rotor, and can better control the motor. As a result, a sensored setup tends to run smoother, as there are no "bad guesses" about the rotor position. It may also feel like it responds faster, due to this.

Mind you, some sensorless setups can also be very smooth, but if you want the ultimate throttle control, sensored is the way to go.

 

[Mentolio]

As far as adjusting this particular truck’s slipper: I think it would be easier to go from tight to loose, rather than the other way around. I went loose to tight when I installed the brushless system, and it took longer than when I went back to brushless, tight to loose. May just be the newness of this truck to me, and the way it is adjusted (which I find to be a bit of a PITA).

 

[Jerry-rigged]

Yeah, it is a PITA. I leave the shaft out and adjust the slipper as 2wd, get it right, then reinstall the center driveshaft.

 

[GuyFromCanada]

Yeah, it is a PITA. I leave the shaft out and adjust the slipper as 2wd, get it right, then reinstall the center driveshaft.

Now I remember you saying that...good idea.

 

[GuyFromCanada]

Alright, so I made some good progress tonight and I'm updating all my threads. I definitely know what the slipping sounds like in extreme cases now. To tighten it up, I removed the drive shaft, attached the wrench they provide, apply moderate downward pressure (about half shock compression) and roll the truck forward until the wheels start skidding. I know it isn't very precise, but it seemed to work while still allowing a small amount of slippage.

I think the Arrma video for slipper adjustment is similar, but it wasn't clear from the video how tight it should be using that method.

Thoughts?

 

[Jerry-rigged]

Sounds like a good way to get to a close starting point, but you still need to run it. Different motor, different gearing will need different slipper settings.

 

[GuyFromCanada]

Yeah, I did a full session afterward and things sounded pretty good I think. That said, You're right about the spring design, it seems very easy to go from slipping to locked...

 

[larrys4227]

Per Arrma, the correct slipper adjustment (assuming for brushed) is to tighten all the way to snug. Then back off 1/2 turn. On my brushless 3900KV-22t pinion, I am backing off 3/4 turn for insurance. Going reverse and slamming the throttle forward, your just asking for trouble. I'm making the extra 1/4 turn on the slipper for that unusual occurance when I might be on the throttle when landing a jump. I want it to slip in that instance and take the brunt of the driveline force. Like my spur gear ..... my slipper is original.